How does glass get a patina?

Question

Like when you find older bottles, and instead of being worn down, they'll have this irridescent sheen on them. What causes that, and is there any way I can imitate it?

Answer 1

Iridescence on glass that has been buried is caused by the weak organic acids in the soil etching the surface over long periods of time. Iridescence in modern glass making is often made by the use of metallic elements and/or the use of chemicals mainly acids on finished articles.

Answer 2

.  Some of what you see may be from impurities in the glass or the method used to create the bottles.  I expect that a glass blower could tell you exactly how to reproduce it.