Re: Gelatine is also '' a '' gum but not Gum.

Judy,

Regardless of the proof anyone might cite, I am certain that the same
top to bottom hardening that we see in carbon is also the primary
mechanism for colloid hardening in gum. However, the exact manner in
which this happens is obviously much more complicated in gum since
there are issues of surface adhesion not present in carbon.

The specific proof that you cite, "depending on mix and exposure,
it's often possible to wipe off the TOP of a gum layer, while the
bottom, the part in/on the paper holds," would not be convincing to
me in the least since the same operation will cause the same result
if applied to a carbon image during development. For whatever reason,
highlight density is always the most difficult to clear in carbon.

Sandy

>On Tue, 4 Apr 2006, Sandy King wrote:
>>In general, although there are some important working differences,
>>they all work on the same basic principle, ie. exposure (and
>>hardening of the gelatin) is from top to bottom, with no transfer
>>of the image (as in carbon and carbro).
>
>Actually, the distinction generally given in "the literature" is
>that because carbon hardens from the top down, transfer is necessary
>(although "direct carbon" without transfer has its own litany),
>while gum (as far as has been established to date) hardens at the
>surface of the paper.
>
>Among proofs of that I could cite, I mention only that,.
>
>Paul Anderson devised a theory that gum latched onto paper texture,
>and illustrated it with a diagram showing the dear little gum
>molecules hanging on for dear life. This has been cut and pasted
>into manuals from that day to this, but, evidence suggests, although
>it may happen in whatever cases and conditions, it doesn't have to.
>
>J.
Received on Tue Apr 4 22:23:00 2006