Re: Gelatine is also '' a '' gum but not Gum.

>
>
>In all my experience of making gum prints and carbon prints for
>printing and gravure, the coating hardens from the top down. But
>then I believe that it one should use as few coatings as possible to
>achieve one's effect rather than building up the contrast with many
>layers. If one uses very thin layers one may not be able to tell the
>difference between hardening at the surface and otherwise.
>
>It may also be significant that using the Autochrome tissue, I have
>no trouble in clearing highlights in carbon prints. But that is on
>fixed out RC paper. A couple of years ago. after many years of
>practice, the tissue stopped transferring effectively to home made
>transfer paper made following a recipe that I had used for many
>years. We are now conducting a further trial with Dick Sullivan's
>tissue.
>
>It all shows how inadvisable it is to place too much faith in manuals.
>
>Terry

If anyone seriously questions the mechanism of colloid hardening they
should make the following test. Just apply a very thick coating of
heavily pigmented coating of gum to the surface of the paper. When it
dries, expose from the rear, though the paper support. If you do
this, using the right amount of bichromate in the emulsion, and and
exposing correctly, you should get a print with a lot of Dmax, nice
mid-tones, and nice clear highlights, rather similar to a carbon
transfer print. Exposures will be quite long, however, as the paper
adds a lot of transmission density, and the paper texture will also
likely cause some loss of sharpness.

Exactly how a gum image is formed on a paper surface when exposure is
from the top is a more complicated matter, however, since it
involves, in addition to exposure, other issues such as surface
adhesion.

Sandy
Received on Wed Apr 5 08:12:01 2006