Nitrites, silicates, borates and azoles may also be used to prevent corrosive attack on metal. Other possibilities include polyphosphates, silicates, borates, and various antioxidants. The most common compositions are based on lead borates (leaded borate glass or borosilicate glass). All borates can be considered derivatives of boric acid, B(OH). Borax is also easily converted to boric acid and other borates, which have many applications. Borates are boron-oxygen compounds, which form boron oxyanions. Boron does not occur naturally as a free element, but in compounds such as borates. Cellulose insulation uses borates for fire retardation. Chinese boron producers have been unable to meet rapidly growing demand for high quality borates. Sedimentation is used by Rio Tinto Minerals to refine raw ore into refined borates. Bulky pyrazolyl borates can be prepared from 3,5-disubstituted pyrazoles, such as the dimethyl derivative. The borates in cellulose insulation provide added control against mold. Acylation of borates is possible in the presence of an acyl halide. Borates are a non-renewable mined product. The scope of organoboranes and borates as reagents for organic synthesis is extremely wide.