http://en.wikipedia.org/wiki/Surface_plate
Plate material
[edit] Granite
Before the Second World War, metal was the standard material used for surface plates, however, after the war efforts of various countries put a strain on the availability of metal, a monument and metal shop owner (Wallace Herman) in Dayton, Ohio started using granite in place of metal for his surface plates. Today most surface plates continue to be made of black granite with less common surface plates being made of pink granite or diabase. The quartz content of surface plate determines the wear ability of the granite surface plate with higher quartz content being more resistant to wear. Pink granite contains much higher levels of quartz and is often thinner than black granite which reduces the load bearing capabilities of the surface plate.

Cast iron

A cast iron surface platePrior to World War II, most all surface plates were made from ribbed cast iron with the ribbing used to increase strength while reducing the weight. The cast iron was aged to reduce stress in the metal in an effort to decrease the likelihood of the plate twisting or warping over time.

Cast iron surface plates are now frequently used on production floors as a tool for lapping granite surface plates to achieve certain grades of accuracy. The metal allows itself to be impregnated with the lapping media over a large flat surface.

زهر or حديد زهر is "cast iron"

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Note added at 22 mins (2010-12-01 19:44:13 GMT)
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Note added at 29 mins (2010-12-01 19:50:57 GMT)
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شنكرة is "marking out"
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