Looking for something in particular?

A phase diagram is essential to a metallurgist's "toolkit'. They tell us in what state and where we will find each element within a material when in "equilibrium" (i.e. it's lowest energy and most stable state) for a given composition, temperature (and pressure). This can be reached by heating at a given temperature or by cooling very slowly. A phase is defined as a portion of a system with the same structure, properties and composition, and that is different to the rest of a system. Why are they relevant to a jeweler? A phase diagram is important because it helps to tells us: Will the alloy be single phase or two phase? Can the alloy be age hardened? At what temperature should we anneal our metal to restore ductility? Will adding more alloying additions introduce a deleterious phase? Explaining Phase Diagrams Single Phase Solid Solution: Gold–Silver Phase Diagram For some compositions, the phase diagram is quite simple – the two elements form a complete single phase solid solution across all possible compositions. Gold & Silver form such a solid solution, where the silver sits as a substitutional soluite on the gold lattice or vice versa. The lines on the diagram illustrate the boundary between two regions, and the regions between the lines are for a particular stable phase or combination of phases: Liquidus – The temperature at which the liquid begins to solidify on cooling or when the material is entirely liquid on heating. Solidus – The temperature at which the solid begins to melt on heating or when the material is entirely solid on heating. The region between the solidus and liquids is a two-phase region where the liquid and solid co-exist. This is the melting range. Normally, a narrow melting range is desirable. We can determine the proportion and composition of solid and liquid at a particular temperature using the phase diagram. It is not necessary to know, but you can read more here. Two-phase solid solution: Gold–Copper Phase Diagram Gold and copper form a series of "ordered" intermetallic phases, and the liquids and solidus have a complicated shape (a pronounced dip in the middle). Compared to Gold-Silver, the melting range is narrow. On cooling, gold and copper initially form a single-phase alloy at all compositions. But at lower temperatures, around 400˚C, ordered intermetallic phases are formed. These low-temperature phases are stronger, harder, and less ductile. They are relevant to why we quench after annealing. A binary eutectic system: Ag-Cu phase diagram Compared to gold and silver, silver and copper have limited solubility. As we've seen elsewhere, silver atoms are very different in size from copper atoms. In the central region of composition "space," there is a two-phase microstructure. A silver-rich phase and a copper-rich phase will form, and the proportion of them depends on the overall composition. Both phases contain both copper and silver. These two-phase microstructures are often stronger and more difficult to work with. The key feature of the silver-copper system is that it contains a eutectic point. At this point, the solidus and liquidus meet. Two solid phases form from a single liquid at a single temperature. Normally, only pure metals melt at a single temperature; melting generally occurs over a range in alloys, Eutectic compositions are typically used as solders because these alloys have a far lower melting point than the two or more pure metals from which they are made. Three-element alloys – A ternary phase diagram When there are three elements, we must plot a ternary diagram. In this case, the phase diagram forms a 3D graph. The triangular cross-section of the prism gives us the composition at a particular temperature. These isothermal triangular sections can be plotted at every temperature and vertically stacked to form a 3D graph in the shape of a prism. We can use Gold-Copper-Silver as an example. This alloy system forms the basis for most karat gold alloys. The ternary diagram shows the stable phases at all compositions for a particular temperature (an isothermal section). We can re-cut the prism horizontally to find the possible microstructures at a particular temperature for all compositions, or we can cut vertically to find the possible microstructures at a particular range of compositions for all temperatures. Horizontal sections show that depending on composition, 18, 14, 10, and 9K gold-silver-copper alloys can be either single-phase or two-phase. Three possible sections would be for 10K (37.5 wt.% Au), 14K (58.5 wt.% Au), and 18K (75 wt.% Au). Again, we can see the significant region of two-phase structures at these three karats. Summary Phase diagrams plot the expected phases at a given temperature and composition in the "equilibrium" case. Phase diagrams can be used to help design alloy compositions and processing treatments to optimize the properties of jewelry alloys.

Increasing the strength and hardness (i.e., scratch resistance) is one of the main areas of research for precious metal metallurgists. Many new technologies are on the horizon, but the widely used fundamental strengthening mechanisms are explained below. A reminder of how metals deform Metals are polycrystalline. In a crystal, atoms are arranged in a pattern that repeats in all three dimensions. The material is made up of many small crystals; these crystals will have the same structure and composition but are oriented differently. They are known as grains. Because of their different orientation, the grains do not pack perfectly, and the regions that separate them are known as grain boundaries. Plastic (irreversible) deformation occurs by layers of atoms sliding over one another. This is known as "slip". Slip leads to a change in the shape and orientation of grains in the microstructure and, ultimately, a change in the shape of the material. A type of crystal defect known as a dislocation is necessary for slip to occur. These types of defects are specific to each crystal. Strengthening mechanisms work by hindering (resisting) the dislocations' motion, so a larger force (stress) is needed for slip to occur. When a force is applied to a metallic object and irreversibly changes shape, it deforms plastically. The layers of atoms in the crystal slide over one another in a mechanism known as 'slip'. This results in a change in the shape and orientation of each grain and an overall macroscopic shape change. Types of Strengthening mechanisms The main forms of strengthening mechanisms for jewelry materials are as follows: Processing: Grain-size control Processing: Cold Working Alloying: Solid solution strengthening Alloying: Two-phase microstructures Alloying and Processing: Precipitate Hardening Grain-size control When a polycrystalline material deforms, each grain must be deformed. The change in the shape and orientation of grains must be consistent with neighboring grains. This means multiple slip events are required, and so a larger force. The smaller the grain size, the larger the strengthening eggiest. This is expressed by the Hall-Petch relationship. Yield strength is inversely related to the grain size required. Thus the jewelry is stronger and harder if it is fine-grained and beneficially; it is also more ductile and less prone to cracking, impurity embrittlement, and the “orange peel” surface after deformation. Cold Working As we work a material, there is a change in microstructure. Namely a reduction in grain size, shape, and orientation. This leads to an increase in the number of defects (incl. grain boundaries). These defects hinder the motion of dislocations and therefore hinder further deformation. This is known as work hardening. The extent of cold working depends on the alloy. Some alloys are easier to work harden than others. This depends on their composition, as well as other factors. For example. Gold-Copper alloys work harden more than Gold-Silver ones. Two-phase microstructures The general presence of a second phase, with a similar volume fraction to the primary phase, helps to generate a strengthening effect. This provides a mild strengthening effect as the different phases will deform differently and so hinder one another. Solid-solution Strengthening When an alloy exists as a single phase, it typically forms a "solid solution". The added element will occupy a random position in the crystal lattice. In most precious metal alloys, the added element will substitute for another atom in the crystal lattice. If the element differs in size, then the lattice will become strained and distorted. The strain induced by the substitutional atom makes it more difficult for the layers of atoms to move. A larger size mismatch means a stronger strengthening effect. This is known as "solid-solution" strengthening. Why is copper better at strengthening gold alloys than silver? Alloying gold with copper leads to a stronger strengthening effect compared to silver because there is a greater size mismatch (see table below). This means the lattice is distorted more, and so the layers of atoms in the crystal have much more difficulty sliding over one another. Precipitate Hardening If the element has a dissimilar size, very different chemistry, or we add too much of it, then it may be energetically favorable to form a second phase. This phase may have either a different composition, crystal structure, or both. Precipitation is where a second phase (with a different composition) forms within another. In a solid, these precipitates tend to form preferentially at grain boundaries. Adding a second phase with a different crystal structure makes it difficult for these dislocations to move. As a result, the material is more resistant to deformation. When we anneal the alloy to restore ductility, the metal has a single-phase microstructure. The alloy is then quenched (cooled quickly in water), so the microstructure is "frozen-in". At high temperatures, the atoms have the energy to rearrange themselves. By cooling quickly, they don't have sufficient time to diffuse (move) around and form second phases. The alloy is then aged (heated) at a high temperature within the two-phase field of the phase diagram. This allows precipitates to grow and coarsen. The size and distribution of these precipitates dramatically affects their strengthening effect; it must be carefully controlled during working and annealing (processing). Achieving the optimum or "peak" hardening is a function of temperature and time. As we anneal for longer, the precipitates grow larger. When the precipitates are small, they provide little resistance to deformation. The precipitates can be easily deformed or "cut". When the precipitates are too large (coarse). The precipitates do not interact strongly with dislocations, and so the strengthening effect is very weak. There is a fixed amount of solute that can form precipitates. As a result, as the precipitates get larger, there are fewer of them. The best distribution is achieved when there is a distribution of many small precipitates – a fine microstructure. Summary table

Silver is synonymous with jewelry and all things luxurious. Still, silver jewelry is not pure silver. Pure precious metals are simply too soft to be regularly used for jewelry, as they would wear away and deform. Instead, we use alloys. Silver alloys are a mixture of silver plus one or more other elements, such as copper. These other elements are not impurities. They are carefully chosen to make the material harder and more durable (more scratch and wear resistant), more tarnish-resistant (see below), more lustrous, and easier to work with for the jeweler. There are four main types of silver jewelry on the market: Sterling (925), Britannia (958), Fine Silver (999), and Silver plate. Sterling or Fine? What's the Difference? Fine silver – 999 – has the highest fraction of silver. It is 99.9% silver by weight. It is essentially pure. However, fine silver is very soft and not suitable for most jewelry. It is seldom used in earrings and other pieces that experience little wear. It has a desirable bright, silvery-white color, is hypoallergenic, and tarnishes far slower than other silver alloys. Sterling silver – 925 – is the most common form of silver jewelry. It is mixed with up to 7.5% of other elements, usually copper. It is much harder and more durable than fine silver, hence its' more widespread use. Sterling silver is widely regarded as hypoallergenic. Although less bright than fine silver, it is still extremely desirable. Britannia silver – 958 – is 95.8% silver by weight, making it "finer" than sterling silver. It is not particularly common as it remains quite soft and lacks durability. Britannia silver is a very traditional English material, dating back to the 17th century. Continental silver is 80% silver by weight and was commonly used in Europe. It has various names; it is cheaper and more durable but has a less lustrous appearance. It is not so frequently used. Silver plate is where another metal, such as copper or nickel-copper alloy, is coated with a thin layer of silver. It contains far less silver. The plating will wear and discolor over time, exposing the metal underneath. How do I know the difference? Hallmarking or Stamping is one of the oldest forms of consumer protection. In many countries (incl. UK, the US, and Europe), the precious metal content must be stated and marked on the jewelry piece. Silver is usually hallmarked or stamped by a number. These hallmarks are given below: In some countries, like the UK, the "fineness" hallmark (e.g.,925 for Sterling silver) will be accompanied by various other marks. See here. In the US, silver-plated items must be hallmarked with the percentage silver and clearly stated as plated. No minimum thickness of the coating is required, but it should have "reasonable" durability. See here. Legal hallmarking standards do change, and this information is not guaranteed to be up to date. Tarnishing of Silver Perhaps the biggest disadvantage of silver is that it tarnishes. Tarnishing is the discoloration of the surface due to a reaction with chemicals in the nearby environment from perspiration, oils, household cleaning products, and general dirt. Silver reacts readily with sulfur in the environment. A black coating will form on the surface, typically a mixture of sulfide and oxides. It is not harmful and can be easily removed. If you are looking for silvery-white jewelry that does not tarnish as quickly and is more durable, you may want to look for white gold, platinum, or palladium jewelry, although these are often significantly more expensive. Looking after silver To preserve your silver, it is advised to clean it well before storage to remove any residues that may accelerate the process. You should also avoid contact with chemicals such as household cleaning products, makeup, and those found in swimming pools, etc. Various cleaning and storage products are available to help preserve silver's lustrous appearance over time. Tarnish-resistant Sterling Silver The tarnishing of silver has been a perennial challenge to many metallurgists and jewelers alike. There are many so-called "tarnish-resistant" silver alloys. Ultimately, none are "tarnish-proof", and your silver will discolor over time at various rates depending on its composition and the conditions of the surrounding environment. Tarnish-resistant silvers typically contain silver, copper, and some germanium, which helps form a protective coating. Alloys with a lower copper content are often more tarnish-resistant. The most common example is Argentium, but many other tarnish-resistant sterling silvers are on the market. Be Careful! Silver filling and Silverplate These types of jewelry are not pure silver but instead have a silver coating of different thicknesses. The silver filling is a much thicker layer than the silver plate. The thicker layer means it is far less likely to wear away over time, exposing the "base" metal (often brass) underneath. Silver plate is not recommended to be bought for general wear but instead for display. Rhodium-coated Silver Rhodium is a lustrous, pristine-white metal that is sometimes used as a thin coating. However, this layer is easily scratched and will wear away over time (and quite quickly), revealing the duller silver underneath. A reputable jeweler will always disclose the use of rhodium plating. Nickel Silver (aka German Silver) Nickel silver does not contain any silver! It is instead a mix (alloy) of Copper, Nickel, and Zinc, but is described as Nickel Silver due to its appearance. It is far cheaper and is often used as a "base" metal for plate silver.

An orange peel surface can form during bending or working after annealing. This is a result of recrystallization and "over-annealing" the sample. When the grain size becomes too large, the grains cannot accommodate shape change during working or annealing. To avoid this, we need to ensure the grain size remains small. You can either anneal the sample at a lower temperature (it still needs to be high enough for recrystallization to occur), or for a shorter period of time. This will reduce the grain size so the microstructure can better accommodate shape changes, resulting in smaller surface roughness. Want to understand more? Read about recrystallization here.

Before this, have you read: "What is alloying? Surely, pure gold is best?" Hallmarks of Gold It is wrong to think of Hallmarks as a measure of purity. They are a measure of fineness rather than the weight fraction that is a particular precious metal. Alloys of different Karat have different properties; other elements are deliberately added to improve consumer wear and manufacturing properties. The main hallmarks of gold are 24 Karat, 22 Karat, 18 Karat, 14 Karat, and 10 Karat; their basic details are outlined below: The basic alloy system Almost all karat gold alloys are based on the Gold-Copper-Silver ternary alloy system. While Gold and Silver form a single solid solution, Gold and copper form several ordered intermetallic phases at low temperatures. The binary phase diagrams are shown below: The ordered Gold-Copper intermetallics are one of the reasons why we water quench 18K and lower karat golds that contain high copper contents after annealing. This prevents these less ductile phases from forming and keeps the whole system in the single-phase ductile condition, which we wanted to form by annealing. Type I, II, and III Classification Alongside deciding the karat and color (see below), the composition determines the proportion of the second phase. This determines how hard and difficult to work an alloy is. Alloys are sometimes classified into Type I, II, and III. Type I are single-phase and easy to work with, while type III alloys have second phases, making them difficult to work with. Beyond silver and copper Typical further gold alloying additions include Zinc, Silicon, and Iridium. Colour of Gold Alloys Pure gold has a characteristic "golden" yellow appearance. But by alloying gold, even with just copper and silver, a wide range of colors can be achieved across the different karats of gold. This can be illustrated on a color triangle or ternary diagram: Pure gold is at the top corner of the triangle, copper on the bottom right, and silver on the bottom left. Horizontal lines illustrate the gold content for 18, 14, and 10 karats on this triangle. At each karatage, all possible alloy compositions can be plotted. On the left side, we can see that if we come down to 9K, a gold-silver alloy is actually white. On the right side, a 10K gold-copper alloy is very red, but we cannot get a good red at high karats, such as 22K. Alloys of gold containing both copper and silver lie between the two extremes on the appropriate lines at any karatage. The amount of each depends on the position of the line and is in proportion to the relative lengths of the lines on either side of the point. White gold alloys are typically based on Gold-Nickel and Gold-Palladium compositions. A detailed discussion of white golds can be found here. Summary The key points include the following: Gold is primarily alloyed with copper and silver The color of gold can be varied from white to yellow and red by tuning the copper and silver content. Copper forms ordered intermetallic phases with gold that must be avoided on cooling to maintain ductility for working. 18K and lower karat gold alloys are then age hardened. Various alloying other additions are made to clean melts and ensure bright castings as well as to finely tune the microstructure (grain refinement).

Once a gold alloy has been annealed to restore its' ductility and water quenched, it can often be hardened by annealing at a low temperature (say 400˚C). A similar effect can often be achieved in Sterling Silver. This allows a highly workable "fully-soft condition" alloy, which, once semi-finished (shaped), can then be hardened to ensure excellent scratch resistance and durability. The material can then be polished and finished. How do these alloys get their strength? These alloys are "precipitate hardened". The presence of a second phase within the microstructure (notably at grain boundaries) has a significant strengthening effect by inhibiting deformation. Plastic (irreversible) deformation occurs by layers of atoms slide over one another. This is known as "slip". Slip leads to a change in the shape and orientation of grains in the microstructure and, ultimately, a change in the shape of the material. A type of crystal defect known as dislocations is necessary for slip to occur. These types of defects are specific to each crystal. Adding a second phase with a different crystal structure makes it difficult for these dislocations to move. As a result, the material is more resistant to deformation. What happens during the processing? When we anneal the alloy to restore ductility, the metal has a single-phase microstructure. Copper and Silver exist in the gold crystal lattice as substitutional solutes. This provides some strengthening effect. The alloy is then quenched (cooled quickly in water), so the microstructure is "frozen-in". At high temperatures, the atoms have the energy to rearrange themselves. By cooling quickly, they don't have sufficient time to diffuse (move) around and form second phases. Practice: Avoiding "over-aging"? Achieving the optimum or "peak" hardening is a function of temperature and time. As we anneal for longer, the precipitates grow larger. When the precipitates are small, they provide little resistance to deformation. The precipitates can be easily deformed or "cut". When the precipitates are too large (coarse). The precipitates do not interact strongly with dislocations, so the strengthening effect is very weak. There is a fixed amount of solute that can form precipitates. As a result, as the precipitates get larger, there are fewer of them. The best distribution is achieved when there is a distribution of many small precipitates – a fine microstructure. Particular alloys Gold Alloys 18K and 14K Gold-copper alloys can be increased by age-hardening treatments. Gold and Copper can form an "ordered" intermetallic phase. The gold and copper occupy particular positions in the lattice; gold and copper sit in alternate layers. By quenching the alloy after annealing, we prevent this phase from forming on slow cooling. We can then re-anneal (age) the alloy at a lower temperature to carefully control the precipitation and form a finely dispersed array of precipitates. Typically, we heat at about 280-300°C for 3-4 hours. 18K: Annealed at 550°C (1022°F), aged at 360°C (680°F) for 1 hour 14K: Annealed at 650°C (1202°F), aged at 260°C (500°F) for 1 hour Higher fineness gold alloys have insufficient copper alloying to form these ordered precipitates. Silver Alloys Silver alloys can also be hardened, but soldering hinders the use of this strengthening technique. Platinum Alloys Most 950 platinum alloys are not amenable to age-hardening treatment, but some heat-treatable alloys have been developed. These alloys contain indium or gallium, which form small second-phase precipitates. Hardness of up to 360 HV can be achieved. Summary The key points are as follows: Metals deform plastically by "slip". Layers of atoms can slide over one another caused by defects called dislocations. Second-phase precipitates can hinder the passage of dislocations, so they have a strengthening effect. The size and distribution of precipitates significantly affect their strengthening effect. A fine well-dispersed array of precipitates has the best strengthening effect. This can be achieved by quenching an annealed alloy and aging at a high temperature within the two-phase field. The time and temperature of aging are important to controlling the precipitate distribution.

Before you read this, make sure you've read: "What is a metal? The basic structure". Ductility formally describes how easily a metal can be drawn into wires. In practice, it describes how much we can deform a metal before it breaks. Heating an alloy to a high fraction of its melting temperature after melting can dramatically improve and restore its ductility after extensively working material. It is common practice amongst goldsmiths and silversmiths. A brief summary of working There are two main types of working (deformation) used to shape a piece of metal; Hot working and Cold Working. When we deform a metal, shape change occurs as layers of atoms slide over one another, causing a change in the shape and orientation of each grain and, ultimately a change in the macroscopic shape of the object. Cold working is when we deform the material at low temperatures (typically below half the alloy's melting point). At this temperature, we see: A macroscopic shape change, A change in grain structure – grains become elongated and reoriented, An increase in strength – metals become harder due to work hardening, A reduction in further ductility – Grains are heavily deformed and cannot deform much further. Further working will cause the metal to fracture. Hot working is where we deform a material at a high temperature. We are working and annealing at the same time. The materials are softer and easier to deform, and the material can be deformed more before it breaks. Annealing a cold-worked alloy When we heat metal to a high temperature, but below its melting point, the atoms have more thermal energy. As a result, they can slowly rearrange themselves into a more stable structure within the lattice structure. The elongated grain structure formed during cold working is a high-energy state. When heated, the atoms will form undeformed equiaxed grains that grow in the place of the deformed ones during annealing. This is known as recrystallization. The temperature we heat to, and the extent of cold working affect the final microstructure and properties of the alloy: Below a critical temperature, no recrystallization can occur. As a result, there is no change in grain structure. The hardness does not change significantly. Above this critical temperature, recrystallization occurs, and undeformed grains grow in place of elongated ones. There is a dramatic reduction in strength as the work hardening is "undone" and ductility is restored. At high temperatures, the undeformed grains continue to grow. Larger and fewer grains are more energetically favorable as fewer high-energy grain boundaries exist. Hardness continues to fall (due to increasing grain size), and ductility will also eventually begin to fall – there are fewer different grains to accommodate any shape change. Remember – We need a minimum amount of cold work necessary for recrystallization to occur, typically about a 12-15% reduction. Otherwise, there is no driving force (the reduction in energy) for the microstructure to change. Why do we cool the alloy in water after annealing? Cooling in water (quenching) is much faster than cooling in air. Fast cooling helps to prevent any further undesirable changes to the microstructure that may occur if the alloy is allowed to cool slowly. Particularly to avoid losing any ductility. Particularly for alloys with substantial alloying additions (e.g., low Karat Golds), it may be favorable to form (precipitate) second phases, such as copper-rich phases, which can dramatically change the properties. These often make the alloy harder but less ductile. The sample may crack during subsequent work. By cooling quickly, there is not enough time for the atoms to rearrange themselves while they have enough energy; the microstructure is "frozen". Practice: Controlling annealing to optimize the properties Finer recrystallized grain size is desirable; it has higher strength (still lower than the cold-worked sample), ductility, and toughness. This can be achieved by cold working the alloy more before annealing. The key factors in annealing are: The initial amount of cold work – This leads to a smaller final grain structure as there is more driving force. A lower temperature and/or time is necessary. The temperature of annealing – A higher temperature speeds up the process but also leads to larger final grain size. So what are the preferred conditions? A lower temperature and annealing for longer give more control. The change in microstructure and increase in grain size is slower, so we can avoid a large grain microstructure (which is less hard and, in extreme, less ductile). We do not want to over-anneal our sample! Practice: The effect of composition The composition of an alloy is important. Some compositions can be work-hardened more than others, and the temperature necessary for recrystallization generally scales with the alloy's melting point and inversely with its hardness (a higher temperature is needed for harder alloys). While we can measure the temperature, in practice, Goldsmiths and Silversmiths use the color of the alloy as a guide to its' temperature. What happens during Hot Working? During hot working, recrystallization occurs while we are working. As a result, the material remains soft and ductile as any work hardening and change in microstructure that occurs during working are quickly reversed by recrystallization. Key Points The key points are as follows: When we deform a metal, layers of atoms slide over one another, leading to a change in the shape and orientation of the grains and a macroscopic shape change. Due to the change in microstructure, the alloy's hardness increases, and further ductility is reduced. This is known as work hardening. By reheating the alloy to a high enough fraction of its melting point, the microstructure can return to large, equiaxed grains. Strength is reduced, and ductility is restored.

Structure of Metals All materials are made of atoms, which are made of a nucleus of protons and neutrons surrounded by electrons. An element has a specific number of protons and electrons. Pure metals are a series of elements found naturally. They are grouped together because of the type of bonds these atoms typically form. The so-called precious metals (Platinum, Palladium, Gold, and Silver) fill Group 8 and 1B of the periodic table, as they share similar characteristics. The way that atoms interact and arrange themselves affects their physical and chemical properties. The properties and behavior of any metal can be related back to its structure and composition. Nearly all metals and alloys are crystalline. In a crystal, atoms are arranged in a regular structure that repeats itself in all three dimensions. This can be described using a lattice. Different atoms will arrange themselves in different ways; there are different types of crystal lattice. Metals with different lattice structures behave differently. A material's properties are controlled by the structure of the material. The structure of a material depends on its chemical composition and how the material is processed. They are all interrelated! The structure of a material can be split into three main length scales: Atomic structure – How the atoms arrange themselves. Can be seen in only the most advanced electron microscopes Microstructure – How small crystals (grains – see below) arrange themselves. Metallurgists regularly study this using optical and electron microscopes, and it dramatically affects their properties. Macroscopic shape – The shape of an object we see without eyes and what a jeweler works with. Atomic Structure As discussed, most metals are crystalline. A crystal is a solid in which atoms are arranged in a regular order that repeats itself in all three dimensions. The three main types of lattices are hexagonal close-packed (HCP), face-centered cubic (FCC), and body-centred cubic (BCC). Gold, Silver, Palladium, and Platinum are all fcc, owing in part to their similar size and chemical characteristics (determined by the structure of electrons in the atom). FCC metals are extremely ductile (can be easily drawn into wires), and atoms are densely packed together. Microstructure Nearly all macroscopic metal objects are made up of many small crystals. It is polycrystalline. It is extremely difficult to form one large perfect crystal – on cooling; many crystals will want to nucleate from the liquid and grow. In a pure metal, these crystals will have the same structure and composition but are oriented differently. They are known as grains. Because of their different orientation, the grains do not pack perfectly, and the regions that separate them are known as grain boundaries. The size and shape of grains and features at the grain boundaries (such grain boundaries often attract impurities and are much weaker than the grains themselves) form part of the microstructure. Changes to the microstructure during manufacture and processing dramatically affect the mechanical properties of a metal. General properties of metals Pure metals typically have the following properties: Good thermal and electrical conductivity High surface reflectivity (polish or lustre) Malleable (good ductility, capable of being heavily deformed) Heavy, i.e., high density Toughness Reasonable strength and hardness Remember, these properties do not define metals. Metals are a group of elements that typically form "metallic" bonds. Some electrons in a metal are "delocalized," which gives rise to many of these properties. What is an alloy? The four precious metals are far too soft when pure to be used for jewelry. Instead, metallic elements are mixed together to form alloys. Alloying is typically used in jewelry materials to: Improve strength and hardness (i.e., scratch resistance) Change the color Improve castability (how easy it is to form an object from the liquid metal) Reduce cost The extent of alloying is carefully controlled. Hallmarking is used to define the weight fraction of a particular precious metal or "purity". "purity" is misleading because many of the alloying elements are deliberately added! Microstructure of alloys In pure metals, the crystal structure, composition, and properties of the system are the same everywhere. The system has a single phase. The fact there are grains with different orientations is not important to defining the phase. It forms a separate part of the description of the microstructure. It may be energetically favorable for an alloy to have two or more phases. That is, regions with a different crystal structure and/or composition. Key Points The key points to remember: Metals are a group of elements defined by the typical nature of the bonding between atoms. Most metals are crystalline, where atoms are arranged in a regular, repeating structure that repeats. Almost all metal objects are polycrystalline; they are made of lots of small crystals with different orientations called grains. They do not pack perfectly. The regions between them are called grain boundaries. Regions of the material with the same composition, crystal structure, and properties are described as a phase. A material may have multiple phases co-existing. The chemical composition, crystal structure, and grain structure control the properties of an alloy. They can be changed by processing (deforming or heating) an alloy; they are all interrelated!

Before you read this, we recommend reading "What is a metal? The basic structure". An alloy is a mixture of two or more pure metals. Alloying additions (deliberately added elements) can be chosen for many reasons. These include: Hardening and strengthening: Alloying can dramatically improve strength in several ways. This is the primary reason for alloying in Jewelry. Control physical properties: Control the melting temperature and temperature range of melting. Reduce cost: Precious metals are expensive. Color change: Most alloys have a silvery-white appearance, but gold alloys are a great example of how alloying can dramatically change color. Deoxidizing agents: Help to prevent other elements from reacting with oxygen () and remove dissolved oxygen (which causes porosity during casting). Improving fluidity: Alloying can help the liquid alloy flow better and therefore improve form filling (how easily the liquid fills the mold) Grain Refinement: Reducing grain size can help improve strength and appearance. Microstructure of Alloys Pure metals form single-phase microstructures. An alloy can have a microstructure with one or more phases. When an alloy exists as a single phase, it typically forms a "solid solution." The added element will occupy a random position in the crystal lattice. In most precious metal alloys, the added element will substitute for another atom in the crystal lattice. If the element differs in size, then the lattice will become strained. If the element has a dissimilar size or very different chemistry or we add too much of it, then it may be energetically favorable to form a second phase. This phase may have either a different composition, crystal structure, or both. A two-phase system can form via either: Phase separation – where two phases form from one original phase (solid or liquid) normally during cooling. Precipitation – Where a second phase (with a different composition) forms within another. In a solid, these precipitates forms at grain boundaries. Precipitation of a second phase is the most common and is an extremely useful method of strengthening. Two-phase alloys are stronger but typically less ductile. Strengthening effect of alloying Crystalline structures deform plastically (irreversibly) by a process known as slip. Layers of atoms in the crystal can easily slide over one another to accommodate the shape change. As a result, grains re-orient and change shape, leading to an overall shape change of the material. In single-phase alloys (where the alloying addition is a substitutional solute), the strain induced by the substitutional atom makes it more difficult for the layers of atoms to move. A larger size mismatch means a stronger strengthening effect. This is known as "solid-solution" strengthening. In two-phase systems, the layers of atoms are different in the different phases, and so have different strengths and cannot "slide" in the same way. This is known as precipitate hardening; the "second phase" precipitates from the melt during cooling (or typically during annealing) and hardens the alloy. Solid-solution strengthening is limited in its effect; we can only substitute so many atoms (there is also an upper limit on size mismatch) before the strain in the lattice is too great, and it is more energetically favorable for a second phase to form. Precipitate hardening can have a much greater strengthening effect, but these alloys are far less ductile. The strengthening effect can be carefully optimized during working and annealing (processing). Changing the color Metals, except for a few, are typically a silvery-white color. However, the color of gold alloys is extremely sensitive to composition. Adding Nickel or Palladium in substantial quantities turns the alloy white, adding copper gives it a reddish hue, and adding silver gives the alloy a yellowish/green hue. For example, a wide range of colors can be changed by changing the proportions of gold, silver, and copper. Pure gold is at the top corner of the triangle, copper on the bottom right, and silver on the bottom left. Horizontal lines illustrate the gold content for 18, 14, and 9 karats on this triangle. There are no such things as white gold mines! Notice that alloys with lower karatage have a wider range of possible colors. Changing casting conditions Alloying additions make the alloy less dense (most elements are lighter than gold) and can dramatically change its melting point. Most alloys melt or solidify over a temperature range, whereas pure metals melt at a single temperature. In general, the melting (liquidus) temperature falls as the alloying additions increase. The temperature at which an alloy melts and the size of the temperature range over which the alloy melts affect the processing conditions, the microstructure of the alloy, and, therefore, its properties. Impurities Of course, not all elements present in these alloys are deliberately added. Many elements may be there as impurities. These tend to segregate and accumulate at grain boundaries. This results in embrittlement, as they hinder "slip" and plastic deformation; the material will just crack along the grain boundaries (inter-granular fracture). Typical impurities include lead, silicon, sulfur, tin, selenium, and bismuth. Oxygen and hydrogen can often dissolve into the liquid alloy and then evaporate on cooling, which results in casting porosity. Key Points The key points are: An alloy contains two or more elements. An alloy can consist of one or more phases. These phases have either differing compositions, crystal structures, or both. They form part of the description of the microstructure alongside the grain structure. Alloying helps to strengthen precious metals; pure metals are very soft and are not suitable for most jewelry. Alloying can also be used to make casting easier, improve appearance, change color or reduce cost.

Tarnishing is a discoloration of the surface, often black in color, and this is due to a corrosion reaction. General corrosion is not a major concern for precious metals, but low-karat golds and silver do often undergo tarnishing. Other alloys, such as 21K and 22K golds, have been shown to tarnish, but this is less common. The tarnishing layer is often an oxide or sulfide caused by a chemical reaction between the metal alloy and the environment. Causes of Tarnishing Tarnishing requires the presence of sulfur, moisture and oxygen, and often chlorine. Sulfur is normally present in the atmosphere, typically as sulfur dioxide, and particularly in industrially polluted atmospheres. In consumer environments, sulfur-containing substances include: Human perspiration – sulfurous compounds and sodium chloride (salt) will be present in human sweat and saliva. These exacerbate any corrosion. Food – Onions, eggs, fruit juices, pickles, and spices contain a lot of sulfur. The kitchen is not safe for tarnish-prone jewelry. Perfumes and deodorants, household cleaners, etc.: Many sprays contain sulfurous propellants and active agents. Jewelry storage – Glues and linings to boxes often contain sulfur compounds which can slowly leach or evaporate. Many papers are bleached in chlorine. Microporosity in cast jewelry – Surface microporosity may trap pickling solutions, many based on sulfuric acid, during final finishing. During use, they may absorb perspiration, perfumes, etc., leading to localized tarnishing, which can spread. In general, tarnishing is unsightly but is not a hazard to health, although it can result in black smudging of the skin, which is undesirable but not harmful. Tarnishing reaction in silver For both sterling and other fineness silvers, tarnishing is an accepted problem. It is caused primarily by sulfur-based corrosion reactions but can also occur in chloride-rich environments. The presence of moisture is also important. The corrosion mechanism is complex but is based on the reaction: $\rm 2Ag + H_2S + \frac{1}{2}O_2 \to Ag_2S + H_2O$ Silver sulfide, which is black, is thermodynamically more stable than silver oxide. Copper in the silver alloy can also be involved, and we should note that copper oxides are black or red. The alloy microstructure and composition play a role in such corrosion reactions. Where two phases are present, or there is chemical segregation in the casting, electrolytic cells can be set up on the surface in the presence of a corrosive environment, and corrosion is accelerated. Tarnish-resistant Silver There are many new Sterling and other silver alloys on the market that claim improved tarnish resistance. Most are based on modified silver-copper alloys with minor alloying elements added, such as germanium and silicon. These generally act to form transparent oxides on the surface in preference to the black silver-copper sulfide-based tarnish layers. There are also a number of efforts to add gold and platinum-group metals to improve the tarnish resistance and strength. Ultimately, all silver will tarnish, we can only delay the onset and lower the rate of tarnish, and different alloys will perform better in different environments. Tarnishing in Low-Karat Gold For the karat golds, the silver and copper content are involved in the tarnishing reaction since gold is the noblest of all metals and does not corrode. The areas with lower gold content will corrode preferentially in two-phase alloys, so single-phase alloys are preferred. To prevent tarnishing in low-karat gold alloys, there are some general, broad rules: Tarnishing occurs more readily as the gold content is reduced. Tarnishing occurs more readily as the silver/copper ratio increases at the same gold content. In two-phase alloys, the silver-rich phase tarnishes preferentially. Multi-phase alloys tarnish more readily than single-phase alloys of the same gold content. Special attention should be paid to heat treatments. Zinc additions generally improve tarnish resistance, often through stabilizing a single-phase structure. However, in the absence of silver, high-zinc low-karat golds are more susceptible to tarnishing. The susceptibility of alloys to tarnishing will also depend on the severity of the corrosive environment. If severe enough, even high-karat gold can tarnish!

Deforming and shaping metal is an integral part of making jewelry. Bending, rolling, and forming in the absence of heat are referred to as "cold working" an alloy. Understanding how metal behaves when we are shaping it is an integral part of avoiding failure and cracking. A quick review of the structure of metals Metals are crystalline. Atoms are arranged in a pattern that repeats in all three dimensions. This can be described by a lattice. Jewelry metals are polycrystalline. This means they are made up of many small crystals. These crystals will have the same structure and composition in a pure metal but are oriented differently. They are known as grains. Because of their different orientation, the grains do not pack perfectly, and the regions that separate them are known as grain boundaries. An atomic view of deformation Plastic (irreversible) deformation occurs by layers of atoms sliding over one another. This is known as "slip". A type of crystal defect known as a dislocation is necessary for the slip to occur. These type of defects are specific to each crystal. Strengthening mechanisms work by hindering (resisting) the motion of dislocations, and so a larger force (stress) is needed for the slip to occur. Slip leads to a change in shape and orientation of grains in the microstructure, and ultimately a change in shape of the material. As a material deforms, it gets harder, stronger, and more difficult to deform further. More defects hinder slip further, so a larger force is required. Further ductility is limited because the grains can't deform much more, and the defects generated hinder deformation (they interlock, which stops any further deformation). The extent of hardening depends on the alloy composition. For example, gold-silver alloys harden slower than gold-copper alloys. Effect of deformation on the microstructure In addition to a change in the shape and orientation of grains, slip gives rise to characteristic deformation bands within the grains. Such sliding occurs over several crystal planes in a complex way that gives rise to these bands. Such deformation is also visible in the overall macrostructure. The microstructure isn't uniform as it appears fibrous. Most cold-working processes result in uneven deformation through the cross-section. For rolling or extrusion, most deformation occurs at the surface, especially if the reduction in thickness for each pass is small. Effect of deformation on the properties When we deform a metal, we see several things: A macroscopic shape change. A change in grain structure – grains become elongated and reoriented. An increase in strength – metals become harder due to work hardening. A reduction in further ductility – Grains are heavily deformed and cannot deform much further. Further working will cause the metal to fracture. As we deform a metal. there is a reduction in grain size. For practical purposes, “large” will usually mean grains of the order of millimeters or larger, and “small” will refer to grain sizes of the order of tenths or hundredths of a millimeter (1–100 microns). As we reduce the grain size, then there are more grain boundaries. This means there are more defects in our microstructure. These defects hinder the slip mechanism (layers of atoms sliding over one another – movement of dislocations), so a larger force is required to deform the microstructure. This is a process that occurs during work hardening. To restore ductility, we must anneal the sample to reduce the number of dislocations and restore the grain structure. After annealing, the grain structure and strength relationship remains true. Hence, annealed metals are softer as the grain size is higher. A fine-grain microstructure is harder and stronger than one with large grains. This is given by the Hall-Petch relationship. In addition to being stronger and harder, once annealed fine-grained metals are (compared with coarse-grained annealed metals): More ductile and tougher (less prone to cracking) Less prone to impurity embrittlement Less prone to "orange peel" surface after deformation. ASTM Values You may also hear of grain sizes referred to in terms of an ASTM numerical value. This is a comparative method of measuring grain size. The higher the number, the smaller the grain size. Key Points Deformation of metals occurs by distorting the crystal lattice. Layers of atoms slide over one another by a type of defect known as a dislocation. Deformation leads to the work hardening of the metal. The metal becomes harder and more brittle due to the work already done. At a microscopic level, working a metal leads to a change in the shape and orientation of the grain structure. Reducing the grain size increases a metal's strength (hardness) and toughness (resistance to cracking). Cover Image courtesy of Chris Ploof.

Note: In the US, this is referred to as 'Stamping" Hallmarking is one of the oldest forms of consumer protection. The Goldsmiths' Company in London has been "hallmarking" for over 700 years. In fact, the term "hallmarking" comes from the fact that jewelers had to take their items to Goldsmiths' hall for them to be assessed and marked. Since 1972, an international convention has been signed on hallmarking, which makes the whole process of understanding a lot easier. 18K Gold is not pure gold! Nearly all jewelry sold is not pure gold or silver. These pure metals are far too soft and so the jewelry would not be durable and damage easily. Instead, we use alloys. An alloy is a mixture of two or more elements. Specific additions can help to improve the properties. This includes strength and hardness, but we can also change the color (for example, white gold) and other useful properties! They are not impurities, but deliberately added! Why do we hallmark? Because we use alloys rather than pure metals, there is a need to know what the "quality" or fineness of the jewelry alloy is. Hallmarking is a legal guarantee of what proportion (by weight) of the alloy is a particular precious metal. What is the advantage of a hallmarked item? A hallmark means the article has been tested (in the UK, it is performed by an independent body). It guarantees that it conforms to all legal standards of purity (fineness). It also guarantees provenance by telling us where the piece was hallmarked, what the article is made from, and who sent the article for hallmarking. What do all the marks mean? By the international convention, three marks are required: Sponsors Mark An indication of who took the piece to be hallmarked. Millesimal Fineness Mark – An indication of the fineness of the lowest standard alloy in the piece of jewelry e.g. if a piece was made of 18K and 14K Gold, then the piece would be hallmarked 14K. This guarantees that the quality of the whole article is no less than the fineness indicated. Assay Office Mark Where the piece was tested. The common control mark is used in countries that are subscribed to the convention. This is the one with the scales. In these countries, all four are used. Note, in the UK, there are often additional marks, namely the "traditional fineness mark" (for sterling silver it is a lion) and the "date mark" (a single letter) which tells us the year in which it was marked. So which mark tells me the fineness of my piece? The fineness mark is the key indicator of what proportion is made of a particular precious metal. The convention marks are as follows: The number indicates the fineness. For example, 800 means 80% of the weight of the alloy is made from this precious metal. The shape of the "shield" around the number tells you the type of metal. Remember, this is a mark for the fineness of the lowest standard alloy, so some bits may be made of higher fineness, but none of lower standard. Why is there more than one fineness stamp? Sometimes a jewelry item will be made of more than one precious metal. These are known as "mixed metal" and each precious metal can be hallmarked. According to the London Assay Office (July 2022), the rules are as follows: To be marked, the two metals should be distinguishable by color. e.g. Yellow gold and sterling silver would be marked, but White gold and sterling silver would not Each precious metal must be at least the minimum legal fineness for that metal, i.e. silver 80%, palladium 50%, gold 37.5% and platinum 85%. The main hallmark will be for the component with the least precious metal. Any other hallmarks will be for parts made of a different precious metal with higher precious metal content If an item contains a clearly visible non-precious "base" metal part, the word METAL must be applied next to the Hallmark on the precious metal part. Where practical it is also stamped on the base metal part. What about plated items? Under the convention, any plating is not hallmarked. This is different in the US, where a plating will be marked. If the metal underneath is precious, then this will be hallmarked. The rules for plating are as follows: If the metal underneath is the same precious metal as the plating, then the plating must be of equivalent or higher fineness. Any precious metal used for plating must equal or exceed the minimum fineness standard. This means gold platings must be 10K, silver platings 80% (by weight), etc. Precious metal cannot be plated with a non-precious metal. Disclaimer: This is a general guide and does not serve any legal purpose. The legal requirements may change and differ from the time when this was written.