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.
187. It occurred twice in my works that on collecting the precipitated gold it was found to be brick red, instead of having the usual brown color. It was also deficient in quantity. On the first occasion I could not account for this, but on the second I satisfied myself that the workman had not thoroughly mixed the precipitant with the solution, and being consequently deceived by a test, he had not used iron sulphate in sufficient quantity.
188. If the solution of gold contains lead chloride, the iron sulphate causes a precipitation of lead sulphate, which renders the subsequent washing of the gold difficult. This may be prevented by precipitating the lead as sulphate by means of sulphuric acid, or any soluble sulphate by which the gold will not be affected, as sodium sulphate. After settling, the liquid must be transferred to another vat, in which the precipitation of the gold is effected as usual.
In some cases the leach contains substances the character of which has not been determined, and which affect the purity of the precipitated gold. If the lixivium is allowed to stand several hours, a deposit is formed on the sides of the vat. The purified solution is then drawn off into another vessel, and the solution of iron sulphate is added.