Fascinating stuff! Here is a short description of what is happening during the firing process. It is not a 100% infil product after the firing process. Very creative!!

"This is Brad Woods, I invented Filamet. The percentage we list is by mass, not volume. The loss of volume is compensated for by a phase our polymer goes through during heating. If you make a print, fire it and cut it in half you can see what's up. Quite simply our polymers "foam up" at a stage after it becomes liquid. This foam and escaping gasses force metal against the walls of the investment creating thick outside walls, and fill the cavity with a geometric form that resembles Styrofoam. Unlike styrofoam the cell walls are very thick, adding strength to the structure."