Johann Beckmann, A History of Inventions, Discoveries…
“ The greater part of the plumbago at present used in commerce, but which, as far as I know, is fit only for iron-black, comes from Spain, where it is dug up in the neighbourhood of Ronda, a town in Grenada, a few miles distant from the sea; but, in regard to the antiquity of these pits, I have found no information. In commerce, it is called potloth; and the mills, such as those at Bremen, where it is ground fine, are named potloth mills, an appellation which in all probability has been borrowed from the Dutch, among whom potloot signifies as much as potters’ lead. ”
