More info on "R" typing.

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.