What is casein?
Conway Stewart produced a lot of pens from casein as casein offered colors and designs that could not be produced by hard rubber.
Casein is not vegan! It’s made from rennet and skimmed milk, which then goes through a formaldehyde treatment to harden the material. The material is hygroscopic, so water exposure must be minimalized and the pen should absolutely not be soaked in water.
When exposed to heat and light, just like hard rubber, casein can degrade. The surface becomes dull and lacks lustre. This is called surface crazing, and if observed closely, the cause can be seen as many thin, shallow cracks.