insulating riser without clay slip in perlite vermiculite

I guess I need to post a thread for this specific topic for future reference.
When insulating a fire brick riser and filling the space between the riser and the surrounding material. (sheet metal duct in my case) the RMH book says to mix clay slip with perlite at a ratio of 1 measure of slip to 6-9 parts perlite. Then cap the enclosure around the riser with cob. I suppose vermiculite (all I can get right now) could be substituted for perlite with only minor performance differences.
If you were using either is it absolutely necessary to mix in the clay slip? Other than a couple reasons like using some kind of mesh screen to encase it around the riser or not having it drifting all about because of it's super lightweight nature. I see no reason to add the clay slip.
Reason 1 not to is that all the water in the slip has to be cooked out.
If the perlite is enclosed in a sealed enclosure around the riser that would seem to work fine.
Lets try to clarify this for every body to learn from in the future.

John; The sheet metal surrounding your riser is expected to burn off, the clay slip will hold your perlite in place after it fails. Vermiculite holds water , perlite does not. The heat will quickly dry any water anyway. Try looking at a landscape store for perlite, I know that home depot sells 2' bags for 17 $ but i find it in 4' bags for 22 $ at the landscape place.

thomas rubino wrote:John; The sheet metal surrounding your riser is expected to burn off, the clay slip will hold your perlite in place after it fails.

That is true of a RISER made of steel where the heat is intense. I don't think it's true of the area inside the barrel where temps are much much lower. Erica Wisner actually suggested what I did which was to use 2 8" duct pieces to make a 16" one. I've ran the 14" duct on a smaller riser for over a hundred hours with zero signs of heat damage.

Vermiculite holds water , perlite does not. The heat will quickly dry any water anyway.

I swear I read the opposite, that's why most recommend the perlite (including the Rocket heater book). If I only used 2cu ft that would still require 1.5+ gal of slip. A lot to dry out.

Try looking at a landscape store for perlite, I know that home depot sells 2' bags for 17 $ but i find it in 4' bags for 22 $ at the landscape place.

For a 4cu ft bag of vermiculite I paid ~$22 at the farm store, perlite is a couple $ more but they had to order it in. A very small bag of perlite was ~4$

John, with the riser you've made, just chuck vermiculite between your heat riser and tube, and do a little clay or else on top, to stop it flying. Wear a dust mask when pouring it. Squash/pack your vermiculite a bit, it doesn't matter. Best performance from it, is when it's un-mixed. Peter said to me, to use 2 inches aproximately. I would advise you to cut the stubs of brick sticking out of your heat riser, to stop theses forming a heat bridge to the outside tube.

Merry Christmas to all ! Christmas eve and I still need heat, so no rest for the wicked
Max,
Thanks for the input. I went ahead and mixed a little slip with about a gallon of vermiculite to go in the bottom of the insulating sheet metal duct just to make sure it didn't come out the bottom seam any at all.
I was working very slow (a lot of back pain) when rebuilding the riser. As it was it took me 4 hours. I didn't have any more time to cut off all the bricks to keep it a square on the outside. It's near impossible to do it after the fact. There is an inch or more from the very corners of the protruding brick ends to the surrounding duct which is now 16"(previously 14"). After filling with vermiculite I put a thin cap of cob to seal it in.