Princeton : Neptune : Synthetic Squirrel : Watercolour Brush : Series 4750 : ... Princeton, who over twenty years ago created a synthetic version of the sable brush, have created synthetic squirrel bush. Princeton’s synthetic squirrel brushes, like their synthetic sable cousins, were designed to surpass their natural counterparts. Princeton’s Neptune™ brushes hold colour excellently and come in a large range of shapes, including mottlers and quills. Neptune™ squirrel brushes have traditional solid wood watercolour handles.