5/8″ Type X gypsum board provides one-hour fire protection when used on both sides of a steel or wood frame partition. The main additive that helps produce Type X board is glass fiber. On exposure to fire, gypsum loses chemically combined water. As a result, the gypsum particles shrink, lose contact with each other, and the board falls apart. The network of glass fibers in the core of Type X board helps maintain the structural integrity of the board when exposed to fire. Type C board is produced when vermiculite ore is added to the board formulation in addition to the glass fibers. Vermiculite ore expands when exposed to fire. Because the ore expands at the same time that the gypsum is shrinking, the board is dimensionally stable for an extended period of time. In other words, a Type C board is superior to Type X. Type C not only maintains the board integrity at high temperature but also keeps the board from shrinking.The majority of fire-rated assemblies use Type X board. In some asse