60 mounted as set forth above are placed in any suitable cvd reactor one example of such a reactor is described and illustrated in ser no 933 077 filed 20 nov 1986 now u s pat no 4 772 498 issued 20 sept 1988 a uniform thickness of silicon carbide is chemically vapor deposited onto upper planar surface 52 outer sidewall 53 lower planar surface 54 sidewall 58 annular surface 65 and walls 69 and 72 as well as the exposed surfaces of the mask not shown and mask 60 i e smaller diameter surfaces not shown and 63 and sloping sidewall not shown and 62 to a depth equal to the desired thickness of holder 1 the substrate is withdrawn from the cvd reactor and the masks are removed from recess 55 and cylindrical space 57 at least a substantial portion of the graphite of uncoated inner surface 70 and lower surface 59 and adjacent to inner sidewall 58 are removed by machining for example sic coated disk 50 is bored from the center of inner surface 70 out to wall 69 by a vertical milling machine then a diamond grinding wheel is used to grind smooth the surface of the silicon carbide coating adjacent juncture 69 coated disk 50 is flipped over and the graphite above uncoated surface 59 is milled to within a fraction of an inch of the silicon coating on upper planar surface 52 and annular surface 65 and the remaining fraction of an inch of graphite is grit blasted away to result in support 30 shown in fig5 graphite core 46 at.