Gum bichromate

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Gum bichromate is a photographic printing process. It can be repeated on the same sheet of paper for a multichromatic result. Traditionally it is contact method, though any method which selectively emits ultraviolet light can be used. The process coats plain paper with a photo reactive chemical mixture, exposes an image, then goes through a simple development stage.

It was introduced commercially in 1894.

Contents

[edit] Darkroom technique

[edit] Materials and equipment

  • sheets of good quality drawing paper, size and texture appropriate for image
  • gum arabic
  • water color pigment
  • potassium dichromate crystals
  • tins of four or more water color powder pigments
  • camel hair brush and paint brush to give brush marks
  • sun (or UV light source)
  • hand board and tape (to stretch paper flat while drying)

[edit] Recipe

  • Gum: powdered gum arabic (350 g)
  • hot water to make 1 liter (add powdered water color pigment, according to the image color and density required
  • Sensitizer: potassium dichromate crystals (50 g)
  • Warm water (500 ML) - store sensitizer in dark tinted container

Mix 2 parts gum to 1 part sensitizer just prior to use

[edit] Prepare paper

  1. Soak paper 5-10 min in water, tape it down onto hardboard and let dry.
  2. Mark out image area lightly with a pencil.
  3. Mix solution under a safe light (red/orange).
  4. Coat paper with mixed solution, quickly and evenly in a crosshatch pattern.
  5. Allow paper to dry in dark

[edit] Printing in one color

coat and dry several sheets of paper, choose a softly lit image with stong tonal gradation.

  1. Place intermediate negative with sensitized paper (clamp down with a piece of glass).
  2. Test exposures, 30 secs to 2 minutes with film negatives (paper negatives take several hours).
  3. Cover paper in cold water face down until orange dichromate and gum pigment diffuses out.
  4. Choose the time that produces the best results and expose a second sheet, and start over with best exposure time.
  5. After processing full print, brush away unwanted shadows with a small soft water color brush
  6. Tape print to board and let dry

[edit] Printing in three colors

  1. Make three color separations (blue, green, and red filters) use panchromatic film
  2. Mix three pigmented gum solutions (yellow, magenta, and cyan) add sensitizer before applying each to the paper
  3. Coat with sensitized yellow gum and expose to the blue separation.
  4. Process and dry, recoat with magenta gum to print to the green separation.
  5. Repeat in cyan and red separation.

[edit] References

  • Langford, Michael, The Darkroom Handbook, New York: Dorling Kindersley Limited, 1981, p. 321-323.

[edit] External links

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