Ox Gall liquid is a wetting agent and is used to improve flow when mixed directly with watercolours.
A few drops of Ox Gall are added to a jar of water and this is used to dilute the water colour.
Ox Gall is also used on very hard sized papers to reduce surface tension. If a paper is resisting a water colour wash, leave it to dry before covering the paper with the diluted Ox Gall. Once this is dry the painting can be continued as normal.