What substance is used to impregnate wood to make it more fire-resistant?

The use of mineral salts to impregnate wood for enhanced fire resistance is based on the fire-retardant properties these substances possess. Mineral salts, such as ammonium phosphate, ammonium sulfate, and borates, react with the cellulose in wood when heated, releasing water vapor and forming a char layer that helps to inhibit combustion. This treatment makes the wood less likely to ignite and helps it resist flames, effectively increasing its fire safety characteristics.

Other substances listed serve different purposes. For instance, formaldehyde is primarily used in the production of various adhesives and resins and is not intended for enhancing fire resistance. Acids, while they might have some interaction with certain materials, are not typically used for wood treatment in the context of fire resistance. Polyurethane is a type of synthetic polymer often used as a finish or coating for wood but does not inherently provide fire resistance and can even be flammable. Thus, mineral salts are the preferred choice for effectively making wood fire-resistant.