An X-ray absorption method was utilized to determine rapidly and precisely the thickness and weight of inorganic coating layers applied to paper. The mass absorption coefficient μ/ρ was calculated for kaolins and substituted in the following formula

where I is the X-ray intensity after passing through a medium whose thickness is x centimeters, I0 Is the X-ray intensity before passing through a medium, μ. is the linear absorption coefficient, and ρ is the density of the absorbing medium.

The film thickness was calculated and related to coat weight. Coated sheets of variable basis weight, utilizing different adhesives and coat weights, were used. The method is rapid and coating thicknesses can be determined with an accuracy of plus or minus 0.5μ and coat weight of plus or minus 0.2 lb. It is believed that the method can be adapted for production control on the paper coating machines.