This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The difficulty of making oil colours adhere to zinc is well known. Some time since, Prof. Bottger published a process which consists in applying with a hard brush a mordant composed of 1 part copper chloride, 1 copper nitrate, 1 sal-ammoniac, and G4 water, to which is added afterwards 1 hydrochloric acid. The zinc immediately becomes intensely black, which changes in drying (12 to 24 hours) to a dirty whitish grey, on which oil colours may be laid, and to which they will adhere firmly.