rate of about 35 ml min to about 45 ml min after addition the dispersion was stirred for about an additional 30 minutes the suspension was then dried in a forced air oven the coarse particulates obtained were ball milled milled for about 8 hours to obtain a fine particulate the milled material was then placed into a crucible and heated to a temperature of about 850 u00b0 c for two hours after this treatment the white powder obtained had a mean particle diameter of from about 4 microns to about 7 microns the resulting white powder was silanated by tumbling about 100 g of the white powder with about 4 weight percent silane and 0 5 weight percent acetic acid solution the mixture or slurry was then dried in an oven at about 90 u00b0 c to obtain filler example 3 in powder form into a glass container was placed about 200 g of 0 4 micron barium glass and 1 000 g distilled water the barium glass was dispersed in water by stirring with a mechanical mixer for about 2 hours after dispersion the barium glass had a mean particle diameter of from about 0 6 microns to about 0 7 microns the barium glass dispersion was then pumped into about 700 g of a stirred colloidal mixture of silica and zirconia in water about 15 solids with a mean diameter of from about 115 nm to about 150 nm at a rate of about 35 ml min to about 45 ml min after addition the dispersion was stirred for about an additional 30 minutes the suspension was then dried in a forced air oven the coarse particulates obtained were ball milled milled for about 8.