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

On Mon, 24 Nov 2003, Sandy King wrote:
>
> 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.

I don't do it that way... I mix the dichromate solution to be as dilute as
I need it to be (starting from a saturated stock solution) and use the
same total amount of liquid and gum at least for any tests... Which
strikes me as much simpler than measuring dry dichromate, let alone
handling it for each print. I think possibly the reason this works for
gum, if not for carbon, is, that as I understand carbon, exposure has to
be right on the money. I can estimate exposure for gum (actually the
negative is as controlling as the % solution) -- if it's over-exposed it
will usually be OK with a longer soak.

> 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.

To put it mildly. And, as noted, no particular reason to do so. Or none
that I can see.

And I seem to have lost the message where you wrote that the pictorico
measured .15 per step on the densitometer... apparently claiming that
that meant it was slower than the Stouffer. But isn't the Stouffer .15
per step??? You said something about comparing by having them overlap. I
couldn't follow that. Would you explain again-- ?

TIA,

Judy

>
>
> >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.
>
>
> --
>
Received on Thu Nov 27 00:06:54 2003