Transparent browns, especially on copper resinate from ancient easel paintings, have been studied microscopically. Thin sections from fifteenth and sixteenth century easel paintings of the low countries have been examined in transmitted and ultraviolet light. It is established that copper resinate is nearly always discoloured. A second cause of browing is due to varnishes and fixatives, but in a third series of samples the presence of a distinct glaze with brown dye can be distinguished.