How Stone Lithography Works

Etching the Stone

Once the artist finishes, it is time to etch the stone to prepare it for printing. The process of etching fixes the artist's image on the stone -- makes it a part of the stone, in reality, through a chemical reaction involving nitric acid.

Photo courtesy Toby Michel
Etching chemicals and tools include gum arabic, nitric acid, rosin, talc, brushes, cloths and protective gear.

The process is described in the following photos.

Rosin protects the finer details of the image from the acid, so they do not burn out when the acid is applied. Toby sprinkles on the rosin...

...and brushes it down tight on the stone. He does the same with talc.

A solution of 4 ounces (113 g) of gum arabic combined with six to seven drops of nitric acid is prepared.

Toby pours it onto the stone...

Photos courtesy Toby Michel
...and brushes it on. The solution is applied more heavily on the dark areas and hardly at all on the light areas.

The nitric acid reacts with the grease (oleic acid) to create oleomagnate of lime. The image literally becomes part of the stone. In the process, the acid sensitizes the dark areas so they accept ink and reject water, and desensitizes the light areas so they reject ink and accept water. Both the sensitization and desensitization happen in one step.

Toby buffs the gum arabic down with cheese cloth, using
perhaps eight cloths in five minutes, in a process that is "like waxing a dining room table."

After it dries, Toby wipes the stone with paint thinner.

Photos courtesy Toby Michel
In this photo, you can see that the image has "gone gray" and become part of the stone. Toby rubs in asphaltum, a naturally occurring petroleum substance that seeps out of the ground in certain areas. It is a thick, greasy fluid. This gets a greasy substance into the image.

Next we'll find out about inking the stone.