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I've used stainless needles as reinforcing material, that works well. I think regular steel reinforcing will tear it apart, but I haven't tried.

its been a bit since I posted updates..man i love this stove! such a simple design that works so good! Followed Matts examples for the 8" core and heat riser..i used 8" duct pipe for the center and 14" duct pipe for the outside. Now..one of my biggest concerns was how in the world do you know about your soil..how much sand..etc..well Living in southern middle tn that good ole red clay seemed to have a great mix of clay and sand aleady in it. just added straw and water and started packing...my best advice for those out there that like to research like I do...just do it...if you make a core like Matt shows and follow the basic instructions..i dont think you can mess this up. I did go ahead and build a partial manifold out of backer board but really it was just to hold the "cob" in place..put in a clean out right there and fed the exhaust through 20' of greenhouse...lol..the first firing i didnt have the exhaust riser on...just wanted to see what would happen...well it didnt draw great..but it did draw...and the greenhouse filled with smoke..not like from a traditional wood stove...vapor..it was like a fog..lol. i put the 90 degree elbow and riser up on it and away she went...talk about steaming up the cob! I have some pictures and will take some more and post them. I have not finished the bench but am using the method Matt did for outdoor benches by using backer board for the sides... since this in a greenhouse and will be getting watered..
Want to thank Matt for all the help and ideas and really everyone that posted ideas...just go for it guys...learn as you go..

Great post Cletus, and I love the attitude. So many folks try to figure it all out in their heads before getting dirty. As you found out, you will answer 99% of your questions just by getting muddy. Love to hear that it's working so well for you, congrats man!

I made an all square wood form for a 4" core. It's one of two 4" experimental cores I've made and neither were great. I'm guessing it was the size though and not the square. I think a round riser would be better if possible but square will still function. There are so many options for a round form.

I've made square risers with wooden forms many times, they work just fine. There is something to be said about that little grease drum as a container for the finished riser. It makes moving and maintenance very easy as you can move the riser in one piece without fear of it falling apart. Other than that, I don't think there is any real disadvantage to using square wooden forms.

I was reading another post and nobody gave an answer. how long will the barrel last?

Should last for decades in indoor use. There's just not enough action at the top of the riser often enough to break it down, and it can shed head quickly to protect itself. Mine is showing no signs of breakdown after constant use these last three plus years.

thanks for all the info and I started building forms today and will purchase the number 23 castable and use it. think he quoted me 30$ a bag and it will take three bags. will let you know this stuff pours and works out.
Bill

Hello Matt. New to this site, I am unable to find the link to your video on how to cast a riser.
Is it available on youtube ?
Would really like the idea of a cast core, for myself and the community I do volunteer work with.
Thank you.
Ben.