Fires involve combustible metals. Potassium, sodium, aluminum, and magnesium burn at extremely high temperatures. Unless you work in a laboratory or in an industry that uses these materials, it is unlikely you’ll have to deal with Class D fires. They are uncommon in agriculture. Burning metal is very difficult to extinguish, and only Class D extinguishers are recommended on burning metal. Class D fire extinguishers are not used on any other class of fire. The material in a Class D extinguisher is a foam product that puts out the fire by replacing the oxygen near it. Class D fire extinguishers are the most expensive. Class D extinguishing equipment is marked with a yellow star containing the letter D.