Mischna zu Oholot 18:12

There are three different types of baskets: If a worn-out basket is patched on to a sound one, all is determined by the sound one; If a small basket is patched on to a large one all is determined by the large one; If they are equal all is determined by the inner one. Rabbi Shimon says: if the cup of a balance was patched on to the bottom of a boiler on the inside, the latter becomes unclean; but if on the outside it remains clean. If it was patched on to the side, whether on the inside or the outside, it remains clean.