As a dry lubricant softener as I say more below. The plumbagina was a black, very fine powder, that it was used by rubbing with the parts referred to as "spaces", which were a kind of wedge very thin and long, about 10 cm, which could move another piece, is wedged between the two inner sides of the crib , and to get that fixed, allowing that the cradle could gently slide to be pulled by a metal bar, which made this crib to expand all of the parts forming the letters of the words, forming a compact, that served as mould for injection of molten lead, which at the end of the process produced a piece rectangular already-solid lead, which was one of the lines of text to then be able to print. I don't know if it is clear. It was a very nice trade that work with Linotype.
