Gum printing is a way of making photographic reproductions without the use of
silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
halide
In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a flu ...
s. The process uses salts of
dichromate in common with a number of other related processes such as
sun printing
Sun printing may refer to various printing techniques which use sunlight as a developing or fixative agent.
Techniques
Cyanotype
Cyanotype, also referred to as "blueprinting", is the oldest non-silver photographic printing process. It involves ...
.
When mixtures of mucilaginous,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
-containing materials together with soluble salts of dichromate are exposed to ultraviolet light, the protein content becomes tanned and resistant to solution in water. The untanned material can be washed away in warm water leaving a hardened, tanned protein negative.
For gum printing a solution of
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 ...
is mixed with either potassium or ammonium dichromate. The higher the proportion of dichromate, the more sensitive the mixture. However, increasing the concentration of dichromate also reduces the contrast which is very low at best. The right concentration of dichromate is always a compromise between speed and contrast.
Using
ammonium dichromate
Ammonium dichromate is an inorganic compound with the formula (NH4)2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium. It is a salt consisting of ammonium io ...
allows concentrations up to 15% of the active ingredient whereas
potassium dichromate
Potassium dichromate, , is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health ...
is limited to about 10%. Exceeding these concentrations results in deposits of
chromic acid
The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixtu ...
in the dried film which ruins any attempts at printing. The greatest sensitivity expressed as an
ASA speed rating
In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quanti ...
is estimated to be about ASA 0.003.
The resulting mucilaginous mixture is spread on a suitable base and allowed to dry in the dark. A contact negative the same size of the finished print is then placed on top of the dried coating and exposed to an
ultraviolet light
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 radiatio ...
source, typically bright sunshine.
Often more than one negative is used to provide detail in all tonal ranges. Using multiple exposures requires very careful registration. In exposing the paper, the thinnest parts of the negatives will allow the most exposure and cause the areas to be darker. The densest parts of the negative require more exposure.
The exposed print is then developed gradually in a succession of trays of still water (approximately ten-minute intervals) at room temperature until the bath water is clear. The gum is soft and easily removed at this stage. The negative is then carefully dried when the negative image will be visible as raised areas of clear colourless gum. This surface can then be inked using proprietary oil-based printing ink and impressions made using a simple pressure printing press. Each negative can be made to yield several copies or even copies in different colours. However the fragile nature of the dried gum surface usually will restrict re-use to only one or two copies.
See also
*
Gum bichromate
References
The gum bichromate process
Photographic processes
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