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.

More on gum bichromate

An overview of gum printing history and process

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. William Henry Fox Talbot (February 11, 1800 - September 17, 1877) was one of the first photographers and made major contributions to the photographic process. ... A colloid or colloidal dispersion, is a form of matter intermediate between a true solution and a mixture (suspension). ... For the art collective, see Gelitin. ... Acacia senegal plant from Koehlers Medicinal-Plants 1887 Gum arabic, a natural gum also called gum acacia, is a substance that is taken from two sub-Saharan species of the acacia tree, Acacia senegal and Acacia seyal. ... A carbon print is a photographic print produced by soaking a carbon tissue in a dilute sensitizing solution of potassium bichromate. ...

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]. Gum printing is a way of making photographic reproductions without the use of silver halides. ... The alternative process is a non-silver based photographic printing process. ... Cyanotype is an old monochrome photographic printing process that gives a cyan-blue print. ... Platinotype is a monochrome photographic printing process, based on the light-sensitivity of Ferric Oxalate. ... Cyan, magenta, yellow, and key (black) CMYK (or sometimes YMCK) is a subtractive color model used in color printing. ... Cyanotype is an old monochrome photographic printing process that gives a cyan-blue print. ... Duotone is a form of printing halftones with two colors of ink, one usually black. ...

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. In photography, a negative is a rectangle of material (nowadays usually photographic film) coated with chemicals that, upon photographic exposure, cause the material to record the colors or monochromatic shades of the scene in inverse, negative form. ... Photographic Enlarger An enlarger is a specialized transparency projector used to produce photographic prints from film or glass negatives. ... Contact prints are the worst part of photography. ... For other uses, see Ultraviolet (disambiguation). ... A Mercury-vapor lamp is a gas discharge lamp which uses mercury in an excited state to produce light. ... Spectrum of a fluorescent black light source. ... Sol redirects here. ...

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. R-phrases   S-phrases   Except where noted otherwise, data are given for materials in their standard state (at 25 Â°C, 100 kPa) Infobox disclaimer and references Structural formula of sodium metabisulfite Sodium metabisulfite or sodium pyrosulfite (American spelling; English spelling is Sodium metabisulphite or sodium pyrosulphite) is an inorganic compound...

Darkroom technique

Materials and equipment

• Sheets of good quality 100% rag (cotton) white or bright hotpress watercolor paper (i.e. Arches Aquarelle, Fabriano Artistico)
• Gum arabic 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
• Chrome Alum
• Potassium metabisulfite
• [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 for drying
• Pushpins
• Adhesive tape
• [Gram scale]
• Waxed paper
• [Stouffer 21-Step Gray Scale]

Acacia senegal plant from Koehlers Medicinal-Plants 1887 Gum arabic, a natural gum also called gum acacia, is a substance that is taken from two sub-Saharan species of the acacia tree, Acacia senegal and Acacia seyal. ... Potassium dichromate, K2Cr2O7 is used in oxidation reactions. ... This page meets Wikipedias criteria for speedy deletion. ... For the art collective, see Gelitin. ... A crystal of alum Alum, (IPA: ) (aluminium potassium sulfate,) in chemistry, is a term given to the crystallized double sulfates of the typical formula M+2SO4·M3+2(SO4)3·12H2O, where M+ is the sign of an alkali metal (or generally monovalent cation) (lithium, sodium, potassium, rubidium, or caesium... Potassium metabisulfite, Potassium pyrosulfite, K-meta, or the active ingredient in Cambden tablets. ... Wooden clothes-pin A clothes-pin (also C47, bullet, clothes peg, or just peg) is a fastener used to hang up clothes for drying, usually on a clothes line. ... A brass thumbtack A thumbtack (known as a drawing-pin in the UK, India, Australia and New Zealand) is a short nail or pin with a large, slightly rounded head made of metal which is used to fasten documents to a background for public display and which can easily be... Two rolls of adhesive tape. ...

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.

Sizing paper

Sizing paper (pre-shrinking and coating) is necessary if you are printing more than one color or multiple times with the same color to build up density. It’s also advisable to size your paper to help minimize staining. A gelatin coating allows the unhardened dichromate to release from the paper fibers. Without sizing, 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 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

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 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.

Cyan, magenta, yellow, and key (black) CMYK (or sometimes YMCK) is a subtractive color model used in color printing. ...

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. Ammonium dichromate is less soluble than potassium dichromate so it clears slower than potassium dichromate. 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. Potassium metabisulfite, Potassium pyrosulfite, K-meta, or the active ingredient in Cambden tablets. ... Ammonium dichromate, (NH4)2Cr2O7, sometimes known as Vesuvian Fire, is a chemical compound that was used as an oxidizer in early photography and pyrotechnics, and in the formation of tabletop volcanoes. Ammonium dichromate forms orange crystals, and is toxic and corrosive. ... Potassium dichromate, K2Cr2O7 is used in oxidation reactions. ...

References

• Langford, Michael, The Darkroom Handbook, New York: Dorling Kindersley Limited, 1981, p. 321-323.
• Farber, Richard, Historic Photographic Processes, New York, Allworth Press, 1998, p. 150-176. ISBN: 1-880559-93-5
• Crawford, William, The Keepers of Light, New York, Morgan & Morgan, Inc. 1979, p. 199-212. ISBN: 0-87100-158-6
• Scopick, David, The Gum Bichromate Book 2nd Edition, Stoneham, MA, Focal Press, 1991. ISBN: 0-240-80073-7

External links

• Gum Photo technical information
• Alternative Photography process notes
• Illustrated Gum Printing Tutorials and works