Jerevan, I'm still working through the two films. They're almost identical and I couldn't swear that any differences I think I see are inherent to the material or a fallout of my imperfect duplication of conditions. The big thing is that 3M sells retail, both 6.8 mil (sheet film thickness) and 3.9 mil (roll film) Right now, I think that the 3M film may not have the adhesion that the Dupont material does, but that the Dupont film has a greater tendency to make comets (repellency spots.) Again, though, could be my imagination, or there could be something significant I haven't run across yet. You know me, when I find out more, I'll blog my heart out. I do know that both are great to work with. Love 'em.
Ian, I totally agree about grocery store chemicals in the emulsion itself. Given that all the other ingredients together don't touch the cost of silver nitrate, why bother? Having said that, I do have a paper recipe I love that uses sea salt. Goes to show -- no hard and fast rules. Where I think that grocery store chemicals have a real place is in the processing chemistry. Chances are that even if some of the necessary emulsion chemicals get restricted or expensive, it will always be possible to get (and afford) small amounts from lab supply companies. The quantities used in emulsions are so tiny that even an expensive chemical wouldn't cost much in the long run. But, the quantities used in development is a different story. Food grade potassium alum is quite pure. It would be fine in hardening baths. One additional nice thing about the diy/grocery store processing chemistry: likely less environmental fallout. I love the idea of Caffenol, etc.