Casein - An Early Plastic

As I was cleaning my first group of buttons, I started discovering different materials. At the beginning I expected all the buttons to be simply made of plastic. I had no idea they could be any different. Of course, I could tell some were made of metal or mother of pearl, but the rest was just plastic to me.

These buttons smelled suspicious to me...

But then, some buttons looked different. They were lighter or made a different sound when put together in a bag. Some others had uneven surface on the back. I started researching and I learnt that there are lots of other materials in the plastic family, actually, most are early kinds of plastic, which have different properties. And thanks to this information, it is also easier to date the items as made in different decades of the 20th century.