The gelatine layer of a photographic negative or print is subject to swelling in water, particularly as the temperature increases, making it subject to damage during handling. If the temperature rises too high the gelatine
may tear or reticulate even with minimal handling. Hardening is measured by measuring the swelling of the gelatine, or sometimes by measuring its melting point. Optimal hardness seems to be reached at a pH
between 3 and 5 (depending on the hardening agent utilized). Little or no hardening can take place in an alkaline environment. Hardening agents are often added to the fixing bath, particularly for film, in the form
of the salts of chromium or aluminum (chrome alum or potassium alum). Organic agents, such as tannic acid or formaldehyde are also effective. Formaldehyde in particular is often recommended for use prior to high
temperature development or prior to any procedure which might damage the negative through abrasion. Succinaldehyde or glutaraldehyde may be used in place of formaldehyde, though they allow greater swelling of the emulsion.
The precise mechanism of gelatine hardening is not fully understood, but it is believed to be the result of crosslinking between the long chains of protein molecules (polypeptide chains) that make up gelatine. This
crosslinking prevents swelling and increases the melting point of the gelatine.