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Cum inspectează Sobling calitatea materiilor prime pentru bijuterii?
Quality Inspection and Defects Analysis of Jewelry Raw materials
Introducere:
ewelry production requires various raw materials and auxiliary materials, and their Performance directly affects the quality and production cost of jewelry production. Therefore, it is necessary to strictly control the inspection of raw and auxiliary materials in production to avoid inputting unqualified materials.
Overall, the materials used for jewelry production mainly include precious metal materials such as Gold, silver, platinum, and palladium; Filled alloy materials for preparing various carat alloys; gemstone materials such as diamonds, rubies, sapphires, and jade; auxiliary materials used in multiple main processes such as jewelry master mold making, jewelry casting, setting stones, finishing and polishing, electroplating, some of which have a direct impact on the quality of jewelry products.
Tabla de conținut
Section Ⅰ Quality Inspection of Precious Metal Raw Materials
Conținutul principal al inspecției calității matriței principale include forma, dimensiunea, greutatea, structura, calitatea suprafeței, sprue de turnare etc.
Part 1. Pure Gold Nugget
Gold is one of the most widely used raw materials in producing precious metal jewelry. Enterprises generally purchase pure gold Nuggets from refining plants, precious metal suppliers, etc., to prepare materials such as 24K gold, 18K gold, and various carat gold.
1. The purity requirements of pure gold ingots
prepare materials such as 24K gold, 18K gold, and various carat gold.
The purity of pure gold nuggets is the basis for guaranteeing the fineness of gold jewelry. In 1999, the American Society for Testing and Materials (ASTM) issued the standard ASTM B562-95, “Standard Specification for Refined Gold,” and revised it in 2005 and 2012. The standard specifies the permissible range of impurity elements in pure gold nuggets, as shown in Table 4-1, which is the only standard used for high-purity gold Nuggets. Among them, grade 99.5% indicates a gold content of not less than 99.5%; 9995 indicates a gold content of not less than 99.95%, and so on.
For grade 99.5% pure gold, only the minimum gold content needs to be tested, which is the only purity level that requires measuring the gold content. The gold content is calculated using the difference method for other purity levels of pure Gold. In 9995 pure gold, five elements need to be tested, including silver, copper, and palladium, three elements commonly used in gold alloying. The other two elements are iron and lead, which are impurity elements that can seriously impact material processing. In 99.99% gold, many more elements must be tested, including arsenic, bismuth, chromium, nickel, manganese, magnesium, silicon, tin, etc. However, 99.995% of Gold, arsenic, and nickel have been removed.
Table 4-1 ASTM B562 maximum allowable impurity content of pure gold Nuggets
Metal content unit: x10-6
| Pure Gold Grade | 995 | 9995 | 9995 | 9999 |
|---|---|---|---|---|
| Argint Sterling | / | 350 | 90 | 10 |
| cupru | / | 200 | 50 | 10 |
| palladium | / | 200 | 50 | 10 |
| iron | / | 50 | 20 | 10 |
| lead | / | 50 | 20 | 10 |
| silicon | / | / | 50 | 10 |
| Magneziu | / | / | 30 | 10 |
| Arsenic | / | / | 30 | / |
| Bismut | / | / | 20 | 10 |
| Staniu | / | / | 10 | 10 |
| Crom | / | / | 3 | 3 |
| Nickel | / | / | 3 | / |
| Manganese | / | / | 3 | 3 |
Impurity elements in pure Gold are divided into three categories: metal, non-metal, and radioactive. Metal impurities are relatively easy to analyze. Platinum is a common trace element in pure Gold. Still, it is not listed in the standard, mainly because platinum is more valuable than Gold and does not hurt the manufacturing performance of Gold. Other platinum group elements such as rhodium, ruthenium, osmium, and iridium are also not listed. Because analyzing these elements is difficult, expensive, and of little practical use. Therefore, sometimes, a single element is chosen to reflect the amount of this group of elements, such as using palladium as an indicator of the platinum group elements. When the palladium content is high, other platinum group elements need to be tested; when the content is low, there is no need to test. Oxygen, sulfur, and chlorine are often used in some form for gold purification. They can form non-metallic impurities that remain in pure Gold, but these typical non-metallic elements are not listed in the standard. Radioactive impurities such as uranium and thorium can cause safety issues with jewelry, but their levels are generally negligible and are not listed in the standard.
Therefore, ASTM B562 only considers some metallic elements but ignores many others. To ensure the quality of the product, manufacturing companies may request that these elements be listed, as explicitly mentioned in the standard, “the buyer and seller may negotiate certain restricted elements.”
2. Impurity element analysis method for pure gold Nuggets
The gold content in pure gold Nuggets is determined by cupellation, the earliest analysis method. The accuracy of this method depends on multiple factors, including the testing environment conditions, the precision of the testing equipment, the application of the testing method, etc., which can result in significant variations in the results of the same sample within the same batch; the calibration value of the standard fluctuates wildly and is unstable; poor accuracy and precision, among other issues. The London Bullion Market Association (LBMA) requires refining gold assaying capabilities: when the test result is greater than or equal to 99.95%, the permissible error is ±0.005%; when the test result is less than 99.50% -99.95%, the allowable error is ±0.015%.
There are several techniques available for detecting impurity elements in pure Gold. A commonly used method is to dissolve the Gold first, then analyze the content of various elements using spectroscopic analysis methods, including atomic absorption spectroscopy or direct current plasma atomic emission spectroscopy. Inductively coupled plasma spectrometers can be used for solution analysis and, in some cases, can directly analyze solid samples without the need for dissolution. It has two advantages: it avoids the problem of undetectable impurity elements that do not dissolve, and the detection accuracy is not affected by experimental glassware and reagents. There are also other methods to avoid sample dissolution, such as using mass spectrometers and X-ray fluorescence spectrometers, among which mass spectrometers are more suitable for detecting trace elements in high-purity materials.
Although cupellation is the most accurate method for detecting gold content, it is almost impossible to use it to detect impurity elements in pure gold Nuggets because this method involves collecting precious metals from a specific sample, aggregating them into beads, and then comparing the weight of the beads with the original sample, limited to detecting the content of all precious metal elements. While cupellation can determine whether the gold content is 99.5% or 99. 9%, or even 99.99%, cannot identify which impurities are present and their respective quantities. Therefore, ASTM B562 only specifies the minimum gold content of 99.5% when cupellation is used; when the impurity content is higher, the content of the main impurity elements is detected, and the rest is assumed to be Gold. All major impurities must be considered; otherwise, the calculated gold content will be incorrect.
The above detection methods are mainly used to analyze the average content of impurity elements in pure gold nuggets to produce pure gold nuggets. Several detection technologies are more suitable for jewelry production enterprises, especially the scanning electron microscope (SEM) equipped with a dispersive X-ray spectrometer (EDS), which can focus on a specific part of the sample for local detection. For example, if jewelry has defects such as fractures or hard spots in certain areas, probes can be concentrated on these areas to analyze their composition. This is particularly practical because many harmful impurity elements tend to segregate to grain boundaries, lattice distortion sites, etc., resulting in much higher impurity element content at those locations than the average, which may lead to product quality issues. Therefore, jewelry production enterprises need to pay attention to the gold content of pure gold nuggets and be aware that some trace impurity elements may segregate during the casting process, leading to very high local content.
[Case 4-1] Analysis of the composition of pure gold nuggets.
Randomly select pure gold Nuggets produced by different refining manufacturers and use a glow discharge mass spectrometer for detection, analyzing 17 kinds of metal elements; the results are shown in Table 4-2.
Table 4-2 Analysis results of pure gold Nuggets produced by different precious metal refining manufacturers
Manufacturers #1-8, Metal content unit: x10-6.
For samples produced at different times from the same refining plant, impurity element contents were analyzed and detected, as shown in Table 4-3.
Table 4-3 Analysis results of different batches of pure gold Nuggets produced by the same refining plant
Metal content unit: x10-6
The purity threshold required by the reference standard is that only 8 out of 9 refining plants meet the standard requirements, and one company’s product needs to be qualified, containing 200 x10-6 impurities. Silver is the main impurity, much higher than other impurities; for 99.99% pure gold, the silver content ranges from 20 x10-6 to 70 x10-6; for 9995 gold, silver reaches 120×10-6, other elements less than 10 x 10-6, followed by iron and copper, about 5 x10-6, lead about 1 x10-6, and the remaining about 1 x 10-6 elements include palladium, silicon, platinum, etc. The impurity element content in pure gold Nuggets produced by the same refining plant fluctuates more or less at different times. Therefore, jewelry companies should prioritize choosing refining companies with good qualifications when purchasing pure gold nuggets.
3. The impact of impurity elements in pure gold nuggets
Some impurity elements such as lead, bismuth, and arsenic in pure gold Nuggets will seriously deteriorate the Performance of Gold. In contrast, other components, such as silicon, iron, etc., sometimes also bring harmful effects.
3.1 Lead
[Case 4-2 ] Brittle fracture of 18K White gold jewelry
Descrierea defectului:
A specific jewelry company has produced 18K White gold jewelry for many years. During a certain period, there were batch quality problems. After the jewelry was cast and formed, it would break with slight force during the setting or inlaying process, with the fracture morphology as shown in Figure 4-1. This problem had yet to occur before. The factory tried various solutions, including replacing the Filled alloys, changing the Sprue, adjusting the pouring temperature, etc., but the problem needed to be more effectively solved.
Production investigation:
From the morphology of the fracture, the casting does not have obvious shrinkage holes or looseness, indicating that the fracture is not caused by insufficient density reducing the strength; the fracture surface shows no malleable deformation, presenting a typical brittle fracture. Therefore, the production process conditions were investigated. The factory used precision casting with gypsum molds; the ring had two sprues, the gypsum temperature during casting was 650℃, the metal liquid pouring temperature was 1040℃, and the gypsum mold was air-cooled for 15 minutes before quenching. When smelting the ingredients, 50% old Gold and +50% new Gold were used, with the old Gold being used for the third time. For the casting of K white gold jewelry, the above production process conditions used by the factory are relatively standard and should not cause batch brittleness. It is speculated that harmful impurity elements may have been mixed into the metal material.
Upon inspecting the source of the new Gold, it was found that due to urgent production needs earlier, a small amount of pure gold nuggets was purchased from a small refining merchant, accompanied by an X-ray fluorescence spectrum analysis result showing that the purity of Gold reached 99.99%. As XRF is a surface analysis and trace elements are challenging to analyze accurately, it is recommended that the factory extract a small amount of pure gold samples for cupellation analysis at an analysis center. The results showed that the lead content in the pure gold Nuggets reached 110 x10-6.
Cause analysis:
Lead is one of the most harmful elements in Gold, directly affecting its machinability. As early as 1894, it was found that insufficient lead content would make Gold brittle. This is because lead forms intermediate phases such as Au2Pb AuPb2 AuPb3 in Gold, which are phases with low melting points and high brittleness, significantly deteriorating the metal’s processing performance. The gold-lead alloy equilibrium phase diagram in Figure 4-2 shows that when the lead content reaches a certain level, a certain composition of the intermediate phase will form. In actual production processes, even if the lead content in Gold is minimal, due to the low solubility of lead in Gold and its much lower melting point than Gold, lead is prone to segregation during the cooling and solidification process, being rejected by the grain boundaries and forming clusters.
When the lead content in the clusters reaches a certain amount, it will create a lead-rich gold-lead intermediate phase, reducing the material’s malleability. With the increase in lead content, more gold-lead intermediate phases will be formed. When the lead content reaches 600 x10-6, copper-containing and pure gold alloys cannot be rolled. Many jewelry companies consider 50 x10-6 as the upper limit of acceptable lead content
3.2 Bismuth
Bismuth is also one of the most harmful elements in Gold, and its impact on its mechanical processing performance is comparable to that of lead. Figure 4-3 is the gold-bismuth binary alloy phase diagram. Bismuth has almost no solubility in Gold. During the cooling and solidification process, bismuth will segregate and accumulate at the grain boundaries, forming gold-bismuth intermediate phases, significantly affecting the malleability of Gold and causing products to be prone to brittle fracture.
3.3 Iron
The role of iron in Gold should be viewed in two aspects. On the one hand, it can serve as an alloying element. Iron-containing gold alloys have been used in Europe. By combining with other alloying elements, gold alloys formed can achieve a beautiful blue color effect when oxidized at medium temperatures for a long time. In recent years, iron has also been tried as a bleaching element to produce K white gold materials.
On the other hand, iron significantly affects Gold’s casting performance. Figure 4-4 is the gold-iron binary alloy phase diagram. From a thermodynamic perspective, iron can dissolve in pure Gold, but due to its significantly higher melting point than pure Gold, it causes
It is not easy to dissolve into Gold. Suppose Gold contains 100 x10-6 of iron. In that case, it is difficult to achieve uniform composition, resulting in segregation in the casting, leading to the so-called “hard spot” defect, as shown in Figure 4-5.
(From David J Kinneberg et al., Gold Bulletin, 1998)
3.4 Silicon
From Figure 4-6, it can be seen that silicon is almost insoluble in Gold. When the silicon content exceeds 200 x10-6 , Au-Si eutectic silicon phases will form at the grain boundaries, as shown in Figure 4-7, with a melting point of only 363℃, very brittle, and prone to hot cracking. The embrittlement effect of silicon is related to the alloy’s total amount of Gold and silver. With the increase in the total amount of Gold and silver, the alloy’s flexibility decreases, and the brittleness increases when the silicon content exceeds a particular critical value. In other words, as the fineness of gold increases, the allowable amount of silicon decreases. When the nominal silicon content in 14K gold exceeds 0.175wt%, silicon-rich phases will appear at the grain boundaries. When the amount of silicon exceeds 0.05wt% in 18 KY, it is prone to brittleness.
3.5 Iridium
[Case 4-3 ] Hard Point Defect in 18K White Gold Ring
Defect Description:
Hard points were found on the surface during polishing, appearing as large single grains or nest-like small grain clusters. The workpiece is complex to polish brightly, with many scratches, as shown in Figure
Production Investigation:
The factory uses two forming methods, casting, and stamping, both of which have experienced similar defects in their products. The defects appeared not only in recycled materials but also in newly mixed gold alloys. It can be inferred that the defects are not related to the forming methods, and the problem should lie in the metal material or gold melting method. Upon investigation, it was found that the Gold was melted using a melting furnace with inert gas protection, and the gold melting temperature was adequately controlled, ruling out the melting method as the leading cause.
(from David J Kinneberg et al., Gold Bulletin, 1998)
The cause should be found from the method of metal materials. Upon inspecting the pure gold Nuggets and Filled alloys used for metal material preparation, it was found that the Filled alloy materials used were from the original inventory, which had been relatively stable and had not encountered such problems before, whereas in terms of pure gold nuggets, a recent purchase of a batch of pure gold nuggets led to the issue arising after using this batch of Gold. Samples were taken from this batch of pure gold nuggets and analyzed using chemical analysis methods, revealing a relatively high iridium content, reaching 0.03wt%
Root cause analysis:
Iridium has a very high melting point, and if not handled adequately during smelting, it won’t be easy to dissolve uniformly in the gold liquid. Moreover, iridium has a very low solid solubility in Gold, even lower in the liquid state. High-melting-point iridium can preferentially precipitate and aggregate during solidification, leading to uneven distribution. Due to iridium’s significantly higher hardness than Gold, hard points or clusters of hard points are formed when they reach the surface, causing scratches and comet tails during polishing.
4. Gold purification
When excessive harmful impurities appear in pure gold or gold alloy materials, the materials must be considered for purification. There are various methods for purifying Gold, and their primary processes and characteristics are as follows:
4.1 Amalgamation method.
This is a relatively ancient purification method. Amalgamation is the process of mixing Gold, mercury, and water and continuously grinding until no gold particles are left, forming a metallic compound of Gold and mercury. Sulfur powder is mixed with the combined Gold and ground, then heated and roasted in the air to evaporate excess mercury. Base metals first form metal sulfides and later metal oxides. After repeating these operations several times, the material is melted into Nuggets using borax as a flux. Base metal oxides react with borax to form low-melting substances that float on the liquid surface, while pure Gold settles at the bottom.
This method is suitable for processing coarse gold particles captured by mercury. The purity of Gold depends on the thoroughness of amalgamation and sulfurization. When processed well, the purity of Gold can reach above 99%. Due to the use of the toxic element mercury, this method has been largely eliminated.
4.2 Aqua regia purification method.
The crude Gold to be purified is dissolved in aqua regia, and a small amount of hydrochloric acid is heated and added multiple times until no yellow gas is produced. Adjust the pH value, and add reagents such as sodium bisulfite, oxalic acid, or metals like zinc powder or copper. After the production of sponge gold, pour out the liquid, rinse it several times with deionized water, then heat it with sulfuric acid for half an hour, rinse again with deionized water, wash with nitric acid for half an hour, and finally rinse with deionized water. The purified sponge gold can be cast into Nuggets after drying, with a purity of up to 99.95%.
4.3 Electrolysis method
This is a more commonly used method. It uses Gold as the anode, pure Gold or stainless steel as the cathode, and concentrated hydrochloric acid as the electrolyte. Under the action of the electric field, Gold is deposited and purified on the cathode, with a purity of up to 99.95%. However, this method is relatively slow, has a long working time, and requires timely electrolyte replacement during production.
4.4 Granulation by Dropping Method
This is also a commonly used technical method. First, silver is added to the crude gold material to be refined, with a ratio of about ( 2.2-3.0):1 . They are melted together, using borax as a slag-making agent. After the Gold and silver are melted and stirred evenly, they are poured into cold water to obtain granules of a specific size. The granules are placed in a beaker; nitric acid is added to remove silver; the nitric acid silver is poured off after the reaction, and concentrated nitric acid is added and boiled for 40 minutes; this operation is repeated, then rinsed multiple times with hot water until the liquid is free of white color, rinse several more times to obtain a pure gold powder. The purity can reach 99.8% or more.
4.5 Ammonium Chloride Method
This method is more suitable for purifying gold powder. Larger gold pieces must first be granulated into small particles or pressed into thin sheets to accelerate the chlorination rate.
First, use methods such as hydrochloric acid + Table salt + hydrogen peroxide, hydrochloric acid + Table salt + chlorine gas, or hydrochloric acid + table salt + perchloric acid to dissolve Gold into AuCl3 liquid, then heat the solution to remove oxidizing gases. Remove non-metallic substances, wash the residue with water several times, adjust the pH value to 13 with ammonia, use reducing agents such as formaldehyde to reduce Gold, and heat the solution for nitrate evaporation. The purity achieved by this method can reach 99.95%.
Part 2 Pure Silver Nugget
Pure silver is divided into three grades according to its chemical composition: IC – Ag99.99, IC – Ag99.95, and IC-Ag 99.90.
Table 4-4 Range of permissible impurity elements in pure silver Nuggets (Unit: %)
| Silver Grade | Ag | Cu ≤ | Bi ≤ | Fe ≤ | Pb ≤ | Sb ≤ | Pd ≤ | Se ≤ | Te ≤ | Total Impurities ≤ |
|---|---|---|---|---|---|---|---|---|---|---|
| IC - Ag99.99 | 99.99 | 0.003 | 0.0008 | 0.001 | 0.001 | 0.001 | 0.001 | 0.0005 | 0.0005 | 0.01 |
| IC - Ag99.95 | 99.96 | 0.025 | 0.001 | 0.002 | 0.015 | / | / | / | / | 0.005 |
| IC - Ag99.90 | 99.9 | 0.05 | 0.002 | 0.002 | 0.025 | / | / | / | / | 0.1 |
Same as Pure Gold, lead, bismuth, arsenic, etc., are also very harmful elements in pure silver. Figures 4-9 and 4-10 are the silver-lead alloy phase diagram and the silver-bismuth alloy phase diagram, respectively. Their solid solubility in pure silver is minimal, making them easy to crystallize.
Same as Pure Gold, lead, bismuth, arsenic, etc., are also very harmful elements in pure silver. Figures 4-9 and 4-10 are the silver-lead alloy phase diagram and the silver-bismuth alloy phase diagram, respectively.
Their solid solubility in pure silver is tiny, and they tend to polarize at grain boundaries, forming low melting point intermediate phases that result in brittle materials. Silicon has almost zero solid solubility in pure silver, as shown in Figure 4-11, and is mainly used as an antioxidant element in silver alloys, but when the silicon content exceeds a certain level, it will cause material brittleness.
In the quality inspection of pure silver, detecting trace impurities is the most critical measure of pure silver quality. However, using atomic absorption or spectrophotometry, the national standard specifies the analysis of only lead, copper, iron, selenium, palladium, antimony, tellurium, and bismuth. This method can only determine impurities one by one, and the procedure requires multiple steps, making the analysis complex and time-consuming. In international trade, the detection requirement for trace impurities in pure silver is 23 kinds. Therefore, some testing institutions have attempted to use Inductively Coupled Plasma Atomic Emission Spectrometry to continuously determine impurity elements in pure silver, achieving good results. This method can provide reasonable detection limits, minimal matrix interference, a wide linear dynamic range, simplicity, accuracy, and reliability.
Part 3 Pure Platinum Nugget
The international standard “ASTM B561:2005 Refined Platinum Specifications” specifies pure platinum’s purity and impurity element requirements. The standard “GB/T1419-2004 Sponge Platinum” also adopts similar provisions, as shown in Table 4-5.
Lead, bismuth, and other impurity elements are very harmful. Their solid solubility in pure platinum is almost zero. During smelting and solidification, they are easy to aggregate at grain boundaries, forming low-melting brittle intermediate phases, seriously deteriorating the processing performance of the alloy.
Table 4-5 Range of permissible impurity element content in pure platinum Nuggets (Unit: %)
| Platium Grade | SM-Pt99.99 | SM-Pt99.95 | SM-Pt99.9 | |
|---|---|---|---|---|
| Platium content ≥ | 350 | 90 | 10 | |
| Impurities ≤ | Pd | 0.003 | 0.01 | 0.03 |
| Rh | 0.003 | 0.02 | 0.03 | |
| Ir | 0.003 | 0.03 | 0.03 | |
| Ru | 0.003 | 0.003 | 0.04 | |
| Au | 0.003 | 0.01 | 0.03 | |
| Ag | 0.001 | 0.005 | 0.01 | |
| Cu | 0.001 | 0.005 | 0.01 | |
| Fe | 0.001 | 0.005 | 0.01 | |
| Ni | 0.001 | 0.005 | 0.01 | |
| Al | 0.003 | 0.005 | 0.01 | |
| Pb | 0.002 | 0.005 | 0.01 | |
| Mn | 0.002 | 0.005 | 0.01 | |
| Cr | 0.002 | 0.005 | 0.01 | |
| Mg | 0.002 | 0.005 | 0.01 | |
| Si | 0.002 | 0.005 | 0.01 | |
| Sn | 0.002 | 0.005 | 0.01 | |
| Si | 0.002 | 0.005 | 0.01 | |
| Zn | 0.002 | 0.005 | 0.01 | |
| Bi | 0.002 | 0.005 | 0.01 | |
| Ca | - | - | - | |
| Total Impurities ≤ | 0.01 | 0.05 | 0.01 | |
Note：
a. The control limits and analysis methods for elements and volatile substances not specified in the Table shall be determined by mutual agreement between the supplier and the demand side.
b. Ca is a non-mandatory test element.
Part 4 Inspection Methods for Precious Metal Materials
After the jewelry company purchases precious metal materials from the market, it needs to conduct an incoming inspection, and the inspection method is shown in Table 4-6.
Table 4-6 Inspection methods for purchased precious metal materials
| Inspection items | Metoda de inspecție | Inspection content | Inspection tool | Acceptance criteria |
|---|---|---|---|---|
| Invoice | Verification of supplier information, model number, identification and amount on invoices | Full inspection | Manual verification | Consistent with contract requirements |
| Packaging | Check if the packaging is intact | Full inspection | Sensory examination | In accordance with contract requirements |
| Weight | Detecting precious metal materials Weight | Full inspection | Electronic scale Weighing | Implement standards "Quality Tolerance for Precious Metal Jewelry Measurement" Regulations |
| Conținut | Detect precious metal content | Full inspection | Use fluorescence Spectrometer or chemical Analysis method | Execute standard of Gold Chemical Analysis Method, Silver Chemical Analysis Method, Determination of silver content by silver chloride precipitation-flame original Atomic absorption spectrometry method》, "Jewelry Gold Content Determination X-ray Fluorescence Spectroscopy" |
Section Ⅱ: Quality inspection content of filled materials
Inlaid jewelry, various carat gold alloys, silver alloys, platinum alloys, and palladium alloys jewelry have always accounted for a large proportion. These alloy materials are prepared from pure precious metals and other elements to form intermediate alloys. For example, 18K gold is prepared from pure Gold and intermediate alloys, commonly known as Filled materials. The quality of Filled alloying directly affects the quality of jewelry products. Currently, jewelry manufacturers use a variety of Filled alloying materials, and the Performance of Filled alloying materials produced by different suppliers sometimes varies greatly.
Even if the same supplier provides Filled alloying materials, performance fluctuations often occur, affecting production. Therefore, companies must inspect the quality of a new Filled alloying material when choosing it. Performance evaluation mainly includes physical properties, chemical properties, mechanical properties, processing properties, safety, and economy. Taking K gold Filled alloying as an example, the specific content is as follows.
Part 5 Physical Properties
K gold jewelry belongs to the category of precious metal jewelry, and it also emphasizes the effects of surface decoration. Therefore, paying attention to and rationally designing material physical properties is essential, mainly reflected in aspects such as density, color, magnetism, and melting point.
5.1 Density
The selection range of Filled alloying elements for gold jewelry is wide. Each alloying element has its atomic mass and corresponding density. Different alloy compositions will have different densities. For example, in a gold-silver-copper-zinc alloy, the density of silver is 10.5g/cm3, and the density of zinc is 7.14g/cm3. When zinc is used instead of silver, the density of the alloy will decrease. For a piece of jewelry with a fixed volume, the alloy’s weight is reduced, and the same quality alloy can use less Gold.
5.2 Color
As jewelry, color is an important physical property. Jewelry gold alloys are generally divided into colored Gold and white gold alloys based on color. By changing the alloy composition ratio of K gold, materials of different colors can be obtained. The most commonly used colors of K gold include K yellow, K white, and K red series. Recently, a few unique colors of K gold materials have also been developed.
Visual estimation is a simple method for estimating and describing the color of alloys. Still, this method relies on the subjective perception of the naked eye, making it difficult to clearly explain the various hues of gold colors, such as yellow, green, white, and red, in language. To quantitatively describe the color and color stability of gold alloys, the jewelry industry has introduced the CIELab system for color measurement of alloys based on chromaticity principles. This system uses three coordinates L*, a*, b* to describe colors, which are stable and reliable. The system is also an effective tool for quantitatively describing the discoloration of alloys. To determine and compare the colors of alloys more simply, some countries have established color standards for gold alloys and corresponding color charts for comparison. Switzerland, France, and Germany successively established 18K gold color standards: 3N, 4N, and 5N. Later, Germany added three standard colors for 14K Gold: ON, 1N and 8N. Their positions in the color coordinate system are shown in Figure 4-14.
【Case 4-4】The whiteness difference of 18K White Gold
Problem description:
Complaints were received from customers about 18K white gold jewelry exported by a particular factory. After wearing it for some time, the local plating was worn off, exposing the yellowing metal base, which had a significant contrast with the color of the plating, and a return was requested.
Reason analysis:
White Gold, as a substitute for platinum, requires good whiteness. Therefore, most white gold jewelry is rhodium-plated on the surface. Rhodium plating is usually very short, commonly known as “flash plating,” and forms a skinny layer. After a period of use, it is easily worn off, revealing the original color of the base metal. In many cases, there is a vast contrast between the color of the metal body and the color of the plating. When determining the metal material, the supplier and the demand side only generally specify it as 18K white gold. In alloy color, a qualitative description method is used, which can easily lead to disputes between jewelry companies and customers due to inconsistent judgments. In response to this common issue, MJSA, and the World Gold Council
In cooperation, after using the CIELab color coordinate system to detect the color of 10KW, 14KW, 18K White gold samples, the definition of the yellowness index of K aur alb was uniformly stipulated using the ASTM yellowness index, defining that the yellowness index of “K White gold” should be less than 32, and dividing K White gold into 1st, 2nd, and 3rd grades according to color, as shown in Table 4-7.
Table 4-7 White level of K white gold
| Color Grade | Yellowness index YI | Whiteness level | Plating rhodium |
|---|---|---|---|
| Nivelul 1 | YI< 19 | Very white | Not needed |
| Nivelul 2 | 19 < YI < 24.5 | White is acceptable | Poate fi placat sau nu |
| Nivelul 3 | 24.5 < YI < 32 | Slabă | Must need |
This grading system allows suppliers, manufacturers, and retailers to use quantitative methods to determine the color requirements of K White gold. When YI exceeds 32, it cannot be called K White gold.
Since nickel and palladium are the main bleaching elements, the higher their content, the whiter the alloy’s color. However, the corresponding production difficulty or cost will increase. Therefore, jewelry companies often need to consider the issues of color and processing performance comprehensively when choosing Filled alloy materials.
5.3 Magnetic
As precious metal jewelry, K gold jewelry generally wants the alloy to exhibit something other than magnetism to avoid consumer doubts and complaints about the authenticity of the material.
【Case 4-5】18K white gold ring with magnetism
Problem description:
A jewelry company produced a batch of 18K white nickel rings, which were returned and complained about because the rings have strong magnetism.
Cause analysis:
In nature, iron is a well-known metal element with magnetism. In addition, there are a few other elements with magnetism, such as cobalt, nickel, and gallium. Nickel is commonly used as a bleaching element in White Gold. The addition of nickel sometimes makes the gold alloy exhibit a certain magnetism. Precious metal jewelry with magnetism often faces consumer doubts and complaints, so efforts should be made to eliminate its magnetism.
Whether a substance exhibits magnetism not only depends on its composition but also its microstructure. Sometimes, with the same elements but different structures or at various temperature ranges, there may be differences in magnetism. The gold-nickel alloy phase diagram shown in Figure 4-15 can illustrate this point.
Figure 4-15 Magnetic transitions of gold-nickel binary alloy
The phase diagram shows that the gold-nickel alloy is a single-phase solid solution below the solidus line and above a specific temperature, which is rich in gold ɑ1 and rich in nickel ɑ2, both non-magnetic. A two-phase region begins to appear when the single-phase solid solution region is slowly cooled to a specific temperature. When the temperature drops to about 340℃, a magnetic transition occurs. When the composition of nickel-white Gold falls within the range of magnetic transition, the alloy may exhibit magnetism.
Due to the slow cooling process of nickel K White gold after casting and the component segregation generated during casting, a two-phase structure will appear under the casting conditions and undergo a magnetic transformation to produce magnetism.
Solution:
Under the condition of unchanged alloy composition, to eliminate the magnetism of nickel K White gold, it is necessary to control the alloy structure, that is, to obtain a non-magnetic single-phase solid solution through heat treatment. The cast structure can be heated to the single-phase solid solution zone, kept at this temperature to achieve a certain degree of uniformity in composition, and then rapidly cooled (such as quenching) the alloy to maintain the single-phase solid solution stable at high temperature to room temperature, thereby eliminating the magnetism of the alloy.
5.4 Melting point
The gypsum mold casting process mainly produces k gold jewelry. Due to the poor high-temperature thermal stability of gypsum, thermal decomposition will occur when the temperature reaches 1200℃, releasing SO2 gas, causing casting porosity. Incomplete calcination of gypsum mold leaves residual carbon in the mold, or severe oxidation of the metal liquid forms a large amount of copper oxide, significantly reducing the decomposition temperature. Therefore, to ensure the safety of gypsum mold casting, it is necessary to control the alloy’s melting point. Generally, the melting points of K yellow gold and K red gold are around 900℃, so there will be no significant problems with gypsum mold casting. However, for K White gold, due to the use of high-melting-point nickel and palladium as bleaching elements, the alloy’s melting point is higher than that of K yellow gold and K red gold, posing a risk of gypsum mold thermal decomposition. When the nickel and palladium content is very high, gypsum mold cannot guarantee production quality, and expensive acid-bonded casting powder must be used, which significantly increases production costs.
Part 6 Chemical Properties
The chemical properties of K gold alloys mainly manifest in their resistance to tarnish and corrosion, which are crucial for jewelry. The corrosion resistance of alloys varies with composition. Ordinary strong acids do not corrode 18K gold, and 14K gold also has good corrosion resistance but may leach out copper and silver from the surface under solid acid attack. Gold alloys below 9K are not resistant to strong acid corrosion and may tarnish in adverse environments. However, the noble metal content is not the sole factor affecting tarnishing. Tarnishing is a comprehensive result of chemical composition, chemical processes, environmental factors, and microstructure. In low-carat K gold, when the Filled alloys are conducive to increasing the Gold’s potential, forming a dense protective film, and improving the alloy’s microstructure, it is still possible to obtain an alloy with excellent chemical properties and good anti-tarnishing ability. Among the three main series of K gold, K rose Gold is prone to surface tarnishing due to its high copper content, requiring beneficial alloying elements for improvement.
Part 7 Mechanical Properties
Jewelry pieces must maintain high brightness for a long time, requiring an increase in the alloy’s hardness to meet abrasion resistance requirements. Some structural jewelry components, such as ear studs, ear hooks, brooches, and springs, require good elasticity and enhance the alloy’s hardness. However, Gold has low hardness and strength, making it challenging to meet the setting requirements. One of the purposes of K gold plating is to enhance the material’s strength, hardness, toughness, and other mechanical properties. Among the three typical types of K gold,
Nickel-bleached K white gold has high strength and hardness, with more excellent elasticity, requiring a balance between strength, hardness, and flexibility. K rose Gold may undergo an order-disorder transformation and lose malleability, necessitating the consideration of the Filled alloys and manufacturing process.
Part 8 Processing Properties
When designing the Filled alloy metal, full consideration should be given to the requirements of different processing technologies on Performance. For example, different smelting methods have different effects on the oxidation resistance of alloys. Different smelting methods such as oxyacetylene flame melting, induction heating melting in air, melting in a protective atmosphere or under vacuum for the same alloy will yield inconsistent results. Similarly, in jewelry production, methods such as casting, stamping, and welding are employed, each technique having specific performance requirements for K gold in certain aspects, which also determine the selection of alloy element types and amounts. When choosing the filled metal, the process operability of the alloy should be fully considered to avoid operational issues caused by a narrow process range. Processing performance is mainly viewed from casting Performance, malleable processing performance, polishing performance, welding performance, and recyclability.
8.1 Casting Performance
The casting performance of the alloy significantly impacts the surface quality of cast jewelry. The quality of alloy casting performance can be evaluated from aspects such as the fluidity of the molten metal, the tendency for shrinkage cavities and porosity, and the tendency for deformation cracking. It is required that the K gold used for casting has small crystal spacing, low tendency for gas absorption and oxidation, good fluidity and fill ability, and is not prone to form dispersed shrinkage and generate deformation cracks, which is beneficial for obtaining jewelry castings with complete shape, clear contours, dense crystals, and sound structure. Step-shaped, flat plate-shaped, and mesh-shaped specimens are generally used to test the casting performance of the Filled alloys, as shown in Figure 4-16. Among them, step-shaped specimens are mainly used to test hardness and step surface quality, flat plate-shaped specimens are used primarily to detect grain size and porosity tendency, and mesh-shaped specimens are used to evaluate fluidity.
Figure 4-16 Casting Performance Test samples
8.2 Malleable Processing Performance
Malleable processing technology has been widely used to produce K gold jewelry. In addition to using drawing, rolling, and other mechanical methods to produce sheet metal, wire, pipe, and other profiles, it is also frequently used for shaping jewelry, such as turning on machine tools, stamping with stamping machines, and hydraulic pressing. To ensure the quality of malleable processed products, besides correctly formulating and strictly following the operating process specifications, the malleable processing performance of the material itself has a decisive impact. K gold materials must have good malleable processing performance, especially when carrying out drawing, rolling, stamping, and hydraulic pressing operations. The hardness of the alloy should be manageable, and the work hardening rate of the alloy should be slower to facilitate operation; the material is required to have good flexibility. Otherwise, cracks are prone to occur, as shown in Figure 4-17.
8.3 Polishing performance
Jewelry has precise requirements for surface quality, and most jewelry must be polished to achieve a mirror-like surface brightness. This requires not only the correct execution of the polishing operation process but also the alloy itself, which has an essential influence on the properties. For example, if the workpiece structure is dense, the grains are refined and uniform, and there are no defects such as pores and inclusions if the workpiece has coarse grains, shrinkage, and porosity defects, it is easy to appear orange peel, polishing pits, comet tails, and other phenomena. If there are rigid inclusions, scratches and comet tail defects are also likely to occur, as shown in Figure 4-18.
8.4 Reusability
The casting process yield is generally around 50% or even lower for the jewelry process. Each casting will bring many reused materials such as a pouring system, scrap, etc. Jewelry companies always hope to use as much reused materials as possible based on production cost and efficiency. Due to inevitable issues such as volatilization, oxidation, and gas absorption during the alloy smelting process, the composition of the alloy will change with each casting, affecting the alloy’s metallurgical quality and casting Performance.
The deterioration of Performance during the repeated use of the alloy is not only related to the operating process but also closely related to the reusability performance of the alloy itself.
The reusability performance of the alloy is mainly determined by its gas absorption and oxidation tendency, as well as its reactivity with crucibles and casting materials. The lower the gas absorption and oxidation tendency, and the lower the reactivity with crucibles and casting materials, the better the reusability performance.
8.5 Welding Performance
In jewelry making, it is often necessary to divide the workpieces into simple small parts for separate production and then weld these small parts together. To obtain good welding quality, in addition to using the correct solder, it is also necessary to assess the welding performance of K gold. If the welded part has good thermal conductivity, the heat does not easily accumulate at the welding site during welding heating. Still, it quickly conducts to the entire workpiece, which could be more conducive to the melting of the solder. Suppose K gold is prone to oxidation during heating. In that case, the formed oxide layer will reduce the wettability of the solder, prevent the solder from infiltrating the weld seam, and lead to problems such as weak welding and false welding.
Part 9 Safety
Jewelry is in direct contact with the human body for a long time, and its safety is an essential factor that jewelry materials must consider. Harmful elements to the human body, such as cadmium, lead, and radioactive elements, should be avoided in the Filled alloys; allergic reactions caused by jewelry contact with the skin should also be avoided. For example, in K white gold jewelry, nickel is widely used as the primary bleaching element, but there is a problem when using Ni white gold; some people may have allergic reactions to Ni after contact. Therefore, the EU and some other countries have strict limits on nickel release rate in jewelry, and nickel-containing jewelry must meet the standards for nickel release rate.
Part 10 Economy
K gold is an alloy material composed of Gold and Filled alloys, and the price of solder is an essential factor affecting production costs, especially for low-carat K gold, which requires a large amount of solder for alloying. Therefore, in selecting solder alloy elements, the principle of comprehensive material sources and low prices should be followed, and expensive precious metals should be avoided or used as little as possible to reduce alloy costs.
Part 11 Inspection Method of Filled alloys
When a jewelry production enterprise introduces new Filled alloys, it should conduct comprehensive tests to ensure that its Performance meets the requirements before it can be put into production. Especially for mass production, caution is required. Production and operation problems caused by inappropriate Filled alloys are not uncommon. The main inspection contents and methods of the Filled alloy are shown in Table 4-8.
Table 4-8 Inspection method of Filled alloys
| Inspection items | Metoda de inspecție | Inspection content | Inspection tool | Acceptance criteria |
|---|---|---|---|---|
| Invoice | Verification of supplier information, model number, identification and amount on invoices | Full inspection | Manual verification | Consistent with contract requirements |
| Packaging | Check if the packaging is intact | Full inspection | Sensory examination | In accordance with contract requirements |
| Weight | Detecting precious metal materials Weight | Full inspection | Electronic scale Weighing | Implement standards "Quality Tolerance for Precious Metal Jewelry Measurement" Regulations |
| density | Inspection of the precious Metal alloy density | Random Inspection | Water density meter | Both parties agree |
| Culoare | Inspection of the precious Metal alloy color | Full inspection | Prepare the corresponding color sample, and compare it Color proof or color measurement with a colorimeter | Agreed by both parties Standard color proof |
| Punct de topire | Inspection of the precious Metal alloy Melting point | Random Inspection | Material, detect melting point using differential thermal analyzer | Agreement between both parties |
| Schimbarea culorii | Check the color fade resistant performance of metal alloys | Random Inspection | Prepare alloy materials of corresponding color Material, soaking in solution, salt spray corrosion, Corrosion atmosphere, polarization curve detection, color fading resistant performance of alloys | Agreement between both parties |
| Duritate | Check the Metal alloy hardness | Random Inspection | Prepare the corresponding alloy material , use a macro or micro hardness tester to check Hardness test | Agreement between both parties |
| Turnare | Inspection of the casting Performance of metal alloy casting | Random Inspection | Prepare the corresponding color alloy material , use screens, steps, flat plates, etc. for testing Sample testing of casting performance | Agreement between both parties |
| Malleable processing | Check the alloy Shaping & processing performance | Random Inspection | Preparation of alloy materials of the appropriate color, using rolling presses, hardness testers, etc. to test processing behavior | Agreement between both parties |
| Random Inspection | Random Inspection | Random Inspection | Random Inspection | Agreement between both parties |
| Welding | Inspect the Alloy Welding Performance | Random Inspection | Prepare corresponding colored alloy materials Material, detect welding performance using flame, laser, arc, hydrolysis And other methods to detect welding performance | Agreement between both parties |
| Lustruire | Inspect the Metal alloy polishing performance | Random Inspection | Configure the corresponding color of the alloy material, use mechanical cloth wheel, mechanical grinding, etc. Way to test polishing performance | Agreement between both parties |
| Reusability | Check the alloy recycle Performance | Random Inspection | Configure the corresponding alloy material , using investment casting process to cast samples, reused several times, comparing each casting quality | Agreement between both parties |
| Safety | Check the metal alloy safety | Random Inspection | Configure the corresponding alloy material, using artificial sweat immersion method to check Measure metal release rate | Execute product destination Harmful metal content in the ground Quantity or release rate standards |
Section III Quality Inspection of Auxiliary Materials
A large number of auxiliary materials are used in jewelry production, which has different degrees of impact on the quality of jewelry products, among which is the significant effect of investment powder, boric acid/borax, crucibles, and other auxiliary materials.
Part 12 Investment Powder
Investment powder is among the most essential auxiliary materials in jewelry casting molds. Requirements for the Performance of investment powder: good replication performance, complete replication of wax mold details; stable thermal and chemical properties, not easy to decompose, not easy to react with molten metal; stable and appropriate thermal expansion performance, maintaining the dimensional stability of cast jewelry; suitable and uniform particle size. The inspection method of investment powder is shown in Table 4-9.
Table 4-9 Inspection Methods for Casting Powders
| Inspection items | Metoda de inspecție | Inspection content | Inspection tool | Acceptance criteria |
|---|---|---|---|---|
| Invoice | Verification of supplier information, model number, identification and amount on invoices | Full inspection | Manual verification | Consistent with contract requirements |
| Packaging | Check if the packaging is intact | Full inspection | Sensory examination | In accordance with contract requirements |
| Humidity | Check whether the casting powder is dry or damp | Random inspection | Grip tightly and then release | Loose powder, no agglomeration |
| color | Check the color of the casting powder | Random inspection | Randomly with a steel spoon Observation after extraction | Pure white, no stains |
| Technological performance | Examine the relationship between water-gypsum ratio and strength, fluidity, setting time, etc. | Random inspection | Preparation with different water powder ratios Slurry, poured flat sample | Both parties agreed |
Part 13 Boric acid, borax
Borax and boric acid are not the same thing. Borax is a compound of boric acid ten sodium tetraborate decahydrate, molecular formula: Na2B4O7 • 10H2O, English name Borax, soluble in water alkaline. Boric acid’s molecular formula is H3BO3, the English name for Boric acid, and it is a weakly acidic solution. Boric acid and borax are widely used in jewelry production and are known as “fairy powder” in the industry.
13.1 Borax prevents oxidation of diamonds in diamond processing.
During the actual cutting and grinding process, when the surface temperature of a diamond reaches above 600℃, the oxygen in the air can cause changes to the outermost layer of carbon atoms of the diamond. In this oxidation process, the diamond directly burns and transforms into gaseous carbon dioxide, leaving a thin, circular, ring-shaped white opaque burn mark on its surface. When the diamond surface is locally deprived of oxygen and reaches temperatures above 1000℃, it may transform into its allotrope – graphite, leaving brownish-black burn marks on the diamond surface (this situation is scarce). The appearance of burn marks dramatically affects the clarity of the diamond, thereby reducing its value. Repair requires re-polishing.
The unique thermophysical properties of borax can essentially solve the oxidation problem that occurs during diamond grinding. The solution is as follows: dissolve borax in hot water to form a supersaturated solution, then soak the cleaned diamond (diamonds have an oleophilic nature, easily absorb oil, and oil stains on the surface will damage the protection of borax on the diamond surface) in the supersaturated borax solution, and finally grind the diamond with borax solution. During the grinding process, the high temperature generated on the diamond surface due to the accumulation of grinding heat causes changes to the borax attached to the diamond surface.
Borax protects diamonds in two ways: first, borax absorbs heat and undergoes a dehydration reaction, lowering the temperature of the diamond surface; then, borax begins to melt, and the molten borax uniformly flows onto the diamond surface to form an isolation layer, isolating oxygen from contacting the diamond surface, thereby preventing the appearance of burn marks. Although heating diamonds in a low-oxygen environment to 2000 ~3000℃ will turn them into graphite, and this transformation process begins at 1000℃, the transformation of diamonds into graphite is extremely slow, and the instantaneous high temperatures generated during diamond grinding mainly prevent the appearance of black burn marks on the diamond surface under the molten borax layer. Therefore, diamond oxidation can be effectively prevented with the supersaturated borax solution’s protective effect.
13.2 Boric acid plays a role in preventing gemstone discoloration in wax casting.
In wax casting, gemstones are subjected to high-temperature baking in the burnout furnace for a long time with the mold, and the high-temperature metal liquid during casting will also cause thermal shock to the gemstones, making them prone to discoloration and loss of luster. In production, a boric acid solution is generally used for protection.
【Case 4-6】Poor quality borax powder causes diamonds in wax-inlaid products to become cloudy.
Descrierea defectului:
The diamonds in the 18K White gold jewelry of wax-inlaid diamonds have a high proportion of cloudiness and discoloration over time, as shown in Figure 4-19. The proportion has suddenly increased from 0.15% to about 0.5% and has been fluctuating at a high level, with no apparent regularity in the areas of discoloration.
Production condition investigation:
The diamonds used are of medium grade, the same as before; the gypsum temperature is 670℃, and the metal liquid temperature is 1040℃; a particular brand company produces the casting powder used; the casting powder contains saturated boric acid water. From the above situation, the production conditions are within the normal range, ruling out defects caused by improper production conditions. The diamond quality is the same as before, also ruling out that. Therefore, the problem is likely to be with the gypsum powder.
Finding the source of the problem:
The gypsum powder has been consistent.
The temperature and humidity of the storage warehouse are average for the same batch of incoming goods. Recently, a different brand of boric acid powder was used, and the problem may lie with the boric acid powder, as it did not provide adequate protection.
Solution:
All the newly prepared boric acid water from the new brand was discontinued and replaced with the old brand of boric acid powder, resulting in the proportion of diamond haze returning to its original low level.
13.3 Boric acid and borax act as fluxes in jewelry soldering.
Jewelry processing requires solder joints to be uniform, firm, and free of cracks, bubbles, shrinkage holes, etc. However, due to the small and delicate nature of precious metal jewelry, the solder joints are fragile, causing the solder （or solder rod） to have difficulty evenly entering. Solder compositions often contain silver, which tends to oxidize and turn black when exposed to air at high temperatures. This results in a noticeable color contrast between the solder joint and the jewelry component. By utilizing the fluxing agent role of borax in the soldering process, these two problems can be effectively addressed.
There are currently two different views on the role of borax as a fluxing agent: one view is that when jewelry components dipped in borax solution or solder rods coated with borax powder come into contact with a high-temperature flame, the borax undergoes a dehydration reaction first, followed by melting. The molten borax flows uniformly onto the metal surface at the solder joint, forming a thin layer. Under sustained high temperatures, the solder melts, and guided by the “thermal bridge” formed by the borax, the solder drips evenly to all parts of the solder joint. In industry jargon, this “thermal bridge” effect of borax makes the solder “flow well,” meaning that borax enables the solder to flow evenly. The other view is that when heated, the fluxing agent (such as borax) melts and interacts with the liquid metal, causing the slag to float upwards, protecting the molten metal and preventing oxidation.
13.4 The role of boric acid borax in precious metal smelting slag making
Crystalline borax is dehydrated by heating at high temperature to form anhydrous borax before use. It is known from the composition of borax that it is a solid acidic flux, which can form borate slag with many metal oxides. The alkaline components in borax can react with silica in slag-making ingredients to form silicates. Borax slag-making has two significant advantages: first, its slag-making ability is more vital than that of silica, and it can decompose some refractory minerals, such as chromite; second, as a borate, borax has a lower melting point than the corresponding silicate, and adding borax to the ingredients can significantly reduce the melting point of the slag.
Part 14 Crucible
Depending on the different properties of jewelry materials, different crucibles are used. Commonly used crucibles include graphite crucibles, including high-purity graphite crucibles; ordinary graphite crucibles; ceramic crucibles, including quartz crucibles, corundum crucibles, magnesia crucibles, mullite crucibles, lead oxide crucibles, silicon carbide crucibles, etc. The requirements for crucibles in smelting mainly include refractoriness, density, thermal stability, reactivity with molten metal, etc.
14.1 Graphite Crucible
Graphite crucible can be used for melting Gold, silver, and copper alloys. Figure 4-20 shows some typical crucible shapes. Graphite crucible has high refractoriness, good heat transfer, high thermal efficiency, low thermal expansion, good thermal shock stability, and resistance to slag erosion. It provides specific protection to the molten metal, achieving good metallurgical quality.
Table 4-10 Physical and Chemical Properties of High-Purity Graphite
| Volume density (g/cm3) | Porosity (μΩm) | Compressive strength (MPa) | Tensile Strength (MPa) | Resistivity (μΩm) | Ash content (%) |
|---|---|---|---|---|---|
| ≥1.7 | ≤24 | ≥40 | ≥20 | ≤15 | ≤0.005 |
Table 4-11 Physical and chemical indicators of coarse graphitegold Nuggets
| Maximum particle size (mm) | Volume density (g/cm3 | Porosity (μΩm) | Compressive strength (MPa) | Modulus of elasticity (GPa) | Thermal expansion coefficient (10-6/℃) | Ash content (%) |
|---|---|---|---|---|---|---|
| 0.8 | ≥1.68 | ≤7.8 | ≥19 | ≤9.3 | ≤2.9 | ≤ 0.3 |
14.2 Ceramic crucible
To meet the smelting requirements, ceramic crucibles should have high refractoriness, high density, good thermal stability, low reactivity with molten metal, and good chemical stability. According to the properties of jewelry metal materials, the most widely used ceramic crucibles are quartz and corundum.
The main chemical component of quartz crucibles is silicon dioxide and purity significantly impacts its Performance. The raw materials determine purity, and the raw materials for quartz crucibles require high purity, good consistency, and uniform particle size distribution. When the harmful components are high, it will affect the crucible’s melting and temperature resistance and may also cause bubbles, discoloration, peeling, and other phenomena, seriously affecting the quality of quartz crucibles. Therefore, there are strict requirements for impurity elements in quartz, as shown in Table 4-12.
Table 4-12 Requirements for Impurities in Raw Materials for Quartz Crucibles
Metal content unit: x10-6
| Denumirea elementului | Al | Fe | Ca | Mg | Ti | Ni | Mn | Cu | Li | Na | K | Co | Bi |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Conținut | 11.6 | 0.3 | 0.5 | 0.5 | 1.0 | 0.01 | 0.05 | 0.01 | 0.7 | 0.43 | 0.42 | 0.03 | 0.04 |
A well-fired quartz crucible has typical physical and chemical properties: bulk density ≥2.90 g/cm3; refractoriness≥1850℃; apparent porosity ≤20%; thermal expansion coefficient about 8.6 x 10-6/℃; thermal shock resistance 1300℃; maximum continuous use temperature 1100℃, for a short time 1450℃. Quartz crucibles can be used for melting K white Gold, nickel, silver, and other materials.
The corundum crucible is composed of porous fused alumina with complex and refractory qualities, resistant to high temperatures, not resistant to acid and alkali, resistant to rapid cooling and extreme heat, resistant to chemical corrosion, and high density after slurry molding. It can be used for smelting materials such as K White gold, nickel silver, stainless steel, etc. The physical and chemical indicators of the corundum crucible are shown in Table 4-13.
Table 4-13 Performance indicators of corundum crucibles for jewelry casting
| Articolul | Indicator | ||
|---|---|---|---|
| Compoziție chimică | Al2O3 | > 99 | |
| R2O | ≤ 0.2 | ||
| Fe2O3 | ≤ 0.1 | ||
| SiO2 | ≤ 0.2 | ||
| Volume density (g/cm3) | ≥3.80 | ||
| Open porosity (%) | < 1 | ||
| Flexural strength (MPa) | > 350 | ||
| Compressive strength (MPa) | > 12000 | ||
| Dielectric constant E(1MHz) | 2 | ||
| Fire resistance (℃) | > 1700 | ||
| Maximum operating temperature (°C) | 1800 | ||
| Continuous use temperature (°C) | 1600 | ||
| Thermal shock resistance/times ( 300℃ rapid cooling) | >7 | ||
Part 15 Silicone Rubber
Jewelry lost wax casting requires using rubber molds to make wax molds. The quality of the rubber mold determines the quality of the wax mold. Correct selection and use of jewelry rubber are essential. Two main types of rubber can be used to make soft molds: natural rubber and silicone rubber. Natural rubber has high tensile strength, up to 21 ~ 25MPa, and a long service life but poor molding performance, requiring a lot of mold release agents and poor wax mold quality. Compared to natural rubber, silicone rubber is more inert and does not react with silver or copper, reducing the need for surface electroplating with nickel or rhodium on the original model. The surface of the rubber mold is smooth, has self-lubricating properties, requires less mold release agent, reduces quality issues caused by the accumulation of these substances on the rubber mold, and makes it easy to remove the wax mold. Since introducing silicone rubber into the jewelry industry, it has become the main jewelry rubber. According to its vulcanization method, silicone rubber can be divided into high-temperature vulcanized and room-temperature silicone rubber.
The strength of high-temperature vulcanized silicone rubber is generally between 7 ~ 10MPa, with good malleability, ease of press, and ease of cutting the mold. Silicone rubber molds can maintain the original shape better than natural rubber molds during wax injection, making them more capable of withstanding changes in injection pressure. In addition, silicone rubber molds generally fit more tightly, reducing the flying edge of wax parts and being suitable for making delicate and complex parts. The service life is lower than that of natural rubber, which is usually used several hundred to thousands of times.
Room-temperature vulcanized silicone rubber (RTV) does not require heating and pressurized vulcanization and is suitable for fragile, brittle, and low-melting-point originals. In addition, it does not shrink and can accurately control the size of the wax mold, which is crucial for operations such as setting stones and assembling components. However, RTV has a long curing time and low tensile strength, usually only 0.7-1.4MPa, making it prone to tearing and damage, with a short service life. Be careful when cutting the mold in combination to avoid damaging the rubber mold. Many RTV rubbers require precise mixing in proportion, with a very short working time, usually 1-2 minutes, while some RTV rubbers can have a working time of up to 60 minutes. Usually, RTV rubbers need to be vacuumed to remove air bubbles. Some malleable materials may hinder the vulcanization of RTV silicone rubber, which can often be solved by electroplating the jewelry master mold. RTV rubber molds are unstable and sensitive to moisture, accelerating their deterioration when exposed to humid air.
The performance comparison of natural rubber, high-temperature vulcanized rubber, and room-temperature vulcanized rubber is shown in Table 4-14.
Table 4-14 Comparison of Jewelry Mold Material Performance
| Mold Material | Vulcanization Temperature (°C) | Curing Time | Tensile Strength (MPa) | Shrinkage rate (%) |
|---|---|---|---|---|
| Cauciuc natural | 140 - 160 | ≤ 45 min | 21 - 25 | 0 - 4 |
| Cauciuc siliconic | 140 - 160 | ≤ 45 min | 7 - 10 | 2.6 - 3.6 |
| RTV silicone rubber | 140 - 160 | 18 ~ 72 hours | 0.7 - 1.4 | 0 |
Jewelry silicone rubber for soft mold manufacturing should meet performance requirements such as corrosion resistance, aging resistance, good recovery performance, elasticity, and softness. The contents and methods of incoming inspection are shown in Table 4-15.
Table 4-15 Silicone rubber inspection contents and methods
| Articolul | Content and Acceptance Criteria | Inspection Method | Inspection content | Inspection records |
|---|---|---|---|---|
| checking information | Check the model, label, and amount on the invoice | Full inspection | Check the supplier information on the invoice | After checking, in Invoice signed Name confirmed, Record |
| Packaging | Full inspection | Check if the packaging is damaged | ||
| Cantitate | Full inspection | Count, check the invoice | ||
| Calitate | Rubber press test | Random inspection | Select typical product for compression molding |
Part 16 Jewelry wax raw material
In investment casting, the quality of jewelry wax molds directly affects the quality of the final jewelry. To obtain suitable jewelry wax molds, the wax material should have the following process parameters: the melting point of the wax material should be moderate, with a specific melting temperature range, stable temperature control, and suitable flowability; the wax mold is not easily softened or deformed, the heat stability should not be lower than 40℃, easy to weld; to ensure the dimensional accuracy of jewelry wax molds, the wax material is required to have a small expansion shrinkage rate, generally less than 1%; the wax mold should have a sufficient surface hardness at room temperature to ensure that there is no surface abrasion in other processes of investment casting; to remove the wax mold from the rubber mold smoothly, the wax mold can bend without breaking, and it can automatically restore its original shape after removing the mold. Jewelry wax should have good strength, flexibility, and elasticity, with a bending strength greater than 8 MPa and a tensile strength greater than 3 MPa, minimal component changes during heating, and low residual ash content during combustion.
The elemental composition of wax materials includes wax, grease, natural and synthetic resins, and other additives. Wax is the matrix, adding a small amount of grease as a lubricant; various resins are added to make the wax mold rigid and elastic while improving the surface gloss. Adding resin to paraffin wax hinders the growth of paraffin wax crystals, refining the grains and enhancing their strength
Popular jewelry waxes come in various shapes, such as beads, flakes, tubes, and threads, with colors including blue, green, pink, and other categories. The quality inspection of jewelry wax feed generally includes the contents and methods as shown in Table 4-16, and other performance indicators may be tested by professional institutions as needed.
Table 4-16 Inspection Contents and Methods of Jewelry Wax
| Articolul | Content and Acceptance Criteria | Inspection Method | Inspection content | Inspection records |
|---|---|---|---|---|
| Verification of materials | Check the model, label, and amount on the invoice | Full inspection | Check the supplier information on the invoice | After checking, in Invoice signed Name confirmed, Record |
| Packaging | Full inspection | Check if the packaging is damaged | ||
| Cantitate | Full inspection | Count, check the invoice | ||
| Calitate | Melting point ±3℃ | 1 sample of each batch | Testing with a soldering iron |
Part 17 Electroplating original solution
In jewelry electroplating, the plating solution is a key component in the electroplating process. The composition of the plating solution determines the properties of the coating. Different plating metals use different plating solutions but generally include main salt, conductive salt, complexing agent, buffering agent, wetting agent, stabilizer, etc. Factories typically use commercial electroplating original solutions to formulate and open the cylinder.
The inspection method for the purchase of the original electroplating solution is shown in Table 4-17.
Table 4-17 Inspection contents and methods of electroplating original solution
| Articolul | Content and Acceptance Criteria | Inspection Method | Inspection content | Inspection records |
|---|---|---|---|---|
| Verification of materials | Check the model, label, and amount on the invoice | Full inspection | Check the supplier information on the invoice | After checking, in Invoice signed Name confirmed, Record |
| Packaging | Full inspection | Check if the packaging is damaged | ||
| Cantitate | Full inspection | Count, check the invoice | ||
| Plating Trial | Open the cylinder for a small test | Sampling | use 500ml to do test plating |
