Re: Dreaded fungus

From: Richard Knoppow <dickburk@ix.netcom.com>
Subject: Re: Dreaded fungus
Date: Wed, 17 Dec 2003 22:00:06 -0800

> Most hardeners of the
> sort found in fixers harden the emulsion only when it is
> wet. I think formaldehyde may actually harden it when dry.

Note that any hardner hardens the gelatin and gelatin is hardened
whether it's wet or dried. The difference is that the antiswelling
effect of hardening is seen when the emulsion is wet, not when dry.

Gelatin always retains some amount of water, depending on the RH of
the air. Hardening does not affect much in gelatin's absorption of
water at high RH. (OTOH, at very low RH, emulsion may shrink and
become excessively curly and brittle, whether it is hardened or not.)

Formaldehyde is not a very desirable form of hardening agent in
emulsion making for several reasons, but it is a quite effective
hardening agent. Yet, its hardening has little influence on the
absorption of water vapor from air at high RH.

Dry strength is more strongly influenced by the gelatin
structure. When the emulsion is dried slowly at chilling low
temperature, the gelatin is in a gel structure, while when dried fast
at high temperature, the gelatin is in sol structure. The gel
structure is the desired form in terms of dry mechanical
properties. The gel structure also responds to hardening agents better
than the sol structure. Although modern coating facility design is a
tight trade secret, patents in this area generally describe very fast
coating of very thick emulsion on film or paper, and the coated
material is immediately chill set and dried in dry, cold environment.

--
Ryuji Suzuki
"Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)