... )light grey,thin wall,hollow,light weight. 2)Volume weighs is 250 - 450kg /m3,granularity diameter is about 1 mm , Hollow glass ... micropheres is a hollow ball collected from coal powder,its coloure is light grey,thin wall,hollow,light ... ,80-120 mesh,150—200 mesh,over 200 mesh Hollow Glass Microspheres Collected From Coal Powder