Complex sentence: Van Eyck's mixture may have consisted of piled glass, calcined bones, and mineral pigments boiled in linseed oil until they reached a viscous state--or he may have simply used sun-thickened oils (slightly oxidized by Sun exposure).
Split into simpler sentences: Van Eyck's mixture may have consisted of piled glass, calcined bones, and mineral pigments boiled in linseed oil until reaching a viscous state. Or he may have simply used Sun-thickened oils (slightly oxidized by Sun exposure).