Starrett 80635 Crystal Pink® 18 x 24 x 4 Granite Surface Plate Two Ledge, Grade B - Toolroom. Starrett Crystal Pink® Granite has the highest percentage of quartz of any granite. Quartz is more than twice as resistant to wear as the other minerals in granite so higher quartz content means greater wear resistance. The longer a surface plate holds its accuracy, the lessoften it will require resurfacing, ultimately providing better value.Accuracy Under Load: Starrett Crystal Pink and Superior Black Granite plates have a thickness capable of supporting a total normal load equal to 50lb. for each square foot of surface area loaded in the center of the plate without deflecting the plate along a diagonal of more than one-half theflatness tolerance. This is the accepted rating in the U.S. Federal Specification GGG-P-463c. In the situations where abnormal loading conditions are anticipated, Starrett can engineer and modify surface plate thickness to meet virtually any requirement. Call MSI-Viking Gage for assistance with custom orders.Ledges and Clamping: Surface plates without work clamping ledges are recommended for sustained accuracy and reliability. Ledges are for work clamping purposes only. If excessive torque is used when applying clamps to ledges, it can adversely affect measurements taken near the plate edges. If clamping is important, T-slots and threaded metal inserts may be installed in the surface.