Bleaching and clarifying bath for Sw reversal Self preparation

Oliver · 2005-08-23T20:26:23+00:00

Hallo Ich habe im Netz mehrere verschiedene Rezepturen zum Ansetzen des Bleich- und Klebades gefunden. Welches Rezept benutzt Ihr und wo ist der Unterschied...

Oliver

Hello I’ve found several different recipes online for preparing the bleaching and bonding baths. Which recipe do you use, and what’s the difference between them? Are the solutions from Fomakit also available individually? Kind regards, Oliver

ChristianKolinski

There are essentially two recipes for a bleach bath: dichromate or permanganate. Dichromate is carcinogenic and now very hard to come by (keywords: certificate of competence, proof of disposal...), but it has the advantage of being quite stable in solution. Permanganate is less of a concern in that respect, but it goes off within a very, very short time and requires a bit more effort (you must filter the solution after preparation, as even in Aqua-demin a little brucite always precipitates out, etc.). What different clarification bath recipes have you come across?

Oliver

Hello So far, I’ve only found two recipes for Bleach baths and Fixing baths. 1. The one from F OTOIMPEX 2. The one on the Super8site website In both cases, the Bleach bath is prepared using potassium dichromate. Kind regards Oliver Krosse

ChristianKolinski

Hello Oliver Most (in fact, all) suppliers and retailers now only sell dichromate to schools, universities and laboratories. Ilford, for example, switched its reversal formula to permanganate a good two years ago; see: http://ilford.com/html/us_english/pdf/reversal_web.pdf Unlike Ilford, however, I would recommend always preparing a fresh batch of the permanganate solution. Even in deionised water, the stuff breaks down gradually over time; remember the problems with the Tetenal reversal kit (which was also switched from dichromate to permanganate; unfortunately, their permanganate solution had already gone off before it was sold. That’s why it’s off the market, and probably why Foma supplies the permanganate in individual sachets). Christian

cfb_de

Hello Christian, That would be yet another typical example of how German over-engineering is supposed to replace the individuality of customers (who are treated as incapable here), only for it to backfire :-) Perhaps a quick tip regarding the potassium permanganate bath. In the laboratory (chemistry, not photography), a very similar method is commonly used. I have achieved a slightly longer shelf life (up to around three hours) there using the following procedure: - Use degassed water for the preparation (boil under reflux for a few hours under inert gas) - Use degassed, clean sulphuric acid (freeze a few times, vacuum, allow to thaw and repeat) - Use *clean* potassium permanganate; the commercially available sort usually already contains up to 1% manganese dioxide - Prepare the solution in an inert gas counter-current setup, filter under inert gas and bottle. Store in a dark place. Admittedly, this requires more equipment and isn’t easily achievable in every laboratory. But it works reasonably well. Decide for yourselves whether the effort is too much for you. The important thing is not to have oxygen or reducing agents (e.g. fat) in the mixture from the start, and above all not to begin with potassium permanganate in the mixture. Dichromates are much safer to handle, although they really have no place in the hands of inexperienced people (risk of chemical burns, carcinogenic potential, contaminants on surfaces used for other purposes, disposal). Best regards, Franz