Abstract: A microwave heating oven has a microwave-shielded loading passage and a first heating zone for containing the material in solid form. A second heating zone maintains the material in its liquid state and provides an exit passage for removing the material while in its liquid state. The oven finds particular application in the processing of materials for photographic emulsions where the emulsion material is placed in the first heating zone in solid form. The second heating zone is located directly under the first and is separated from the first by a perforated partition. Subjecting both heating zones to microwave radiation causes the solid emulsion material in the first zone to melt and to pass through the perforated partition to the second zone where the emulsion material is removed as a liquid.