The inserts can be perceived as glass but they are not as perfect. Glass would not be practical as it would be fragile, and wood expands and shrinks with the weather and would crack them. Resin is extremely strong. I used polyester resin with the first posts, but epoxy resin has become easily obtained. There are many examples now to be found on the internet.
Epoxy resin has a big advantage in that the constructions does not have the strong resin smell and it is extraordinarily strong. There are also disadvantages over polyester. The process of making the inserts is lengthy, as each opening has to have its individual mould. Anyone wanting to know more should contact me under CONTACT.