3.6 Gum Bichromate Print
In gum bichromate printing the paper is coated with gum arabic which carries a
pigment, and is sensitized with a bichromate. On exposure to UV light the
bichromate causes the exposed gum arabic to harden and become insoluble in
proportion to its exposure. The areas not exposed to light remain soluble.
The print is developed by floating it face down in water. The unexposed
portions dissolve taking the pigment along. The insolube portions remain on
the paper.
The print can be manipulated while developing, allowing the printer to make
many local modifications. Any watercolor pigment can be used allowing the
printer to choose the color of print. It is also possible, though very
challenging to make full color prints using color seperation negatives.
In order to get a good print it usually necessary to multiple print, that is to
coat, expose and develop the same print repeatedly to develop a full tonal
scale. This requires some sort of registration technique.
Gum bichromate printing can be very demanding because of its flexibility. The
final gum bichromate image is usually somewhat soft. It is not possible to
make a gum print with very fine detail.
3.7 Carbon Print
A tissue is coated with a gelatine solution carrying a pigment sensitized with
a dichromate. When it is xposed to light the gelatin hardens and becomes
insoluble. The gelatin hardens only on the surface, to counteract this the
exposed and washed tissue is placed firmly on a final support and then
peeled away. This leaves the image flipped over on the final support and
exposes the unhardened gelatin.
The transfer tissue is the temporary support that holds the gelatin pigment
mixture during exposure. After exposure the gelatin and pigment are transfered
to the final support. There are no current manufacturers of transfer tissue,
though some sources may still have some available (see the notes at the
end of this section). The pigment transfer tissue can be made by hand.
This makes carbon prints some what complicated.
After the pigment tissue has been coated it is sensitized with a dichromate, and
left to dry. The pigment tissue is the exposed to UV light. The dichromate
causes the gelatin to harden where it is exposed to light.
After exposure the tissue is soaked for a few minutes to allow the excess
dichromate to dissolve out and to allow the tissue to flatten. The tissue and
final support are placed face to face under water and aligned. The two sheets
are then carefully pulled out of the water, placed on a flat sheet (glass) and
lightly squeegeed together. The sheets are then weighed down for about 20
minutes. The two sheets are then placed in water at about 40 degrees C. The
soluble gelatin and pigment will begin to ooze out. After another minute
carefully peal the pigment tissue off the final support. At this point there
is no visible image on the final support. The final image is developed by
agitating in all directions the final support in water in a number of baths
until the image is fully developed. The image is immersed in cold water to
harden the gelatin
The resulting image is made up of only gelatin and pigment, making the carbon
print one the most archival processes.
The following comments which refer to possible sources of tissue were made
on this list.
_______________________________________________________________________________
A few years ago visited Dr. Green and he had freezers full of the
Haefstaengl tissues. He was the sole U.S. distributer and was warring
with Haefstaengl because they were selling the tissues mailorder to
anybody within his territory. Dr. Green sells a book hes written on
monochrome and tricolor carbon and carbro. I would recomment it for anyone
interested in the processes.
Dr. Robert F. Green, M.D.
Gallery 614
0350 R.R. 20
Corunna, Indiana 46730
(219) 281-2752
Claude Seymour
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For the person who needs carbon tissue: I just called Autotype USA, 2050
Hammond Dr., Schaumberg, Illinois 60173 (Tel. 1-800-323-0632). They still
carry rolls of Carbon Tissue. I have not ordered from them since 1988
when I purchased a large quantity. They have been very helpful to me in
the past.
Sarah C. Pierce
UTSA,San Antonio, TX
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Their stuff would be suitable for photogravure only. There might be an
indicator dye to monitor development, but I doubt that one could use
such material to produce high quality permanent carbon transfers.
Luis Nadeau
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3.8 Carbro Print
In carbro printing a pigment tissue similar to that used in Carbon printing is
brought in contact with a bromide print. The gelatin in the pigment tissue
loses its solubility through the chemcial reaction between the sensitizer on
the pigment tissue and the silver in the bromide print. The tissue is then
transfered to the final support and developed in the same manner as a carbon
print.
The advantages of a carbro print is that the pigment tissue is never sensetive
to light and a UV light source in not necessary since the bromide print is made
using traditional silver-gelatin methods
Most bromide prints made today are coated with a hard surface to protect the
fragile gelatin underneath. It is not possible to use these papers in carbro
printing. Common unsupercoated papers currently available are: Kodak
Polycontrast Rapid RC matte surface, Luminos RD Matte Bromide, Agfa Portriga
Rapid No. 118, SupreBrome Royal Portrait Matte Paper, Ilfobrome Semi-Matte
paper.
3.9 Polaroid Transfer
The Polaroid transfer uses Polaroid peel apart color films in an unconventional
manner. The Polaroid pack is peeled apart during processing, the postive image
is transfered to an alternative surface. The surface the image is tranfered to
imparts a different quality to the image.
A receptor sheet is prepared, this will hold the final image. If paper is being
used it is thoroughly soaked, other media can be used such as silk and other
textiles. The polaroid negative is exposed, by shooting live or by projecting
a previously made slide onto the polaroid. The film as processed in the back
as normal. The film is peeled apart after about 10 seconds. The negative
containing all the dyes is then placed on the prepared recieving sheet. The
negative is then firmly pressed onto the receptor sheet using a roller or
your hands. It is critical to use even pressure. After waiting 90
seconds to two minutes the negative is peeled off the receptor. The dyes
have now been transfered to the recieving sheet.
Polaroid makes a variety of peel apart films in different sizes. Transfers
will not work with black and white polaroids or with the sx70 style instant
polaroids.
Polaroid has produced a publication on Polaroid Transfer. They can be reached
Toll Free at 800-225-1618
see part 3