Sentence:  The Sleeping lubra is an oil painting on board, and the underpainting, which has been laid in as a transparent wash, seems to have been mixed with some incompatible substance that has caused the colour to marble, possibly the result of clear enamel being dropped directly into the <WSD>turpsy</WSD> film of oil paint. 
Meaning of the word marked with <WSD> tags:  heavily diluted with turpentine