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We know a ceramic blanket makes the best heat riser insulation.
So why not use the blanket AS the heat riser?
Yes, it may not be rigid enough by itself.
What about soaking it in a wet clay slip. Or maybe use fireplace cement.
Then wrap it around a form, possibly cardboard, of desired length & diameter.
Then use dry blanket as additional insulation.
Burn it out & attach it to your core.
This seems to me to be the best possible heat riser one can make.

I was thinking along similar lines. All the voids / air pockets in ceramic blanket give it R-value. To preserve those voids I was thinking of constructing the riser as follows. Start by encasing a sacrificial cardboard/sono tube form with a layer of fireclay to serve as the riser's smooth interior, maybe a sheet of clay about a 3/8" thick, and let it dry hard. Then wrap that "ceramic tube" with the blanket, and encase it with a tin outer shell of perhaps some HVAC metal pipe, or a couple of stacked 16 gallon grease barrels. The tin pipe (barrels) would make for a fairly strong and smooth riser exterior surface to protect the ceramic blanket during annual cleaning.

If heat fracturing of the long ceramic riser tube becomes a problem, the tubes could be made in shorter sections that sleeve together, for multiple expansion joints etc.

How about this:
Get one of those wall texturing spray guns at Home Depot
Instead of gypsum fill it with thick clay slip
Spray the inside of a strip
Fix into a cylinder with stainless wire holding it in shape, sprayed clay on inside
Spray the outside of cylinder with slip
Put on a BBQ or rocket core or other rocket stove & fire up to completely dry out
If the clay will keep it in shape, the inside of the ceramic blanket free of clay, remaining totally insulative
While the dry clay & stainless steel wires would keep cylinder shape & extreme heat would not distort

OR
Do the exact same as above, except instead of spraying with clay slip, spread on a thin layer of fireplace cement
And then do the same on the outside, still using stainless steel wires to help keep in cylinder form

And before the fireplace cement dries, pour on loose vermiculite or perlite
That might give the combustive gasses inside of the heat riser a little needed turbulence

You guys will enjoy these threads:

viewtopic.php?f=6&t=313

viewtopic.php?f=6&t=316

viewtopic.php?f=6&t=320

viewtopic.php?f=6&t=325

There may be a bit more, but you get the gist of it. I sure wish Nutcase would come back and let us know how things progressed, if they did.

You can buy rigidizer for the blanket, by the way. I think the formed ceramic fiber risers are the best riser solution, personally, but there sure are a lot of options. Lots of them are good, and I love the experimentation and process. Great thread Nirky.

Thanks for the links, Matt. I didn't think I was the first one to have this idea.
It looks as Nutcase was successful at getting a lot of data.
I would not have had the gonads to attempt a blanket-made core!
You say there ARE already such things as formed ceramic heat risers?
Commercially-made? If so, where would I get one?

Ask your refractory supplier, where ever you got your blanket. They may be able to get one for you. I, ugh, I'm not interested in selling people stuff, but I have made them available on my Walker Stove site since apparently they are really tough to source locally for some folks. Gah. That feels weird, but, yeah, I can sell you one if you can't find one near you. They are kinda spendy.

Fear not, I'd not harm our relationship by purchasing from you.
I did some checking on the eBay seller who sold my ceramic blanket,
and checked eBay & other refractory outlets in general.
Found ceramic boards.
Ceramic blanket rigidizer colloidal liquid.
Insulating castable 2500 degree mortar:
http://refwest.com/browseproducts/GREEN ... --BAG.html
Is there a better brand or type of insulating castable which you prefer?
I prefer making things myself, if it's possible. This looks very possible. Even doable.
And I'm excited to try.

That looks like it has all the right numbers Nirky. I don't think it's worth recommending brands, just properties, since the different distributors will buy in bulk and repackage under their own brands. That looks good man.