This method describes a procedure for determining the brightness of clay or other mineral pigment that has been pulverized under controlled conditions and made into a uniformly compacted pigment plaque. This method is for use with minerals normally used in the manufacture of paper and is not intended for highly colored pigments.
The instrument employed has the same spectral, geometric, and photometric characteristics as that described in TAPPI T 452 "Brightness of Pulp, Paper, and Paperboard (Directional Reflectance at 457 nm)." The brightness scale applicable to this method is the same as the brightness scale described in TAPPI T 452.
In contrast to TAPPI T 534 "Brightness of Clay and Mineral Pigments (d/0o diffuse)," which uses an instrument with an integrating sphere to provide hemispherical illumination and perpendicular observation, this method utilizes an instrument which uses 45o illumination and perpendicular viewing. NOTE 1: Brightness values obtained using this method will not agree with those obtained using TAPPI T 534.
The specimens must be prepared with close adherence to the instructions found in the Appendices.
This method utilizes sample preparation apparatus which is identical to that required for TAPPI T 534. The measurement procedure is also similar.