Potassium cyanide may not have killed any modern wet plate practitioners yet, but alternatives to it really ought to be thoroughly explored before anyone begins to seriously consider it, even as a last resort. Cyanide may be fast, clearing collodion in only a few seconds, but in the words of the old road safety slogan, surely its better to arrive alive, even if an alternative fixer takes slightly longer. I also wonder if the supposedly clearer highlights are worth the risk when more controlled and safer reduction could subsequently be undertaken with a more conventional reducer like potassium ferricyanide (which despite the apparently similar name, is classified as only a low toxic hazard, as long as it is kept away from concentrated acids).
PhotoEngineerss constructive suggestions above regarding thiocyanate and thiourea really should be followed up.
Ammonium thiosulphate once used to be expensive and hard to find, which explains why my 1950s era
Focal Encyclopedia of Photography recommends a 40% solution of ammonium thiocyanate with 5 per cent formalin as a rapid fixer. It is said to function in about 30 seconds or even less if it is used hot up to 120 degrees F or 50 degrees C. It is also reported to have a low freezing point, taking only a few minutes to fix below 32 degrees F (0 C). Thiocyanate is not widely used for film because it softens gelatin excessively (the
Focal Encyclopedia says to keep immersion times short); hence the call for formalin - nasty stuff, and unnecessary for collodion.
Grant Haist,
Modern Photographic Processing, chapter on the fixing process, p. 596-598 provides more information on thiocyanates; i.e. you might want to use a two bath system, or fresh solution for each plate - there is a chance you might you get a water-insoluble opaque silver salt when the cleared emulsion goes into the wash.
Thiourea is also not much used for fixing film because it softens gelatin. However, once again, this should not be a problem for collodion, although clearing complexes could again be an issue, raised by Haist, p. 601-602. In deference to Kirks safety concerns, another safer suggestion follows.
Cysteine hydrochloride is an organic fixing compound Ive experimented a little with a little since I first raised it in the monobath thread. PhotoEngineer cautioned to be careful of reticulation, but Ive experienced no problems at all with Harvey A. Hodes cysteine monobath (see (there was a url link here which no longer exists)). It has a pH of only 9.8-10, and Hodes specifies temperatures for black and white film to 35 degrees C. Perhaps Photo Engineers caution was based on past experience with extremely rapid processing at very high pHs and temperatures, which, again, may not be a problem for collodion. However I am beginning to wonder if cysteine hydrochloride is really worth the trouble; going back to the patent I noticed that Hodes cysteine monobath has slightly longer processing times than his thiosulfate variant. I also tried Alan Fittermans cysteine fixer; at pH 6 (see (there was a url link here which no longer exists)), it seems to have been formulated for stability, and seems to me to have no particular advantage over a conventional sodium thiosulphate fixing bath. In its favour, on the other hand, cysteine hydrochloride is completely non-toxic (I bought 100g at a health food store), dissolves rapidly in water or a sodium hydroxide stock solution, and is then ready for immediate use, although an alkaline solution (which you need for rapid fixing) doesnt seem to keep all that well. It does start to smell a little in a strong hydroxide solution, but is probably relatively innocuous compared to some of the other mercaptos. I dont have any equipment for accurately measuring pH to pursue it further; but perhaps an alternative, safer, alkali, to sodium hydroxide might be preferable. Im also not too sure how to assess film softening, other than through the highly scientific fingernail-film leader scratch test. Unlike film, in any case, softening at high pHs is probably not going to be a problem with collodion.
The classic comparative study of film clearing compounds is Haist, King, and Bassage Organic Silver-Complexing Agents for Photographic Monobaths
Photographic Science and Engineering, v. 5 no. 4 July-Aug 1961, p. 198-203. They found thiourea quite slow to clear at 70F (65 sec), reducing to 19 sec at 130F (pH 6.5 and 10.5 are virtually identical; higher pHs werent feasible because of fogging); cysteine hydrochloride is marginally worse than ammonium thiosulfate at pH 10.5 and 70F (44 vs. 33 secs) and marginally better at 130F (15 vs. 18 sec). These are all based on stock solutions of 0.25 molar; the film used was Kodak Fine Grain Positive Film. Ammonium thiocyanate wasnt included in this study.
PhotoEngineer undoubtedly knows far more about fixing than me; but perhaps I could suggest that his Superfix, posted in the formulas section of APUG at (there was a url link here which no longer exists), may be worth a try with collodion. Perhaps PhotoEngineer could persuade Mark Osterman to try a sample of his next generation Superfix MMX (thats 2010 in Roman numerals, or is past that yet
?) the next time he meets him for lunch! In the meantime, perhaps it might also be worth trying a conventional ammonium thiosulphate rapid fixer concentrate mixed with a 40% ammonium thiocyanate solution. Even if it doesnt completely match potassium cyanide, it, or some similar synergistically compounded rapid fix, is sure to be far safer.
Philip Jackson
