Talcum powder is a white or light gray powder used on a lithograph plate to make the ink or coloring material hold and to protect the lines on the image from the acid of the platemaking solution.
It is a fine powder produced by pulverizing talc (hydrated magnesium silicate). Talc is also known as agalmatolite, a mineral used for coloring toys. It is resistant to acid and is soft and smooth, so it does not damage the plate surface. It can be ground into finer powder than resin, which is used for the same purpose, and can thus be applied on the finer details of the image.
Before using talcum powder, resin powder should first be applied over the image area for protection (lithograph crayons, tusche, printing ink, etc.) for protection. Then, by rubbing talcum powder gently over the plate surface using a soft cloth or cotton wool, the powder attaches itself to finer sections of the image that the resin powder could not reach, to provide greater acid resistance. The image is therefore protected when the acidic platemaking solution is applied thereafter.