"Lew" wrote in message
...
I'm just reading an article in the April issue of B&W
magazine (vol 8, #42, p.46) in which a tri-color carbro
process is described. While I believe that the writer
botched the description it has reminded me that I don't
have a clear understanding of the differences between the
carbro, carbon, and bromoil processes. I suppose that this
is a tedious request for some members of this list, but if
any would care to weigh in, I'd appreciate it.
(I've posted this to as
well.)
-Lew

I also recommend the source given by Micheal
Gudzinowicz. FWIW, the three color carbro process was very
widely used for high quality color prints for reproduction
purposes up to perhaps the late 1940's after which it was
supplanted by dye transfer and Kodachrome.
Carbro and and Bomoil are variations of carbon and oil
process which use matrices of differentially hardened
gelatin. In the Carbon process the gelatin comes in the form
of a thin gelatin "tissue" containing a pigment. While
actual carbon may have been used early on it is actually
never found as one of the pigments. In oil printing a
coating of gelatin is differentially hardened so that it
absorbs varying amounts of transparent oil paint.
The matrices are made from Gelatin sensitized with
potassium dichromate. When exposed to strong light the
dichromate hardens the gelatin in proportion to the amount
of light. In carbro and bromoil another method is used to
expose the sensitized gelatin. A bromide print, i.e., a
normal silver gelatin print, is rolled into contact with the
sensitized gelatin and left for a time. A reaction between
the silver image and dichromate causes the same differential
hardening effect as exposure to light. After the appropriate
time the print is stripped off and the gelatin further
processed as it would be for which ever printing process is
being used. The advange of using a bromide print is that
enlarged prints can be made from small negatives without
having to make enlarged negatives for what otherwise are
contact printing processes. I think the FAQ on the
Alternative printing site gives much more detail.
Three color Carbro was used for making prints from color
separation negatives. Up to the late 1940's these were
usually made from one-shot color separation cameras. The
process was very fussy requiring tight control of the
negative exposure and contrast and the intermediate prints.
Because the hardness of the gelatin tissue was important the
labs specializing in this work were referigerated.
For the best quality advertising work one shot cameras,
matched films or glass plates, and carbro printing materials
were supplied as a system by three companies: National
Photocolor and Devin-McGraw in New York, and Thomas S.
Curtis in Los Angeles. The well known Autocolor materials,
made in England, were not considered reliable enough for
commercial work.
The Kodak Dye Transfer method, while still fussy, is much
less so than three color carbro. It uses silver halide
emulsions which are differentially hardened by the
developer. Pancromatic emulsions were supplied as well as
non color sensitized ones, so that matrices could be made
directly from color negatives. An improved version of this
process became available in the late 1940's and, along with
Kodachrome and similar multi-layer color films, completely
supplanted three color carbro within a few years.
Carbro and Carbon prints have the advantage of being made
with pigments rather than dyes so they can have very long
life (100 years plus if the pigments are chosen right).


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Richard Knoppow
Los Angeles, CA, USA