Definitions - Solidus and Liquidus

I was asked yesterday what solidus and liquidus meant. This made me think of all the terms we use in silversmithing or metallurgy which we assume people understand but may mean different things in different contexts. So I thought that it might be of interest if I defined and tried to explain some technical terms.
If we start with the basics; pure metals have a single melting point. This is the point at which they change from a solid to a liquid. For silver this is 960.8C (1761F), copper 1083.4C (1982F) and germanium 937C (1719F); so all the main constituents in Argentium silver alloys have very similar melting points.Similarly the boiling point of a pure metal is the temperature at which it changes from a liquid to a gas. For silver this is 2163C (3925F), copper 2560C (4640F) and germanium 2830C (5126F).

One metal which is commonly used in silver alloys which has a very low boiling point is zinc. Its melting point is 419.5C (787F) but its boiling point is only 911C (1672F). If you are making an alloy of silver and copper which also contains zinc you can see that at a temperature when you have the silver and copper molten you are already above the boiling point of zinc. This is why zinc is such a difficult constituent to control in silver alloys and why we refer to zinc loss on melting, the zinc does literally boil away!

If we now think about what happens when we add copper to silver we will be able to understand what we mean by solidus and liquidus. As we have said, pure silver melts at 960.8C (1761F) and you would think that by adding copper to it which has a higher melting point of 1083.4C (1982F) the overall melting point would increase, but that is not the case it actually falls.

To explain this we have to think of what is happening to the silver atoms when we add copper to them. The atoms of silver and copper have different sizes (silver is larger than copper) and in a crystal of pure silver the bonds between each atom are of equal length. As you add copper to silver the smaller copper atom replaces a silver atom in the crystal structure and as a consequence increases the length of the bonds between itself and the silver atoms because of its smaller size. This longer copper-silver bond is not as strong as a silver-silver bond and so it is more easily broken when we apply heat to the solid silver-copper alloy when we want to melt it.

This is why we have a melting range with alloys, the weaker bonds are broken first at the point the alloy starts to melt and all the bonds are broken when it is completely molten. So the point at which melting starts (or if you think of the molten metal cooling, the point at which the molten metal becomes completely solid) is called the solidus. The point at which the metal is fully molten (or again to think of the metal cooling, the point at which the completely molten metal stops being completely liquid) is the liquidus. So these two terms, the solidus and liquidus define the melting range of the alloy.

The best definition though is that of the temperature range between the solidus and liquidus when the alloy is not completely molten; that is called either the ‘mushy’ or ‘pasty’ range. You have to smile when you can legitimately call the state of a metal ‘mushy’!

Coming soon... Argentium Guild Forum

Following requests from some of our Members, we will be setting up an exclusive 'Argentium Guild Forum' (members only). Please watch this space for further details.

Different Silver Alloys

One thing I am always asked is, “how is Argentium silver different from other silver alloys”? With there being so many different alloys now available I thought I would try to categorise and summarise each alloy type’s particular quirks.

Traditional sterling silver – This is the simple 92,5% silver, 7.5% copper combination. A good basic alloy with good hardness. For silversmiths the problems of firescale and tarnish are well documented.

Spinning silvers – Historically these alloys had part of the copper content of the traditional sterling silver composition replaced with cadmium (usually about 2%). This gave an alloy which was about 10-15HV lower in hardness than traditional sterling silver which had excellent deep drawing and stamping characteristics. Cadmium containing silvers are now prohibited by worldwide legislation and attempts to create similar alloys by simply replacing the cadmium with tin or zinc failed because the oxides of the tin and zinc, formed when torch annealing, were very hard to remove. Some manufacturers now use Britannia silver (95.8% silver, 4.2% copper) as a spinning silver because of its lower hardness.

Deox silver alloys – These are alloys developed primarily for casting applications and their properties were recently reviewed in an excellent paper presented at the 2010 Santa Fe Symposium by Joerg Fischer Buhner (click here to download pdf). Some of the copper content of these alloys is replaced with zinc and/or silicon with the aim of giving bright, firestain free castings. While the silicon and zinc additions do limit the formation of firestain it is not always a complete success. The higher silicon content alloys can be more difficult to cast consistently and cannot be fabricated easily limiting their use; whereas zinc is well documented to fume at typical investment casting temperatures. These alloys are an improvement on the traditional silver composition for investment casters but have no significant benefits for the practicing silversmith working with sheet and wire.

Platinum group metal additions – these are additions of either gold, palladium or platinum which replace some of the copper content of the traditional sterling silver composition. Aside from the considerable cost implication of replacing copper with a precious metal it has yet to be demonstrated that an alloy which contains an addition of one of these elements has a significantly improved tarnish resistance compared to the deox alloys detailed earlier. Gold additions make the alloy more yellow; platinum creates difficulties on melting and increases the potential for hard spots to form and silver alloys containing palladium have shown sensitivity to ultraviolet light when tested under ‘showroom’ conditions.

Alloys containing germanium - the mechanism by which the germanium content of Argentium silver alloys protects against tarnish and firestain by forming a transparent germanium oxide is something that I will discuss in another blog. Our work has shown that the composition of the Argentium silver alloys with about a 1% germanium addition optimises the mechanical working and casting characteristics of the Argentium silver alloys while giving exceptional tarnish resistance and firestain resistance. Other alloys may contain germanium, but these are limited to contents below 0.5% germanium because of our patent protection and they have to rely on supplemental additions of zinc and tin to try to match the performance of Argentium silver alloys.

New Members

We would like to welcome all of the new Members of the Guild.  We have had a wonderful response to the website going live and look forward to the site evolving with information and showcasing the wonderful range of work being made in Argentium. 

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