Re: Dichromate dilution and speed

Okay, I think I need to hit the reset button on this discussion.

I missed something somewhere, which is understandable since I've been
trying to write a book and so haven't been watching the list every
minute for the last couple of months.

But I'm really confused here. I thought that the whole thing about
dilution and speed was a dead issue and that I had spent a day making a
willing fool of myself by proving something that it turns out no one
disputes after all. And I couldn't figure out why you wouldn't stop
beating me about the head and shoulders over dry gum and Sam and
everything.

Now I think I get that you're saying that it"s Sam who is making the
claim that dilution of dichromates doesn't necessitate longer exposures,
and that the issue is still very much on the table. If that's the case,
(1) why did Christina retract her statement, and (2) does Sam, or do
you, have some actual data comparing the diluted dichromate to saturated
dichromate with everything else held constant? Or is all this just
speculation based on the idea that Sam's exposure times "seem" to be
low? I want to see some comparison data.

My job as a statistician mainly consisted of trying to stop people from
explaining differences that didn't actually exist. When I've seen some
real data that shows quantitatively that for Sam the dilution doesn't
affect the speed as much as it does for me, THEN we can talk about the
reasons why that might be so. Otherwise, it's all speculation and I just
don't have time for it.

But before I go, a couple of thoughts regarding some of the
speculations:

1. Diginegs, maybe... but I'd have to see it. And just having someone
say "my diginegs print in such and such a time, and I think that's
pretty short".... that just doesn't cut it. It's got to be an actual
comparison with the saturated dichromate, holding everything else
constant, using diginegs and then contone negs. This 2x2 comparison is
the only way to judge (1) whether there's an actual difference to
explain and (2) if the difference is due to digital negatives.

2. I think the gum thing is a wild goose chase. But I do understand now
the source of some of the difficulty between us. You actually believe
that gums of the same baume could be wildly different in percentage
solution; I don't; furthermore even if they were I think it's quite
immaterial. I just don't think that dog will hunt.

But if we take this line any further, we need someone, like Dick
Sullivan perhaps, who actually knows something about this, to come in
and set things straight; none of us so far know enough to have any idea
what we're talking about.

But from your description, it seems likely that Sam and I use basically
the same mixture, except for the dilution of the dichromate.

3. Sizing .... not likely, since I don't size. But I thought Sam said
that it's just for one-coats that he doesn't size.

4. Hmm. I'm sure there was something else, but I'm too tired to look
back through all this.

At any rate, you get me some data, and then we'll talk, okay? Bye.....
Katharine

Sandy King wrote:
>
> Katharine Thayer wrote:
>
> >Sandy King wrote:
> >>
> >> Katharine Thayer wrote:
> >>
> >
> >>
> >> Now, if there is a relationship between Baume and percent solution,
> >> and you can tell me what that relationship is, then you will have
> >> answered my question. I could then calculate the final gum percent
> >> solution of your coating and compare it to Sam's coating, and/or to
> >> the percentage of colloid in my carbon tissues.
> >>
> >
> >There are many people who don't have a hydrometer and mix their own
> >powder at 30%, whether in the belief that 30% yields 14 degree Baume, or
> >the knowledge that 30% works fine, I I can't say. Whether their success
> >at doing this shows that that relationship is true, or whether it means
> >there's more latitude in the specific gravity requirements for gum
> >printing than is generally understood, I can't say. I've always assumed
> >that the relationship is not 100% reliable, and that's why the specific
> >gravity is specified rather than the percent solution. The feeling that
> >the relationship is inexact is one reason I've always let someone else
> >mix my gum and verify that it's 14 degree Baume, since I don't have a
> >hydrometer.
> >
> >But I think the exactitude you demand is beyond what's reasonable for
> >gum. It's like insisting that numbers be given to the 5th decimal place
> >when there simply isn't that much precision in the measurement.
> >Katharine
>
> You are putting words in my mouth. I am not demanding any level of
> exactitude for gum. I am merely trying to suggest how one might test
> for maximum speed with a minimum amount of dichromate, maintaining
> good contrast. There must be some rational explanation for the
> reason some people find it necessary to use saturated solutions of
> ammonium dichromate to get enough printing speed while Sam Wang uses
> about 1/10 of that amount and his printing times are quite short.
>
> What I have tried to do is suggest that the speed of gum is not just
> a matter of the percentage of dichromate in the coating solution, but
> also of the percentage of gum in the coating. You will recall that
> yesterday I made this remark about carbon printing.
>
> "In carbon printing a tissue made with a pigmented gelatin solution
> that consists of 5% gelatin would be less sensitive by a factor of
> 2X-4X than one made from a pigmented gelatin solution that had a 10%
> gelatin solution, and one of 20% has more sensitivity over the 10%
> solution by a factor of another 2X to 4X."
>
> In other words, even when using the same amount of dichromate in the
> sensitizer, there is a dramatic increase in effective printing speed
> of carbon when comparing tissues that are made with a very high
> gelatin content when compared to those made with a low gelatin
> content. This relative comparison is true regardless of the type or
> amount of pigment in the tissue.
>
> Now, if you tell me that you are starting with a 30-35 % stock gum
> solution and you dilute this 1:1 with water and then add a bit of
> pigment in aqueous dispersion it is clear that your final coating
> solution is approximately a 15% gum solution. This is approximately
> what Sam Wang works with since he mixes his dry gum arabic 1:2 with
> water, then dilutes this again 1:1 with water to give a coating
> emulsion. This is, at least by analogy with carbon tissue, a coating
> with a very high percentage of colloid and one would expect it to be
> fairly fast. This may explain why he is able to use such very low
> levels of dichromate in the coating solution and still get such fast
> printing speeds.
>
> That is why I am trying to understand the approximate percentage of
> gum arabic in the 14 Baumé gum that most gum printers seem to use. If
> the percentage is around 30% then your final coating solution,
> assuming you dilute 1:1 with the dichromate solution, would be
> similar to what Sam is using. But on the other hand, suppose that
> the actual percentage of gum in your 14 Baumé is only about 15%, and
> you then dilute that 1:1 with the dichromate solution. That would
> give you a final coating solution of less than 7% gum. A thin colloid
> coating like this, at least in theory by analogy to what happens in
> carbon printing, would be much slower than a 15% coating, by a factor
> of between 3X-5X.
>
> Obviously there are many different ways of making good gum prints and
> I can understand that someone highly invested in time with their own
> way of printing is not likely to be interested in making dramatic
> changes. But for someone starting out in the practice it makes sense
> to me to use the most efficient method possible, without compromising
> results of course.
>
> Sandy King
>
> --
Received on Mon Dec 1 22:04:51 2003