Re: Hardening gelatin with formaldehyde

Dr. Bruce E. Kahn wrote:

> Just a few other points about using formaldehyde hardening of gelatin
> to throw into the fray ...
>
> In the conventional AgX photographic world (as opposed to alt),
> formaldehyde hardening caused a number of other problems (other than
> the ones discussed here). It is a reducing agent and can create fog,
> it's volatility makes it hard to control, it reacts with magenta
> couplers (definitely not an alt issue), and can cause afterhardening.
> This later point may be important for those of you here. Basically,
> it means that you can't stop it from doing it's thing. Even when you
> think that the hardening is finished it can actually keep going.
> Overhardening is bad! So if you are interested in doing alt for
> archival reasons, you probably shouldn't use formaldehyde hardening
> of gelatin.

Hi Bruce,

On the other hand, excess formalin is more easily washed away, compared with
glutaraldehyde (at least that's the case in tissues). It's also
volatile, so it is likely to stay in the paper for only a limited time.
Furthermore, the amounts used in hardening alt-photo papers is smaller
than what is needed to fully fix ("harden") all of the gelatin in the
first place. At least for my own purposes, there is never any light- or
oxidation-sensitive components in the paper when I harden it. Also, the
paper is thoroughly washed prior to coating the sensitive emulsion.
Finally, formalin hardening has the benefit of having proved it's value
and stability for more than a century. This is more than can be said for
most current practices. Most of them are not even *intended* to last
that long!

On a more technical note, formaldehyde (or rather methylene hydrate,
which is what is present in an aqueous solution), introduces
cross-linking by causing the formation of methylen bridges between
proteins. As a result there are no free aldehyde groups introduced in
the material. Glutaraldehyde, on the other hand, introduces free
aldehyde groups at both ends of the polymer chain (only short-chain
polymers of glutar aldehyde are involved in fixation). These aldehyde
groups are permanently bound in the emulsion (or tissue, or whatever is
being fixed). The introduction of reactive groups may reasonably be
expected to cause reduction of long term stability. Formaldehyde, on the
other hand "does its thing", and then leaves the emulsion in the washing
(even the bound formaldehyde will eventually be washed out, half is gone
in 24h).
(ref. for most of the last paragraph is: Kiernan (2000): Histological
and Histochemical Methods. Theory and Practice. 3rd ed.)

> And since we are getting more scientific and technical here (which I
> am delighted, but very surprised to see)
Me too!

Best wishes,

Tom Einar
Received on Thu Mar 18 06:17:30 2004