Wash a small quantity of very runny POLLYFILLA (or similiar) into the bottom of the tankard - if there's a hollow base do it upside down. Let it soak in and wipe the excess away. leave the tankard in a warm place for it to set. Repeat if neccessary.

Another good sealant is PVC glue - but you do need the sort that contains a small ammount of PVC - not just the solvent.

Do not try solder a glass-bottomed tankard as you will quite likely crack the glass, and anyway solder will not adher to glass.

My current quart tankard is cast pewter, so I'm not likely to have this problem, but the above method worked fine with a glass-bottomed one I used to use.