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Granulation is a decorative technique whereby multiple gold or silver spheres are attached to the surface to create a specific decoration. Technically, this can be done with or without soldering, but "true' granulation is often associated with not using solder. While possible with traditional soldering, the join formed with traditional granulation (reaction soldering) is much more delicate and hidden. The earliest evidence of the technique dates back to around 2500 BC in south Mesopotamia. Today the technique is commonly used in both gold and silver jewelry. Gold granulation is usually practiced in higher purity alloys, from 18–22K (750–917). "True" Granulation, or reaction soldering, does not involve a conventional soldering alloy, but uses a copper salt/glue. It is a form of surface or localized alloying that happens on heating. Granulation in Gold and Silver The granules and the "back plate", onto which the granules are attached, must be carefully prepared. For the granules: Gold or silver granules are made by cutting a coil from a fine wire wrapped around a mandrel. The wire is then placed in a crucible filled with charcoal powder and heated. As the wire melts, it will form a sphere. A spherical shape minimizes the amount of surface area, and therefore the energy of the alloy. This is often described using the concept of surface tension. The charcoal powder helps to form perfect globe shapes, and helps prevent oxidation. The granules can then be pickled to remove any surface oxide which may have formed. For the granulation itself: The granules can be positioned onto the surface using a water-based glue containing a copper salt and then dried. They can then be heated and the granules fuse to the piece. Science of granulation – Reaction soldering On heating: (100˚C/212˚F) – Copper salt is oxidized to Copper (II) Oxide. (600˚C/1112˚F) – The glue burns and carbonizes. This creates a reducing area and converts the copper oxide to a metallic copper. A thin copper layer forms on the surface of the surface and granules. (850–890˚C/1562–1632˚F) – The copper diffuses with into the gold or silver surface. This leads to a reduction in the melting point of the alloy. The two surfaces now have a lower melting point than the rest of the alloy. The surfaces then melt very quickly, long before the rest of the alloy, and so a metallic bond is formed between the two pieces. The heat is removed and the join solidifies, with the rest of the alloy "untouched". Thus, we get a self-soldered joint known as reaction soldering or transient liquid-phase bonding. Forming a solid connection To ensure a strong bond between the surface and the granules, it is necessary to form a thick enough join between the two of them, known as a "neck". The correct ratio between energy and time, between the heat and the duration of heating is crucial: Insufficient heating is insufficient to form a strong connection Overheating will causes the granules to melt into one another and into the substrate. The joining phase takes the least time of the entire process of granulation, but at the same time it is certainly the most difficult. Granulation by plating Granulation in high-karat gold, and sometimes in silver, is often performed using extra steps. Instead of using copper salts with the glue, the copper is applied directly to the granules. The copper can be applied by electroplating, using an acidic solution. This allows careful and precise control of the amount of copper applied. Fully pre-copper plating the granules and the substrate also has some disadvantages, because we cannot remove the excess copper which isn't used to make the connection: Excess copper causes the surface to sweat. The whole sample needs heating for the granulation process to work. Heating an entire sample for decorative granulation is not feasible. Since the granules are not glued onto the surface, the little balls will roll off as soon as the object moves. Even if this is achieved, upo n heating, the granules will heat quicker than the bulk object. They will quickly come to their melting point and melt away while the surface of the object will not be warmed up enough to make the connection. It is often easier to heat the object and granules with a large propane/compressed-air flame, traditionally used for annealing and soldering silver hollowware. Using the torch in the “right” way allows to bring only a very small area to the required temperature for granulation. Common problems Read more by David Huycke (Santa Fe Symposium):
- Why is there porosity in my casting?
Porosity in casting is a common defect and can usually be attributed to one or two main causes. It is often easily rectified. Here's a quick question and answer to try and help you solve the issue. #1 – What do the pores look like? Are the pores round and spherical or are they more tree-like in shape? Round and spherical pores are indicative of gas porosity. Go to #2. Otherwise, go to #3 #2 – Gas porosity – are they throughout the sample or just at the surface? If they are located at the surface only (shown left), the porosity is likely caused by the evolution of gas due to a reaction at the mold wall. In general, clean burnout of the mold, use of cleaned scrap (especially recycled casting sprues) in the melt charge, and lower casting and/or flask temperatures will reduce the probability of gas porosity. For more details, see: D. Ott, Handbook on Casting and Other Defects in Gold Jewellery Manufacture (London: World Gold Council, 1997). If they are located throughout, the porosity is likely caused by dissolved gas in the alloy. Ensure high-quality start material (>99.9% high-purity metals or Pure master alloys) and that the material is all dry before melting. #3 – Shrinkage porosity? If caused by shrinkage, then this is common for investment cast products where there is essentially insufficient liquid able to fill the mold during solidification. It is important to carefully consider mold design, specifically the diameter and positioning of sprues.
- Welding in Jewelry
Welding involves joining two pieces of metal together by melting them with or without filler material. The pieces being joined must be in good contact and heated locally to melt the joint surfaces together. The metal will then solidify, forming a metallic bond between the two parts. Microstructure of a weld The microstructure of a weld has some key features: The melted metal will solidify a classic dendritic-cast microstructure Around the weld is a heat-affected zone (HAZ) which absorbs heat from the molten metal during solidification. The size (width) of the weld zone will depend on the heat source used and the properties (conductivity) of the weld materials. A large gas torch, for example, will produce a larger (wider) weld zone than a laser, where the heat energy can be finely focused and is more intense. Properties of the Heat-Affected Zone The area around the weld will not be melted but heated to a high temperature. As a result, we may see a great deal of grain coarsening and potentially the transformation into meta-stable high-temperature phases. Grain coarsening will ultimately lead to a softer, less hard area Meta-stable phases are typically brittle. So a weld zone may have mechanical and corrosion properties that differ from the normal parent metal unaffected by the welding. We may also expect some surface quality degradation due to melt turbulence and oxidation, possibly with some porosity due to expelled gases. Types of Welding Tungsten Inert Gas (TIG) and electrical resistance spot welding use electric current to generate heat: TIG welding – the current strike an arc between the tungsten electrode and the workpiece. Plasma arc welding – A plasma is formed from the ionized gas surrounding the electrode, generating considerable heat. Laser Welding – Heating can also be accomplished by an electron beam or by a laser beam. Why is Laser Welding so popular? Laser welding has become very common in the jewelry industry. This high-energy source allows very fine and deep welds. This means: Repairs can be made close to set gemstones without damaging them (no removal necessary) due to a small heat-affected zone. It is quick and precise. It can be done in the air (no special atmosphere or flux is necessary), and there is little fixturing required. Several welding operations can be done on the same workpiece without fear of re-melting earlier joints. Filler metals can be used if desired, for example, in filling open porosity; these are normally of the same alloy as the workpiece, thus giving no problems with color matching or karatage mismatch. It is particularly suitable for pure precious metals (e.g., 24-karat gold). Laser welds tend to be stronger and more ductile than soldered joints, with little porosity. There are no toxicity problems as are associated with cadmium in solders. On the other hand, laser welding can be more time-consuming than soldering, and the welds may have a bulbous overlap surface that will need grinding and polishing. As stated earlier, it is also a line-of-sight method. Laser welding is often used for decoration by granulation and is commonly used for making chain links in situ.
- A guide to buying Gold
Gold is the protagonist of jewelry materials. Gold jewelry can be traced back more than 6000 years! Nowadays, gold comes in various forms, colors, and karats – each has its unique properties. Of course, like all other precious metals, gold is far too soft to be used as jewelry for everyday wear. Instead, we use alloys. Gold alloys are a mixture of gold and one or more other elements, typically silver, copper, nickel, and zinc. These help to improve the strength, hardness, durability, and workability (for manufacture) and to tune the color of the gold alloy. What does Karat mean? Karat is a measure of fineness (proportion of precious metal by weight) in a gold alloy, with 24K being essentially pure gold. The most common fineness for jewelry is 18K, then 14K, and 10K. 24K and 22K are sometimes sold, but these are not durable enough for everyday wear in use for rings, necklaces, etc. N.b. This is different from carat, which is the measure of the size/weight of a gem. There are some general rules associated with properties and karatage: Tarnish resistance decreases as the karat falls, Appearance is duller in white gold as the karat falls, and lower karat yellow and rose golds have a deeper hue, Hardness, wear resistance, and durability increase as the karat falls. We recommend you always ask a reputable jeweler about the metal before purchase. All jewelry should have a hallmark or stamp to indicate what it is and its fineness (or karat, for the specific case of gold). This can either be a symbol or a number, such as "14K" or "585". The requirements for hallmarking vary from country to country. We recommend only buying expensive jewelry from a reputable jeweler or manufacturer. Skin Allergies and Gold Except in very rare and specific circumstances, pure gold is considered hypoallergenic. There are a few specific cases where gold can cause a skin allergy. Gold alloys, on the other hand, can often cause contact dermatitis (skin allergies) due to the presence of certain "base" metals. The common cause is nickel, which is often present in large quantities for many white gold alloys. The nickel content in most 18K and 22K alloys is too low to cause skin irritation, except for the most sensitive of skin. As for 18K Nickel-white golds and all lower karat golds, they can often be problematic. If you have sensitive skin, it is often wise to choose nickel-free alloys, e.g., palladium-white gold alloys. The color of Gold Pure gold is a stark yellow color. To many people, its color is unfamiliar due to the rarity of 24K gold jewelry. There is no such thing as white or rose gold, but rather white-gold and rose-gold alloys. The color and hue of gold jewelry can be changed by alloying additions. Subtle changes in composition can give precise tuning of color. For example: Nickel and Palladium are strong bleachers, turning gold white Silver gives gold a greenish hue Copper gives gold a reddish hue Selecting the color of gold for your jewelry should be based primarily on your preference. There are some minor differences in properties which are explained below. White Gold White gold has become extremely popular. Two main types of white gold alloys are Nickel-white gold and palladium-white gold. These are gold alloyed with nickel and palladium, respectively, as these elements bleach the color. Palladium-white golds are preferred because they are hypoallergenic; the absence of nickel means they do not cause skin irritation and contact dermatitis. However, they are more expensive. It is important to be aware that white gold alloys are sometimes coated with Rhodium to give them a bright-white color. Over time, this will wear away, and the bright white color will be irreversibly lost. The white gold underneath will have a duller and yellowish appearance compared to the rhodium. Higher karats tend to be duller, as there are fewer "bleaching" additions. A jeweler may be able to reapply the rhodium coating. Rose Gold Rose gold was first introduced to the Russian Imperial court by Carl Fabergé himself. In recent years, it has become extremely popular. Rose and Red gold are predominately gold-copper-silver alloys, with copper giving the reddish hue. One often has to move to a lower karat to achieve darker colors, such as 18K to 14K. This has the added advantage of improved durability. Other colors of Gold? You may often hear of other colors, such as purple and blue. These are made by mixing gold with aluminum (purple), indium, or gallium (blue). These materials are extremely brittle, so they can only be used for decorative purposes. They remain an area of research. Gold-plating (incl. Vermeil/silver-gilt) Gold plating, rolling, or filling is also common, where a thin layer of gold is coated onto an alloy. Gold fill lasts much longer than plating, as the layer applied is much thicker (it must be 5% of total weight), followed by rolling (2.5% by weight) and then plating. Vermeil, or silver-gilt in English, is sterling or pure silver which has been gilded with gold (coated with a thin layer). Besides being cheaper, silver-gilt objects are far lighter and have a long-standing history of use. In the US, gilt must have a gold content equivalent to 2.5 micrometers of fine gold.
- An introduction to Mokume Gane
Mokume Gane is a name for a Japanese metal-working technique developed some 250–300 years ago. The name refers to the visual appearance, translating as wood grain metal. Three or more sheets of non-ferrous metal (not iron-based) are laminated together without the use of solder, reduced, carved, and then forged to produce patterns that often resemble wood grain. The technique is thought to develop from decorative metalwork, notably swords, in Feudal Japan. It is widely attributed to metalsmith Denbei Shoami. The method was long-forgotten in the "west" but has seen a recent revival in the late 20th century. Making Mokume Gane (Traditional Method) Preparation To make mokume gane, by any method, we must first make a billet. This involves layering sheets of the various metals. The sheets must be well-prepared to make sure they are clean. Any surface oxides, dirt, or defects (surface roughness, etc.) will be trapped in the interface once the billet is made and weaken the material. Traditionally, sheets had to be forged from cast ingots and then worked by hand with files, scrapers, and abrasive stones to develop the smoothest, flattest surfaces possible on both sides of each sheet. Firing and Forging the billet Traditionally, the sheets are stacked between two iron plates that are bound with heavy iron wire, and folded iron sheets are used to hold the stack together during firing. Firing the mokume game billet was performed in charcoal–fired forge (a hodo). Charcoal not only acts as a fuel source, but it also generates a reducing environment; this prevents the oxidation of the metal since charcoal (carbon) preferentially reacts with any oxygen. The billet was then carefully heated to a uniform temperature. The temperature should be low enough to fuse the metal but not melt it. This is a traditional method is a form of transient-phase liquid bonding (see below). Empirically, the temperature is sufficient when the visible edges of the billet will exhibit a shine or flash. The billet is then removed and hammered on an anvil to ensure the bond is tight. Patterning the billet Once bonded, the billet needs to be forged down to a suitable size for the desired object: The first step involves reducing the billet to a thinner, more useful shape. Traditionally this was done by hand. In most cases, a fault in the billet here requires scrapping! The billet came then be carefully carved (with small chisels) and forged (flattening) to produce patterns that will slowly emerge as the process is carried out. Finally, the worked billet can be carefully cut, shaped, and finished to produce Mokume Gane objects. While many artists continue to produce wood-grain effects or patterns, the endless combination of twisting, forging, and carving leads to unlimited possibilities. For the manufacture of rings, the stock is rolled to the square wire, which is then can be twisted (or not!), carved (or not!), and then forged and rolled to the proper width and thickness for ring making. Alternately, rings can be made from flat sheets in the traditional soldering method or made from washers created from a flat sheet and converted to a ring using a series of dies and punches. Deep drawing mokume gane patterned sheet or un-patterned is also possible, with subsequent steps to pattern and finish the ring. A laborious but worthwhile process! Making Mokume Gane (Modern Method) While the essence of Mokume Gane manufacture remains unchanged, there are still a few useful developments to the modern methods: Nowadays, commercially produced billets, plate forging rod and sheet is available for those not desiring the loss that accompanies making proper mokume gane (approximately 50%). This gives the aspiring artist materials to work with without the extra steps of creating the bonds. It is more common to bolt together the plates of steel and a torque plate to clamp the stack for firing. Gas-fired and electric kilns are used to better control the heat. A reducing atmosphere is still necessary. This is often achieved by packing charcoal around the billet and wrapping in foil. Hydraulic presses have become a common choice for forging billet and the final objects. Pressing can be done either hot or cold. Tools like rotary files called burs or drills are more often used to carve patterns today. Metal Choice – Colorful and malleable The choice of metal is careful. A craftsman selects the metal based on various characteristics: Color – not just the natural bulk color but also the various patinas that can be developed through treatment. The number and thickness of layers are also important to the overall design look. Malleability – how easily the metal can be shaped without fracture. Ideally, two metals should be processable similarly, so they work well together in a billet. Hardness – Best results are often achieved when alloys of similar hardness are put together. Chemical compatibility. At the interface, a mixed composition will form. We want to avoid the formation of intermetallic and other compounds that make the billet brittle and may cause the interface to fracture. A wide range of metals and alloys can be used, including copper, gold, steel, palladium, platinum, and silver. Some favored combinations are given below: Science of joining #1 – Solid-State Diffusion Bonding Modern techniques typically use solid-state diffusion bonding, which is used for joining unlike materials together in a wide range of industries. This differs from the traditional technique, which is similar to liquid-phase diffusion bonding. Traditionally, the stack is heated to the point at which some or all of the alloys are above their solidus temperature (begin to melt). The liquid fuses the layers together and solidifies upon cooling. Careful heat control and skilled forging are necessary. Solid-state diffusion bonding can occur when the stack is held at a temperature just below where either metal would melt or any liquid phase would form. Atoms in one metal can move into the other metal and vice versa. Doing so is energetically favorable. Although slowly, the inter-diffusion forms a metallic bond between the layers. Science of joining #2 – Liquid-Phase Diffusion Bonding Liquid-phase diffusion bonding produces a billet with high strength that can be manipulated greater without the risk of delamination and with fewer necessary annealing steps during working compared to solid-state diffusion bonding. Furthermore, liquid-phase diffusion bonding occurs over a far shorter time period. Liquid-phase diffusion bonding occurs, as the name suggests, with the interface in a liquid state rather than a solid state: Diffusion can occur similarly to solid-state diffusion bonding when the metal is heated. This leads to a change in composition near the interface of the two metals, and so we are likely to form a composition with a lower melting point (closer to the eutectic point). The metal (of new composition) at the interface will melt with continued heating. At the liquidus temperature, the liquid phase flows and helps fill any voids and gaps at the interface. On cooling, the interface solidifies, forming a strong bond. Compared with solid-state diffusion bonding, this process occurs at higher temperatures for shorter time periods. It's practical applications are discussed below. Want more? Read more about Mokume Gane, written by James Binnion, Chris Proof, Stewart Grice, and others: For a how-to guide for small manufacture: For modern techniques discussing Mokume Gane via solid or liquid-phase diffusion bonding: We thank Chris Ploof for his support in writing this article.
- Platinum Jewelry: A quick guide
Platinum and Jewelry: A brief history Platinum is an extremely rare precious metal that is commonly used in engagement rings, wedding bands, and other fine jewelry. It is quite common to see this lustrous white metal on the red carpet too! A brief guide to Platinum Jewelry Is it pure platinum? Precious metals are too soft, when pure, to be used as jewelry, especially rings and wedding bands that are worn daily. Instead, we use alloys, a mixture of two or more elements. The other elements improve properties such as hardness, making jewelry more wear-resistant, durable, and shinier for longer. Hallmarks and Stamps of Platinum Like all jewelry, items are stamped or hallmarked by the manufacturer or an assay office to indicate the fineness – i.e., how much platinum is in the jewelry. Platinum is typically sold at 950 or 850 fineness, meaning that 95% or 85% of the piece by weight is pure platinum. Platinum is usually alloyed with copper, cobalt, ruthenium, and sometimes a little bit of indium or gallium (to make it harder). In the US, a stamp such as Pt950 or Plat 950 will be stamped on the jewelry to disclose the fineness. In the UK and Europe, there are a series of marks which you can learn about here. We always recommend checking the hallmark or stamp when buying a piece of jewelry! What are the advantages of platinum? There are many advantages to platinum jewelry: It is a stunning bright white! Platinum jewelry, when polished, is extremely shiny and a beautiful bright white It is hypoallergenic! Unlike some jewelry metals, notably nickel-white golds, Platinum very rarely causes any skin irritation. Nickel is often the cause of contact dermatitis from jewelry, and is absent from platinum jewelry. It does not tarnish! Platinum and its alloys do not react readily and so do not tarnish over time (or noticeably) It is very durable! Although they will scratch, platinum rings are extremely durable (the metal doesn't wear away) and will hold gemstones far better than gold or silver. It forms a patina Over time, wear and scratches will develop a matte "patina" on the surface. If you don't want this then polishing at a jeweler will remove this and restore the original shine Why is platinum jewelry expensive? Despite pure platinum being cheaper to buy on trading markets, it is more expensive to buy platinum jewelry: Fineness Platinum jewelry is typically 95% platinum by weight (950 fineness), whereas 18K Gold is only 75% Gold by weight. Mass Platinum is denser than gold, so the same-size ring requires more material. Workmanship Platinum is harder to work with than gold or silver. It has a higher melting point, and it is far harder to shape Want more? For a comparison with White Gold jewelry, read here: For more information about platinum jewelry, we highly recommend visiting the platinum guild website:
- White Gold, Platinum or Silver? A guide to white metals
Platinum, White Gold, and Silver all look very similar to the eye. However, these three metals are all quite different, with different properties. Wait, how can gold be white? Platinum, Palladium, and Silver all naturally have a shiny white appearance. Yet pure gold has a yellow appearance. So how can they all be "white"? Except for fine jewelry (fine gold and silver), all jewelry metals are not pure metals but a mixture of metals. These are called alloys. The precious metals are simply too soft, when pure, to last the test of time as jewelry. We add other metals such as copper, silver, and rare-earth elements to improve the hardness and durability – these additions can also change the color. In large quantities, adding palladium and nickel to Gold makes the resulting alloy white. Zinc is also added. Similarly, adding a large amount of copper turns gold a rose color. So how much gold, platinum, and silver are in each piece of jewelry? Hallmarks or Stamping tell us the proportion of each precious metal in a piece of jewelry when it was tested by the company or an assay office (depending on the country). It is always important to check the Hallmark or Stamp when buying jewelry. Read here. For white gold, we commonly see 10K, 14K, and 18K standards (41.7%, 58.5%, and 75% Gold by weight, respectively). Platinum is typically sold at 950 Fineness (95% Platinum by weight). Sterling silver is the most common, at 92.5% Silver by weight. Common Questions Be Careful! Plated Jewelry White gold is often plated with a thin layer of metals such as Rhodium to give it an even brighter appearance. While this may look very nice to start with, it will not last forever. Over time, the plating will wear away and eventually expose the metal underneath. In the case of white gold, the color may be noticeable. Any reputable jeweler will be able to tell you if the piece is plated or not, and it is always good to ask to avoid surprises.
- Diffusion Bonding – New or Old Technology?
What is diffusion bonding? As said earlier, diffusion bonding is used for joining unlike materials together. If two clean oxide-free surfaces are heated to a high temperature, atoms in one metal may move (diffuse) across the interface. This forms a bond between the two pieces of metal. Compared to other bonding techniques, the bond area will be very narrow due to the inter-diffusion effect. The alloy composition of the bond region will be a profiled mixture of the two alloys being joined. Defects in diffusion bonding The diffusion rate of different atoms in different metals differs. Ultimately, atoms from one metal may move across the boundary quicker than atoms from the other. As no other metal is involved, this can lead to a row of pores across the interface. Kirkendall porosity actually dates back to some of the earliest studies of metallurgy, specifically proving how atoms actually diffuse! Some elegant experiments show that increasing the diffusion time increases the interface's porosity level. Read here Where is diffusion bonding used? It is often used to join materials of different colors, such as in making billets for mokume gane and for the manufacture of multi-colored rings. Diffusion Soldering Diffusion soldering is scarcely used but has great potential for jewelry manufacture. Between the two surfaces, an interlayer is placed. The join is heated (above the melting point of the interlayer but below the solidus of the alloys on either side of the join and the one that forms at the join), and the interlayer melts and diffuses into the adjacent solid metal. This results in a coherent, homogeneous bond with no obvious interlayer. Due to the diffusion of the interlayer into both surfaces, the join has a higher melting point than the bonding temperature (we have a different alloy). On a phase diagram, we can see how the join becomes liquid and then solidifies again at a constant temperature. Where is diffusion soldering used? This technique was developed for high-karat golds (see Jacobson). It uses a few micrometers of tin, which is deposited as a thin film on a gold foil (for easy use). Diffusion soldering is performed at 250˚C/482˚F, which is slightly above Tin's melting point, and a small pressure is applied to ensure contact. The soldering occurs in less than a second. In the second step, the joint is heated to 450˚C to homogenize the joint. This technique is advantageous in that bonding is accomplished at low temperatures. There is no loss of strength and hardness in the workpiece, and the joint is strong. It could also be used to join unlike alloys such as white and yellow gold or platinum and karat gold, for example. Want to read more about Diffusion soldering?
- An introduction to soldering
Soldering involves joining two pieces of metal by melting and solidifying a filler material between them. In soldering, the pieces being joined do not melt (only the filler metal, the solder, is melted). Strictly speaking, this process should be called Brazing since it tends to be carried out above 450˚C (840˚F), the arbitrary temperature separating brazing from soldering. Soldering Technique To make good soldered joints, we rely on capillary forces to encourage the flow of molten solder along the joint gap, so a thin joint gap is preferred. Thin joint gaps also give rise to stronger joints. For good soldering: Focus the heat on the parent metal, and allow the solder to melt and flow along the gap, preferably towards the hotter end. We need clean, oxide-free surfaces to ensure good wetting of the solder over the to-be-soldered surfaces. Solder fluxes appropriate to the soldering temperature are recommended. Choosing the correct solder For jewelry materials, the solder chosen should have the following properties: The solder should have a lower melting range than that of the parent metals being joined. As a general rule, the liquidus temperature of the solder should be ≥25˚C/45˚F below the solidus temperature (where the alloy begins to melt) of the parent alloy. The solder melting range (the difference between the liquidus and solidus temperatures) should be suitable. Many solders are often based on eutectic compositions (i.e., they have a low melting range). The molten solder should have good fluidity to enable it to flow into the joint gap. The color should match the parent metal as closely as possible, so no solder lines are visible. This is not always easy to achieve, especially for high-karat or high-copper gold alloys. The fineness/karatage should match the parent metals where possible but should certainly conform to hallmarking/stamping regulations where lower fineness/karatage solders may be allowed. The solder should be chemically and metallurgically stable, and compatible with the parent metal. One does not want brittle phases to form in the joint area or for the solder to tarnish in service preferentially. The soldered joint should have reasonable properties (e.g., strength, ductility, wear and tarnish resistance) compatible with the parent metals being joined. Solder alloys should preferably be relatively easy to cast and work to the desired form, but some solders are often hard and difficult to work. Their incorporation in recycled scrap should not lead to later problems such as embrittlement, as is the case for lead-tin-based solders. Solders should be free of toxic elements, notably Cadmium. Although widely used in some parts of the world, a full range of cadmium-free solders are available, up to 22K. As a result of these requirements into account, it is perhaps not so surprising that many solders are based on the parent alloys in composition but with alloying additions to lower their melting range. Typically: Colored karat-gold solders are typically based on gold-copper-silver alloys, often close to eutectic compositions, because they have the lowest melting temperatures. Platinum and Palladium have no real color-match problems – they may be platinum, palladium-based, or white-gold alloys. As with gold, solders are based on parent silver alloys with alloying additions to lower melting ranges for the silvers. The selection and evaluation of various solders have been discussed by Grice. Solder is often used as wire or small coupons (paillons) cut from strip. In some cases, other forms may be more useful: For items produced by stamping, a solder-flush strip or sheet (where a layer of solder is bonded to the sheet) is often used for one-half of the jewelry; the two matching halves are then jigged together and passed through a belt furnace to effect the soldering. For chain making, the use of solder-cored wire makes post-soldering easier than the traditional solder powder method, although in situ laser welding is now finding favor. Step-wise soldering and Solder Grades In many cases, a series of solders with a range of melting temperatures enable step soldering in fabrication and safe soldering in repair situations. Starting with a higher melting point solder and progressively working downward permits the soldering of a number of joints in close proximity with little risk of previous joints being re-melted. This gives rise to the terms hard (highest melting point), medium, easy and extra easy, for example, to describe such solder grades. Sadly, solder manufacturers have no uniformity in applying these descriptions. A medium karat-gold solder from one manufacturer may have a similar or lower melting temperature to one from another manufacturer described as easy. Soldering Fluxes – A guide Conventional fluxes are based on borax (sodium tetraborate) with other additions such as boric acid and silicofluorides, usually made with potassium salts rather than sodium to prevent the yellow glare. Such fluxes remove surface oxides and contaminants by the dissolution of the oxides before the solder melts and then by excluding oxygen ingress to prevent further oxidation during solder flow. Ceramix-based fluxes are a more recent development that can be easily removed with hot water and brushing after soldering. If left on the jewelry, they may lead to localized corrosion. Should I use Solder paste? Solder paste has many advantages over conventional solder paillons to make joints. Solder pastes are simply normal precious metal solders in powder form, mixed with an organic binder to make a paste, which volatilizes off to leave no residue. Often, for torch soldering in air, they will also contain some appropriate flux. The fluxless version is usually used for belt furnace soldering under a protective atmosphere. They can save a bench worker a great deal of time since positioning is less critical, and so productivity is far higher. It is also easier to control the amount of solder used. However, solder pastes are more expensive and have a shorter shelf-life. Solder pastes can be purchased in all the usual colors (for gold), a full range of compositions (i.e., easy, medium, hard grades, etc.) in the full range of karats/finenesses. Solder pastes are available for gold, silver, platinum, and palladium jewelry. They are usually purchased ready for use in plastic syringes of various sizes (weights) and needle/nozzle sizes.
- Which Join? Soldering, Brazing, Welding and Bonding
Welding, brazing, and soldering are all methods for joining two or more pieces of metal. The key difference among these processes is the temperature used to create the joint. Diffusion bonding is a more modern technique involving heat but no filler material. Welding The two pieces of metal are melted to create a consolidated joint. Welding may or may not involve the use of filler metals or shielding gases. Welding is becoming increasingly popular with TIG (tungsten inert gas), electrical resistance (tack or spot) welding, or laser. Brazing and Soldering Filler metal is melted between the two materials, but the two pieces of metal are not. The liquid filler metal wets the base materials through capillary action. When the liquid filler metal solidifies, it is bonded to the base materials, creating a joint. Brazing is a higher temperature equivalent of soldering. The cut-off temperature is 450˚C/840˚F. Bonding Heat and pressure are applied to the metals being joined, but no melting occurs if we are joining dissimilar metals. This is used in Mokume Gane. Cover Photo of a Pulse Arc Welder. Source: The Bench by Cookson Gold
- Firestain in Silver
Firestain is a common problem in silver manufacture, associated with corrosion. It commonly becomes a problem during polishing since it is difficult to remove. What is Firestain? Most sterling silver is based on silver-copper alloys. When heated in the air (or an oxidizing atmosphere), the copper can oxides. This oxidation occurs on the surface and penetrates deep into the material, forming a dark black oxide known as Firestain. In silver, oxygen diffuses into the surface regions faster than copper can diffuse out to the Hence, it penetrates deep into the surface, forming a copper oxide scale. While pickling in a hot acid bath can remove the surface oxide layer, it cannot remove the deeper copper oxide. The result is a soft, nearer-pure silver surface with a silver plus copper oxide layer underneath. If heated further, e.g., during soldering, the oxygen can quickly diffuse through the silver layer, causing further oxidation beneath. Polishing reveals dark, disfiguring stains. Deep firestain can also cause cracking during working and during soldering. Preventing Firestain Fire stain was traditionally prevented by: Protect the metal surface throughout heating by using flux This is time-consuming and only practical for larger work Working in a protective non-oxidizing atmosphere This is expensive and only feasible for small pieces of work Heavy polishing or electrochemical stripping in a cyanide solution This is wasteful and dangerous. Fire-stain resistant alloys The development of Silver-Copper-Germanium alloys, known as Argentium, has been a major development in silversmithing. Many other competing alloys, with comparable alloys have also been developed. Germanium has a greater affinity for oxygen than copper or silver and fulfills a protective function in the molten state and the wrought/cast alloy. The alloys are two-phase, with a copper-rich and a silver-rich phase. The germanium forms a protective and transparent oxide layer on the surface, which prevents firestain during soldering and annealing. In addition to being a powerful deoxidizer, germanium also helps to improve the fluidity of the melt, which is desirable for investment casting. These alloys can be age hardened, although some properties differ from usual silver alloys. Firestain caused by Zinc Other alloying additions to silver, such as zinc, can also oxidize quickly, forming an internal oxide scale. For zinc, this scale is often whitish in color and so is less obvious to observe.
- Corrosion-related cracking in low-karat gold?
Despite the vulnerability of the low-karat golds to this phenomenon, it is not widely observed today, which tends to suggest that the manufacturers are at least aware of the potential problem and take steps to minimize its occurrence. Stress corrosion cracking can occur in some lower karat-gold alloys (8–10K and less frequently in 14K, but particularly nickel-white golds) due to the combination of a corrosive agent and an applied or residual stress. The corrosive agent is typically a chemical in the local environment. Cracking or, in the worst cases, spontaneous failure only occurs by this method in the presence of both a stress and a corrosive agent. It may happen during the following: Manufacture Due to fumes from pickling Service Exposure to chemicals during consumer use, even after many years of satisfactory service. The mechanism of stress-corrosion cracking is complex and not discussed in full here. Causes of stress The residual stress is commonly introduced during manufacture. It is important to stress-relief (or fully anneal) potentially susceptible alloys before the point of sale. A suitable treatment for stress-relied anneal would be 250-350°C/480-650°F, typically for 30 minutes, and has little effect on hardness or grain structure. Unfortunately, stresses can also be introduced in service with the customer (for example, chains stretched by children pulling them) or by the jeweler when rings are re-sized without a subsequent annealing treatment. Crack initiation is likely at a defect, scratch, or even a stamped mark, where stress is concentrated in the surrounding material. Causes of corrosion Chlorine or chlorides are a major agent; acids and pickling solutions are the usual source at the manufacturer. For the user, the list is almost endless: detergents and household cleaning fluids, inks, swimming pool water, sea water, and coastal atmospheres, some foodstuffs, perfumes, deodorants, and human sweat can all act as the source of corrosion. The failure usually manifests itself by a characteristic intergranular fracture. The susceptibility of alloys to stress corrosion is particularly determined by the composition and by metallurgical structure, but the subject is a complex one with initial attack determined by local differences in composition and initiating at a defect or stress raiser. For further discussion, see: W.S. Rapson, “Advances in Knowledge Relating to Gold Alloys and Their Use in Jewellery,” The Santa Fe Symposium on Jewelry Manufacturing Technology 1995, ed. D. Schneller (Boulder: Met-Chem Research, 1995): 65-82. G. Normandeau, “Stress Corrosion Cracking Evaluation of Cast Products and Stamped Four-Claw Settings,” The Santa Fe Symposium on Jewelry Manufacturing Technology 1991, ed. D. Schneller (Boulder: Met-Chem Research, 1992): 323-352. J.J.M. Dugmore and C.D. DesForges, “Stress Corrosion in Gold Alloys,” Gold Bulletin 12, no. 4 (1979): 140-144. S. Grice, “Failures in 14Kt Nickel-White Gold Tiffany Head Settings,” The Santa Fe Symposium on Jewelry Manufacturing Technology 2002, ed. E. Bell (Albuquerque: Met-Chem Research, 2002): 189-230. C.C. Merriman et al., “Environmentally induced failure of gold jewelry alloys,” Gold Bulletin 38, no. 3 (2005): 113-119. C.W. Corti, “Jewelry —Is It Fit for Purpose? An Analysis Based on Customer Complaints,” The Santa Fe Symposium on Jewelry Manufacturing Technology 2018, ed. E. Bell et al. (Albuquerque: Met-Chem Research, 2018): 163-176.