of the invention described herein however it is desirable to use pumices containing silica content in the range of 80 or more specifically expanded pumicite which is described hereinafter constitutes a volcanic dust comprised of a silicate rather than a silica when expanded the pumicite changes from it 39 s flat irregular particles or shards to countless tiny glass sealed hollow spheres or air cells increasing in volume from ten to fifteen times it 39 s original size the preferred pumicite composition includes silicon dioxide sio 2 in the range of 80 or more iron oxide fe 2 o 3 in the range of 2 and trace quantities of sulfur trioxide so 3 and magnesium oxide m g o of about 3 moisture content comprises about 3 maximum the physical form of pumicite is an extremely fine white powder and is chemically inert it 39 s thermal conductivity k value is less than 3 btu per hour per square foot per degree fahrenheit per in 75 u00b0 f mean it has a water absorption characteristic in the range of 700 to 800 pounds per 100 pounds pumicite which is expanded in accordance with the explanation set forth hereafter has a density of from 4 8 to 6 pounds per cubic foot the expanded material does not require grinding since it is in the size range of 325 mesh while retaining the micro spherical configuration of the particle the extremely fine pulverulent of pumicite substantially exceeds other forms of rhyolites including obsidian and pitchstone its pulverulence substantially exceeds perlite also this is the case because pumices as mentioned above are.