I wonder how this machine could ever have worked. Each sort of paper needs a different speed on my drum dryer, depending on weight and other material properties of the paper base. I don't think that the emulsion is that critical and I got good results with MG FB as well. But I have to admit that the older ORWO and Brovira, which I still have, are more trouble-free in this regard.
When I saw the picture of your machine, my first thought was: what a tiny little thing! My drum dryer weights more than 100kg (>220 lbs) and has a huge oil-filled drum that is back-heated (i.e. the paper gets heat from both sides). A small drum has necessarily more surface curvature which causes more tension to the emulsion. I've never used such a tiny drum dryer, but it seems logical to me that the more the paper is bent, the more likely the emulsion will crack.
Hey... that's another good point!
I've about resigned myself not to sweat trying to get this to work. The current customer(s) will have to just get semi-gloss (which I really don't like). When the availability of funds and one of those temp-controllable double-sided dryers intersect, I'll resume my efforts with one of those.
That increased curvature thing might have more to do with it than anything else.
I worked one or two places that had the huge chrome-drum dryers, and someday, when I build my dream darkroom I'll try and find one, but I've never really used one. Alas, when they were available to me, I was too young, in too much of a hurry, and saw no reason to anything other than RC and usually machine-processed RC as well. Ick.
Oh -- and on a minor sub-thread, I've checked around and although other forms of toning are reported to react adversely to heated drying/glazing, selenium appears to weather it with no ill effects (according to various discussion group reports).
Thanks Thilo. I think that curvature idea may be the missing piece of the puzzle. I think that my dryer might have worked just fine with most of the contemporary papers back when it was made, but I've heard that most papers now have way thinner emulsions than those of 20 or more years ago.