Hardness - Why It Matters

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 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.

In a presentation to the 22nd Santa Fe Symposium on Jewellery Manufacturing Technology (May 2008) Dr Chris Corti of the World Gold Council discussed the Role of Hardness in Jewellery Alloys. He concluded that all precious metal jewellery 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 jewellery.

One way of assessing the relative hardness of materials known to most people is the Mohs test; a comparative scratch test where a material high on the list will scratch those lower on it in the list; the ‘hardest’ material being diamond rated as a 10 on this scale. 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, aluminium and zinc. So 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 comparison using a test which is standardised, repeatable and gives a quantified hardness value is to press a hard indenter into the material under test under controlled conditions of time and force and to then measure the size of the indentation produced. Under this type of testing protocol hardness can be defined as “the resistance of a material to plastic deformation” (ASM Metals Reference Handbook). For metals and alloys the Vickers Hardness test is probably the most widely used technique, in this test a pyramidal shaped diamond is pressed into the metal under test. The smaller the impression left by the diamond, the harder the material and the greater the quoted hardness value, HV. (This HV value is also sometimes quoted as DPH, Diamond Point Hardness.)

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. To increase hardness, historically, copper was added to silver. 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 alloys is replaced with elements such as tin or zinc to give the ‘deox’ casting alloys or a degree of firestain resistance then the fully annealed hardness will fall (to 50-55 HV in some cases).

To meet the 100 HV criteria proposed by Dr Corti for sheet or wire products is relatively easy. Traditional sterling alloys can be supplied in the quarter to half-hard condition which gives sufficient hardness to withstand handling damage while retaining sufficient ductility to form complex shapes. For the alloys with zinc or tin additions to meet the 100 HV hardness criteria they need to be supplied in the three-quarter hard condition. This leaves them with limited ductility and they can only 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 a silver alloy. This is where the ability of Argentium silver alloys to be hardened by a simple low temperature heat treatment at about 300C (570F) 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. The heat treatment of Argentium silver alloys is something I will return to in a later post.