Process 4 : Gum Bichromates

Based on the light sensitivity of dichromates, this is an early photographic print process developed in the 19th century. Its organic nature and painterly rendering of images are uniquely inspiring and interesting. It is a multi-layering process but a single layer can also do the job in respective cases. Colorful images can be achieved by the careful use of layers.

Tanima, resident artist at Goa cap, is exploring this alternative printing process.

This process of printing can sometimes be combined with other alternative processes like the cyanotype or platinotype.

Chemicals :

  • Gum Arabic solution (pre-mixed solution recommended)
  • Water color pigment in powder or tubes (tube colors recommended)
  • Ammonium, sodium, or potassium dichromate crystals (potassium dichromate recommended)
  • Gelatin
  • Chrome alum
  • Potassium metabisulfite
  • Dropper bottle for dichromate solution
  • Cone-top fabric paint bottle for each color/gum solution
  • Contact printing frame or vacuum exposure frame
  • Sponge or brush for applying emulsion
  • Camel hair brush and/or paint brush to give brush mark
  • adhesive tape
  • Gram scale
  • Waxed paper
Process :

A heavyweight cotton watercolor or printmaking paper than can withstand repeated washing and soaking is ideal. Each layer of pigment is individually coated, registered, exposed and washed. Separation negatives of cyan, magenta, and yellow or red, green, and blue are used for a full-color image.

Contact printing is the procedure involved and no enlarger for this printing process. The negative of desired print size is sandwiched between coated paper and glass and exposed to a UV light source.

The print is then faced down in a bath of room temperature water in order to extract the the soluble gum, excess dichromate and pigment. Once the layers are completely dry, a clearing bath of sodium metabisulphate is what is used to soak the print in and extract any left over dichromates and the print is then archival quality.

Sheets of good quality 100% rag (cotton) white or bright hotpress watercolor paper (e.g. Arches Aquarelle, Fabriano Artistico) are ideal for printing.

Trials and errors. Failures are also important and are essentially part of any process. Happy Tanima after bettering at the process! a start ..

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