Readings were standardized for 100% reflectance with barium sulphate.
The reference was a barium sulphate standard plate.
Pressed barium sulphate was used as a working standard for the calibration of 100% reflectance.
Supplies of barium sulphate, mainly from home sources, are sufficient to meet all requirements.
It is composed of zinc sulphide and barium sulphate.
Higher levels of the vein also contained quantities of baryte (barium sulphate), although at the time this had no commercial value.
