A colleague and I want to model how a slab of PCM (wax) solidifies as it cools off. What I have noticed in my experiments is that temperature sensors on the top and bottom of the slab say it should be solid, as they read well below the melting point. But when I peek at the sample, the center is still liquid. In order to get the full cooling effect of the PCM, it needs to be completely solid. How can we model the time for slabs of varying thickness to completely solidify? What we need is a starting point for this investigation. How can we get a first order approximation as simply as possible? Can we forget about the fact that the slab is part solid and part liquid, and just say that "heat in = heat out" and the solidification time is just the time to lose all the heat we stored in the slab? For example, one model we thought of: if we melt the slab using 10W and it takes 1 hr. to liquefy, then we remove the heat source, the slab will lose heat according to Newton's law, assuming ambient temperature is 20 degrees C. How many hours will it take to lose all of its stored heat. Assume the slab is 1 cm thick, 2 cm wide, 30 cm long. Assume the PCM is Rubitherm RT-28 (http://www.rubitherm.de/english/download/Techdata_ RT28HC_EN.PDF).