Alternate Materials for Core

I have seen where metal degrades, and even firebrick and fireclay can break down in the temperatures inside a RMH.

I found these materials online...

http://www.hightemptools.com/castablerefractory.html

I would think the last one on the page would serve well. a 55 lb. bag for $77 doesn't sound too bad. You'd just have to cast it as a liner for the core.

I've been thinking about similar things and wondering about materials for Core 3.0/4.0/whatever version we're up to. I wrote about the possibility of a hybrid core, making the inner liner out of split fire bricks for strength and durability and packing in 8:1 perlite/clay insulation around them to hold them in place and super insulate the center of the core. If this Kast-o-lite stuff is as impressive as they claim, an inner core liner made of refractory cement wrapped in high insulation perlite clay could be an amazing combo. A buddy of mine tried something like this, he poured a core but "painted" a bunch of refractory cement on the wood inner form then packed something like 15:1 perlite/clay around it. He said his crumbled and broke, which makes sense, it was way too thin.

I used dimensions from Christine Baker's thread back a bit ago on a 6" core to estimate volume of material needed (5.3" wide, 24" long, and I made this shorter, 7.3" tall), for a 1" liner, it would be 1 bag of cement, for a 2" liner, it would be around 3 bags. Since it's super hard and it will be surrounded in insulation, maybe the 1" liner would be enough? Use the standard strategy for pouring a core, make an outer box and an inner form, but I'd make the feed tube and heat riser tubes tall (6" above the top of the burn tunnel). When you pour the core, you could still only put 1" of refractory cement around the inner form, let it dry, then take off the outer box, build a second outer box for the final size of the core + insulation, pack in the insulating mix (8:1 perlite/clay), set the dry core liner into the box and pack insulating perlite/clay around it all the way to to top of the outer box, covering the previously uncovered parts of the feed tube and riser tube. I lined my feed tube with split fire brick so I wouldn't have any problems with crumbling material, something like that could work, or maybe you try to paint on a thicker coat of refractory cement onto the upper parts of the feed tube and riser tube, then pack the insulating mix around it and hope that's enough?

So, the question is, would this be enough of an improvement to make it worth the extra effort? The 3000 degree rating sounds impressive, the hardness would be great, the low heat conductivity combined with surrounding it with a high insulating wrap means it should direct way more heat back into the fire than your typical fire brick build and even more than a 1:1 perlite/clay mix cast core.

You ready to go build one? You've got me curious, I'm just out of places where I need a core. And while I'm loving the experimentation and design thinking, my core works really well and I'm not sure how much it would improve what I've got. If I didn't have a heater and I was getting ready to build one, I think I'd go for the extra effort and do the refractory liner.

Have you worked with the material at all before? I was surprised how heavy and awful to work with the 1:1 perlite/clay mix was, I wonder what the refractory cement is like to work with around forms and packing in and the like ...

I haven't worked with it before. Here's where I'm coming from, though...

I like the idea of RMHs a lot. But the steel barrel is ugly to me, and I've been reading that they only last one season, due to heat degradation. A lot of people are experimenting with fire brick or refractory fire clay. The drawback of firebrick is expansion, and the careful brick-laying needed to accommodate that. Fire-clay sounds better to me.

But firebrick and fireclay both begin to break down beyond about 2200-2500 F. I'd hate to have to rebuild my core. I'm lazy like that.

I'd like to build something that will withstand the test of time. The same methods one uses in building a RHM with fireclay should work with this kind of refractory forge cement.

The outer steel barrel is not subject to degradation; I have heard of them lasting twenty years, and have not heard of them degrading and having to be replaced. It is in the combustion core exposed to flame that they break down quickly.

If you want to cover the barrel with something more attractive, there are lots of examples - there is a thread here something like "Beautiful RMHs". Anything thicker than a bare barrel will reduce the immediate heat radiated, and may reduce the heat pumping action that pushes the exhaust through the ducting. There are conflicting opinions on that last point.

Castable refractory for the combustion zone does seem to be the way to go for best efficiency, and if the kast-o-lite is as durable as claimed it sounds like a winner. I will be testing these variables myself.

Folks say that steel risers can't handle the heat, but that comes with a few technical notes. Something thin, like sheet metal duct, would die after 1-2 burns. I have 1/4" steel for my tube, it's been burned daily for two months, and aside from a little blistering on the bottom 8", it's fine. One of the key factors is that the heat drops off significantly the higher up the riser you go. With a cast core, the first 6" of the heat riser are the cast core material, which protects the steel tube riser. Another trick I'm working with now is to use a metal interior liner for the riser, but mainly as a shape holder. My system is 6", so I have a 6" steel tube with 3" of perlite/fire clay insulation packed around it, and a 12" duct holding that together. If at some point the steel tube in the center of the riser fails, the clay/perlite will have hardened and the riser will be fine. The 12" duct on the outside will be fine forever, temps inside the barrel after the heat riser don't get high enough to hurt metal (except maybe right above the riser, depending on how small the gap is between the top of the riser and the top of the barrel).

Once you have a core liner, some insulation, a riser and something to redirect the exhaust (typically a barrel), you're free to decorate/make it pretty how you please. My poured core is still in the wood box I poured it in, even after 6 hours of burning, I haven't seen a scorch mark on any of the wood. If you had a refractory cement core with much better insulation around it (my entire core mix is Matt Walker's 1:1, you could use 8:1 or something like that) I'm betting it would get warm to the touch at most. And while Glenn brought up the concern about covering the barrel in cob or the like potentially slowing down the heat pumping action, if you covered the barrel with 8:1 perlite/clay that insulated the barrel and kept the heat inside, you could decorate however you want and not worry about passing to much heat to any mass surrounding the barrel.

If a steel riser of any thickness survives long term, there's a simple explanation. Temperatures suitable to blacksmithing and smelting are not being reached.

When I did pottery, I used cones to measure temperature. Most firings were at cone 5. Some required cone 10. I didn't have to guess about the temperature achieved. I would suggest putting a cone near the top of the riser. They are available in a wide range of temperatures, from around 1000F to 2350F. Start off at about 1500. This is just above glass slump temperature.

Dale Hodgins wrote:If a steel riser of any thickness survives long term, there's a simple explanation. Temperatures suitable to blacksmithing and smelting are not being reached.

Dale, I'm wondering about the ideal temperature for RMHs. I've done a little Googling on the subject of clean burn temperatures, and I found that 2000 F is about all that's needed. I'm wondering if the designs that many people have been replicating are just getting too hot? Firebrick or Fireclay are fine at that temperature.

I was just reading more about fireclay here:
http://www.mineralszone.com/minerals/fire-clay.html

It seems that it should be able to do the trick for us. I think the main difference between firebrick and fireclay is that firebrick has been fired under factory controls and conditions whereas we have to do our own firing for fireclay.

In reading on that page about the manufacturing process of firebrick from fireclay, I'm interested about the weathering, adding of grog, and water that is done. I wonder if some of the RMH builders who have experimented with fireclay haven't been going through these processes, which is what causes their experiments to crack. Also, their burns aren't temperature controlled.

I defer to Ernie and Erica concerning ideal temperatures. I was simply stating that steel is unsuitable.  The environment inside a stove can be highly corrosive.  Even if slumping and melting temperatures are not reached, metal can still fail. Masonry materials are much more resilient.

I aspire to casting a "super-riser" some day. In the mean time, mine is composed of $32 worth of 2800* insulating fire brick. It has only been working intermittently, & for a fairly short time, but it was relatively easy to fashion and is presently holding up just fine. I'll let you know if & when it fails.