Re: Dichromate dilution and speed

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

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