>Martin, that's an impressive pharmacopeia. Perhaps you know if there's
>some reason for using salicylic acid in preference to, say, formaldehyde,
>glyoxal, or thymol, other than (presumed) lower level of toxicity?
When I started gum printing fifteen years ago there was no modern book
about alternative photo process and no reprints available in french. I have
some difficulty with English, so I was looking about french historical
books. I have examined coolection of books in Library and buy also
historical books about the
gum. I found in this literature five option and tested 3 of them;
-using fresh mixed gum (described by Puyo, Maskell, Demachy) which work
very fine with the disadvantage of short conservation.
-using formaldehyde,(described by Emery) this substance is toxic, have
cumulutive effect and seem be cancer-producing. I used this option many
years ago , but have some difficulty to develop prints coated with gum
oldest more that 6 / 9 months. I have not definitive explanation of this
evolution. Formaldehyde is a preservative but also hardener, maybe I make
error when dosing formaldehyde? ( Was the gum too acid or harden ? ) The
toxicity of this substance and the difficulty to work with gum oldest more
that 6 / 9 months decide me to abandon this option.
-using salicylic acid,(described by L.P.Clerc, H. Schneeberger)this method
give me satisfaction and I keep this option.
I don't have made test with the two another possibility described in this
litterature
-using thymol,(describe by L.P.Clerc)
-using phenol,(describe by Emery) but this substance is very toxic
>Does the "acid" have any effect on pH of the Gum?
When the gum has dissolved (in cold water most of the time about 10 days )
the solution turn slowly sour, and the viscosity change. In this moment the
fermentation must be fixed by additing preservatives . Without additing the
preservative the gum become too acid and are soluble with much difficulty
when coating.
> Perhaps not in those
>amounts, but what comes to mind is that apparently gelatine size needs
>some alkaline environment to prevent the hardener (formaldehyde, glyoxal)
>links from reversing, in which case the gelatine washes away.
I have never observe this kind of problem.
Sorry for my English.
Martin
--------------------------
marbecka@club-internet.fr
--------------------------