refractionary

In his description of the hard, finely divided (and preferably metallic) earthy particles that create a permanent and beautiful color, de la Follie drew on enamel or ceramic coloring processes: His examples may have been found at the faience factories for which Rouen was famous and that he had investigated on behalf of French government agencies. 19 It is a general law, de la Follie stated, that the more a body contains of these metal-like earthy substances, the more refractionary it is.

The Creation of Color in Eighteenth-Century Europe 2006