Some people say that they have had success with repeated bleaching and redevelopment in a staining developer.
For intensification by redevelopment you want a rehalogenizing bleach. You certainly don't want to use any bleach that contains thiosullfate since this would destroy any image.
Chromium intensifier is not considered to be archival.
All very interesting stuff.
As I have plenty of frames to "play with" from this roll, and as I coincidentally had some Ferricyanide arrive this morning (as I'm going to try some lith printing and wanted to be armed for two-stage), I thought I'd try a bleach and redevelop.
So I've just made a rehalogenating bleach stock of 7.5g/100ml each of Potassium Ferricyanide and Potassium Bromide, diluted some 1+9 and clipped three frames from the dead roll. I bleached this clip in a shallow tray with continuous agitation for about 5 minutes. There remained some image apparent, but mostly gone, including all edge markings. I chose these three frames as they had three sisters originally exposed within a few moments of each other, so I have a before- and after- comparison.
I then gave it a thorough wash.
Then I knocked up some print developer (this is Fotospeed's recommendation for redevelopment after using their Cl10, what the rationale might be I don't know but I thought I'd try it) and redeveloped the clip in that, again with continuous agitation in a shallow tray. Within 3 or 4 minutes there was noticeable redevelopment and a significant increase in density.
They are washing at the moment, and I'll check tomorrow to see how they look.
I wondered whether I'd get much more improvement from a commercial product?
i'm sure if you buy these ingredients anywhere you'll end up on a list from several government agencies.plus, the stuff is right out dangerous in the wrong hands. i would write this off to fate and experience and move on.all the best.
With all due respect, only the uranium nitrate is not readily available from UK from photographic suppliers, as Chamaeleo pointed out in the first reply to my OP.
Many of the others mentioned in this thread are in the formulae for commercial photographic products, and Ferricyanide is often named in APUG threads as a component of bleaches.
In the process of not writing it off the experience, on this occasion, I am actually gaining experience and knowledge.
Consider - How did you gain your (considerable, judging by your work and publications) knowledge and experience?
The funny thing (and government types miss this point) is that when you purchase uranium salts they are made from depleted uranium. What you get is only mildly radioactive and you cannot make a bomb from it. In this respect uranium nitrate is no more dangerous than lead nitrate unless you want to poison someone. Your local hardware store has many things which are just as dangerous and readily available.
The trick about chromium intensification is that the chromium bleach builds up density while it bleaches, the development step afterwards recreates silver density while (hopefully) not removing chromium density. If you bleach with ferricyanide instead, then redevelop, you will not gain much.
Which brings up Ralph Lambrecht's point: Ferricyanide is mostly harmless, but Dichromate is quite toxic and overall nasty. Yet Fotospeed is apparently able to sell it without repercussions. I have read a lot about photographers being harassed by British/US police (or rent-a-cops) for taking pictures, but never for possession or use of photo chemistry.
ah thank you Rudeofus, that's a bit of information (about the chromium bleach) that I hadn't properly understood.
Reviewing the negatives this morning, there is a marginal improvement. Worth the effort to understand the process, but not significant enough to make much difference to a scan or print.
I think next time I need to order some bits and pieces I'll include some chromium intensifier and have another go. I'd still be interested to know more about the Speedibrews intensifiers.
It's not just Fotospeed by the way who are able to supply Bichromate. You can buy by the pound on eBay.