I was asked via PM how I packed the cement mixture between the ducts. Here's the story.
It's been a number of years since I did this so my details may be a bit off.
The 8-inch duct has to be made with a seam on the bottom so you can open it to fit the 6-inch duct inside. Once the duct is inside, screw the 8-incher back together. I also used an 8-inch endcap with a 6-inch hole cut in it to help line the ducts up.
I had two of these contraptions butted together - one was for the feed tube and the other for the riser. Since the 6-inch T is shorter horizontally than the 8-inch T, I had to use a short length of straight 6-inch duct between the two.
Once I got the dimensions and lengths test-fitted, the two T's were separated and I began to pack the void with the vermiculite/cement mixture, being careful to keep the 6-inch duct fairly centered (you can see on mine that there is more cement on the bottom than the top which made fitting the second T assembly more difficult).
To pack the mixture I used a piece of 3/4-inch plywood cut to be narrow and slightly longer than the 8-inch T. You have to make sure you can "reach" everywhere within the assembly with your packing rod; you don't want to leave any voids which will create weak spots and/or leaks.
You want to pack one assembly first - I first filled it from the open "end" and then laid it down to finish packing the "feed."
** NOTE once you start, you can't stop until you're completely finished; the cement will harden and when you start up again the new cement mixture won't stick to the dry/hardened stuff. **
Once the first assembly is packed full, attach the second assembly to the end of the first and continue packing. You want the entire assembly to be a monolithic structure, with all of the cement bound together.
Once everything is packed and put together it will be ready to burn.
I recommend making the heat riser separately so it can be more easily moved around and so riser length and insulation mix can be modified in the future. To make them separable, simply "cast" the riser
after the core is hardened.
I
did not wait for mine to dry or harden before firing - I stuck a 6-inch duct on one of the T's started a fire. I let the first fire burn until the whole thing quit steaming - which took quite a while. I roasted hot dogs over the fire while waiting for the core to harden.
A few things I've learned since then.
The 6-inch duct
will burn out much more quickly than expected. Fortunately that's why the cement is there.
The contraption will be pretty heavy, depending on the ratio of cement to vermiculite you use. The heavier it is the stronger it will be, though also less insulative. The vermiculite will compress as you pack so you can use more vermiculite than initially assumed. Alternately you can use perlite which won't compress and therefore end up being more insulative.
The core doesn't necessarily have to be super-insulative; I built a box full of vermiculite/ash/dry grass clippings/clay insulation around the core to keep the assembly good and hot.
The space below the heat riser needs to be contoured - the unused portion of the T (the "void" beneath the heat riser) should be filled with whatever mixture you have left and curved to direct the airflow upward. If you leave the open void there the contraption won't draw and burn nearly as well as it should.
You may choose to use the "void" in front of the feed tube as an ash clean-out. I definitely don't recommend it, though; that void will make it extremely difficult to maintain airflow into the stove and will cause fire creep up the feed tube. The only way to make it workable is if you can make some type of removable "plug" that extends all the way to the side of the vertical feed.