Gel medium, also called Flemish or Maroger medium, is an oil painting supply traditionally used by Jan Vermeer and other famous Dutch painters. The versatile medium, when combined with oil paints, produces thick brushstrokes that catch the light and add depth to a painting. Flemish medium is also ideal for producing thin glazes to build up shadows on a painted object. You can find ingredients for Flemish medium at art supply or hardware stores, with the exception of lead monoxide, which is available from chemical supply companies.
Mix the litharge with a small amount of linseed oil, using a palette knife.
Heat the remaining oil over low heat in an outdoor or well-ventilated area using a covered enamel cooking pan. Add the litharge mixture when the temperature reaches 160 degrees Celsius.
Increase the heat gradually to 250 degrees Celsius over a 2.5-hour time span. Stir the mixture every 15 minutes with a wooden spoon.
Remove the mixture from the heat and let it cool down to 200 degrees Celsius. Gradually add the mastic crystal tears while stirring.
Add the turpentine slowly, stirring constantly.
Strain the medium through cheesecloth into a wide-mouth jar once it reaches 140 degrees Celsius. Let it cool before covering it.
If using pale drying oil, there is no need to use additional litharge.
Litharge is extremely poisonous. Do not ingest the product or handle it with your bare hands. Do not use dishes used to mix litharge for food or drink.