Abstract: A gate valve for controlling the flow of high temperature fluids includes a housing having an opening defining an inlet and an outlet for the fluid. An annular collar surrounding the opening includes a transverse passageway therein. The blade for controlling the fluid flow through the opening is made of fused silica. An actuator mechanism coupled to the blade moves the blade in the collar passageway to control the fluid flow through the opening. The fused silica blade exhibits superior mechanical characteristics, can be readily made in large sizes, and minimizes valve leakage.