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#21
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There is a good explanation on how and why Hypo-clear works by Richard Knoppow et alia here: https://groups.google.com/g/rec.phot.../c/jUo3VYGJQEQ
TLDR: ion exchange forces the hypo (thiosulphate) ions out of the paper by replacing them with sulphite ions - many sources of sulphite will work but sodium sulphite seems to be the best. Fortuitously, the sulphite also does the same job on some of the fixing by-products. I use a heaped tablespoon of Sodium Sulphite per litre, (approx 2% solution!) but have noticed some precipitation lately so I may have to make up some proper HCA if my water has gotten even harder of late... Of course, you can just wash for longer instead, but I find the reduced washing time suits my process as the 6-7 minutes it takes to expose and process each new fibre print fits perfectly to move each processed print down the "5 min rinse - 5 min HCA - 10 min wash" sequence with film-strength fixer per Ilford (made even easier now by my new washer ) In practice they end up with something closer to 30 mins final wash as I wait for all the slots to fill before removing the "oldest" print to dry. From what I read, using two-bath fixing (per Kodak) does much the same job of removing by-products in the 2nd fixer bath, but that's too much faff for me . In practice, an N% solution is N grams of dry chemical plus water to make 100ml (technically, to make 100g of solution but this is photography, not chemistry!) - so a 2% solution would be 2g diluted in say 90ml of water and then make up to 100ml once fully dissolved. For small percentages like this just add the chemical to 100ml of water (or more likely, 10 times that to one litre in practice) - more than close enough for our purposes I suspect. Last edited by Bob; 5th May 2021 at 06:39 PM. |
#22
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Quote:
This was copied from a post by Ryuji Suzuki who used to write in the APUG web site. Not all of this is particularly relevant, as the alkaline wash was more important in the days of hardening acid fixers. <start of quote> Why sulfite? Besides sulfite, sulfate, bicarbonate were commonly seen in literature reporting on the subject. Bicarbonate is a reasonably effective washing aid. A 2% solution of sodium bicarbonate (baking soda) may be used, although, sulfite bath is preferred whenever possible. Sulfite is superior in that sulfite can desorb not only unreacted thiosulfate but also sparingly soluble and adsorbed argentomonothiosulfate complex. Sea water or solution of common salt is occasionally said to have the same effect as washing aid. For practical use, it is advised to ignore this information. While sodium chloride may have very small enhancement in washing compared to distilled water, it is vastly inferior to sulfites. Also, residual chloride salts can be harmful to the image; a very thorough water washing process is required following the sea water bath, negating the purpose of using a washing aid bath. Alkaline baths There are several studies that investigated the washing accelerating properties of alkaline bath. For example, Crabtree compared sodium hydroxide, ammonium hydroxide, sodium carbonate, Kodalk (sodium metaborate), and distilled water. This result was probably valid at that time, but the factors involved in washing property of fixed film and paper are somewhat different nowadays, because today's emulsions are highly hardened during manufacturing and use of hardener is becoming uncommon in manual processing. Therefore, it's probably not very meaningful to discuss those methods in detail. If there is anything worthy of noting, it is probably that Crabtree found that ammonium hydroxide was most effective of those. |
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