FotoDave@aol.com
Wed, 03 Nov 1999 22:35:56 -0500 (EST)
> Sure you have to get the hardener into the gelatin film somehow. I'm not
> too sure what you are getting at with this?
I think I misread your earlier post again. You mentioned you agreed that the
gelatinous gelatin can be hardened but didn't think that dry gelatin can be
hardened appreciably, so I mentioned that the dry gelatin actually went
through the gelatinous stage in hardening.
But now I re-read your post, and I think what you meant was even when the wet
gelatin is hardened, after it dries, it is still doubtful that when dry, it
is harder than the dry gelatin that has not been hardened ("harder" in the
sense of more abrasion resistance).
The reason that I kept misreading you, I believe, is because we are thinking
about
two completely different aspects of hardening. You emphasis is that one
cannot harden the gelatin in any way such that the dry gelatin film is much
harder than the dry unhardened gelatin.
But that was not what I was thinking at all! The subject under discussion is
hardening gelatin sizing (not the strength of dry gelatin film). Maybe some
also think in that direction because they think that the purpose of hardening
is to make the dry gelatin film more abrasion resistance, but as my
explanation shows, that is NOT how size hardening works and NOT the purpose
of hardening the size. The purpose is to "harden" it in the sense that it is
more resistant to moisture and swelling.
I am now repeating myself for those who are interested but still didn't find
my earlier post clear enough. Basically anyone who has prepared gelatin for
sizing or carbon tissue knows that almost as soon as you add the gelatin into
water, it swell. It doesn't even have to wait for 10-15 minutes as many books
say. After you heat it, you coat it on your paper as the size.
When you put down your gum emulsion, for example, since the emulsion contain
water, you in effect are re-wetting and resoaking the gelatin, and as before,
you almost immediately get the gelatin soft and swollen again. It is bad
enough if you don't touch the emulsion, but you use your coating brush to
brush, move, massage your emulsion, so you are basically mixing your emulsion
with the soft gelatin! That is main reason for staining with unhardened
gelatin.
With the hardened gelatin, however, the gelatin does not swell and become
soft. It is in this sense "harder." Given enough time and long soak, the
so-called hardened gelatin will also swell a little bit. However, for
emulsion coating, long before it swells again, the emulsion has dried.
This swelling minimization is effectively done by hardeners and is good for
sizing. Whether it is achieved through more cross linking or stronger bonds
or other mechanism, I do not know; neither was I thinking or addressing about
the issue in my previous posts at all!
Dave S
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