Response to Speedibrews Celer Reverser ?

Pete says: Crikey Ed, did you find that formula for bleach on a stone tablet? Modern sequestering agents like EDTA (EthyleneDiamineTetraAcetic acid) are much safer and probably easier to obtain. It's what all these new-fangled colour reversal processes use.

Actually EDTA will not accomplish the bleach step. EDTA is a chelator of divalent cations. What you need for a bleach is a strong oxidising agent. In color reversal (E6) there is no bleach step between the developers. There is a first developer that is basically a black an white developer, followed by the second developer that incorporates the color couplers. When the two developers are finished in E6, the silver image is totally black and is subsequently bleached away leaving the color image. In black and white reversal processing you need to remove the negative image completely before developing the positive image. Permanganate and Dichromate will oxidize the metalic silver (Ag(0)) to the silver cation (Ag(I)). In the case of reversal processing bleaches the counter ion is sulfate. The silver sulfate is soluble and thus washes out of the emulsion. If you look through your formulas at the reagents used for chrome intensification, you will see that the bleach there is potasium dichromate and hydrochloric acid (HCl). In that case the counter ion is chloride and the resultant AgCl is insoluble and remains in the emulsion to be redeveloped in the intensified negative.

One note on the bleach formula listed in Ed's posting. It is probably safer to use bisulfate instead of sulfuric acid in the bleach. The reason that I say that is that sulfuric acid is a liquid and for a home darkroom situation having it around lends itself to the possibility of spills and potential injury. Sodium bisulfate is a solid and this easier to contain and clean up in the event of a spill. My formula for dichromate bleach is 9.6 grams potasium dichromate plus 66 grams sodium bisulfate in 1 liter water. I didn't design this recipe myself, I got it from somewhere that I cant remember. The original is probably floating in cyberspace somewhere. Remember, though, this bleach solution is plenty nasty, even though you used bisulfate instead of sulfuric acid. Use rubber gloves and avoid contact with this particular solution.

Concerning the choice of first developer, to get optimal results you need to match your first development conditions to the film you are using. As an example, my favorite film for reversal procesing is TechPan. Everywhere you go you will see admonitions that TechPan is too contrasty to use for anything except lettering or line art. But by twiddling with the first developer I have gotten TechPan to work very well for general pictorial reversal processing and I use it almost exclusively. Using a D19 based first developer, though, it would never give acceptable results. So, the moral of this passage is to get optimal results, you need to put in the time experimenting. But that is what makes this an art form after all, isn't it?

[ Previous | Next ]