These discs are made of sharp durable aluminum oxide grain on a heavy "C" weight paper backing with a full phenolic resin bond system and a non-loading zinc stearate treament. These open coat discs deliver a fast cut and exceptionally long life. Aluminum Oxide Abrasive Grain - Aluminum oxide is extremely tough and has a wedge shape for high penetration of tough materials without excessive shedding or fracturing of the grain. Aluminum oxide is suited particularly well for grinding high tensile strength materials i.e. carbon steels, alloy steels, tough bronze and hard woods. Aluminum oxide is used whenever toughness ( the ability to resist fracturing ) is the main consideration. Aluminum oxide products are typically brown in color.