Stone or pottery molds which have been used for metal casting often exhibit dark stains or discolorations of a metallic nature. In the National Museum of Antiquities of Scotland, a survey was made of the molds in the collection. Most of the molds were clay molds. Of the stone molds, the early ones were found to be made from sandstone or granite, and the later ones from soapstone. The stains in early molds were found to contain less zinc and more tin than those in later molds. Experimental clay-mold castings of known composition bronzes and brasses were performed and the stains produced on the molds studied. Although those experiments provided some information about the factors which influence the amount and type of metallic stains produced on mold surfaces, they demonstrated that it is not easy to work back from the composition of the stain to the composition of the cast alloy.