Re: chrome alum and plate coating

["Christina Z. Anderson" <zphoto@montana.net>]

Thank you Phillipe. I found the Z-6040 at the website and it will be a good
experiment to try, with the z-6040 added and without, both with chrome alum.
Chris
----- Original Message -----
From: Monnoyer Philippe <monnoyer@imec.be>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, May 02, 2002 6:33 AM
Subject: RE: chrome alum and plate coating


> 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