an agate burnisher like this will do the trick http://www.cooksongold.com/Jewellery...rcode-999-AOA3, To work harden, use a raw hide hammer and a steel bench block and well, hammer :) Will also flatten the piece for you as quite often charms warp in the kiln. Some peeps swear tumbling work hardens pieces, but am not convinced, lots say it doesn't...