I've used a microwave on my Col Conk soap, but would be wary on something like MWF that's not glycerin-based. Another way of melting soap that I have been told is to put it in a zip-lock baggy and put it in a sinkful of boiling hot water. Should turn it into a nice soft mush, which you can then squish into your bowl. I'm possibly going to try it on my Old Spice puck in the future.