Fire-retardant wood treatment (FRT) is the process by which pressure impregnates wood with fire-retardant chemicals. It is a protective treatment that reduces the combustible nature of wood, minimizes the effects of fire, and is suitable for interior and exterior wood, dimensional lumber, engineered lumber (excluding wood wafer products), and finish materials. Two types of FRT exist today: one involves a blend of a nitrogen-phosphorus organic compound and boric acid, and the other involves the use of ammonium polyphosphates with additives (boric acid, borax, moldicides, and others). Formulation compositions used to treat wood are proprietary and differ between manufacturers.