If your tankard is made of "Black" pewter ie old pewter which contains lead, NEVER try and clean it out on the inside if you intend to drink out of it - just wash it gently and wipe it out with a soft cloth. If you are only drinking beer out of it, it will aquire a fairly insoluble coating, but if you scour it back to clean bare metal you will be allowing lead to be leached out into the beer which you are drinking. NEVER drink cider, shandy or anything acidic out of leaded pewter - the acid (probably citric) will disolve the lead and you will be drinking Lead Citrate - which is poisonous.

Modern "White" pewter doesn't contain lead, but I still don't recommend drinking cider out of it - despite the chorus of "John Barleycorn" which suggests "Put your cider into old tin cans".