Rice husk, also known as rice hull, is the hard protective covering of grains of rice. It is formed from hard materials, including silica and lignin, to protect the seed during the growing season. Rice husks are a by-product of rice production during milling, and each kilogram of milled white rice results in roughly 0.28 kg of rice husk. Rice husks can be put to use as building material, fertilizer, insulation material, or fuel. Some of the common products from rice husk include solid fuel, carbonized rice husk produced after burning, and the remaining rice husk ash after combustion. Combustion of rice husks affords rice husk ash, which is a potential source of amorphous reactive silica, used in materials science. Rice husk ash has long been used in ceramic glazes in rice-growing regions in the Far East, such as China and Japan. Rice hulls are a low-cost material from which silicon carbide "whiskers" can be manufactured, and the SiC whiskers are then used to reinforce ceramic cutting tools, increasing their strength tenfold. Rice husks can also be used in brewing beer to increase the lautering ability of a mash. In Kerala, India, charcoal from rice husks was universally used for over centuries in cleaning teeth, before toothpaste replaced it.