FYI -
I've made up a couple of small foundries using a perlite & fire cement mix. 4 parts perlite, 1 part fire cement. The stuff I've got is good to (pause for a maths break) about 2800F. A 2" layer of that running a charcoal foundry only just starts to hit paper scorching temperatures on the outside. A large bag of perlite costs me about $4-5, 2kg fire cement around $9. Also does pretty well for a tin can forge lining.
I'll be making up another batch today to line my new forge.
Full details on the mix here -
http://www.john-wasser.com/NEMES/MakeICR.html
HTH
Peter