Camfield Wills (Harry) and I have discussed this problem. He has an
exceptional collection of Woodburytype material, Carbon/Carbro, (and Dags.),
some of which can be seen in his volume "History of Photography - Techniques
and Equipment" Hamlyn, 1980. We reckoned that there might be traces of lead
and possibly lubricating oil in the Woodburytypes. However,a clue to
effective identification might lay in the high degree of burnishing that
Woodbury printing paper required in order to produce a smooth impervious
surface. The smoother the surface, the less the gelatine printing ink
dragged in the highlights. The ideal printing surface was glass. I have some
exquisite high key Woodbury lantern slides which show no trace of mottling
in the highlights. (That isn't to say however that this problem would not
have occurred on, say, a 10x8 glass plate. I don't know of any examples.)
Oh intaglio!
John Malcolm