In the early days of developing fire-retardant treatments, researchers at the Forest Products Laboratory (FPL) investigated about 130 treatments. Combinations of chemicals were used to obtain the best performance for both fire resistance and other performance properties, such as corrosion, leaching, gluing, finishing, and cost.
Chemicals tested included ammonium sulfate, mono- and di-ammonium phosphates, ammonium chloride, zinc chloride, borax, and boric acid. The phosphates were identified as the most effective. These chemicals were used in the first generation of commercial fire-retardant-treated wood in the United States.
The 1940s test pictured above shows an attic section in which the rafters, roof boards, inside of the end wall, and the top ply of the flooring, were impregnated with a moderate degree of fire retardant and exposed to a 5.25-lb magnesium bomb. The treatment completely stopped the spread of fire on exposed surfaces, but the untreated subfloor was ignited by the excessive heat transmitted through the flooring.
(Excerpt from John Koning’s book Forest Products Laboratory 1910-2010: Celebrating a Century of Accomplishments.)