What type of insulation is around the riser?
The insulation is "Roxul". Ernie gives it a good endorsement. It's much quicker than perlite.
What kind of brick is good to use? Look for brick that's rated for 2200 F. I have a Firebrick place here in Minneapolis, MN. All the bricks I bought were new.
The riser are "half" bricks
What is on the bottom of the barrel, holding it up? Sheet metal? It looks like a nice tight cut to fit the burn chamber.
That is another barrel cut to fit into place. It's the newer design that Ernie and Erica have came up with, and the design they recommended to us. It can be a pain in the ass to get the barrels to fit together. Otherwise go with the method described in Ianto's book.
What did you use to seal the brick where it meets the barrel base? That's
Cob. I'll be cobbing in a bunch of
local ledge rock into the base. What you see here is an unfinished aesthetic thermal mass, but the functionality of it is there.