Early milled silver

[DrLarry]

Could anyone please give me a quick explanation of the manufacture of early milled silver?  I have read some of the strawing process which suggests that some of the black flow lines in the early silver coins are the result of putting the hot planchets once die pressed onto to damp straw.  Were the planchets hot when introduced to the press?  there are some interesting old images of the milling process but I cannot find out the exact process, or just have not looked hard enough. I have read the books on the changes that Sir Isaac newton seemed to have some responsibility for when keeper of the mint but there is not a lot of detail.  I am particularly interested in the later 17th C and early 18th C although as I see the same in GII coins up to mid 18th C.  Some help please.  THanks  

[Unwilling Numismatist]

Do you mean haymarking ?

[Rob]

I have always put haymarking down to insufficiently mixed copper to make the alloy, this being the usual admix to obtain the required purity. The melting point of copper is about 100 degrees C higher than that of silver, so it is not enough just to melt the silver, throw in the requisite amount of copper and expect the alloy to be thoroughly mixed because the temperature isn't necessarily high enough. Whilst I have no definitive proof that this is the reason for the black spots, it is probably significant that there is far less haymarking seen on gold coins which can be explained by the fact that the two metals are only 20 degrees apart in their melting points. Copper (II) oxide is black.

If it was due to a process other than mixing metals such as the treatment of blank flans, I would have expected to see similar percentages of coins with haymarking irrespective of the metal.

[DrLarry]

Thank you for your help I suppose it is a sensitive process even if simple to those stoking the mix. In igneous petrology we use similar phase diagrams to understand the crystallisation of a magma it is always affected by pressure, temperature and impurities especially gases in the mix and metal alloys must be affected by the impurities I assume.  I never really fully understand the Straw process. Thanks