Description
This materials have important uses in photography and in photographic screen printing, where it is used as an oxidizing agent together with a strong mineral acid.
Potassium dichromate has important uses in photographyand in photographic screen printing, where it is used as an oxidizing agent together with a strong mineral acid.
Gum bichromateprinting was one of the very first stable photographic printing processes, dating back to about 1850. A solution of gum arabicand potassium dichromate, once applied to paper and dried, will harden when exposed to ultraviolet light.
Chromium intensificationor Photochromos uses potassium dichromate together with equal parts of concentrated hydrochloric aciddiluted down to approximately 10% v/v to treat weak and thin negatives of black and white photograph roll. This solution reconverts the elemental silver particles in the film to silver chloride. After thorough washing and exposure to actiniclight, the film can be redeveloped to its end-point yielding a stronger negative which is able to produce a more satisfactory print.
A potassium dichromate solution in sulfuric acidcan be used to produce a reversal negative (i.e., a positive transparency from a negative film). This is effected by developing a black and white film but allowing the development to proceed more or less to the end point. The development is then stopped by copious washing and the film then treated in the acid dichromate solution. This converts the silvermetal to silver sulfate, a compound that is insensitive to light. After thorough washing and exposure to actinic light, the film is developed again allowing the previously unexposed silver halide to be reduced to silver metal.
The results obtained can be unpredictable, but sometimes excellent results are obtained producing images that would otherwise be unobtainable. This process can be coupled with solarisationso that the end product resembles a negative and is suitable for printing in the normal way.
CrVI compounds have the property of tanninganimal proteinswhen exposed to strong light. This quality is used in photographic screen-printing.
In screen-printinga fine screen of bolting silk or similar material is stretched taut onto a frame similar to the way canvas is prepared before painting. A colloidsensitized with a dichromate is applied evenly to the taut screen. Once the dichromate mixture is dry, a full-size photographic negative is attached securely onto the surface of the screen, and the whole assembly exposed to strong light – typically about half an hour in bright sunlight – hardening the exposed colloid. When the negative is removed, the unexposed mixture on the screen can be washed off with warm water, leaving the hardened mixture intact, acting as a precise mask of the desired pattern, which can then be printed with the usual screen-printingprocess.
