RE: chrome alum and plate coating

[Monnoyer Philippe <monnoyer@imec.be>]

Dear Chris,

As your varnish is hydrophobic, the gum layer has to come appart from
the surface.And that's what happened. To have a good coating of a
colloid based (gelatin) emulsion, one has to have an hydrophilic base
surface, to lower the surface tension difference between the liquid
emulsion and that surface.
This can also be reached by adding a surfactant like tween. Surfactants
have both behavior (hydrophobic and -philic) at each molecule end. They
can that way make an interface between opposite materials.
But this is only for coating. This does not mean that your dried
emulsion will remain on the plate upon process.
For sticking properties, you have two solutions. The first is subbing
the glass plate with a very diluted hydro-alcoholic solution of gelatin
+ hardener. I should check my litterature to tell you the compositions,
but this should be around grams of gelatin per litre water.
The second solution is the addition of a functionalized surfactant that
will make a very strong adhesion both to the glass surface and to the
gelatin. Therefore I recommend silicium based surfactants like Z-6040
from Dow Corning. A part of it binds chemicaly to the gelatin (or
colloids like gum, albumin etc ...). The other (silicium based) sticks
to the glass surface. Use less than 1% in the emulsion. This gives you a
strong adhesion.
You can find it at
http://www.dowcorning.com/applications/Product_Finder/PF_details.asp?sel
Industry=006&prod=01011278&type=PROD&pgroup=00000261&country=USA
As emulsion making will become an old technology unfortunately, I want
keep it alive... Well, there are a lot of similar epoxysilane
surfcatants that should work as well @Dow.

Finally, you should keep something in mind: when you guys address an
hardening related problem, you have to be aware that the hardening
action of the different hardeners are very often slow. With chromium
alum for example, used at a concentration proper for an emulsion
manufacturing, this takes weeks or months for the hardening to be
completed. In order to compare your results between each other, always
mention the hardening duration, pH and concentration vs. colloid
(gelatin).
One can also obtain faster (some days) hardening with formaldehyde
(which hardens proteins, like the surface of your eyes in the darkroom
for example) at basic pH if I remember well. Glyoxal should be even
faster and is solid which is good for handling w/o vapors.
And of course, higher doses means faster reactions.

Another last crucial step in glass emulsion coating : clean glass
surfaces !! Wash it roughly with soap and rinse (with gloves), then
start a washing procedure that you can find in a lot of old photo
manuals (avoid dichromates which are toxic and prefer a wash in
concentrated aqueous sodium hydroxyde solution - corrosive !! wear
glasses and stronger gloves than latex-). Then rinse several times with
mechanicaly rubbing its surface. Last rinse in distilled water. Let dry.
Look your plate and smile.

I hope this might help you.

Cheers,

Philippe

-----Original Message-----
From: Christina Z. Anderson [mailto:zphoto@montana.net]
Sent: Wednesday, May 01, 2002 05:15
To: Alt Photo List
Subject: chrome alum


      Sandy King (or anyone else in the know), a while back you said you
usually use chrome alum for hardener, sometimes potassium alum, too.
You
mentioned mixing 2 g of chrome alum (the purple stuff)  per 1000 ml of
gelatin solution.  This is about 1/2 tsp.  How much *gelatin* do you use
in
your 1000 ml--my formula calls for 2 tsp Knox gelatin, does yours, too?
     I'm starting to work with tiles and glass to get liquid emulsion to
stick, and a student wants to try to sub glass with the chrome alum
solution
for gum; she tried it already with varnish and a mild sanding and the
gum
totally came off.  She was inspired by my telling her a while back about
Sarah Van Keuren's students doing gum on glass. Short of a sandblaster,
we're going to keep trying.
Chris