Use good modern glazing that is used in glazing (puttying) old-style wooden windows. Use Linseed oil to make it soft and roll it into a snake. wrap it into the class and pewter frame on the outside of the bottom. Push it in with a piece of pegwood sharpened to a point. Scrape off the excess. Let dry four days and four nights. Wash in the sink by hand and NOT the dishwasher and renew it once every few years if you have to. Thus said my boss who is a jeweler with fifty years' experience behind him.