b 2 o 3 p 2 o 5 bao and li 2 o essential for the present invention is the volume fraction size and aspect ratio of the mica phase in the resultant sintered glass ceramic the larger the mica plates and the higher their aspect ratio the lower the volume fraction of mica that is required to attain machinability an aspect ratio i e ratio of thickness to length of the mica plate of u2267 2 is preferred at least thirty volume percent 30 vol of mica is required to attain machinability at least thirty volume percent 30 vol of the residual glass phase is required for sinterability therefore the mica content is between about 30 and about 70 volume percent and preferably between about about 40 and about 60 volume percent some of the compositions of the present invention are extremely reactive and will dissolve additives e g pigments and fluorescing agents during sintering of the cad cam blocks in this case it was found that using coarser additives e g pigments or fluorescing agents substantially alleviated this problem normally these additives pigments and fluorescing agents have an average particle size of about 4 to about 8 microns for most reactive compositions it was found to be critical to use additives of average particles size equal to or exceeding 15 microns it is believed that at the same time particles exceeding 60 microns will compromise strength thus.