Rosin (colophonium) solution to make the soot transfer easier
In case you are interested in preparing this solution: 20th century punchcutters used to employ a substance to make the punch surface sticker to help the soot transfer from the cellophane to the punch. P. Rädisch and H. Drost used to use pine resin (rosin or colophonium) disolved in natural (pine) turpentine. When I was at the Imprimerie National I saw the little bottle Nelly Gable had and checked the label. It read: Colophane 2,5 gr / Alcohol 60 gr. So, in the IN they used alcohol instead of turpentine to dissolve the rosin. It's a basic varnish. Then, I purchased some pine resin (easy to find in internet and good art supplies stores) and prepared 2 bottles, one using acetone as solvent and the other using alcohol. Both solvents will work well, but acetone has the advantage of dissolving the resin faster and when you apply it to the metal, the solvent evaporates quicker leaving a very sticky coating which is what we want. I will make some tests with good quality turpentine as soon as I get a bottle.
Note: Plasticine (children's modeling clay) is also used, but it leaves a much less stickier coating. The rosin solution probably also acts as fixer once it dries.