U of S | Mailing List Archive | alt-photo-process-l | Re: "Boric acid" and gum

Re: "Boric acid" and gum

Well, damn, Ryuji,
Back to glut being the best hardener on the market, huh :)?
Thanks for turning me onto it.
I'll email your post comments, below, to the chemist.

To see if borax would do, what you needed to do is to make a
boric acid solution, titrate with NaOH to the end point pH of
about 9.2 at 25C, and use this as if it were borax
solution. (because it is identical to borax solution.) In
reality, the active agent in the crosslinking mechanism above
is borate, not boric acid, and the pH of 9 or higher would be
most effective. (At pH of 9.18 at 25C, 50% is in borate form
and 50% is in boric acid form.)

Crosslinking of PVA or other polyhydroxy polymers with borate
is via condensation reaction, which is quite different from
how common gelatin hardeners work. As it is condensation
reaction, the crosslinking is easily reversible by changing
the pH or adding more water.

In photography, hardener is defined as an agent that
suppresses swelling of the polymer binder when immersed in
water or other processing solution. Even if you can make a
thin film layer of gum-borate coating, it will swell (and
dissolve away, as you noted) once immersed in water (as I
described above) and it won't qualify as a hardener, even
though borate crosslinks the polymer.

Ryuji Suzuki