Dust explosions are caused by combustible dusts dispersed in a gas, usually air, and ignited. Many combustible dusts, ranging from very common to highly specialized materials, are capable of causing explosions. The appearance of a dust explosion can be broadly compared with that of a gas or vapor explosion occurring under similar circumstances. For a cloud dispersed in the open air the result of the ignition is a flash of flame, usually developing little hazardous pressure. The flames produced from burning of gases, liquids, or solids are potent sources of ignition for dusts. Thermal ignition theory is based upon the fact that the ignition of a dust in a hot environment depends upon the balance between the heat generation produced by reaction of the dust with the air and the heat loss arising from cooling to its surroundings. Measurement of the minimum ignition temperature of a dust cloud in air is measured in a version of the Godbert-Greenwald furnace apparatus.