The hectograph or gelatin duplicator is a printing process which involves transferring from an original sheet prepared with special inks to a gelatin pad. The special aniline dyes for making the master copy came in the form of ink or in pens, pencils, carbon paper, and even typewriter ribbon; the pencils and pens are sometimes still available. Various other inks have been found usable to varying degrees in the process; master sheets for spirit duplicators have also been pressed into service. After the image is transferred to the inked gelatin surface, copies are made by pressing paper against it. When a pad had ceased to be useful, the ink could be sponged off the top of the gelatin and the pad re-used for the next master.

The gelatin process is useful for print runs of somewhere between twenty and one hundred copies, depending upon the skill of the user and the quality of the original. At least eight different colors of hectographic ink were available at one time, but purple was the most popular because of its density and contrast.

Hectography, requiring very limited technology and leaving few traces behind, has been deemed useful both in low-technology environments and in clandestine circumstances where discretion was necessary. The process also lent itself to small runs of fanzines in the earlier 20th century.

Before the popularization of spirit duplicators and the mimeograph, there were mechanized hectography machines which used a drum, rather than a simple flat pan of gelatin.

While the hectograph process is almost entirely obsolete for printing on paper, it's still used for making temporary tattoos on human skin. Tattoo artists use hectograph pencils to draw pictures on paper and then transfer them to the recipient's skin.

See also: Duplicating machines.