This section is from the book "Leaching Gold and Silver Ores. The Plattner And Kiss Processes: A Practical Treatise", by Charles Howard Aaron. Also available from Amazon: Leaching Gold And Silver Ores.
197. The matte obtained in melting the silver precipitate is usually quite rich, and it seems to be impossible to completely desilver it by means of iron alone, at least in the presence of copper. The bullion is about .800 fine, even when there is a good deal of copper in the precipitate, because very little copper is reduced from the sulphide by iron at a white heat. It is, however, necessary that enough sulphur be left, in the roasting, to convert the copper into sulphide, for if too much oxide is formed a part of it may be reduced by the blacklead pot, metallic iron, or by charcoal if present, and thus contaminate the silver. Antimony and arsenic, though their sulphides are reduced by iron, yet volatilize, or combine with a further portion of iron and remain in the matte. Lead, if present in the precipitate, cannot be kept out of the bullion. Gold, when melted in the presence of sulphur and iron, forms a black, brittle mass, containing much iron, but the gold sulphide obtained with the silver precipitate is mostly reduced in the melting, and the metal is found alloyed with the silver.