A by-product of mixing paint in a plastic bowl. Once the acrylic dries completely it is waterproof, so I tend to mix one colour on top of another - so the layers build up in unused paint over weeks, if not months, until eventually it is thick enough to separate from the the original bowl. A bowl made of paint! I suppose I could carry on using them to mix paint in but I'm becoming quite attached to them as objects in themselves each with their own characteristics and a 3 dimensional record of the time spent creating 2 dimensional paintings.
