Gum bichromate
Encyclopedia
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
Mungo Ponton
Mungo Ponton FRS was a Scottish inventor who in 1839 created a method of permanent photography based on sodium dichromate....

 discovered that dichromates are light sensitive. William Henry Fox Talbot later found that colloids such as gelatin
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...

 and gum arabic
Gum arabic
220px|thumb|right|Acacia gumGum arabic, also known as acacia gum, chaar gund, char goond, or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal...

 became insoluble in water after exposure to light. Alphonse Poitevin
Alphonse Poitevin
Alphonse Poitevin was a French chemist, photographer and civil engineer who discovered the light–sensitive properties of bichromated gelatin and invented both the photolithography and collotype processes....

 added carbon pigment to the colloids in 1855, creating the first carbon print
Carbon print
A carbon print is a photographic print with an image consisting of pigmented gelatin, rather than of silver or other metallic particles suspended in a uniform layer of gelatin, as in typical black-and-white prints, or of chromogenic dyes, as in typical photographic color prints.In the original...

. In 1858, John Pouncy used colored pigment with gum arabic to create the first color images.

Gum prints
Gum printing
Gum printing is a way of making photographic reproductions without the use of silver halides. The process used salts of dichromate in common with a number of other related processes such as sun printing....

 tend to be multi-layered images sometimes combined with other alternative process
Alternative process
The term alternative process refers to any non-traditional, or non-commercial photographic printing process. Currently the standard analog photographic printing process is the gelatin-silver process, and standard digital processes include the pigment print, and digital laser exposures on...

  printing methods such as cyanotype
Cyanotype
Cyanotype is a photographic printing process that gives a cyan-blue print. The process was popular in engineering circles well into the 20th century. The simple and low-cost process enabled them to produce large-scale copies of their work, referred to as blueprints...

 and platinotype
Platinotype
Platinum prints, also called platinotypes, are photographic prints made by a monochrome printing process that provides the greatest tonal range of any printing method using chemical development....

. 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
Cyanotype
Cyanotype is a photographic printing process that gives a cyan-blue print. The process was popular in engineering circles well into the 20th century. The simple and low-cost process enabled them to produce large-scale copies of their work, referred to as blueprints...

 layer. A simple duotone
Duotone
Duotone is a halftone reproduction of an image using the superimposition of one contrasting color halftone over another color halftone. This is most often used to bring out middle tones and highlights of an image...

 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
Enlarger
An enlarger is a specialized transparency projector used to produce photographic prints from film or glass negatives using the gelatin silver process, or from transparencies.-Construction:...

 is used, but instead, a contact print
Contact print
A contact print is a photographic image produced from film; sometimes from a film negative, and sometimes from a film positive. The defining characteristic of a contact print is that the photographic result is made by exposing through the film negative or positive, onto a light sensitive material...

ing frame or vacuum exposure frame is used with an ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light source such as a mercury vapor lamp, a common fluorescent black light
Black light
A black light, also referred to as a UV light, ultraviolet light, or Wood's lamp, is a lamp that emits ultraviolet radiation in the long-wave range, and little visible light...

, or the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. 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
Sodium metabisulfite
Sodium metabisulfite or sodium pyrosulfite is an inorganic compound of chemical formula Na2S2O5. The substance is sometimes referred to as disodium...

 is used to extract any remaining dichromate so the print will be archival.

Materials and equipment

  • Sheets of good quality 100% rag (cotton) white or bright hotpress watercolor paper (e.g. Arches Aquarelle, Fabriano Artistico)
  • Gum arabic
    Gum arabic
    220px|thumb|right|Acacia gumGum arabic, also known as acacia gum, chaar gund, char goond, or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal...

     or pre-mixed 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) NOTE: Read the MSDS thoroughly before handling.
  • Gelatin
    Gelatin
    Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...

  • Chrome Alum
    Alum
    Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate with the formula KAl2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB2.12H2O.-Chemical properties:Alums are...

  • Potassium metabisulfite
    Potassium metabisulfite
    Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder with a pungent sulfur odour. The main use for the chemical is as an antioxidant or chemical sterilant. It is a disulfite and is chemically very similar to sodium metabisulfite, with which it is...

  • Dropper bottle for dichromate solution (Part B, Recipe II below)
  • Cone-top fabric paint bottle for each color/gum solution (Part A, Recipe II below)
  • Contact printing frame or vacuum exposure frame
  • Sponge brush for applying emulsion
  • Camel hair brush and/or paint brush to give brush marks
  • Sun, metal halide ultraviolet light source, or bank of several fluorescent black lights
  • Developing tray & tongs
  • Small graduated cylinder
  • Large graduated cylinder
  • Clothes line and pins
    Clothes-pin
    A clothespin is a fastener used to hang up clothes for drying, usually on a clothes line. Clothespins often come in many different designs.-Design:...

     for drying
  • Pushpins
    Thumbtack
    A drawing pin , thumbtack , or push pin is a short nail or pin with a circular, sometimes domed, head, used to fasten items such as documents to a wall or board for display. Various designs and names are used. They are inserted and removed by hand, hence the terms "thumbtack" and "push pin"...

  • Adhesive tape
    Adhesive tape
    Adhesive tape is one of many varieties of backing materials coated with an adhesive. Several types of adhesives can be used.-Types:Pressure sensitive tape...

  • Gram scale
  • Waxed paper
  • Stouffer 21-Step Gray Scale

Recipe I

  • 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

Recipe II

  • Part A
    • 20mL Prepared Gum Arabic Solution (14 degree Baume liquid solution from fine art or graphic arts supplier)
    • 2-3g tube water color with high density of pigment (i.e. Winsor&Newton, MaimeriBlu)
  • Part B
    • 13g Potassium dichromate or 25g ammonium or sodium dichromate
    • 80mL distilled water at 125 degrees F
    • Approx. 20mL distilled water at room temp to make 100mL solution.

Part B is a saturated solution of dichromate salt. Use only one of the three 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
  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
Potassium metabisulfite
Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder with a pungent sulfur odour. The main use for the chemical is as an antioxidant or chemical sterilant. It is a disulfite and is chemically very similar to sodium metabisulfite, with which it is...

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.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK