Thanks. I have money enough to outright buy a 5-gallon bucket of the stuff from a masonry supplier, but the whole idea of permaculture pretty much runs counter to doing that, so I'll try your method and eventually post results. I've successfully built several "rocket stoves" but this RMH has got me thinking, if it's going to be a failure right out of the chute, why bother? I may forego heating a mass and just build a finned, short horizontal run of final flue pipe and put a fan on it. No sense going crazy, if there's even a chance of failure lurking about.
I built my entire PvdB batch-box RMH (household stove) totally "refractory cement" free, making every thing from fire brick mortar to "claycrete-cob" from masons sand and dry milled fire clay, the latter sourced from HWI.
One thing to consider about casting a core from perlite-fireclay is that, as long as the inner layer is relatively rich in clay (maybe half & half with perlite), the heat of the fire will turn the inner part to pottery and it will no longer be water-soluble... also much stronger. The top of the feed tube will not get this benefit, though if you can temporarily extend the feed (perhaps with some bricks) you may be able to get the real feed tube hot enough to fire to pottery.
If you have or can get use of a pottery or ceramics kiln, you could put the whole inner core after drying in it and fire it for real. Making it in sections that fit in the available kiln would be a good move, if necessary. Firing to standard bisque temperature (cone 06 or thereabouts) would be quite sufficient.
While some of the alternative options are certainly not permie friendly, there are a few that will work well. There is a metal casting forum that has a list of homemade refractory compounds that can give some ideas:
It took me quite a while to hunt it down, but it turned out there was a pottery supply store in a small hole on the rear side of a strip mall where you would never find it on your own. The lady running the place was very nice, and although confused with what I was doing (making an aluminum melting furnace) she was very helpful. I picked up a 40 pound bag of fire clay, 10 pounds each of bentonite clay and alumina, all for something like $50 several years ago. You can find cheaper online, but then the shipping makes it much more expensive. It may take a lot of digging through numerous sources (masonry supply, pottery supply, heating oil furnace supply) to find a place close enough to be worthwhile to drive there. I know of one place online that sells smaller quantities of refractory cement for reasonable prices with shipping here:
I haven't used them but came across them in the Alloy Avenue forums from the first link. There are lots of people doing casting or knife making that use castable refractory, so it would be a good idea to seek these people out locally if your other efforts fail. The people at those forums are a friendly bunch and there might be someone with more local information to point you in the right direction.
There is a lot of info on those forums about hot faces, insulation, flux, and other things that can be directly applied to both a furnace and a rocket stove/mass heater. I would strongly encourage anyone interested in using homemade refractories or making multiple rockets to go there to learn of other people's successes and failures to save yourself a lot of money and grief. For a one-off rocket you are much better off using a commercial product and following the instructions as closely as is reasonable in order to have the best chance of success.
I believe my optimal mix was 2 or 3 parts Perlite to 1 part Rapid Set Mortar (RSM). I followed the manufacturers directions on water based on the amount of RSM I was using in the batch. I also discovered after a few batches that the most effective way to get a good distribution on all the materials was to actually mix all the water in the Perlite and then add the RSM in to coat the granules of pelite that were holding water. I did not end up with pockets of perlite that would not bind using this method. I'm planning to use this mixture to build my first Batch Box in the near future as the furnace is still in great condition after years of use.
I gave it 3 days to cure and then used some grey furnace cement thinned with water enough to brush on with a cheap brush. That sealed it up and gave a really nice inside finish. It should work well on my heat riser.
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