Gum bichromate

Gum bichromate is a 19th-century photographic printing process based on the light sensitivity of dichromates. It is capable of rendering painterly images from photographic negatives. Gum printing is traditionally a multi-layered printing process, but satisfactory results may be obtained from a single pass. Any color can be used for gum printing, so natural-color photographs are also possible by using this technique in layers.

History and process overview

Gum bichromate, or gum dichromate as it is also known, is a photographic printing process invented in the early days of photography when, in 1839, Mungo Ponton discovered that dichromates are light sensitive. William Henry Fox Talbot later found that colloids such as gelatin and gum arabic became insoluble in water after exposure to light. Alphonse Poitevin added carbon pigment to the colloids in 1855, creating the first carbon print. In 1858, John Pouncy used colored pigment with gum arabic to create the first color images.

Gum prints tend to be multi-layered images sometimes combined with other alternative process printing methods such as cyanotype and platinotype. A heavy weight cotton watercolor or printmaking paper that can withstand repeated and extended soakings is best. 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. Some photographers prefer substituting the cyan emulsion in the CMYK separations with a cyanotype layer. A simple duotone separation combining orange watercolor pigment and a cyanotype can yield surprisingly beautiful results.

Low density photographic negatives of the same size as the final image are used for exposing the print. No enlarger is used, but instead, a contact printing frame or vacuum exposure frame is used with an ultraviolet light source such as a mercury vapor lamp, a common fluorescent black light, or the sun. The negative is sandwiched between the prepared paper and a sheet of glass in registration with previous passes.

The print is then floated face down in a bath of room-temperature water to allow the soluble gum, excess dichromate, and pigment to wash away. Several changes of water bath are necessary to clear the print. Afterwards, the print is hung to dry. When all layers are complete and dry, a clearing bath of sodium metabisulfite is used to extract any remaining dichromate so the print will be archival.

Darkroom technique

Materials and equipment

Recipe I

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

Recipe II

Part B is a saturated solution of dichromate salt. Use only one of the three kinds of dichromate crystals. Mix the dichromate crystals with the 80mL of warm water until dissolved. Then add enough distilled water to make 100mL of solution. Store finished solution in a light-tight bottle. Keep away from food, children, pets, etc.

Stretching and sizing paper

Stretching (pre-shrinking) paper is necessary if you are printing more than one color or multiple times with the same color to build up density. If your paper is not sized (most watercolour papers are, though to varying degrees) it’s also advisable to size the paper to help minimize staining. A gelatin size (coating) prevents the unhardened dichromate from permeating the paper fibers. Without stretching, the paper will change shape between layer printings. Make lots of sized paper at one time. Label all paper with pencil after sizing.

  1. Preshrink paper for 10–15 minutes in hot water
  2. Hang to dry
  3. Mix gelatin by slowly adding powder to warm water
  4. Add 25g of chrome alum to gelatin and stir NOTE: carefully read the MSDS for chrome alum. It is a suspected carcinogen
  5. Apply warm gelatin mixture to paper with sponge brush, or dipping the paper in warm gelatin solution.
  6. Hang to dry
  7. Wash paper for 10 minutes in cool running bath to remove excess chrome alum
  8. Hang to dry

NOTE: If pigment stains paper after printing, repeat step 7 with remaining sized paper. This is why it’s important to make notes in the margins or on the back of your prints. Indicate each step including how many coats, length of baths, and hardening agent for archival and personal purposes. (Other hardening agents not recommended are Glyoxal, Formalin, & formaldehyde.) In this way you can easily trace your steps to find mistakes or places to improve your printing.

Sensitizing paper

  1. Mix part A & B
  2. Coat with sponge brush in low incandescent light
  3. Dry in dark or nearly dark

Printing in one color

coat and dry several sheets of paper, choose a softly lit image with strong 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 again 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

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.

Printing in four colors

Before you begin, coat a small sheet of paper with emulsion and let dry. Make a test exposure on the small sheet using a Stouffer scale to determine correct exposure time for your light source.

  1. Scan your image or open an existing digital image file.
  2. Using a photo editing program, convert the image file to CMYK.
  3. Separate the individual CMYK color plates into four grayscale digital images (i.e. split the channels in, e.g., Photoshop).
  4. Label each image with its corresponding color.
  5. Invert the images to create four negatives.
  6. Print each of the grayscale images (Cyan, Magenta, Yellow, and Black) onto inkjet transparency film in black and white. (Thin images work better than dense ones.)
  7. Lay two of the negatives (i.e. Black and Cyan) together and align the corners of the image, then use two pushpins on the outer edge to create registration holes.
  8. Repeat the registration process with one of the registered negatives (i.e. Cyan and Yellow or Cyan and Magenta) for the other two negatives.
  9. Coat and dry a gelatin sized piece of paper with the least dense color (Yellow) emulsion first.
  10. Pushpin the negative to the paper creating the registration holes for the other three colors.
  11. Tape negative edges to paper and remove pushpins.
  12. Insert paper and negative into a contact frame or vacuum printing frame and expose according to Stouffer scale results.
  13. Remove negative and float paper face down in a tray of still water for 30 minutes.
  14. Gently move print to fresh tray of still water to rinse for 10 more minutes.
  15. Hang to dry.
  16. Repeat steps 9 through 15 for Cyan, then Magenta, and finally the Black negative.

Archival wash

When the print is completely dry and no more layers will be applied, a potassium metabisulfite clearing bath is used to remove excess dichromate trapped in the paper. This can be seen on the reverse of the paper as a yellow stain where pigment was applied on the verso. In a well-ventilated room mix 250g Potassium Metabisulphite in a tray of water and soak the print for 1–5 minutes. This bath may soften the image so care must be taken when washing away the metabisulphite in the next step. In a separate tray, gently run cool water for 10–20 minutes with the print face down to remove all metabisulphite solution. Hang to dry.

References

External links

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