Re: Gum a la Sam Wang

On Sat, 22 Nov 2003, Christina Z. Anderson wrote:

> > Cutting my am di down to 20% of what I normally use did not require a long
> exposure as I presumed. Even with the thinner digital negative, I thought
> cutting down the dichromate that drastically would warrant maybe a 6 minute
> exposure, for instance. With regular negatives enlarged in the darkroom I
> usually exposed layers from 1-6 minutes with the *30%* am di--usually closer
> to the higher number. With digital negatives this 6% solution requires a
> 1-4 minute exposure.

I've always found that digital negs on paper or other NON-FILM material
take about half or 2/3 the exposure of negs on film, all other things (eg
emulsion & mix) being equal. I concluded that the difference is due, not
to difference in contrast and density of the film negs v. digital negs,
since the densitometer finds them comparable-- I develop lith film for gum
to a contrast range of about 0.9, & print out digital negs at about the
same range.

My assumption has been that the substrate for the digital negs is more
transparent to UV than film is, that is, it transmits UV more completely.
And somewhere in distant memory I hear a "voice" on this list saying the
very same thing. In fact I might have even done a test to prove that --
exposed the various materials with just the gum emulsion & no negative
image, in silver *or* ink... but that was as noted long ago, only dimly
recalled. What I can say with certainty is that my digital negs
densitometer more or less like the film negs & print more quickly.

I suspect, BTW, that Sam's weaker dichromate percent may work better with
digital neg (those dots just have to be ON or OFF) than with continuous
tone, where highlights might tend to be too soft & runny. But I cannot
warn strongly enough against using dry dichromate more than essential.
The same effect can be gotten with a diluted solution. Only the math is
different.

Judy

Judy

> Hope this is clear. The lower density and contrast of
> the digital negatives "fits" Sam's 6% solution dilution for good contrast,
> using his curve.
>
Received on Sat Nov 22 23:39:11 2003