Alloy of iron and silicon. Comes as a powder with a black grey metallic lustre. Reacts with and dissolves in alkali solution generating hydrogen gas. Shows no reaction with acids. Mainly used as fuel in red thermite (high temperature ignition composition), where the heat of combustion generally increases with the Si content. Commercial standards discern 6 classes of the material, of which class one shows the highest Si-content (88-93%) and is therefore recommended for fireworks use. Differences between classes are quite large: e.g. class 3 shows only about 45% of Si.