Assaying of precious metals

Photos to be added later

Before the invention of modern analytical methods, it was a skilled job to tell how pure gold and silver were, and never very accurate either.  The practise was called assaying, hence we still have the Assay office which checks the purity of precious metals.  

The task was originally vested in the Wardens of the Company of Goldsmiths in London by Edward the first, in 1300AD.  However the methods used had hardly changed in the previous thousand years, but between 1300 and 1600 assaying was established on a proper scientific basis.  

A period example comes from Chaucer, who mentions the assaying of silver by fire and hammer, the fire having several possible effects; causing oxidation of impurities (The colour telling the skilled assayer what the impurities were) melting of a base metal alloy, or else by cupellation which consumed all the impurities leaving only silver.  A hammer could be used to beat out a thin strip of metal, how well it elongated giving an idea of how pure the metal was.

Other methods available include the use of a touchstone, a black stone on which the suspect metal was scratched, leaving a mark the colour of the metal used.  Copper leaves a red line, gold a gold one, silver a silver one which is in turn slightly different from tin, although the two metals looks superficially the same. 

I can demonstrate the use of the hammer and touchstone to show the different colours, and cupels used for the purification of silver.  Ultimately the public can see how important physical properties are, and discuss the value of coins and precious metals.  Many problems were caused in the medieval period by adulteration of coinage, and of course forgery was an ever present problem, issues which are still live today.