U of S | Mailing List Archive | alt-photo-process-l | Re: dichromate gelatin size

Re: dichromate gelatin size

Thanks, Marek. I'm gonna try that today.


On Apr 7, 2008, at 10:48 PM, Marek Matusz wrote:

Diana and others that have asked for details.
Paper size with gelatin dichromate.
A few words of introduction at first. Only gelatin and dichromate are used. There are no volatile chemicals involved. I did try using chrome alum, but found it not to be adequate at low concentrations. At higher concentrations it would solidify a beaker of gelatin solution to a nice solid mess. My method generates chromium (III) salts in situ and uses little dichromate, unlike some described in the literature before. As a matter of fact after the sun exposure there is little if any dichromate left in the paper. I use both single and double coat. Here is the recipe.
18 grams of gelatin is soaked in 500 cc cold water for about 30 minutes. Then the solution is warmed up to about 45-60 degC, or until gelatin goes into solution. I never let the solution boil, but I do not know if it makes a big difference. That makes about 3.5% gelatin solution. Next I add about 25 cc of 3% potassium dichromate. If you have ammonium dichromate, I would add the same amount. If you already have a saturated (30%) ammonium or potassium dichromate solutions add about 2.5 cc. You can easily see how little dichromate is used compared to traditional sizing and printing methods.
About 25 to 30 cc of size is needed for 22x30 sheet of paper. My method for Fabriano Artistico is to pour the sensitizer in the center and spread it with a 3 brush. Spread it quickly at first and brush it smooth. You should be able to have the entire sheet of paper wetted and smoothed. If parts of the paper go dry before you are done, then you need a bit more size. If it puddles up you will need less. In that case it just requires longer brushing to spread it around. I have never ever been able to spread gelatin with a foam roller to achieve even coat.
My drying time is about 2 hours or overnight if convenient. If second coat is needed I will do it at this point. Next is exposure to sun to convert Cr(VI) salts to Cr(III) salts to crosslink the gelatin. 15 to 30 minutes is OK. Cloudy day or direct sun light makes little difference. Exposure is long enough to decompose all the Cr (VI). The paper should have a distinct tan appearance with all the yellow dichromate colour gone. Last step is a soak to shrink the paper and wash it out. 30 minutes at room temperature (70-80F) is sufficient. There should be no yellow dichromate washing out. Most of the tan tint disappears at this point as well. I have used this method to size papers for tricolor gum bichromate printing as well as multiple-layer monochromatic gum prints.
Marek Matusz

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