Ritcheson's aluminum stretcher bars are museum quality and professional grade. Kiln-dried pine is connected to a specially milled 21in aluminum bar to assure that paintings will never twist, warp, bend or change from dryness or humidity. The recycled aluminum stock provides a structural frame of total strength and stability. The pine outer bar offers a smooth, rounded edge and a solid surface for stapling. Bars can be cut to any length and assembled with steel corner inserts.