Unblinking Eye


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.

Negative Hardeners:

Kodak Special Hardener SH-1
(for use prior to intensification or stain removal)


   500 milliliters

Formalin (37% solution formaldehyde)

     10 milliliters

Sodium Carbonate (Monohydrate)

       6 grams

Water to make

       1 liter

Harden negatives for 3 minutes in this solution, then rinse and fix for 5 minutes.

Kodak Prehardener SH-5
 (for high-temperature development)

Solution A


Formalin (37% solution formaldehyde)

       5 milliliters

Solution B



   900 milliliters

0.5% solution 6-Nitrobenzimidazole Nitrate

     40 milliliters

Sodium Sulfate (Anhydrous)

     50 grams

Sodium Carbonate (Monohydrate)

     12 grams

Water to make

       1 liter

Combine solutions A and B, mix thoroughly, and presoak film for 10 minutes with moderate agitation.  Then drain, rinse for 30 seconds in plain water, and place in developer.  To minimize swelling at high temperatures, add 100 grams sodium sulfate per liter of solution.  Rinse with water for one minute, or use a neutralizing bath of hydroxyl amine sulfate followed by a rinse, before development.

Developing Times as a Percentage of
Normal Time without Prehardener at 68F (20 C):
75 F (24 C)          100%
 80 F (26.5 C)         85%
 85 F (29.5 C)         70%
 90 F (32 C)            60%
 95 F (35 C)            50%

Paper Hardener:

Kodak Hardener F-6a
(for use after toning or prior to high-temperature wash)

Water at 125 F

   600 milliliters

Sodium Sulfite (Anhydrous)

     75 grams

28% Acetic Acid

   235 milliliters

Kodalk Balanced Alkali

     75 grams

Potassium Alum (Dodecahydrate)

     75 grams

Cold water to make

       1 liter

To make a working solution, mix 2 parts stock solution with 16 parts water.  After toning in sepia or brown toner, treat prints for 2 to 5 minutes in working solution.  Wash thoroughly before drying.

Thanks to Ron Mowrey for additional information used on this page.

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