Megilp

Megilp /məˈɡɪlp/ also known as Macgilp and McGuilpis is an oil painting medium consisting of a mixture of mastic varnish and an oil medium: such as walnut, linseed, safflower, poppy, black oil (linseed oil) cooked with litharge or white lead etc. Earlier recipies may omit the mastic and substitute wax It makes oil paint thin, glossy, and easy to work, with an extremely short drying time; however, the archival quality is much disputed. Some claim the medium causes paint to become yellow and brittle, while others claim that improperly prepared paint is the cause and that the longevity of Megilp is excellent.

Victoria Finlay cites Joshua Reynolds as an example of a painter whose reliance on megilp has detracted from the longevity of his paintings. Specifically, she indicates his painting Girl With a Baby as one which has changed a great deal over time. Megilp was also used by Turner.