From: Ryuji Suzuki (rs@silvergrain.org)
Date: 09/09/03-11:09:57 AM Z
From: Judy Seigel <jseigel@panix.com>
Subject: Re: Gum Woes
Date: Fri, 05 Sep 2003 16:14:02 -0400 (EDT)
> If you set up to do a whole bunch of paper (having done a test run of the
> materials first to be certain), and ADD A DROP of KREMER ANTI-FOAM to the
> liter of 3% gelatin, then squeegee with a rod against a glass sheet (from
> the back) the sizing goes very quickly & the coat will be perfect. (I'm
> going to do an exact description with diagrams for the forthcoming #9, but
> will answer any particulars asked in advance.) For "rod" incidentally I
> use an acrylic towel bar about 1 inch in diamter. Works splendidly.
What are the ingredients of the Kremer Anti-Foam? Does anyone know?
(For silver gelatin people to compare notes: 3% gelatin seems very
bubbly. I use about 3% to precipitate and ripen the emulsion at 70C,
but at this temperature and concentration the solution is very
bubbly. The bubble laegely goes away when I add more gelatin to make
7-8% the amount of bubble is less but they are still hard to break...
Mine is photographic grade 225 Bloom gelatin from Eastman Gelatine)
Incidentally, in modern factory coating of silver gelatin materials,
it seems that the coating machines have provisions to give mechanical
or acoustic impacts to the coated base to destroy the bubbles very
quickly. I don't think this is very useful for hand coating... also I
don't know if their emulsions contain any antifoaming agent.
> If you start with a gum arabic powder, of course it will be very light,
> but besides the nuisance of mixing, there's the problem of preservative.
> The lithographers gum we buy by the gallon already has preservative (we
> may not want to know what that is, though I suspect it accounts for much
> of the difference between commercial gums).... and no matter what the
> label says, I've found it keeps well, for my purposes remaining more or
> less the same for years.
Bacteriostats and bactericides are also important in silver gelatin
because the gelatin can get rotten otherwise. In emulsion books (afaik
they are all ancient), phenol or thymol is often used, but there are
more effective and relatively safe agents such as
2-phenylphenol. 2-phenylphenol is effective against fungi and gram
negative bacteria. (If gram positive is a concern, it can be
complemented with another agent such as 4-tert-amylphenol.) As a
bactericide, 0.12% in water is the standard dose, but for bacteriostat
I think even smaller concentration should be effective. There are
many, very potent chlorinated phenols but many of them are discouraged
because of toxicity.
-- Ryuji Suzuki "Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)
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