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Rolling sheet material can result in edge cracking due to overworking between anneals, exacerbated by the poor-quality surface on the edges. As we work a metal, work hardening results in a loss of further ductility and, ultimately, failure of the material by cracking. The metal should be sufficiently annealed during working steps to allow further work. Preventing cracking As soon as cracking is seen at the edges, the sheet should be trimmed to prevent cracks from propagating into the center of the sheet. Fins and laps can arise during rod rolling, which can open up as cracks at later stages: Fins form when too much material is pushed into the rolling groove, so the excess metal is squeezed sideways. Laps form when newly formed fins are rolled into the rod. To prevent lap formation, use a smaller reduction on each pass and rotate the rod through 90˚ between successive passes.

Extensive manual work, such as hand-raising a sheet, will often involve working on a soft former to prevent surface damage. Such soft formers are often made from lead. Lead forms brittle inter-metallic phases in gold and platinum alloys. Hence, if lead-based residues are left on the surface during annealing (to restore ductility during working), the lead can diffuse into the gold. On further working, the lead impurities can lead to embrittlement and failure of the jewelry item. The use of metallic lead in contact with gold and platinum metals is always risky. If considered essential, the piece must be separated from the lead by interleaving it with a tough grade of paper.

Embrittlement by Impurities Certain impurities and alloying metals can cause embrittlement in all precious metals at low concentrations. This means the material has less inherent ductility and will fail before expected under moderate stress at room or hot working temperatures. Many of these impurities are elements with low melting points. If present in the metal, they tend to lie preferentially on the grain boundaries. At grain boundaries, they form second phases or eutectics with low melting points. They often have extremely low solubility in the alloy itself. Mechanism of cracking due to embrittlement Most low-melting-point contaminants, such as silicon and lead, can cause embrittlement of gold, silver, platinum, and palladium alloys at very low concentrations. The presence of these particles impedes deformation and act as pinning sites. The effect is magnified if the grain size of the alloy is large; there are many particles dispersed in very thin films around the grain boundaries. The effect is less pronounced in fine-grained alloys; they have the same number of particles but a much larger grain boundary area. Often, these contaminants manifest themselves as cracking during metal-working operations. Causes of impurities Embrittling elements can occur in alloys from a number of sources: Impure starting materials Starting materials should be pure metals or pre-alloys (master alloys) Purity should be a minimum of 99.9% to avoid embrittlement by certain rare-earth elements. Gas content (especially oxygen for silver and hydrogen for palladium) should be low. Check specific alloys: E.g., Tarnish-resistant Bright Silver should have selenium and tellurium levels below two ppm to avoid embrittlement. Impure scrap The most common issue – it is a continual source of contamination. Common causes: refractory metals such as investment casting particles, oxides from dirty surfaces, lead-tin solder (and other solders) from repaired jewelry, sulfur contamination, and gas porosity. The use of scrap to make new products should be strictly controlled and, preferably, should be subject to melting and analysis before use in making up new alloy ingots or recycled in investment casting. Pick-up during processing from the atmosphere or contact with other surfaces Carbon can be picked up during melting in graphite crucibles, which is a problem for palladium and platinum alloys and palladium-containing white golds in particular. Silicon crucibles can also contaminate platinum and palladium melts if melted under reducing or neutral conditions. Overalloying Too frequently, melters tend to add “a bit extra” when alloying. Very minor additions to alloys, such as grain refiners and fluidity promoters (e.g., silicon), can be deleterious if too much is added. Customer use Contact with liquid mercury leads to the whitening and embrittlement of karat golds and other precious metal alloys. Mercury forms amalgams with precious metals very quickly. Understanding "Hot shortness" Most of these elements that cause impurity form intermetallics and second phases, which often have low melting points because they form deep eutectics. When these second phases have a far lower melting point than the bulk alloy, it means they melt first. The melting of grain boundaries before the rest of the material is known as incipient melting. Hot shortness is where incipient melting occurs during hot working due to impurities. It is irreversible, resulting in material scrapping. Phosphorous is often a cause of incipient melting in sterling silver, and sulfur often does the same in platinum. Porosity in Casting Shrinkage Porosity Ingot Casting When a cast ingot solidifies, crystallization of the liquid results in shrinkage since atoms in most solids pack more densely than their respective liquid (ice being a notable exception). In casting an ingot, the result of shrinkage is the formation of a central pipe at the top of the ingot. It is necessary to cut off this pipe before working the ingot. Otherwise, a central defect will be introduced, which will elongate on working and is likely to result in subsequent longitudinal cracking. Pipe shrinkage can be minimized by not excessively overheating the alloy above the liquidus temperature. The temperature should not exceed 95˚C so that the shrinkage due to the sample cooling isn't too great. The shrinkage due to crystallization is fundamental and cannot be changed for a specific alloy. High casting should generally be avoided since they encourage large grain sizes. This decreases the ductility of the alloy and, at the same time, it magnifies the effect of any low-melting-point impurities that might be present. Investment Casting In investment casting, the volume of the mold is filled, and so shrinkage often manifests itself as shrinkage porosity. This can be avoided by ensuring the feed of liquid metal to the casting is prevented by premature solidification in thinner sections or in the feed sprue. The positioning and size of the sprues (into which the liquid is poured) are important to minimize their occurrence. Generally, sprues should be placed on thicker sections (whose centers are last to solidify and need the most material), and multi-feed sprues may be needed for complex-shaped castings. Gas Porosity Gases such as oxygen and hydrogen can readily dissolve in molten alloys. Upon cooling, the solubility of the gas decreases, and so it is ejected as gas bubbles. This can lead to gas porosity in the casting. Gas porosity is often due to sulfur dioxide, oxygen, or other gases. Compared to shrinkage porosity, the pores are usually fine and round, whereas shrinkage pores resemble a dendritic shape. The gas itself may arise from: Dissolved gas in the start material or moisture Gas dissolved during melting – Too high a temperature, non-protective atmospheres, a flux, or gas-based melting. Sulfur dioxide usually originates from the chemical breakdown of gypsum-based material used to make the investment mold. If the gas is present in the molten alloy before casting, such gas porosity will manifest itself throughout the casting, whereas if it occurs as a result of a chemical reaction with the investment during casting, it will manifest itself as a layer of porosity at the surface. To minimize porosity, it is best practice to ensure clean burnout of the mold, use cleaned scrap in the melt charge, and minimize the casting and/or flask temperatures. Gas porosity affects the tensile strength and ductility of castings. Porosity can also appear later in fabrication operations as surface blisters, defects, cracks, or internal porosity. Porosity and blisters Silver is especially prone to surface defects arising from porosity due to poor de-oxidation. Initial working may flatten the pores and cause small laminations and cracks on the surface. It can also close the porosity only for annealing operations to allow the gas to expand and reappear as pores and blisters on the surface. Annealing silver containing dissolved oxygen or copper oxide inclusions in hydrogen-containing atmospheres can result in hydrogen absorption and the formation of pores due to the “steam” reaction between hydrogen and oxygen. It can also occur in karat gold: $\rm 2H_2+O_2\to {H_2O}_{[gas]}$ $\rm H_2+Cu_2O\to2Cu+ {H_2O}_{[gas]}$ Inclusions and other Defects Inclusions Inclusions are undesirable foreign particles that act as stress raisers and give rise to cracks or failure during subsequent working. They can be incorporated into the melt as oxides, refractory or even metallic particles from the casting equipment, by erosion of the crucible, furnace lining, stirring rods, or a reaction between a non-inert atmosphere and base metal alloying additions or grainers such as copper or zinc, or indium respectively. Surface defects Surface defects can be the starting point of cracks. They often arise due to poor melting and casting. A splash during casting that solidifies and sticks to the mold wall, surface inclusion, surface oxidation, or mechanical damage can all be the cause. To avoid this, all ingot surfaces should be inspected and defects removed and cleaned away before working. If necessary, the surface might have to be milled to ensure it is clean and flat.

Cracking & Overworking If metal is cold-worked without annealing, continued working will ultimately lead to failure by cracking. If a cylinder of metal is loaded in tension, the metal will respond as follows: The Elastic Regime – It will stretch and become longer for low loads. When the force is removed, the sample will return to its original dimensions. The strain is proportional to the force, i.e. Hooke's Law. The material behaves elastically. At a certain load or stress (force per unit cross-sectional area), the material begins to deform plastically. This deformation is permanent. Because volume is conserved during deformation, as it becomes longer, it becomes thinner. As we deform the material, the material becomes stronger and harder, known as work hardening. In the plastic region, the stress starts to fall at some point. This is due to necking. During necking, work hardening still occurs. If we continue, the specimen under test starts to crack up, and it eventually fractures. The cause of work hardening As the material is deformed, the material becomes stronger and harder. This is called work hardening. While work hardening helps to harden jewelry, improving durability. It also ultimately leads to failure. Deformation occurs through the motion of dislocations. More deformation requires more dislocations. As the number of dislocations increases, they impede the motion of one another, thereby increasing the necessary stress. At some point, the necessary stress required for further dislocation motion and deformation exceeds the necessary stress for cracking, and so cracking occurs. When we anneal the alloy, alongside an increase in grain size, recrystallization reduces the dislocation density. Removing dislocations restores ductility and allows further working. Avoiding overworking Overworking can occur in all forms of metal working – sheet and rod rolling, wire and tube drawing, blanking, stamping, coining, spinning and raising, milling, turning and machining, or simply bending by hand. The extent an alloy can be worked before failure depends on the microstructure. Materials in age-hardened condition or with hard second phases will have inherently lower ductility and tend to crack more easily on further working. Materials should be annealed roughly after about a 70% reduction in thickness before annealing. However, there are considerable variations: Nickel-white gold hardens rapidly. Annealing is necessary before a 40% reduction in thickness. Fine gold and some of the high-karat golds can be worked well in excess of 90% reduction in the area before annealing becomes necessary. Cracking & Incorrect Annealing Residual internal stresses and Quench Cracking Non-uniform deformation can often lead to residual stresses in wrought items. In many processes, there is a tendency for the deformation to be focused on the surface regions, and there is less work done in the center. Residual stresses can be circumferential (round the sample), longitudinal (along the length), or radial (pointing outwards from the center). On annealing samples, they can sometimes spontaneously crack. This is attributed to residual internal stress. Residual stresses can be relieved by a low-temperature anneal without reducing the hardness. A common example of this effect is fire-cracking in Nickel-white gold. Nickel-white golds work harden quite rapidly, so they require more frequent annealing. They must be quenched after annealing to prevent the formation of two immiscible phases and any resulting hardening effects due to the second phase. Nickel-white and yellow karat-gold alloys, particularly those containing silicon additions, are prone to quench cracking. Repeated heating and cooling generate internal stresses as the outer layers cool more rapidly than the middle of the metal during quenching; the stresses generated can be large enough to cause cracking in such inherently brittle alloys. To avoid the quench-cracking effect and also avoid the hardening effect induced by slow cooling in nickel-white golds, an intermediate cooling rate is performed using a variety of techniques: Forced cooling in air Cooling by placing on an iron plate Quenching into hot water Slow cooling until close to the critical temperature, at which the second phase begins to precipitate and then quench. Individual manufacturers will have their own techniques for particular alloys and sizes of components but still will often face difficulties. Hot Shortness For investment-cast silicon-containing karat gold, the presence of second-phase particles with a low melting point at the grain boundaries leads to cracking. The residual stresses caused by cooling from too high a temperature lead to hot tearing and cracking. Fire Cracking Fire cracking commonly occurs in nickel-white golds during annealing or soldering. Heating, combined with residual stresses from working operations, is sufficient to fracture the component as the temperature increases (and the strength decreases). The remedy is to slowly heat up to 300°C/575°F and hold at this temperature for a time to relieve the stresses before continuing onto the annealing or soldering temperature. Overannealing and Orange Peel Annealing at too high a temperature, for too long a time (or both!) can result in large, coarse grains. Subsequent deformation can lead to premature cracking and fracture (as well as an “orange peel” surface). This is particularly a problem with torch annealing, where the capability to control temperature is limited. Read more about orange peel effects here.

Even in the best workshops, a piece can crack. While a defect or crack is obvious, the cause is often not. The cause of the crack could be due to the behavior of the specific alloy or the history of its processing. Many different phenomena can lead to similar-looking cracks. Consequences of cracking For metallurgists, a crack is a puzzle to solve, but for a jeweler, it has many undesirable consequences: Item is unusable – it must be scrapped Costly repair or replacement Loss of time and perhaps reputation The concern of repeated occurrence if left unsolved The issue of defect and cracking is not unique to jewelry; it is a problem that pervades all metallurgical manufacturing, from submarines to medical implants. Types of cracks There are two main types of cracking: Intergranular fracture The cracks grow along grain boundaries, so the fracture surfaces are rough. This is a ductile metal fracture, and the fracture surfaces are dull. Transgranular fracture The cracks cut in straight lines across and through grains. The surfaces are quite smooth. This is often associated with the fracture of very brittle metals, and the fracture surfaces are shiny. Cracking of jewelry materials can occur during and after manufacture. The problem is that cracking in jewelry materials is usually intergranular in nature (cracks grow around the grains along the boundaries). Still, the cause can vary widely, making tracing the cause difficult. Why do jewelry materials crack? The answer is simply that the imposed stresses exceed the material's mechanical strength, or the material is strained so quickly that the material can't deform fast enough. Cracks often occur and begin at defects. These reduce the stress value at which failure occurs. A defect disrupts the crystal lattice structure, so there are fewer atomic bonds to bear the load at a given point in the lattice – the stress is concentrated. Cracking will occur when the stress concentration is high enough to break the bonds. Hard particles such as inclusions or second phases at the surface (Figure 4) or within the alloy can act as stress raisers, i.e., they amplify the local stresses in the adjacent metal, which means that cracks can initiate and grow more easily around them, even when the imposed stresses are lower than those normally leading to crack formation. Causes of cracking Cracks can be due to several causes, including: Mechanical overworking Embrittlement by impurities, including gases Casting and working defects and inclusions Stress corrosion cracking Quench cracking in castings Fire cracking The various causes of defects, many of which manifest themselves as cracking during working and manufacturing operations, can be attributed to the following factors: Poor quality start materials Recycled scrap often causes contamination and possible embrittlement. Poor melting practice Casting defects such as pipes and/or gas porosity and blisters, incorporation of inclusions, excessive shrinkage porosity, and chemical segregation Poor ingot or material working practice Incorrect working procedure for the characteristics of the specific alloy It can also lead to surface defects, such as laps, that develop into cracks. Incorrect annealing practice Residual (internal) stress is possibly linked to a corrosive environment. This can be generated mechanically or thermally and lead to phenomena such as stress corrosion cracking, quench cracking and fire cracking.

Why do we work after casting? While cast materials may not have the final shape wanted, the microstructures may also need working to optimize the properties of the final product. Two common issues are: Chemical segregation Separation of constituent elements during casting can lead to fast corrosion, softer materials, and embrittlement. Coarse microstructures A coarse microstructure leads to a softer, less wear-resistant material and issues such as "orange peel." Working after casting is done to refine the shape of the final product (e.g., starting from an ingot, wire, plate, rod, etc.) and the overall microstructure. Preparation Before working with ingot materials, we often need to grind off surface defects and crop off the top of the ingot containing the pipe to prevent defects such as surface cracks, oxide/slag inclusions, internal cracking, and porosity from developing. Hot working or cold working? For precious metals, cold working refers to deforming at room temperature since room temperature for each of our four precious metals (silver, gold, palladium, and platinum) is relatively cool compared to their melting temperatures. Deformation occurs by slip, which involves the generation and motion of dislocations. As we deform further, we introduce more dislocations that impede one another, increasing yield strength and hardness but reducing further ductility. Ultimately, the material will crack. Hot working is typically done at temperatures above half the melting temperature (in absolute degrees kelvin) of the alloy. Hence the working temperature is specific to each alloy. If we work a metal when it is hot, the temperature is high enough for the recrystallization to occur during deformation. The dislocations generated from deformation are 'annealed" out just as quickly. This means we preserve the "soft annealed" condition, which can be repeatedly worked before cracking, without any increase in strength or hardness. Metallurgists call this "dynamic recrystallization". Cold-working metal is performed at ambient temperature. It is typically performed to impart additional hardness, improved strength, and a more accurate shape and superior surface. As the temperature is low, recrystallization doesn't occur. The metal becomes harder, and it must be annealed from time to time to avoid overworking. Microstructure and working: A wedding band case-study When a wedding band is rolled and deformed (upset), not all parts of the metal experience the same force, and so the microstructure is not the same throughout the cross-section. It is heterogeneous. Deformation typically occurs the most at the surface, and so the grains and microstructure will be most heavily deformed here. This means the surface will be more brittle, and cracking is often likely to start at the ,surface. When we subsequently anneal, the fibrous-like grain structure is disrupted by equiaxed (round) recrystallized grains. Overworking and microstructure If we repeatedly work a metal without annealing or working in the "hot" state, then the material will become harder and more brittle. If we overwork a material, it can crack or fracture. To avoid overworking in the cold state, we subsequently anneal. The resulting grain size after annealing depends on the amount of cold work, the annealing temperature, and time: More cold working leads to finer grain size Annealing for longer and at higher temperatures leads to a larger final grain size To avoid too large a grain size, it is recommended to anneal only after substantial cold work (e.g. a 60% reduction in thickness). For a given sample, the desired annealing period varies with temperature and annealing time after cold work. Should I hot work or cold work my alloy? There are many advantages and disadvantages to hot working: In jewelry, cold working is often preferred because: Better control of final shape, dimensions, and deformation pattern Ability to work harden alloy, improving service behavior of jewelry Better control of the surface quality of the item However, when cold working, it is necessary to remember to anneal and maintain equipment to the standard necessary for the quality (e.g. roll surface dressing, etc.). Key points The key points are as follows: An as-cast alloy can have an undesirable shape and properties, thereby requiring further work. When we deform the alloy at room temperature, it becomes harder and more brittle. Annealing is necessary to avoid cracking (overworking). The microstructure is heavily dependent upon the amount of work and the temperature and time of annealing. It is preferable to work the alloy when hot for extensive shaping so the microstructure recrystallizes during the deformation. This is known as hot working. For hardening the material, cold working is best.

Casting is the process of solidifying molten metal in a mold to achieve a particular semi-finished shape or to produce ingots, etc., for further working. As seen in "what happens to a metal during casting," the casting process should be carefully controlled to control the microstructure, notably grain size. There are still many other common issues that can occur during casting, which metallurgists can explain. Segregation and coring When a metal alloy solidifies, it usually solidifies as dendrites. This is due to an effect called constitutional undercooling, which is not necessary to explain. Essentially, as seen from the phase diagram, the composition of the liquid and solid in the "mushy zone" during solidification are not the same. Some of the "solute" in the solid is rejected into the liquid. As solidification proceeds, more solid forms from a progressively more "solute-rich" liquid. Hence the composition of the solid changes. This is known as "coring". As seen in the phase diagram, if an alloy has an overall composition of X, when molten, it has a uniform composition. At a given temperature T, where T lies in the "mushy zone", the liquid will have composition P, and the solid will have composition Q. As the temperature drops to T, the solid now forming approaches composition X, while the remaining liquid has composition R (due to the deficit of solute in the solid that first formed). Due to this partitioning effect of solute between the liquid and solid, the composition of the final liquid to solidify can be very different composition to the main solid. This leads to the segregation of solutes at grain boundaries. Segregation creates many issues regarding corrosion resistance and mechanical properties – while small grain size increases strength (Hall-Petch Relation), the grain boundaries themselves are sites of weakness. The structure is often refined and homogenized (made uniform) through working and annealing to alleviate these concerns. Porosity Water and ice have different densities. Ice is a famous but peculiar exception because the solid is less dense than the liquid. In most cases, the solid is denser than the liquid. For a given amount of molten metal will shrink upon solidification. The atoms in a solid are more densely packed in the ordered crystalline lattice compared to the random arrangement in a liquid. When casting, the mold is a fixed volume, and so is the solidification upon shrinkage (typically 5% for gold alloys). This discontinuous "jump" in density makes crystallization a "first-order" transformation. In large ingot castings, a shrinkage cavity can form on top of an ingot or casting. To avoid this, it may be necessary to provide a reservoir of molten metal to allow for this shrinkage. Sometimes the top of the casting can solidify prematurely, leaving molten metal inside, which will also shrink on solidification, leading to internal porosity or a “pipe,” as it is known (see above). Small-scale internal porosity? Smaller-scale internal porosity can arise from shrinkage, too. When dendrites form, the "arms" of the dendrites have a small space between them. Even if the liquid is fluid enough to fill those gaps, the final solidification shrinkage may lead to small porosity, especially if the insufficient molten metal is fed back into this region during casting. Internal shrinkage porosity is typically difficult to avoid for investment casting, especially for alloys with a wide solidification range or castings with complicated shapes (where feeding metal is difficult). Such shrinkage porosity leads to a dramatic reduction in strength – the pores act as starting points for cracks. In this example of shrinkage porosity, the spacing between dendrite arms is clear to see. Alligatoring in ingots If a metal casting temperature is relatively high, and thereby the cooling rate in the mold is high, then the microstructure of an ingot will typically show columnar growth to the center. The columnar growth leads to a continuous "seam" in the center of the casting. This "seam" is essentially a series of grain boundaries. Grain boundaries are a major point of weakness; they are points along which cracks can easily propagate (like mortar in a wall of bricks). This is not a good structure if you are going to roll the ingot to a plate or sheet, as it may split down the middle (known as alligatoring).

Casting is a process for producing alloys of the desired composition and also for specific shapes. These can be either net shapes, as in investment (lost-wax) casting, or stock materials, i.e., ingots, that can be further processed to modify the shape, structure, and properties. In addition to shaping the metal, the way in which the metal cools substantially affects the microstructure of the metal alloy and, therefore, the properties of the jewelry. Casting should be carefully controlled to ensure high-quality aesthetics and properties. Quite simply, casting involves pouring molten metal into a mold. As the metal cools in the mold, it solidifies. The casting conditions and the shape of the mold are carefully controlled to ensure a high-quality casting (excellent surface finish and few defects throughout the material) and excellent mechanical properties of the final piece – e.g. strength, hardness, and toughness. Details of casting The overwhelming majority of casting tends to be static casting, although techniques such as continuous casting have many advantages for large-scale production. Static casting usually involves melting by gas heating, oil-fired furnaces, electric resistance heating, or induction heating; the latter ensures maximum stirring of the alloy constituents. For gold and silver, crucibles are typically clay-graphite, or graphite (fireclay for nickel-white golds as nickel will react with graphite), and casting will usually be into dressed iron molds or water-cooled copper molds. For platinum and palladium, zirconia or zirconia-washed silica crucibles are preferably used. That said, many casters of palladium use silica crucibles without too much problem, provided temperatures are kept to a minimum and reducing atmospheres are avoided. Science of casting Atoms in a liquid are mobile and free to move amongst themselves. The ease with which they move depends largely on the viscosity of the liquid (how easily it flows). Cooler liquids and those that are more densely packed (e.g., composition) are more viscous; the liquid has slower flow, and the atoms are less mobile. Solidification into a crystal involves two key steps – nucleation and growth. For a polycrystalline solid to form, the nuclei (small clusters of atoms with a particular structure) must form, either on their own or with the help of other particles (impurities or those deliberately added, such as grain refiners). These nuclei form randomly and must reach a critical size to be stable. Once these nuclei are stable, they will grow into grains. Crystal growth occurs by atoms in the liquid "fixing" onto the nuclei. They do so at particular sites which involve the least energy – these are called "preferred crystal directions." The grains will continue to grow until they collide with (impinge on) one another and all the liquid has solidified. Pure metals vs. alloys Pure metals solidify and melt at a single temperature, whereas alloys solidify and melt across a temperature range. These are bound by the solidus (the temperature at which the solid begins melting) and the liquidus (the temperature at which the liquid begins solidifying). The solidus and liquidus temperatures vary with composition. During solidification, there is a temperature range at which the casting is partially solid and partially liquid – the "mushy" zone. Casting and microstructure As discussed, nearly all jewelry metals are crystalline. A crystalline object is usually polycrystalline – it is made up of many small crystals or "grains". The size and number of these grains depend on the processing history of the material. Grain size varies from a few millimeters to hundredths of millimeters. Smaller grains usually lead to higher strength (and hardness) and toughness (higher resistance to cracking). It also reduces the risk of "orange peel" defects, etc. Fine grain size is desirable for jewelry. Dendrites – what are they? In alloys, casting often results in tree-like microstructures known as dendrites. The branches of the dendrite form along preferred crystal directions. As mentioned, atoms in the liquid can fix onto these directions with the least energy, so growth along these directions is fastest. Dendrites occur in alloys because the composition of the liquid and solid during solidification may not be the same. These differing compositions mean that little bumps (perturbations) in the solid-liquid boundary during casting may be stable, and so the boundary isn't flat. This is known as constitutional undercooling – you can learn more in our explanation of defects or here. Importance of cooling rate Casting involves melting and the solidification of molten metal as it cools in the mold. When a metal solidifies, atoms, which are mobile and free to move in the liquid, fix onto growing crystals. Atoms in a crystal have fixed positions in the crystal lattice. The cooling rate depends on the shape of the casting/mold and the mold material (higher conductivity means higher heat extraction and faster cooling). As the cooling rate increases, more nuclei will form (there is a larger undercooling below the liquidus temperature – the liquid is less stable than the solid). Once formed, all nuclei will grow at a similar rate. Ultimately, a higher cooling rate leads to a finer microstructure and smaller grain size. Grain structures of ingots Different grains in the polycrystalline ingot will have different orientations. When surrounded by liquid, the orientation of the nucleus (from which the grain grows) is random, but if we have nucleation near a mold wall, the grains may collectively be oriented similarly. This is known as "texture". The grain structure of a whole ingot may be split into three key sections; next to the wall, an intermediate region, and the center of the ingot. When the metal is poured into the mold, the metal near the mold wall will cool first. The wall acts as a favorable site for nucleation, so many nuclei form at once. A thin layer of fine grains is formed, known as the chill layer. Some of these "chill" grains may be favorably oriented and so grow faster than others (the temperature gradient moving into the liquid aligns with the preferred growth direction). Long columnar grains grow inwards from the chill layer towards the center of the ingot. Depending on the cooling rate, the center of the ingot can take different forms: Using a metal mold, the columnar grains extend into the center of the ingot at high cooling rates. This can create issues in processing, known as "alligatoring". At low cooling rates, using a ceramic mold e.g., investment casting, the cooling rate is low enough that nuclei can form in the liquid. This leads to "equiaxed" grain throughout the center of the casting.

Pure metals are very soft. They can only be strengthened by grain refinement and work hardening, which provide a limited hardening effect. This is due to the intrinsic properties of Gold (for metallurgists, it has a high stacking fault energy). For this reason, alloys have been used to increase strength and hardness. In Asia, 22 and 24-karat gold alloys dominate the market. These account for 40% of total gold jewelry fabrication, and their softness is a major weakness. Microalloying helps to solve this issue. What is microalloying? Micro-alloying is the process of alloying with tiny amounts of additions to improve properties dramatically. There are a number of 24K gold and platinum alloys on the market that show high hardness and strength, despite minute additions. We typically micro-alloy gold with light metals such as calcium or rare-earth metals at levels below 0.5 wt.%. Micro-alloys in silver and palladium are also possible. How does micro-alloying work? The alloying elements are only slightly soluble in the main metal. This means a very small fraction of alloying addition introduces a large fraction of the second phase. The large fraction of the second phase can be achieved in one of two ways: Using light metals Light metals such as lithium, potassium, calcium, and magnesium are very light (small atoms) and so have a low density. A low-weight fraction of these elements still introduces a large number of atoms. For example, there is seven times the number of lithium atoms to gold atoms for the same mass. Some of these elements have minimal solubility in gold and easily form intermetallic phases. Hence, a large number of precipitates can be added with a small amount of alloying. Gold-Calcium and possibly Gold-Potassium are the most viable, as their intermetallics have a high gold weight fraction. We need this to have a high volume fraction of precipitates but remain in the "micro" section of alloying. Using rare-earth metals Rare-earth metals have limited solubility in gold. They form eutectics and intermetallic compounds. Examples include cerium, lanthanum, and dysprosium. These 'heavy' rare-earth elements have limited solubility that falls with temperature. At the eutectic temperature, the solubility is around 0.5% At room temperature, there is minimal solid solubility. Such alloys could be age-hardenable by precipitating a fine dispersion of the inter-metallic. There is little effect on the color of these alloys, as the alloying additions are so small. Uses of Micro-alloyed Gold Micro-alloyed golds can be cast, but hardness is optimized when worked (wrought condition) and age-hardened, where appropriate. The high strength makes them useful for lobster claws, difficult chain designs, etc., which would be difficult to perform with conventional 24K gold. Because of its hardness, 24K gold is easier to polish. These alloys do also have some limitations. To prevent microalloying elements' oxidation, melting must be done in an inert atmosphere. These materials cannot also be simply remelted and recycled without losing strength. The precipitates lose their strengthening effect due to oxidation. 24K gold cannot be easily soldered as solders themselves are not 24K Gold. Some 22K gold solders exist, and in some countries, can be used and remain within hallmarking rules. Microalloying of Silver and Platinum Micro-alloyed Silver Most of the same principles of micro-alloying Gold also apply to Silver. Many contain Gadolinium and are heat treatable. Micro-alloyed Platinum Platinum-based micro alloys also exist. In addition to Gadolinium and Calcium, very small additions of cerium or samarium help improve hardness, luster, and tarnish resistance. The platinum content is a minimum of 99% since the Pt-Ce phase diagram shows the same limited solubility as the gold alloys.

Platinum and Palladium are comparably new metals to the jewelry trade. For platinum, we tend to be interested in 950 and 850 fineness alloys, with 5% and 15% of alloying metals, respectively. For palladium, there is most interest in 950 fineness alloys, although a 500 fineness alloy does exist. Compared with Gold and Silver, platinum and palladium show complex interactions with many elements. Other elements show limited solid solubility, multiple eutectics, intermetallic/ordered compounds, immiscibility gaps, etc. Their phase diagrams are not as straightforward. Platinum Alloys Because of their high fineness (8500 or 950), most platinum alloys are single-phase alloys, although some can be age-hardened (precipitates can form) due to the formation of a small fraction of precipitates. Traditionally, most alloys were based on the platinum-copper system. Platinum-Cobalt alloys have become increasingly popular due to their superior flow and castability, and high hardness (due to precipitate hardening). Other alloys, such as 950Pt with 5% Ru, are also common. The age-hardenable alloys contain gallium or indium, which has limited solid solubility in platinum of about 6% maximum but which drops to around 2.5% at lower temperatures. Heat treatment enables so-called age hardening, specifically the formation of Pt3Ga precipitates. This can lead to improvements in properties, but it can cause some issues in porosity. Palladium Alloys Palladium has similar phase diagrams to those of platinum and gold: Silver is fully soluble in all compositions. Gold, copper, rhodium, and platinum have significant solubility, while ruthenium has limited solid solubility. There is little information on commercial alloys. Pd-Ru alloys tend to be the most popular. Ruthenium has limited solubility in palladium, so these are likely two-phase alloys. Hardness values are comparable to platinum alloys, but their melting points are lower. Palladium also has a 500Pd hallmark, which has slightly higher hardnesses than the 950 alloys, lower densities, and narrower melting ranges. Uses of Platinum and Palladium Jewelry Platinum is a common and popular alternative to white gold. It does not tarnish like white gold, is hypoallergenic, and is regarded as more durable than white gold. It is commonly used for engagement rings as the setting in platinum is far more durable. However, platinum jewelry is more expensive because it is far denser than gold, and so more platinum is needed to make jewelry of the same size. Palladium shares many features with platinum, although is far less dense (jewelry is very light) and more expensive! Cover photo from Platinum Guild International.

Most Silver alloys are based on the Silver-Copper binary phase diagram, which shows a limited extent of strengthening via heat treatment. Copper is a much smaller atom than silver, so it is an effective solid-solution hardener, as it is with karat gold. Since the main silver hallmarks have a high fineness, we often operate in the single-phase silver-rich region of the phase diagram. Compared to karat gold hallmarks, silver hallmarks allow a lower extent of possible alloying: Conventional sterling silver (Ag with 7.5 wt.% Cu) is very malleable with reasonable properties, although it suffers from firestain and a tendency to tarnish. Zinc is often added to improve ductility (for chain-making etc.), deoxidize the melt, improve fluidity in investment casting and make the alloy whiter. Non-sterling silver alloys Britannia silver is an alloy of 95.8% silver and copper and is consequently not as hard as sterling silver. It finds little use today. Lower finenesses of silver, such as 800 (80% silver-20% copper plus zinc) or 830 grades, find application in some countries. These have higher hardness and strength but lower ductility (higher proportion of eutectic phases). The higher copper content also imparts a more yellow color, and such alloys are often electroplated with silver to improve the color. Improving Tarnish Resistance Tarnishing is the discoloration of the surface due to the reaction of the metal alloy with chemicals and oxygen in the surrounding environment. The tarnishing of silver is a well-known phenomenon. There are several so-called ‘tarnish-resistant’ silvers on the market, although none are tarnish-proof. These elements form a transparent oxide on the surface that hinders the formation of silver-copper sulfides, which are the cause of the black tarnish. Typically, the alloying additions are germanium, indium, or even silicon. Hardening by heat treatment (i.e., age hardening) is possible in some of these. Avoiding Fire-stain Firestain is caused by internal oxidation of the copper in sterling silver. Many tarnish-resistant silvers also claim to reduce or eliminate firestain, but experiments show this is not necessarily true. Other alloying metals, such as zinc, can form sub-surface oxides. Even though the surface may not appear darkened, subsequent polishing will reveal its sub-surface. It would be desirable to eliminate copper, but additions such as Zinc and Tin do not sufficiently harden silver alloys. Some alloys based on silver-zinc-tin-indium can have a hardness below 50 HV. Heat-treating Silver Alloys Silver alloys show a limited capability to be strengthened by heat treatment. Heat treatment can be achieved by annealing in the single-phase field (750˚C) and then quenching in water. Aging occurs at around 300˚C for one hour. The resulting microstructure is a silver-rich phase with fine copper-rich precipitate particles. Heat treatments and Soldering – The big problem Soldering is the process of joining two metals by melting a metal alloy with a lower melting point and solidifying it between them. It involves heat. The soldering temperature is below the solution-treatment temperature. Hence, soldering cannot be done before heat treatment as the solder would melt. If we solder after heat treatment, the region around the solder will be hot enough to cause coarsening of the precipitates. This will cause overaging and a loss of age hardening.

Before you read this, we recommend reading our general introduction to Gold alloys and an introduction to phase diagrams. White gold for jewelry was originally developed as a substitute for platinum and is currently particularly popular. Pure Gold has a yellow color, but by alloying with other elements, we can dramatically change the color of the alloy. Typically we alloy gold with copper and silver, but white gold alloys are typically Gold-Nickel and Gold-Palladium alloys. Palladium and nickel are strong bleachers of gold; silver and zinc are moderate bleachers, while other metals have a moderate to weak effect. Note there is a limit in the karatage range for white golds up to 21K (<12.5 wt.% Pd). It is currently impossible to make a 22K white gold because the available 8.4% of alloy additions are insufficient to give a good white color. The difference between Nickel and Palladium In the US, nickel is used as a primary whitener of gold. However, its metallurgy with gold is difficult, with very limited solubility of nickel in gold – Nickel would much rather form a second phase than sit in the gold lattice. This is known as an immiscibility gap. Two-phase alloys are hard but far less ductile and difficult to work with. Those working with nickel-white gold are well aware of its many difficulties. Palladium has extensive solubility in gold alloys, and so forms soft, ductile alloys. They are considerably softer and easier to work than nickel golds. This makes them useful for gem-setting purposes. Additions of 10–12% palladium to gold impart a good white color, but palladium is an expensive metal, and the alloys are denser than nickel ones (pieces of equivalent size are heavier – containing more gold). In addition to their high cost, gold-palladium alloys have higher melting points. Rhodium Plating of White Gold Commercial Gold–Nickel alloys tend to be low in nickel content, so they also contain silver and zinc to obtain whiteness. Copper is also added to improve workability. The low nickel and the copper result in a yellowish-brown tint to the white color. Such poor-color white golds are often rhodium-plated to give a good, tarnish-resistant white color that should last for some years before the coating wears away. Many commercial palladium-white golds contain only about 6% to 8% palladium, with additions of silver, zinc, copper, and even some nickel, in an attempt to keep the cost down. These alloys have a less satisfactory white color and will often be rhodium-plated, experiencing the same problem as nickel-white golds. Legal restrictions and skin rashes Nickel-based white gold alloys are cheaper, but nickel is a skin-sensitizing element and so is known to cause a skin rash. European legislation means nickel-rich alloys have been phased out, although low nickel (<6 wt.%) contents are still legal. Commercial alternatives to Nickel and Palladium With the high cost of palladium, alternative white golds have been developed. These often contain manganese and other metals such as iron and chromium. There are several problems: They are difficult to work and cast. They tend to crack. They often tarnish. A poor white color requires rhodium plating. Quantifying Whiteness Commercial white golds can vary significantly in their degree of whiteness. To quantify their witness, there are agreed definitions of "whiteness" using the Yellowness Index. Summary The key points are as follows: White gold has a white color due to the bleaching effect of alloying additions, namely Nickel and Palladium. Nickel White-golds can often cause skin rashes. Palladium White golds do not cause skin sensitization and are easier to work with but are far more expensive and softer than nickel-based white golds. White golds can often be rhodium plated to ensure a bright-white appearance. This effect is limited as the plating can often wear away. The whiteness of an alloy can be quantified using the Yellowness Index. Cover image: White Gold Ring courtesy of Mark Adwar.