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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 Gum printing is a way of making photographic reproductions without the use of silver halides. The process uses salts of dichromate in common with a number of other related processes such as sun printing. When mixtures of mucilaginous, protein-co ...
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 William Henry Fox Talbot FRS FRSE Royal Astronomical Society, FRAS (; 11 February 180017 September 1877) was an English scientist, inventor, and photography pioneer who invented the Salt print, salted paper and calotype processes, precursors t ...
later found that sensitized dichromated
colloids A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
such as
gelatin Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
and
gum arabic Gum arabic, also known as gum sudani, acacia gum, Arabic gum, gum acacia, acacia, Senegal gum, Indian gum, and by other names, is a natural gum originally consisting of the hardened sap of two species of the ''Acacia'' tree, '' Senegalia se ...
became insoluble in water after exposure to sunlight.
Alphonse Poitevin Alphonse may refer to: * Alphonse (given name) * Alphonse (surname) * Alphonse Atoll, one of two atolls in the Seychelles' Alphonse Group See also *Alphons Alphons (Latinized ''Alphonsus'', ''Adelphonsus'', or ''Adefonsus'') is a male given n ...
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 {{unsourced, date= March 2021 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 dig ...
printing methods such as
cyanotype The cyanotype (from Ancient Greek κυάνεος - ''kuáneos'', “dark blue” + τύπος - ''túpos'', “mark, impression, type”) is a slow-reacting, economical photographic printing formulation sensitive to a limited near ultraviolet ...
and
platinotype Platinum prints, also called ''platinotypes'', are photographic prints made by a monochrome printing process involving platinum. Platinum tones range from warm black, to reddish brown, to expanded mid-tone grays that are unobtainable in silver ...
. 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 The CMYK color model (also known as process color, or four color) is a subtractive color model, based on the CMY color model, used in color printing, and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers ...
separations with a cyanotype layer. A simple
duotone Duotone (sometimes also known as ''Duplex'') 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 ...
separation combining orange watercolor pigment and a cyanotype can yield surprisingly beautiful results. Low density
photographic negative In photography, a negative is an image, usually on a strip or sheet of transparent plastic film, in which the lightest areas of the photographed subject appear darkest and the darkest areas appear lightest. This reversed order occurs because th ...
s of the same size as the final image are used for exposing the print. No
enlarger An enlarger is a specialized transparency projector used to produce photographic prints from film or glass negatives, or from transparencies. Construction All enlargers consist of a light source, normally an incandescent light bulb shining thoug ...
is used, but instead, a
contact print A contact print is a photographic image produced from film; sometimes from a film negative, and sometimes from a film positive or paper negative. In a darkroom an exposed and developed piece of film or photographic paper is placed emulsion sid ...
ing frame or vacuum exposure frame is used with an
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
light source such as a
mercury vapor lamp A mercury-vapor lamp is a gas-discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger soda lime or borosilicate g ...
, a common fluorescent
black light A blacklight, also called a UV-A light, Wood's lamp, or ultraviolet light, is a lamp that emits long-wave (UV-A) ultraviolet light and very little visible light. One type of lamp has a violet filter material, either on the bulb or in a sepa ...
, or the
sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. 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.


References

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External links


Movie

Video/ Viméo
(
Wayback Machine The Wayback Machine is a digital archive of the World Wide Web founded by the Internet Archive, a nonprofit based in San Francisco, California. Created in 1996 and launched to the public in 2001, it allows the user to go "back in time" and see ...
archive)
An introduction to the gum bichromate processIllustrated Gum Printing Tutorials and worksGum History, Tips, Process, and Original Image Galleries
{{Webarchive, url=https://web.archive.org/web/20120402081705/http://robertwulfert.com/gum_bichromate , date=2012-04-02 Photographic processes dating from the 19th century Alternative photographic processes