I believe the only difference between Wrap n Zap and regular 100% cotton batting, is that W&Z does not have the poly scrim. It's all cotton, so it won't catch fire in the microwave. So I don't see any reason why you couldn't join leftover pieces. Just don't join them with a polyester iron on tape if you are going to use them in the microwave (bowls, potato bags, etc). Polyester and man made fibers can catch fire in the microwave. Also use cotton thread for items to be used in the microwave.