Knowing the hardness of any metal or alloy is important, as this value gives you an indication of its ability to form and maintain a shape and is also a guide to its durability. The hardness value of a metal can be correlated to other properties such as tensile strength and in general, the harder a silver alloy is, the stronger it is and the more resistant it is to wear and scratching.
One way of assessing the relative hardness of materials known to most people is the Mohs test—a comparative scratch test where a material situated high on the list will scratch those materials lower down on the list (the "hardest" material being diamond, rated as a 10 on this scale). The minerals that form this test are shown in the picture below.
If you use the Mohs scale, most silver and gold alloys would fall between 2 (gypsum) and 3 (calcite), together with other metals such as magnesium, aluminum, and zinc. However, although a comparative test is useful, it does not discriminate between materials or give a definitive hardness value.
The simplest way to get an accurate hardness comparison is to press a hard indenter into the material being tested under controlled conditions of time and force, then measure the size of the indentation produced. This test is standardized, repeatable, and gives a quantified hardness value. (See diagram below.)
Under this type of testing protocol, hardness can be defined as "the resistance of a material to plastic deformation" (ASM Metals Reference Handbook).
The Vickers Hardness test is probably the most widely used technique for metals and alloys where a pyramidal shaped diamond is pressed into the metal being tested. The smaller the impression left by the diamond, the harder the material and the greater the quoted hardness value, HV. The HV value is also sometimes quoted as DPH, Diamond Point Hardness.
In a presentation at the 2008 Santa Fe Symposium on Jewelry Manufacturing Technology, Dr. Chris Corti of the World Gold Council discussed the "Role of Hardness in Jewelry Alloys." He concluded that all precious metal jewelry should have a hardness value of at least 100 HV for satisfactory performance and he also stated that low hardness was often associated with problems seen with some platinum and palladium jewelry.
So what does this mean for silver alloys? Pure silver, in the annealed condition, is too soft (30 HV) to withstand any handling damage and in its fully hard condition (100 HV) has no ductility and cannot be formed without breaking.
Historically, to increase hardness, copper was added to silver and the way in which this affects the annealed hardness of the resulting silver-copper alloys is shown in the graph on the right.
At the traditional sterling silver composition of 92.5% silver 7.5% copper, the hardness values published by the major manufacturers for the fully annealed condition are 75 HV and for the fully hard condition are 150 HV.
Where some of the copper content of the sterling alloy is replaced with elements such as tin or zinc to give "deox" casting alloys a degree of firestain resistance, then the fully annealed hardness will fall (to 50-55 HV in some cases).
* This graph is taken from a diagram on page 251 of Silver, Economics, Metallurgy and Use, edited by Allison Butts and Charles Coxe, whose publication was sponsored by Handy and Harman. For those that are interested in the metallurgy of silver, it is the first book I recommend.
To meet the 100 HV minimum hardness criteria proposed by Dr. Corti for sheet or wire products, it is therefore necessary to either use products which are in a semi-hard condition, or to plan your manufacturing process so that your pieces are not finished in the annealed condition. However, one limit on this type of design is that some pieces will always need to be joined by either soldering, brazing, welding or fusing as the final operation of any manufacturing process. This will leave the pieces in a softened condition.
The way that hardness and ductility vary with % cold work is illustrated in the graphs below.
If traditional sterling silver were supplied in the quarter to half-hard condition, this would give sufficient hardness to withstand handling damage but would significantly reduce ductility and therefore limit the ability of the material, in this condition, to form complex shapes.
For silver alloys containing zinc or tin additions, to meet the 100 HV hardness criteria they would need to be supplied in the three-quarters hard condition. This leaves them with very limited ductility and they could only be used to form the simplest shapes.
For investment cast items we need to look at ways to increase hardness by heat treatment, as the as-cast hardness is typically very close to the annealed hardness of the silver alloy. This is where the ability of Argentium silver alloys to be hardened by a simple, low temperature heat treatment at about 570oF (300oC) really becomes an advantage—giving the opportunity to increase the hardness of an as-cast item to improve its wear characteristics, while retaining sufficient ductility to allow some final forming or setting processes.
This hardening procedure is covered in Cynthia Eid's booklet "Working with Argentium Silver: Tips and Procedures" and Ronda Coryell’s DVD "Techniques in Argentium, Volume 1: Basics." Hopefully the heat treatment of silver alloys, and Argentium silver alloys in particular, is something I will write about in a future post.