A by-product formed in the heating of iron, phosphate rock, silica and coke; this alloy is used to increase fluidity in steel casting.

Red phosphorus may be formed by heating white phosphorus to 250 °C (482°F) or by exposing white phosphorus to sunlight. Red phosphorus exists as an amorphous network. Upon further heating, the amorphous red phosphorus crystallizes. Red phosphorus does not ignite in air at temperatures below 240 °C, whereas white phosphorus ignites at about 30 °C.

White phosphorus, or yellow phosphorus, or simply tetraphosphorus (P4) exists as molecules made up of four atoms. The tetrahedral arrangement resulting in ring strain and instability. The molecule is described as consisting of six single P-P bonds. Two different crystalline forms are known. The α form, which is stable under standard conditions, has a body-centered cubic crystal structure.