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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 procceses such as sun printing. General Name, Symbol, Number silver, Ag, 47 Chemical series transition metals Group, Period, Block 11, 5, d Appearance lustrous white metal Atomic mass 107. ...
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. ...
Chromates and Dichromates are salts of chromic acid. ...
Sun printing is a photographic process in which the final print is produced by conventional lithographic printing processes. ...
When mixtures of mucilaginous, protein-containing materials together with soluble salts of dichromate are exposed to 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. A representation of the 3D structure of myoglobin, showing coloured alpha helices. ...
Gum printing using a solution of gum arabic 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. 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. ...
Using ammonium dichromate 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 in the dried film which ruins any attempts at printing. The greatest sensitivity expressed as an ASA rating is estimated to be about ASA 0.003. 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. ...
In chemistry, chromic acid is a hypothetical chromium (Cr) compound, yet to be isolated, that would have the formula H2CrO4. ...
The American National Standards Institute or ANSI (pronounced an-see) is a nonprofit organization that oversees the development of standards for products, services, processes and systems in the United States. ...
In gum printing, the gum-dichromate mixture is supplemented by high quality powered pigment. 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. Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ...
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, which require more exposure, will have less colour.
The exposed print is then developed in running hot water until pigment stops washing out.
See also Sun printing is a photographic process in which the final print is produced by conventional lithographic printing processes. ...
References The gum bichromate process
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