The chapter called “Gum printing on alternative surfaces” of Sarah Van Keuren’s book “A Non-Silver Manual: Cyanotype, Vandyke Brown, Palladium & Gum Bichromate with instructions for making light-resists including pinhole photography”, written by Dana Leight.
If you have printed in gum bichromate successfully on sized paper, you know that the sizing gives the surface of the paper a ‘tooth’. A surface other than paper, such as metal, glass or plastic, can be printed on with gum if given a tooth by sandblasting, sandpaper or an electric sander, grinding with carborundum, etc. Copper and zinc etching plates can be given an acid aquatint if you are familiar with the etching process. As long as you give a surface enough texture for the pigmented gum to adhere to, you can print on it. I have gotten the best results using a sandblaster on both glass and metal (copper, bronze, brass and zinc), but I have printed on aquatinted etching plates, stone, plexiglas and mylar as well.
You can treat the prepared surface as you would treat paper as far as coating and exposing is concerned. When you coat, make sure to brush the pigmented gum in every direction so it gets trapped in all the pits on the surface. After exposing, it is best to leave the piece in a tray of cool water for 15-20 minutes because a long still development will draw out the details. If the image cannot tolerate being rocked in the tray of water or gently hosed after a few minutes, it will be very easily damaged when you try to coat the surface again with another layer. If the image does not slide off during development, gentle hosing should help to clear out the highlights. Unlike a paper print, you can remove virtually any part of the image with a soft sponge brush while it is wet.
It is a good idea to use a hair dryer to dry the piece after development and to then post-expose without your light resist for several minutes — the added exposure to heat and light will help harden the gum layer, preventing damage to the gum as you add more layers. As you add more layers of gum, the tiny pits on
the surface will begin to fill in and eventually the surface will become built up again so that adding more layers will disturb the previous layers — the image will begin to flake off during coating. I have gotten very interesting results by allowing this to happen and even planning for it, so it is not necessarily a negative aspect. Even after post-exposing a dry plate, you can remove parts of the image with water and a sponge brush if you want to. You can also remove an image completely and start over again by repeating the process of giving it a tooth. Once the image is finished, I use Krylon Crystal Clear Acrylic Spray to protect it from moisture and handling. (Zinc plates will oxidize if left in the air unprotected.)