Re: Dichromate dilution and speed

Joe wrote:

>Sandy,
>
>Do you know to what baume Sam mixes his gum solution? Isn't baume a
>measure of specific gravity?

Sam mixes his gum solution from raw gum arabic, one part gum to two
parts water. This makes a stock solution of something around 30%. I
don't understand the relationship, if any, between Baume and percent
solution.

>
>As an aside, with gelatin do you find differences in bloom make a
>difference in carbon printing characteristics? Perhaps differences
>in baume make for more or less success with gum.

There is no relationship between Bloom in gelatin and printing speed.
Bloom is a measure of the hardness of gelatin, and nothing more. In
practice I have seem some small differences in printing speed of
various gelatin, but the differences are very, very small.

>
>I also think perhaps the nature of the digital negative (on/off vs.
>continuous tone) impacts the results Chris reports. Also, she
>mentions her use of the Edwards exposure unit vs. Sam's use of the
>NuArc. I'd again mention the fact I get much better results using
>the potassium salt and a quartz halogen source or the sun rather
>than UV fluorescent tubes (which I assume are in the Edward's unit).
>I recently acquired a NuArc Mercury 124K (?) unit which I've used
>with some initial success with gum. It appears to print with an
>overall quality similar to the quartz source.

Chris is using inkjet digital negatives (I think) which, in my
experience, print very much like continuos tone negatives. As for the
light source, I very much doubt that it makes much of a difference. I
have print in several processes with both a bank of BLB tubes and a
HID light (NuArc 26-IK) and the differences between the two are not
of any particular consequence. Both emit most of their radiation at a
spike of about 365nm, another at 415nm, and another smaller one at
about 450nm. In fact, the SPD charts of the BL tubes is almost
identical to that of the GW114 mercury vapor lamp of the NuArc so
there is nothing in theory that would explain any significant
difference in printing quality between these two types of lights.

>
>
>I think Sam's results are outstanding, but I really do not think
>using dry dichromate for each emulsion mix is safer than using
>precisely diluted dichromate solution. As far as lessening personal
>exposure and ease of measurement, I would prefer using the liquid.

Whether one uses dry dichromate or not is irrelevant to me. I only
suggested it for testing purposes.

> I think everyone involved in this thread is saying that the amount
>of dichromate and gum and water can be quantified and controlled
>various ways, just how we choose to do the mixing and measurement
>differs. Aren't we really discussing where and when the water goes
>into the mix rather than how much total we add?

I don't really know what we are discussing. In fact, I think we
appear to be discussing entirely different things.

>
>I also vaguely remember reading a statistic which stated the
>industrial discharge of dichromates into the environment was several
>thousand metric tonnes daily. (Actually I think it was more like
>tens of thousands...) I'm reasonably confident the minor amount we
>gum printers discharge into the environment is inconsequential in
>the greater scheme of things, especially if we take care to
>neutralize the hexavalent compounds before we flush them.

Discharge of dichromate is only one of the issues, and not one that
got me initially interested in this topic. The more significant one
that I am trying to understand is why some people get by with a 3%
solution of dichromate in the final coating while others require
saturated solutions of 10X that amount. That fact interests me
greatly.

Sandy

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