Gum printing
Encyclopedia
Gum printing is a way of making photographic reproductions without the use of silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

s. The process used salts of dichromate in common with a number of other related processes such as sun printing
Sun printing
Sun printing may refer to various printing techniques which use sunlight as a developing or fixative agent.-Cyanotype:Cyanotype, also referred to as "blueprinting", is the oldest non-silver photographic printing process. It involves exposing materials which have been treated with a solution of...

.

When mixtures of mucilaginous, protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

-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
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...

 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
Ammonium dichromate is the inorganic compound with the formula 2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium...

 allows concentrations up to 15% of the active ingredient whereas potassium dichromate is limited to about 10%. Exceeding these concentrations results in deposits of chromic acid
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 mixture for glass. Chromic acid may also refer to the...

 in the dried film which ruins any attempts at printing. The greatest sensitivity expressed as an ASA
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...

 speed rating
Film speed
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system....

 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 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.
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