by matt walker » Tue Jul 02, 2013 6:36 pm
Welcome! Sorry about the registration snafu, glad we finally got you on board.
I do think that commercial refractory materials are going to give you a much stronger product than what can be achieved with my DIY mix. That said, it's hard to recommend a thickness for best performance. I'm pretty much an amateur at all this and am learning as I go, so take what I say with a grain of salt.
So, I have been playing with castable refractory for the last year or so and have built quite a few cores in varying thicknesses, and I can't say definitively that thicker is necessarily better. Even heat transfer is the key, and without expansion joints, the core will almost certainly create it's own. Read that last part as "it's gonna crack." Now, my take on it is that cracks are fine as long as the thing doesn't collapse in on itself, so I choose the easy one piece mold approach. The professional way is to create a core made up of pieces with expansion joints, but even doing that I've found that it will still crack. I feel like the heat cycles are too short and too extreme inside these things to expect anything else, but I'm sure somewhere there is a refractory expert shaking his head at me right now.
So, know that going in, plan on using it like a core. As long as it's contained from the outside with cob or something to hold it all together, you should be fine. I think cob is the perfect solution. It can be mixed with Perlite to provide an insulating layer all around the core, and it's fairly elastic so it tends to do a fine job of holding everything together during expansion and contraction cycles.
As for the water, I haven't worked with Kastolite specifically, but I will say follow the manufacturer's directions to a T. The castables are extremely sensitive to hydration variances in my experience. The difference between just right and way too wet is a tiny bit of water.
The professional refractory castings go through a pretty rigorous "dry out" procedure that is basically a slow and steady heating up to being pretty much kiln fired after a few days of consistent, controlled heat. I think they are all different, but the manufacturer should have data sheets up on preferred dry out procedure. That said, since I have no way of doing that, I usually just light em up and watch em crack. LOL. A patient man might find a way to approximate a real dry out, but I haven't found it necessary for my low expectations. Be warned, they also specifically say that flame impingement on the core during dry out will cause spalling, so my mold burn out technique is pretty much a no-no if you want to do it the right way. So yeah, I'm doing it wrong, but it works for me. YMMV.
I have added the fiberglass to most of my experiments, I feel like it just gives a little web across the inevitable cracks my slack process creates and helps keep things from collapsing. I don't know that it's necessary, and I do know that fiberglass vitrifies at way under the temps seen inside these things, so it might be doing absolutely nothing. My theory is that the insulating castable protects the glass fibers buried in it from the highest temps, so some remain to add some structural support across the cracks. I could be totally wrong, but I have the material here from old projects so I use it.
As you can tell, I'm a tinkerer, and I accept a certain amount of sloppiness if the function suits me. If I were a perfectionist and just wanted one heater, I might seriously consider purchasing an off the shelf core from Dragon Heaters. It will be very hard to improve on those since Peter has done such extensive, verified, testing. That said, it's just a heck of a lot of fun to play with this stuff for me, and I think for a lot of folks. So yeah, build it, and be prepared to enjoy it and get a bit wrapped up in the possibilities and experiments.
Finally, as for burning down your house, just be aware that the core is just that, a core only. It will not be insulated enough to set on a combustible material. It will for sure ignite anything under it if it is not protected with a significant amount of high heat insulation and air spaces. There's a lot of ways to do this, and depending on the install location some are more appropriate than others. A raised platform of Durarock or something similar, covered with a significant amount of perlite/cob or vermiculite board or something, is a good starting point. Be sure to test things out somewhere safe until you are comfortable with what's going on underneath the burn tunnel. There is a LOT of heat generated right there.