3M(TM) Paper Sheet 346U, 9 in x 11 in 50 D-weight, 500 per case

Aluminum oxide is a very hard mineral that maintains sharp points on the grains much longer than natural abrasives such as emery or garnet. The abrasive grains are adhered to the sandpaper with some space between them, known as open-coat construction, to minimize the buildup of dust on the surface of the paper, which would otherwise load the abrasive points. Loading often results from sanding soft materials and can clog the abrasive, shortening the usable life of the sandpaper sheet.