Nineteenth-century paint media: The formulation and properties of megilps

Megilp was made by combining a lead-treated drying oil with a mastic varnish and adding the product to paint on the palette, to make a thixotropic medium. Various forms of megilp were used by many artists from the mid-eighteenth to the nineteenth century, both for impasto and for glazing, but undesirable properties of darkening and cracking were soon reported. The present study began with a survey of the published recipes for megilp and the properties described, followed by their formulation in practice, with linseed oil prepared with lead acetate or lead oxide driers combined with mastic varnish in various proportions. These megilp formulations were then combined with modern commercial oil paints, painted out, and aged both naturally and artificially. Their surface morphology on drying and after aging was characterized by SEM-EDX, and thermoanalytical techniques, FTIR and DTMS were used to investigate and compare the different proportions used in various formulations, both for megilps alone and for megilps mixed with paint. It appears that a one-to-one or a resin-rich mixture reacts to form a new entity with properties distinct from the oil or resin alone, regardless of oil source or drier type. Oil-rich mixtures do not form ‘true’ megilps, and break down both in storage and in the paint film.