Raw cobalt ore was found combined with other minerals including copper, bismuth, arsenic, and other sulfides. Preparation of zaffer required careful roasting to separate out those substances. Cobalt roasting furnaces had long chimneys to capture the arsenic and other minerals as they sublimed. The openings in the sides (marked ƒ in the image) allowed periodic collection of those substances. Source: From Johann Gottlob Lehmann, Cadmiologia, oder Geschichte des Farben-Kobolds (Königsberg, 1761-66), plate III.

From The creation of color in eighteenth-century Europe by Sarah Lowengard