A specialized prepress unit consisting of a smooth white typing sheet attached to a backing sheet coated with heavy, waxy aniline dye. Typing or drawing on the top sheet transfers the colored wax onto its reverse side in a mirror image. This top sheet is then clamped to the duplicator drum to print the copies.
Heard on the job
Don't press too hard with the ballpoint when drawing on the spirit master or the wax will smear on the back.