Later, the odor was eliminated and casein was produced in tubes, exactly like oil colors. It dries rapidly and is quite waterproof. Casein can be used as a watercolor but, unlike watercolor, one stroke covers another because the color is relatively opaque. Changes and corrections are easy, not as in transparent aquarelle. Today, tempera, gouache, and casein are almost identical.
Some artists add eggwhite or the whole egg to casein to make it harder.
Polymer A relatively new medium is polymer, the general name for all kinds of plastic colors, which are marketed under various trade names. Prepared from mostly diverse combinations of acrylic and vinyl, these colors are applied with water, but dry almost instantly and dry waterproof. They can be handled like watercolors or oils, in any technique, thick or thin, opaque or transparent. They can be varnished with plastic varnishes, a feature which makes polymer paintings or objects painted in polymer literally scuffproof. Polymer is employed in commercial fields as well as in the fine arts.