Ivorine is an unusual type of plastic made from formaldehyde and casein protein extracted from skim milk. It was invented in 1899 as a replacement for the rubber used in pens, which was difficult to color then, and as a white-colored replacement for school blackboards.
Ivorine also became popular in the figurine and garment industries as it was so easy to color. It turned out to not work so well in pens, however, as it was easily discolored and very susceptible to moisture. Ivorine has also been used as a lower-cost replacement for ivory and is often used in piano keys.