Insulation for firebrick heat riser.

[nemo]

Hello!
Can anyone tell me what would be the best cheap insulation for firebrick heat riser?
I do not have access to perlite and vermiculite.
What I do have is clay, sand, gravel, ash, coal, sawdust.
I have heard about mixing ash with coal and clay, also some clay slurry with sawdust poured over gravel.
I do not know what the proportions are and what is the best insulating mix.
Can anyone help?
Thank you!

[DCish]

If you mix clay with something that will burn out (say, sawdust or chopped straw) you end up with voids left over, making a decent insulator. Donkey uses ash in his clay slip, hypothesis is that the lye it contains helps strengthen it.

[shilo]

ceramic blanket

[nemo]

So DC, the ash is only for strenghtening and does not affect the insulation/conductibility of clay? I mean, can I use it in all my thermal mass or just for casting? Tried it and it does strenghten it. Thanx!

[DCish]

Sorry, nemo, don't know what impact, if any, including ash has on conductivity. I know that dry ash is sometimes used to insulate because it is initially light and fluffy, but that once it settles it no longer does a good job of insulating anymore (too dense).

[Donkey]

In my experience, if you use it as the major portion of your "aggregate", it appears to make very light, insulative bricks.