That's not set up to be a forge it's set up to get as even of heat as I can for heat treating knife blades. Regular fire bricks cost me 75c each and the insulated fire bricks cost me $2.45 each. They were the lightest to be had there/then.
Inside, on the floor, where you can't see, are two insulated fire bricks and simply surrounded by the ordinary fire-place-liner bricks to form the floor.
The insulated fire bricks are light weight almost like styrofoam. :)
No kidding on this, I did this and even showed it to others a couple times... picked up an insulated fire brick with my bare hand with one face of it glowing orange-yellow and turned that face toward myself and felt like I was going to singe my eyebrows. ;)
The first time fired it up after switching to the insulated fire bricks from the old ordinary fire place liner bricks I "almost melted" (non blacksmith ok?;) the O1 kitchen utility knife blade I was trying to heat treat. Not only was it much faster but so bright in there I missed seeing the arrest point and blew right past it. :) (i always heat treat at night)
I was using the same size of flame as before... since then cut it back to 1/4(?) and need to experiment with even cutting it back somemore.
I'd really like to see configurations of insulated fire bricks that are considered good forges by their user! So some day I could copy it. That's the thing about bricks you just re-configure as needed.
I melted and poured in four different melts, 315 pounds of lead using those bricks then put it back the way it is in the picture.
The first few years I was heat treating, that stack of fire bricks took on a dozen different configurations. :)
Alvin in AZ (dogs, beer, ribsteaks cooked over wood;) ps- still working on the spark testing... stalling for a known sample of something in between 1035 to 1060. pps- all I have are 4140 and 5160