Hardeners
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
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| Water | 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
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| Solution A | |
| Formalin (37% solution formaldehyde) | 5 milliliters |
| Solution B | |
| Water | 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 |
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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 |
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Paper Hardener:
Kodak Hardener F-6a
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| 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.