Fine hairs from squirrels' tails are used in brushes for this painting medium whose name tells you it uses H2O
.... [Note: If you have never 
made your own pigment-and-oil paint and use .... quality of Rubens' paint is due 
to the addition of some resin to the oil medium or to the mode of ...... [Note: A "
rigger" is an olden term for a long and thin sable or squirrel-tail "line", or "liner" 
brush.].