Vermiculite, zonolite or perilite-what to use to fill space under flat pan?

[claystroup]

I am going to start bricking my new 6' arch which has about an 8" deep area under where the rear pan sets. I am using a flat rear pan and want to fill that area only leaving about 2" of space to keep the heat on the pan. I have a large roll of 1/2" ceramic insulation to insulate the sides (probably double or triple thickness) but want to fill in the majority of the unused space with either vermiculite, perilite or zonolite. My question is which of these 3 materials is the best to use? The local masonary supply yard has 4 cubic feet bags of vermiculite for $18. I figure it would take about 2 bags. Thanks in advance for any advice - Clay

[psparr]

Anything will work really. Just cover it with the blanket. If not the draft from the fire will make a mess of whatever you use.

[Bricklayer]

I used vermiculite on mine and it worked great. I've heard of people using dry sand also. I went with vermiculite