§ 13 Of Slates
We now come to a different order of stones, blackish in colour and used by the architects only for laying on roofs. These are thin flags produced by nature and time near the surface of the earth for the service of man. Some of these are made into receptacles, built up together in such a manner that the pieces dovetail one into the other. The vessels are filled with oil according to their holding capacity and they preserve it most thoroughly. These slates are a product of the sea coast of Genoa, in a place called Lavagna;90 they are excavated in pieces ten braccia long and are made use of by artists for their oil paintings, because pictures painted on slate last much longer than on any other material, as we shall discuss more appropriately in the chapters on painting.