It's not the lenses which are difficult, it's clear glass. With a piece of pure glass, the rest is some emory and a great deal of patience. Clear glass is als

It's not the lenses which are difficult, it's clear glass. With a piece of pure glass, the rest is some emory and a great deal of patience. Clear glass is also more about knowhow than raw technical difficulty, it isn't the Bessemer process, it's knowing how to flux with soda + add some lead to get the melting point down, then pouring it on lead to get nice flat sheets.