After repeating the prosedures several times here is what I have come up with using Rockland AG Plus emulsion coated on black aluminum trophy plate, coated with spray or brush on poly urithane varnish. Coat the back too so the aluminum will not screw up the chemicals.
Developer, Dektol diluted 1:1, plus 1gm Ammonium Thiocyanate to 3 oz. of working developer.
Exposure for tins,in my experiments have been f:45, 6 sec. overcast. This means a speed of about 12. for ambrotypes use only half exposure.
Develop 90-120 sec. It is a good idea to put a cover over the developing tray to keep the ammonia fumes from beeing a bother. rinse in water a few secons,( some say to go directly to fix ), Then fix about 10 min. in hypo.
Wash 10-15 min. then let dry. The image will darken a little when drying.
The images have all shown a little fog in the shadoe areas but it is'nt too bad. The addition of 1.5 gm Ammonium Thiocyanate to 3 oz. of developer will give a little brighter image but will also increase the fog a little more.
Great information. I'd love to see some of your results. I will be trying this for myself as soon as my thiocyanate comes in. I really like the idea of not being dependent on the not-so-dependable rockland developer.
PhotoEngineer, I would like to hear a little more on reduction sensitization. Any reducing agent that doesn't destroy the emulsion will do it? What are some common reducing agents used for this purpose? Also, how could we reduce fog on these plates?
Josh, I never played around with "R" typing or reduction sensitization. I may have some literature on it. If I do, I'll see what I can post. None of my projects used it.
Fog reduction in these plates may be coming from an amine in the subbing layer, or it may be inherent in the emulsion. Try adding some methyl benzotriazole to the emulsion. Since I don't know grain size or concentration, this would have to be done by trial and error. Make up a 1% solution in methyl alcohol and add it a drop at a time and coat one plate after each drop is mixed in well. This should give a starting point.
The only information I could find is a suggestion to use Stannous Chloride for reduction sensitization. IIRC, it is rather rapid acting and prone to fog.
Stannous Chloride is such a strong fogging agent that it is used as the current reversing or fogging agent in the E6 process, so exercise due caution using it in the darkroom.
I have no information on suggested amount to use, but since it is a surface effect just like Sulfur or Sulfur + gold sensitization, the quantity is dependant on surface area.
I know the fog is not from the plate because the ambro version also had some fog, but keep in mind that this is just experimental and someone out there might take this thing and run with it and find improvements.
The only existing plate I kept for comparison reasons was a little more brighter than the Rockland coated plates. I used my own emulsion on that one and am going to see if the effect is reproducable, although the speed was a lot slower, about .7 to 1.5 where as the the speed of the actual rockland tintype kit is about 1.5.
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It appears that it is not used in any common product today AFAIK. The current products use Sulfur, Selenium and Tellurium as well as these in combination with Gold salts.
Early products which used formaldehyde for hardening often became foggy due to formalin causing reduction sensitization. Formalin can be used to give very high (2 electron type) sensitization, but is not easily controlled as it tends to react with gelatin and is also a gas. This latter method was described in work done in France and was later taken up by Agfa but never commercialized, IIRC.
Formalin was present in old style coatings as hardner and gradually fogged the emulsions. The formalin either did not react fully with the gelain, or the reaction reversed. Formalin also continued to harden coatings well after they were released for sale. This slow 'afterhardening' is described by both Haist and Mees and James in their texts. It shows that formalin was present for quite some time as formaldehyde in some form or another.
I believe that this further illustrates the easy fog formation when trying to use "R" typing in photo products and is probably why it is not widely used.
Finished testing dry tintypes 
Linear Mode
