> Two gum bichromate quesitons:
>
> I've just purchased glyoxal as the hardener for gelatin sizing. The stock
> I obtained is not colorless, but amber. Is that OK? Will the sizing take
> on the color and be left on the print? The salesman told me that this
> color is normal in aging. The stock I have is two years old, but has a
> ten year shelf life. He said he would take it back if I didn't feel ok
> with it.
You are going to dilute that glyoxal: 25 cc to a litre of water to make
the working solution. It won't look amber at all.
> > Is the highly toxic mercuric chloride really necessary for storing the
> gum > binding solution? How long till bacteria and fermentation set in?
You mean your gum arabic? If you've bought ready mixed lithographers gum
arabic 14 baume (or thereabouts) it already has the preservative in it.
If you're mixing your own from powder, you want to add a preservative. A
few drops of glyoxal should do it. (I used formaldehyde in the old days --
about 1 *drop* per ounce, has preserved it for years.)
But note that last fall one or more of our conservator friends from Europe
mentioned that they used salicylic acid. I don't have the amounts in front
of me, but it's got to be in the archive if your tool can dig it up.
As to preserving the WORKING SOLUTION of the glyoxal: This was the only
point on which the glyoxal wasn't as good as or better than the
formaldehyde: Working solution of formaldehyde keeps indefinitely, just
needs filtering from time to time as paper fibres accumulate. Working
solution of glyoxal will polymerize in a few days -- that is, gets globs
in it, little Michelin Tire figures floating & bobbing around.
I had decided you just couldn't keep it (& since you have a gallon of the
stuff you don't have to,but students were tearing through our supply too
quickly), when I discovered by accident that adding an alkali prevents
that. I originally added sodium carbonate for another purpose -- to
preserve the gelatine. That is, an alkali prevents the links formed by the
hardener from loosening in long soaks. Lo and behold, the glyoxal didn't
glom up when the alkali was added.
My first batch used 15 g sodium carbonate per litre -- way more than
necessary, chemprof told me, and it did turn the working solution orange.
But it stayed otherwise fine for months.
Then I tried sodium hydroxide, also orange -- tho again probably too much.
Finally, at his suggestion I used the sodium bicarbonate (something about
the double bi). That's baking soda. Arm and Hammer. Think I posted the
amount. Don't have it in front of me. As I recall, half tsp per litre.
Anyway, this time I added just til pH strip showed pH 7. In 6 weeks the
working solution at school begins to look a little dark, but I don't think
enough to stain paper, and solution otherwise remains good, ie, no
blobs.I'll add also that a litre I mixed at home at the same time is still
light and clear, so it's possible/likely that the dark at school is some
other kind of contamination by students...
Judy