Do you have a source for Magnesium Carbonate powder as top-of-scale?

For measuring the brightness of some materials, some dated industrial methods cite a Magnesium Carbonate block standard to be set to Y = 100% in reflectance. This is a very old top-of-scale standard and HunterLab does not recommend using it.

Contrast Ratio Opacity

When used with industrial and commercial products, there is more than one definition of “opacity” as an optical property.