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

Judy Seigel wrote:

>On Fri, 28 Nov 2003, Sandy King wrote:
>
>> ... However, my purpose in recommending the use of
>> dry dichromate was only for the purpose of testing the actual impact
>> on speed and contrast of different strength solutions. It is
>> difficult for me to see how one could make the test work with diluted
>> solutions without changing the ratio of water to gum in the mixture.
>
>But it's difficult for me to see how I could print gum without diluting
>the gum arabic by at least equal parts water-- or dichromate solution --
>since the gum alone is too viscous to spread properly, or I find it so.
>To use dry dichromate and then add water seems rather inefficient, like
>getting your shoes dirty so you can polish them. As for the math of the
>dichromate solution, who says dry dichromate is the standard ? If this
>interested me, I see no reason why the % solution shouldn't be the
>standard. (And if I wanted scientific numbers, I'd join the astronomy
>club. Whole integers are enuf !)
>
>As for testing minimum amounts of dichromate, I repeat that that's a
>floating figure -- it cannot be quantified without setting pigment,
>concentration, exposure, and development. Oh, and I nearly forgot -- the
>gum! I find gums can have RADICALLY different speeds -- from about 3
>stops to 6, all other conditions identical.
>
>J.

Judy,

You wrote above, "to use dry dichromate and then add water seems
rather inefficient." I know that you do not suffer from reading
incomprehension so I have to assume you have not read Sam's article?

Sam's article makes it perfectly clear that you first add water to
the dry gum, and after it is well mixed the resulting solution
becomes the stock coating solution to which no more water is added.
To make a coating solution you take a small amount of the stock gum
solution and then you add pigment, and *finally*, and just before
coating, you add the small bit of dichromate.

There are two big advantages to working this way, from my
perspective. One advantage is that by adding the pigment with the gum
solution before adding the dichromate you get a very precise idea of
what the final color will really look like. As anyone who has worked
with gum knows, once the dichromate is in the mix colors are very
distorted. Another advantage is that the percent solution of gum in
the coating solution will always be the same. This assures that your
coating solution will have the same consistency and speed
characteristics so the only variable is the type and amount of
pigment.

It is probably true, as you say, that gums can have many different
speeds. That is also true with gelatins. All the more reason to
understand your materials.

And who cares anything about the gum standard? Sam has made a point
of the fact that this is his way of working and if it does not suit
others then so be it.

Sandy

>
>
>
>> I grant that it could be done but it would involve some fairly
>> complicated calculations. Making the test by keeping the amount of
>> water, gum and pigment constant in the experiment would ensure that
>> the only variable would be the amount of dichromate.
>>
>> The main issue as I see it is the one posed by Chris. That is, what
>> is the minimum amount of dichromate one can use and maintain good
>> speed and contrast? The answer to that question has implications,
>> whatever your actual working method.
>>
>>
>>
>>
>> Sandy King
>>
>>
>>
>>
>>
>>
>> >On Thu, 27 Nov 2003, Sandy King wrote:
>> >
>> >> Judy Seigel wrote:
>> >>
>> >> >
>> >> >> 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.
>> >>
>> >>
>> >> Judy,
>> >>
>> >> The reason should be obvious. Protection of *your* environment. The
>> >> way you work results in pollution of the environment with more
>> >> dichromate than is necessary for the application. Just do the figures
>> >> and and show me if I am wrong,
>> >
>> >Sandy,
>> >
>> >I think we're talking about 2 different things... I'm addressing the use
>> >of dry dichromate. What seems implicit in your reply is the assumption
>> >that you use less dichromate if you use it dry -- rather than the same
>> >amount per print only dissolved in water. Why would this be? I've always
>> >diluted the dichromate solution well beyond the standard 26% or even 10%,
>> >hence used less, since I find that coating & developing work better with a
>> >thinner emulsion. I think probably my present practice remains heavier on
>> >the dichromate than 1/2 of one percent, tho I haven't checked YOUR math on
>> >that figure !!! -- but, math aside, perhaps you can explain why suitable
>> >dilution can't be made up & used accordingly.
>> >
>> >Not to mention that in my experience it's next to impossible to measure
>> >out dry dichromate without having fly-around particles, which would by
>> >"dry" system occur with every print -- as would of course be more harmful
>> >to me and my PERSONAL environment than trivial amounts (what isn't
>> >oxidized or hardened into image) entering the water course, that EPA told
>> >me was removed anyway in treatment plant. On top of which, as i do mop up
>> >of work area, and the particles then enter the water stream, I daresay at
>> >least the same amount total dichromate would be ultimately dispersed, not
>> >to mention the effect on my IMMEDIATE environment, which could at some
>> >point remove me from other good works.
>> >
>> >Judy
>>
Received on Sat Nov 29 08:25:02 2003