As indicated, any ller should have a screen grading of 200 mesh or ner, and my preference is for the use of fillers having an average particle size of about 5 to 8 microns or less. ln practice I prefer to subject the salt or other filler to mechanical grinding to reduce its particle size to an average of around 5 to 7 microns, or the filler may be subjected to the action of a fluid energy reductio