Re: Opacity of digital negative substrates, was Re: Gum a la Sam Wang

Judy,

The issue of pigment concentration is just as important in carbon as
it is in gum. I referenced the fact that my tests with carbon for
speed and contrast were carried out with a normal contrast black
tissue.

To carry out the same tests in gum would merely require that you
select for the norm a gum/pigment combination know to give a normal
contrast image , whatever your working procedures. Then adjust the
amount of sensitizer in the mix upward and downward and compare the
results. It would clearly be best to do this kind of test by adding
the dry dichromate directly to the gum/pigment solution because that
way you don't alter the gum percent solution as you would if working
with the traditional method in which one part of a dilute sensitizer
is mixed with one part gum solution. You can see that the later
method will alter the total amount of water in the coating.

One of the obvious problems in carrying out this kind of test with
gum is that you would need to be able to weigh accurately extremely
small amounts of dichromate. For example assuming that you took as
your norm 10ml of coating solution, to run tests with sensitizers in
the 1% to 10% range would require that you be able to measure
accurately from as little as 0.1g to as high as 10.0g.

Sandy

>Somewhere along in here Sandy made an analogy to carbon printing, or
>anyway dichromate exposure with gelatin. I HATE to be wrong in print, but
>I have an idea that analogy doesn't hold. I don't see how you can talk
>about *speed* for a gum print without the variable of pigment
>concentration -- or at least some reference to that -- or at least how the
>"speed" is to be figured, unless of course the referent is gum +
>dichromate tone only.
>
>Although I've done various tests on Pictorico, I haven't printed with it
>enough to speak with assurance of its printing qualities (tho I still
>doubt it's slower than film as seems to be the consensus here -- tho maybe
>it was the black ink that blocked less UV than silver blocks, even though
>it read the same on the Macbeth.???)
>
>HOWEVER, that issue aside, how do you measure "speed" when you're adding
>pigment -- say, SO MUCH pigment that the steps block up, so you only have
>3 instead of 5 or 6? Would you measure by the top step? But even then
>you couldn't count just the substrate, you'd still have to factor the
>variable of pigment & amount of pigment.
>
>?
>
>J.

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