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I think the understanding of copper colouration of lead glass has been known for centuries, when it has been deliberately added (typically 2-3%) to give a turquoise/blue colour. The reverse process - taking copper out of lead metal does not seem to have been used routinely until the mid/late 19th century. This can be done by leaving molten lead at its melting point for a few days. Copper is insoluble in lead and comes to the surface as a dry powder which can be skimmed off. Percy refers to experiments by Baker regarding this process but does not give any details of it being used industrially. The more usual process for removing copper is to stir in flowers of sulphur and requires a powerful stirrer so I have not heard of this being done before the late 19th century or even early 20th century. I can't think of an easy way of taking copper out of red lead so I guess you have to purify the lead metal first. Percy also states that red lead from Snailbeach lead was good for glass making and that from the nearby Bog mine was less suitable. There seems to have been a long-standing tradition by users of non-ferrous metals to specify particular brands of metal for specific applications. This seems to have persisted at least until the end of the 20th century. I would like to know why red lead was used in glass making. Litharge can be used to make glass so why go to the additional trouble of making red lead. It's possible that the red lead process gave a purer product. Richard Smith -----Original Message----- From: Andy Cuckson Sent: Friday, December 22, 2017 5:36 PM To: [log in to unmask] Subject: Copper pyrites in lead - removal of, for glassmakers' red lead Copper pyrites in lead imparts a blue tint to crystal glass, when red lead is used in the glass which contains too much of the impurity. Does anyone know when the phenomenon was first truly understood, and the process for eliminating the copper pyrites from the lead introduced? Andy Cuckson If you need to leave the list, send the following message to [log in to unmask] - leave mining-history --------- If you need to leave the list, send the following message to [log in to unmask] - leave mining-history ---------