These are about 23mm or approximately 7/8". The shanks are glass.
I believe these were decorated using a process that is similar to glass enameling on copper. Fine glass powder (called frit) was applied through a screen with a stencil, and then was fired on the glass button for a couple of moments in a kiln. I have done some of this enameling on glass many years ago, so I am familiar with this kind of work.
These were NOT painted. The design will not wear off very easily, and will not wash off in water.
Vintage glass hand enameled shank buttons in mid century modern assorted designs.