Daler Rowney Ox Gall Solution is an artist’s medium designed to improve the appearance of your paintings and give you more creative options when working with watercolour paint. The liquid improves the wetting and flow of your watercolour. Just add a few drops to the mixing water and it can also improve the acceptance of watercolour on paper, and particularly hard-sized papers. Very little is needed for the solution to work effectively. There is 75ml in each bottle.