Gum Tips

FIRSTHAND TIPS AND FORMULAS

FOR GUM BICHROMATE PRINTING

from Experience.

*Buy pre-sized (European) watercolor paper to start with. Then preshrink it for 15 minutes in hot water, dry, size, dry, harden, dry, size, dry, harden, dry, and pray.

*When drying a multiple-printing print, always hang it from the same corner to avoid registration problems caused by stretching.

*Always use the same drying method throughout the handling of any one particular print.
*Finish a print the same day it is started. The relative humidity and room temperature may change enough from day to day to cause registration problems.

*Expect pigment and/or bichromate stains when printing under humid conditions. The dry air of winter gives the cleanest prints.

*Time your exposures so that the print is of the desired density at the same time that the ammonium dichromate is completely soaked out, (about 1 hr.). Under-developing to save a print will also leave it with a yellow bichromate stain. Overdeveloping to save a print will cause patches of the emulsion to clump together and float away.

Size: 3 packets (24g.) Knox or Plymouth Rock gelatin dissolved in 1 qt. hot water. Refrigerate leftovers for next time. Just warm it up, use it again, then throw leftovers away.

Hardener: 25cc formaldehyde to 1 qt. COLD water. You can get USP grade formaldehyde at any pharmacy if you are over 21, and you must sign for it. Formaldehyde is absorbed through the skin and can be fatal, so use gloves! It helps if you wear a damp surgical mask or towel over your nose and mouth, and if you wear safety goggles to keep the fumes away from your eyes.

Sensitizer: 29g. Ammonium Dichromate. to 75cc hot (125F) water to dissolve, and then add cool water to make 100cc. It isn’t too important that you use exactly 29 grams. What IS important is that you use exactly the same amount every time you mix up a batch.

Pigments: I use Grumbacher from a tube. Process colors are as follows: yellow=Lemon or Hansa Yellow; magenta=Alizarin Crimson; cyan=Thalo Blue; black=Lamp Black. I stir about a third or less of a small tube into 100cc prepared gum arabic and let the excess settle to the bottom allowing at least 24 hrs. for this to happen.

Gum Pigment/Sensitizer solution (emulsion): I use 1 part sensitizer to 1.5 parts gum pigment solution. 1 cc sensitizer plus 1.5cc gum pigment sol. will coat a 5 by 7 inch area.

Exposing: An incandescent sun lamp suspended 15 inches above the printing surface will work for prints up to 8 by 10 inches. My exposures range from 2 to 6 minutes with 3 being the norm. Short exposures will allow you to add to the shadows without disturbing the highlights. Long exposures will allow you to “burn in” annoying highlights, and you can brush the emulsion off of the areas that you do not want to make darker.

Clearing: To remove yellow bichromate stains from a completely finished print, soak the print for an hour or so in a solution of 29g. potassium alum dissolved in a liter of water. This solution can be used, stored, and reused until it stops working. Never clear a print if you intend to coat and expose it any more. The alum will shrink the paper and cause registration problems.

Whether the emulsion is wet or dry, it is affected by light, heat, relative humidity, and time. Theoretically the emulsion begins to expose itself (chemically) as soon as you mix it, so never make more gum pigment/sensitizer solution than you can use right away.