Abstract: A method is provided for preparing a low density porous rigid fused-fiber matrix having a density of between about 3.5 and 5.5 pounds/cubic foot and a free volume of between about 90-98 volume percent for use as a cell-culture substrate or implant material, or in chromatographic separation of blood cells. The method is carried out by forming a slurry containing (I) silica, alumina or silica and alumina fibers having thicknesses between about 0.5 and 20 .mu.m and lengths between about 1 and 10 mm, and having a fiber:liquid weight ratio of between about 1:25 to 1:70, (ii) a thickening agent to give the slurry a viscosity between about 1,000 and 25,000 centipoise, (iii) boron nitride particles between about 2-12 percent by weight of the total fiber weight, and (iv) a dispersing agent when the slurry contains silica fibers, allowing the slurry to settle in a mold to produce a fiber block, drying the fiber block, and heating the dried fiber block to at least about 2200.degree. F.