AN ALLOY’S IDENTITY DOCUMENT
Physical characteristics
TITLE
The quantity (in weight) of precious metal present in the final alloy, expressed as per thousand parts or carats. The characterization of an alloy is performed in dedicated title, since the ratio between metal and master alloy strongly modifies the properties of the obtained alloy; for this reason in the catalogue alloys with the same code in different carats are present.
COLOUR
A fundamental property of matter, it is of great importance in the production of jewellery as it drives emotion, meaning and value. Because of the subjectivity in reading it, precisely defining the colour is very challenging.
Instruments to define colour: the spectrophotometer
It gives a series of numeric values from -100 to +100 on 3 coordinates, readable as spots within a spherical cartesian space. Every spot identifies a colour in the visible spectrum.
The three CIELab coordinates:
Coordinate a*: from green to red
Coordinate b*: from blue to yellow
Coordinate L*: it represents luminosity, the quantity of light reflected by the sample. Luminosity is also strongly dependant on the surface finishing.
Colour shades
GREEN YELLOW. With high silver and/or zinc content, with colour that is light and with greenish hue.
LIGHT YELLOW. Copper content in the alloy grows. It is the most common category, together with rich yellow. The 2N colour standard belongs to this range, in title 750 gold.
RICH YELLOW. Further increase of copper in the formulation. Known also as “arab” or “Hamilton” colour, the 3N colour standard belongs to this range, in title 750 gold.
PINK YELLOW. The list of alloys with highest copper content that still can be defined as yellow. The 4N colour standard belongs to this range, in title 750 gold. These categories are the same for all titles, but with different intervals.
These categories are the same for all titles, but with different intervals.
WHITE. Yellow Index is a numerical value that identifies the intensity of white in the alloy, helping to decide whether the alloy needs to be treated superficially to improve the colour.
It defines three categories of white:
PREMIUM WHITE YI < 19 (Rhodium plating not necessary)
STANDARD WHITE 19 < YI < 25 (Rhodium plating optional)
OFF-WHITE 25 < YI < 32 (Rhodium plating mandatory)
HARDNESS
HARDNESS
A material’s resistance to undergo plastic deformation. The values are measured using the Vickers scale. For jewellery alloys values go from 25 HV for pure silver, up to about 300 HV for a white, high nickel based alloy in cold worked state or after age-hardening.
AS CAST HARDNESS (HV AC)
The hardness before any mechanical working, annealing treatments or aging. It’s a fundamental data for the production process of casting: it will be more difficult to change the hardness of the alloy in the following processing steps.
HARDNESS AFTER AGE HARDENING (HV AH)
For some categories of alloys that need high strength such as spring effect alloys, or particularly soft as those based on silver, it is possible to perform a particular heat treatment to increase considerably the hardness, without need of cold working the metal.
To be defined as age-hardenable, an alloy must increase its hardness to at least 50% more compared to the as cast state.
Gold alloys: the age hardening step consists of maintaining the (annealed) piece for 90 minutes at a temperature between 250°C and 350°C (with best temperature indicated on technical sheets), followed by an air cooling.
Silver alloys: silver achieves maximum hardness when it is age-hardened in two steps:
Step 1: homogenization under inert atmosphere at a temperature close to solidus (700-740°C) for 40 minutes, followed by immediate water quench
Step 2: age hardening at 300°C for 60 minutes, followed by air cooling.
If homogenization is not possible: go for single-step hardening, at 300°C for 90 minutes. Final hardness increase will be approx. 60% of the maximum achievable.
MELTING TEMPERATURES
A pure metal has a single melting temperature, so at a certain temperature it passes directly from solid phase to liquid phase. Alloys instead have a melting range, within which the metal gradually, during heating, melts until it is fully liquid.
• SOLIDUS TEMPERATURE (SOLIDUS), the temperature at which the alloy starts the melting process. It is an important value, for example, during welding, or in processes like cladding.
• LIQUIDUS TEMPERATURE (LIQUIDUS), the temperature at which the alloy is completely molten; on the basis of liquidus, the temperatures for casting and for granulation are defined.
Mechanical characteristics
TENSILE STRENGTH (Rm)
The point at which the sample breaks during the tension test (values in MPa - Mega Pascal)
YIELD STRENGTH (Rp0,2)
The point at which the sample begins to undergo a permanent plastic deformation (values in MPa - Mega Pascal)
ELONGATION AT RUPTURE (E%)
This value expresses the percentage ratio between the length of the piece just before it breaks and its initial length
General characteristics
AS CAST GRAIN SIZE
A statistical measure, expressed in microns, on crystalline grains of internal standard samples in the as cast state. For jewellery alloys, the measure varies from about 1000 microns of pure silver and gold, down to 10 - 20 microns for some gold alloys for mechanical processing with high grain refiner content.
The positive effects of a small crystal grain:
• superior mechanical performance,
• improved shininess and surface brightness,
• high chemical resistance
Grain refinement level
The level of grain refinement in an alloy is proportional to the presence in the alloy of particular chemical elements that improve the microstructure during the solidification and annealing stages. It must necessarily be high in the case of complex plastic deformation processes, but it is preferable also when a compact and shiny surface is requested.
Deoxidation level
The presence of deoxidizing elements is particularly important in the case of the casting, where a clean surface after the casting is an advantage from the production process viewpoint. Depending on the type of casting system different levels of deoxidizers are necessary.
