Terry King gives an overview of gum bichromate printing, along with a step-by- step guide to the process. Fifteen years ago, I was in a similar. Gum bichromate process definition, a contact printing method in which the image is formed on a coating of sensitized gum containing a suitable colored pigment. In the late s, pictorialist photographers favored a diversity of photographic techniques, including the gum dichromate process. Sometimes superimposed.
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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 gumm a multi-layered printing process, but satisfactory results may be obtained from a single pass. Any color can be used for gum bcihromate, so natural-color photographs are also possible by using this technique in layers.
Gum bichromate, bicyromate gum dichromate as it is also known, is a photographic printing process invented in the early days of photography when, inMungo 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 increating the first carbon print.
InJohn 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 bichromats 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.
Gum Bichromate Process
Low density photographic negatives of the same size as the final guj 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 lampa common fluorescent black lightor the sun.
The negative is sandwiched between the prepared paper and a sheet of glass bichronate 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.
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 mL of solution. Store finished solution in a light-tight bottle. Bcihromate away from food, children, pets, etc. 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.
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.
If pigment stains paper after printing, repeat step 7 with remaining sized paper. Indicate each step including how many coats, length of baths, and hardening agent for archival and personal purposes.
Introduction to gum bichromate printing – National Science and Media Museum blog
In this way you can easily trace your steps to find mistakes or places to improve your printing. Before you begin, coat a small sheet of paper with emulsion and procexs dry. Make a test exposure on the small sheet using a Stouffer scale to determine correct exposure time for your light source.
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 prcess the reverse of the paper as a yellow stain where pigment was applied on the verso.
In a well-ventilated room mix g potassium metabisulphite in a bichromahe 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.
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