Oxgall improves the flow of gouache, ink or watercolour from the nib. It also improves the acceptance of them onto paper, add a tiny bit to your colour as a little goes a long way.
 
In watercolor painting, oxgall acts as a wetting agent, reducing the surface tension of water. This allows paint to flow more smoothly and evenly across the paper, making washes more uniform and improving pigment dispersion. It can also help paint adhere better to surfaces that resist water, like heavily sized paper or vellum.
 
Shake well before use.