Holbein Ox Gall Medium is an artist-grade wetting agent that improves flow and enhances wet-into-wet techniques when mixed with watercolors. It also improves the acceptance of watercolor paints on paper, particularly hard-sized papers. - Holbein Ox Gall Medium - 60 ml bottleHolbein Watercolor Mediums, OX GALL 60ML - Holbein Ox Gall Medium is an artist-grade wetting agent that improves flow and enhances wet-into-wet techniques when mixed with watercolors. It also improves the acceptance of watercolor paints on paper, particularly hard-sized papers.