A method of microwave pre treatment of a multi-phase material (200) prior to a subsequent operation on the material (200). The material (200) having a first phase of material and a second phase of material. The method comprises heating the material electromagnetically, preferably with microwaves (202), to produce a power density of at least 10.sup.9 Wm.sup.-3 in a continuous process in which the material (200) moves into and through an electromagnetic, preferably microwave, treatment area (212). The material (200) experiences exposure to microwaves (202), in the treatment area (212) for a time of the order of 1/2 second or less before the material (200) is passed out of the treatment area (212) for subsequent operation.