grade a granite surface plate
The Black Granite Surface Plates is twice as hard as cast iron, non magnetic, low thermal expansion, free from wringing, so there is no interruption of work, free from burrs or protrusions because of the fine grain structure and insignificant; this ensures a high degree of flatness over a long service life and causes no damage to workpieces or instruments. Use these plates in stable temperature environment. Avoid working in direct sunlight because flatness error may occur. Also, do not place a plate in the vicinity of an air conditioner thus the recommended temperature is 20±1°C, Humidity 58±2%.