Most museums don’t use glass at all, at least not any more. They use a plastic material called plexiglass. It can be thin or thick, depending upon the type of enclosure that is being used, the value of the item, and the budget of the institution purchasing the material.
I went to an exhibit of the Faberge eggs many years ago that used bullet proof plexi glass. I have no idea if it was a specific brand. The reason why they use the plastic material, as opposed to actual glass, is because if it breaks, it doesn’t shatter, which could potentially damage the items inside the enclosure.
One of the other reasons, is because glass is highly reflective. No one wants to see their own face when they’re looking at a portrait in a museum. They make non-reflective plastics just for that purpose.