1. Clay rocks are relatively hard, but they can be ground effectively using polycarbonate vials and stainless steel grinding balls.  The key to effectively grinding the samples is not to overload the vials.  For 4 ml polycarbonate vial sets, no more than 350 mg of clay, preferably fragmented, should be added per tube.  For 15 m vial sets, sample mass can be increased up to 3 grams, but again the sample should be in several small pieces.  Remove grinding balls from tubes and load with samples.

2. Add a 3/8" stainless steel grinding ball to each tube of the 24 Well Vial Set and two 7/16" stainless steel grinding balls to the 15 ml vials.  Place caps on the vials.

3. Grind samples for 3 minutes at 1500 rpm using either a Geno/Grinder (model 2000 or 2010) or an HT Homogenizer.  Check the samples to assess the degree of pulverizing.  Fine powder should be apparent, but rocks (of reduced size) may still be present.  Process samples for an additional 3 minutes and check again.  Repeat these steps until the sample is thoroughly ground. 

4. Remove vial sets from homogenizer and extract balls using Magnet Tips (or 24 Pin Magnet for 24 well vial sets).  Most particles should be smaller than 65 microns.

Vlack, Y.  2008.  A Diffuse Spectral Reflectance Library of Clay Minerals and Clay Mixtures within the VIS/NIR Bands.  Master of Science Thesis.  Kent State University