Hardener

From Silvergrain Labs

For processing chemicals

The most common hardener is aluminium alum, aluminium sulfate or aluminium chloride. Besides aluminium, chromium and zirconium salts may be used similarly, but useful pH range is further limited. Aluminium salt hardeners are most effective in pH range of 4.5 to 6.5, and they are used in acid hardening fixers. However, this use of hardener is becoming less common, because films and emulsions are well hardened in the manufacturing process, and there is no need for additional hardening in manual processing. Also, if an aluminium salt is used in the fixer, removal of the fixer in the washing bath will be less efficient, requiring washing aid and prolonged washing time.

For x-ray film development, film is sometimes hardened before or during development. Glutaraldehyde is most common for this application, followed by formaldehyde. However, formaldehyde has tendency to fog the emulsion, so the formaldehyde hardening bath usually contains an antifoggant such as 6-nitrobenzimidazole if it is used before the developer bath.

For emulsion and gelatin coating

Traditionally, chrome alum, formaldehyde, glyoxal are used as hardeners. However, these agents have several disadvantages. In particular, formaldehyde and glyoxal can fog certain kinds of emulsions. Other options of hardners I recommend are:

1. glutaraldehyde (0.5 to 2.5% solution, buffered at pH 3 to 4.5 with acetic acid)

   A very effective, rapid acting, inexpensive hardener that crosslink gelain at amino residue of lysine.

2. 2,4-dichloro-6-hydroxy-s-triazine

   A very effective, medium rapid acting and nonvolatile hardener, which crosslinks gelatin in a similar way to glutaraldehyde. However, this agent cannot be purchased and must be synthesized in-house, which requires partial hydrolysis of cyanuric chloride, a very nasty chemical

3. bisepoxides (such as diglycidyl ether of glycerol)

   A very effective, slow acting, nonvolatine hardener, which can crosslink gelatin at primary amino group as well as carboxyl group.

For raw (not chemically modified) gelatin, glutaraldehyde is an excellent option. Chris Anderson has obtained excellent results with her tricolor gum dichromate process with glut-hardened gelatin sizing, which was significantly better than glyoxal hardened gelatin. However, glutaraldehyde is volatile and must be used in a well ventilated area. The s-triazine is a nonvolatile hardener but it is not commercially available. Epoxides, particularly diglycidyl or triglycidyl ether of glycerol are relatively nontoxic and they recently became commercially available.

For acylated gelatin, amino groups are blocked by acylation; glutaraldehyde or the s-triazine cannot crosslink it. These gelatins can, however, crosslinked at carboxyl group by bisepoxide.