Re: Zimmerman's gum process

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FotoDave@aol.com
Date: 11/10/01-11:09:58 AM Z


<< e. gum printers are by nature incapable of carrying out
    meaningful sensitometric testing,

There might be some truth in it, but the problem might not be the printers
but the process. The main issue is that gum printing is a direct (non
transfer) process, so the speed and contrast depends on the thickness of the
coating. Because of human inaccuracy, this thickness on each sheet of paper
can be different even with the same emulsion mix.

In carbon, your expose the tissue from the front of the tissue, then transfer
it to a support and develop from the "back." If your tissue is slightly
thicker or thinner, it change much on the process, you still expose on the
front, transfer, and develop from the "back" (so the thickness doesn't
matter).

In gum, the process depends on how deep the light has penetrated through the
emulsion. If it is too little, image will still form but the harden layer is
not deep enough to "catch" the surface of the paper, so the whole image will
wash out during development, so the exposure must be such that the thickened
layer get to the paper base.

Now if the emulsion is slightly thicker, the speed is affected because you
need to expose more to get to the base. The thicker emulsion also means
darker layer, so the masking effect will be even more on the shadow, so speed
is again affected.

Now as a human being, it is impossible to manually coat different sheets with
completely same thickness. With experience, one can even deliberately coat a
little faster or a little slower to change the contrast a little.... and that
is *a little.* If changed too much (especially if the emulsion is too thick)
the whole image would be washed off (in sensitometric term, the speed
suddenly becomes way way slow).

But that mainly affects theoretical sensitometry. In practise, since
development is long, one can adjust the development by shorten or lengthen
the development. My usual procedure is to make a test strip using a step
tablet as a guide (it's not 100% accurate especially if you coat the test
strip separately because coating a small test strip is easier and usually
take shorter time and give a thinner coating), develop for one hour; then
expose slightly more for the full sheet and expose it with a step tablet,
and watch the image and step tablet during development.

If you also consider the effect of sizing, then it adds another variable
because sizing is coating a thin film on the surface of the paper. If too
thin, it might not has the effect. If too thick, it closes the surface
toothtiness that scale become shorter. If we do our own external sizing, then
again the thickness cannot be the same for each sheet.
 
So if I may, I would like to modify the statement to read "gum printing is by
nature difficult to apply sensitometry to." :-)

<< f. gum printing is a witch's brewthat does not
    obey any of the photochemical laws, Kosar or otherwise.
>>

It does. It certainly does, but the photomechanical laws does not cover only
the colloid and then dichromate or other sensitizer but must also include the
toothtiness of the paper, sizing of the paper, thickness of the emulsion.
Even if some of these can be maintained constant, the thickness is a part
that is difficult to control as we are human coater, not machine coater.

Dave S


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