In rubber compounding, a finely-divided substance or filler which, when properly dispersed in rubber, produces improved physical properties in the vulcanized product, i.e., greater energy of resilience, greater resistance to abrasion, higher modules of elasticity and tensile strength Certain grades of furnace blacks are the most important reinforcing agents for black stocks. For light-colored stocks, calcium silicate, precipitated calcium carbonates, silica and clay are the most commonly used.