Using concrete sono tube forms to build riser?

Trying to come up with cheap, easy way to build riser. Looked at a number of forums. It seems using clay, sand, perlite mix is best. Usually people use metal forms and keep metal within riser. Would the mixture harden enough under heat to hold form if i were to prefab riser using concrete tubes.  My thought was to create temp r stove outside to burn off cardboard. Then transfer riser to perminent RMH. I was wondering if standard clay, sand, perlite mixture would hold form without metal form? Sono tubes could cost me roughly $17.

Make sense? Thoughts?

Could riser mixture be used for the entire structure of j tube instead of fire brick??
Probably going to by dvd set eventually, but want to build a practice heater on the cheap first.

Hi Ted; Welcome to permies!  Yes , sono tubes work perfectly.  Only issue you could have is if your perlite mix being too wet and you wait to fire it off, I have had the sono tube start to buckle from the water when I did that. Keep your mix as dry as you can, lots of perlite, and use fireclay. Don't wait longer than a day before firing it off. No need to do that outdoors I set mine right where it is going to live and burn it off.  You use a 16 gallon grease barrel as the outer form and the sono tube as the inner form . Caution when buying sono tube they are not standard sized .. all slightly different so they slide together for shipping... bring a tape buy the one that is 8" ID or  6" ID .  16 gal barrels can be gotten at a quick lube for free or very cheap.   Yes you can make the entire core from this mix ... I have done it .. works awesome except .... loading wood into the feed tube you will abrade the sides and will need to do cob repair on a steady basis.  Fire bricks are highly recommended at least in the feed tube.   Search broaudio on youtube, that is Matt Walker from walker stoves. He has a video showing how to cast cores and risers . Consider buying the builders guide by Ernie and Erica Wisner available at amazon and many other places.

I LOVE FORUMS.
First forum I have been on and it took less than a day to get great answers.
I started looking to buy some ground fire clay. But could only find bentonite(sp) and fire clay that was $70 for a 50lb bag. Bentonite doesn’t seem to be the right stuff and $70 seems high.

Ted;  Expect Fire clay to cost 10-20 $ a 50 # bag.  In Spokane I pay $10 in Helena it was $18 and I was told it was $8 in Hamilton.   To find fire bricks and fire clay you need to locate a masonry supply house.  Do not buy firebricks at the big box store. Perlite I find at landscaping stores. A 4 ' bag costs $20 + . Sand and regular clay is generally free.  

Awesome! Thank you much. Just bought rmh dvd set. So that will be awesome.