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.
Thursday, 12 May 2011
Friday, 6 May 2011
Hard Spots in Investment Castings
Every so often I get a request for help regarding ‘hard spots’ in investment castings. Typically this defect can usually be attributed to one of three possible causes.
The first and most frequent cause is a ceramic particle; by this I mean investment powder, which has contaminated the casting. This is usually as a consequence of re-cycling scrap material. To prevent this it is very important that any scrap material is completely clean before it is re-melted. This cleaning process should involve acid dipping and also the use of magnetic pins. It is also possible to get investment powder contamination in a casting if there has been a problem with the flask burn-out and the investment has not reached its optimum strength or if there are any sharp bends or corners in the spruing of the pieces, this can lead to small pieces of investment being washed into the casting as a consequence of turbulent metal flow on casting.
The second possibility is iron contamination, which can be something as simple as a piece of stainless steel in the form of a magnetic pin or piece of burnishing shot getting mixed in with a melt, rather than issues of dust or rusty components which are usually associated with this sort of problem. The iron typically combines with the copper present in the casting to give a hard inter-metallic particle which then acts as the source of the ‘drag marks’ on polishing typical of hard spots.
The final possibility is the presence of a grain refiner in the alloy being cast which has agglomerated at the surface; rather than been distributed throughout the body of the cast piece. Typically this is associated with alloys which contain iridium as a grain refiner. The Argentium family of alloys also contain a grain refiner, boron, at a very low level of parts per million. If there are a large number of hard spots then the small amount of the grain refiner is unlikely to be the primary cause of the hard spots. To prevent this being a concern it is important that any silver casting alloy which contains a grain refiner is well stirred once it reaches the casting temperature. The induction pulse settings on the modern induction casters are very useful in ensuring that the molten metal is a homogeneous mix prior to casting.
When I receive samples of this type of complaint and examine them using a scanning electron microscope then 7 times out of 10 it is a ceramic particle I find (i.e. investment powder), 2 out of 10 it is a piece of iron or steel and the rest of the time it is an issue with grain contamination or poor mixing on melting.
The first and most frequent cause is a ceramic particle; by this I mean investment powder, which has contaminated the casting. This is usually as a consequence of re-cycling scrap material. To prevent this it is very important that any scrap material is completely clean before it is re-melted. This cleaning process should involve acid dipping and also the use of magnetic pins. It is also possible to get investment powder contamination in a casting if there has been a problem with the flask burn-out and the investment has not reached its optimum strength or if there are any sharp bends or corners in the spruing of the pieces, this can lead to small pieces of investment being washed into the casting as a consequence of turbulent metal flow on casting.
The second possibility is iron contamination, which can be something as simple as a piece of stainless steel in the form of a magnetic pin or piece of burnishing shot getting mixed in with a melt, rather than issues of dust or rusty components which are usually associated with this sort of problem. The iron typically combines with the copper present in the casting to give a hard inter-metallic particle which then acts as the source of the ‘drag marks’ on polishing typical of hard spots.
The final possibility is the presence of a grain refiner in the alloy being cast which has agglomerated at the surface; rather than been distributed throughout the body of the cast piece. Typically this is associated with alloys which contain iridium as a grain refiner. The Argentium family of alloys also contain a grain refiner, boron, at a very low level of parts per million. If there are a large number of hard spots then the small amount of the grain refiner is unlikely to be the primary cause of the hard spots. To prevent this being a concern it is important that any silver casting alloy which contains a grain refiner is well stirred once it reaches the casting temperature. The induction pulse settings on the modern induction casters are very useful in ensuring that the molten metal is a homogeneous mix prior to casting.
When I receive samples of this type of complaint and examine them using a scanning electron microscope then 7 times out of 10 it is a ceramic particle I find (i.e. investment powder), 2 out of 10 it is a piece of iron or steel and the rest of the time it is an issue with grain contamination or poor mixing on melting.
Wednesday, 4 May 2011
Guild Members featured in MJSA Journal
Patricia Tschetter's 'Bee Ring' makes the cover page
Congratulations to Argentium Guild members - Patricia Tschetter, Julia Kay Taylor, Ronda Coryell, Cynthia Eid and Novell Design Studio/Kemp Metal Products Inc. for having their beautiful Argentium creations published in May 11 issue of MJSA Journal. The article ‘Silver Bullet’ takes a look at the working properties of Argentium silver alongside a range of other silvers in today’s marketplace. Novell Design Studio/Kemp Metal Products’ new line of Argentium/Gold bonded wedding rings (cleverly titled ‘In Union’) is featured and Patricia talks about her experiences and the benefits of fusing Argentium.
This brilliant article gives silversmiths and artisans clear and concise information to compare working with these silvers and gives some useful tips for using Argentium.
Tuesday, 3 May 2011
Britannia Silver
Given the recent Royal Wedding I thought I would explain the origins of Britannia silver. This is a silver alloy which has a higher purity than the traditional sterling silver grade and is recognised in the United Kingdom and Ireland. It is required to contain a minimum of 95.84% silver, the remainder of the alloy, as with sterling is not restricted.
The Britannia silver standard was introduced by the government of William III in a 1697 Act of Parliament as part of the re-coinage scheme at that time. The intention of the Legislation was to stop the practice of clipping and melting of sterling silver coinage. It was believed that by maintaining a higher purity standard for wrought silver products, there would be less incentive to put the sterling coins of the realm in the melting pot. The new hallmark for this alloy was an icon of the seated Britannia supporting an oval shield compared to the lion passant guardant that denoted the sterling silver standard.
The new Britannia alloy was softer than the sterling silver which it replaced and therefore easier to work with. However its added expense, due to the higher silver content, led to lobbying from silversmiths for the re-introduction of the sterling silver composition with its familiar hallmark which eventually occurred in 1720.
Thereafter Britannia silver has remained an optional standard for silver in the United Kingdom and Ireland. Following the hallmarking changes in January 1999, Britannia silver has been denoted by the millesimal fineness hallmark 958, with the option to also apply the symbol of Britannia.
The Britannia silver standard was introduced by the government of William III in a 1697 Act of Parliament as part of the re-coinage scheme at that time. The intention of the Legislation was to stop the practice of clipping and melting of sterling silver coinage. It was believed that by maintaining a higher purity standard for wrought silver products, there would be less incentive to put the sterling coins of the realm in the melting pot. The new hallmark for this alloy was an icon of the seated Britannia supporting an oval shield compared to the lion passant guardant that denoted the sterling silver standard.
The new Britannia alloy was softer than the sterling silver which it replaced and therefore easier to work with. However its added expense, due to the higher silver content, led to lobbying from silversmiths for the re-introduction of the sterling silver composition with its familiar hallmark which eventually occurred in 1720.
Thereafter Britannia silver has remained an optional standard for silver in the United Kingdom and Ireland. Following the hallmarking changes in January 1999, Britannia silver has been denoted by the millesimal fineness hallmark 958, with the option to also apply the symbol of Britannia.
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