Re: new alt process--gelatin silver

----- Original Message -----
From: "Catherine Rogers" <crogers@optusnet.com.au>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Friday, April 08, 2005 4:52 PM
Subject: Re: new alt process--gelatin silver

> Since we are on the subject of alt print identification -
> or lack of - I was wondering if someone could explain the
> various categories of carbon and carbro prints -
> monochrome and colour.
>
> I have seen displayed what I had understand as being
> carbon transfer prints (monochrome) described as carbro,
> similarly colour carbon prints described as colour carbro
> (on labels by museum staff). My understanding of a carbro
> print was that a carbon tissue image was placed on top of
> the bromide print (hence the '...bro' ending of carbro),
> and that was quite a different thing to a carbon
> (transfer) print where the tissue image is placed onto a
> new piece of paper. So I was wondering how a colour carbon
> image can be a carbro print. It's ages since I made any
> carbon prints (1978 to be exact) - so I may be waaaay off
> track here.
>
> thanks all
> Catherine
>
   Carbro is short for Carbon-Bromide. Carbon is a printing
out process, however, the sensitized gelatin is also
sensitive to reaction products from bleaching a bromide
print which is rolled into contact with it and left for a
while. This is very convenient for making large carbon
prints and for three color prints where the originals are
small negatives.
   Carbon tissue is actually a support with unhardened
gelatin on it. The gelatin has pigment of the appropriate
color in it. While carbon may have once been one of the
pigments it was not widely used when the process was popular
but the name stuck. The gelatin is sensitized with Potassium
dichromate, exposed or printed with a bromide print, causing
the exposed areas to harden. Then it is 'developed" by
washing with hot water to wash away the unhardened gelatin.
   When exposed to a bromide print the gelatin image is a
mirror image so the gelatin must be floated off the original
support onto the final support. This is single transfer
carbro. Some forms of carbon printing require the gelatin to
be transferred twice to the final support to get the image
correct left-to-right. Occasionally a black key is used in
color prints for the same reasons its used in four color
printing: to get better blacks, and to cover up a certain
amount of color mistracking.
   Three color carbro prints were for many years the
standard originals for making photomechanical plates for
color advertising printing. The method was slowly replaced
by dye transfer and by Kodachrome or other tranparency
materials. Color carbro or carbon is evidently an extremely
fussy process requiring close control over many steps.
   There is a long section on color carbon in Freidman's
book _A History of Three Color Photography_

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com