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’!