Great! That does make those refractory brick's price more logical, altough I'm not sure I have found proper bricks for such use where I am. My biggest worries is, aren't those refractory bricks quite fragile? I think I only had light and thin refractory bricks that are quite easy to break... in fact, they broke just by the hight temp in my wood stove (could also be the seller who just swapped for not so much suitable refractory bricks, I don't know).
Those firebricks that you find everywhere for steel box woodstoves are called “splits” because they’re half the thickness of a standard 9”x4.5”x2.5” firebrick.. like if you took a standard brick and split it in half lengthwise.. so splits are 9”x4.5”x1.25”. At least in the US. European bricks are different dimensions depending on the region.
You’re right, too. They’re fragile and crack easily because they’re usually only rated for 2000-2200°F and they use cheap ingredients that can’t be fired higher than that without breaking down. One of the main ingredients in many is the contaminating slag waste from coal burning power plants. Literally the trash left behind after the refractory in the industrial furnace wears out. Companies make them cheap and weak like this because it saves them money and it’s planned obsolescence. It makes customers keep returning every couple years to buy new ones. They’re also a little insulating, so they’re porous.
Porous bricks insulate, but they absorb ash faster and crack apart sooner. Especially if you plan on dismantling and rebuilding the core as you experiment. For playing with rebuilds, I’d recommend buying some of the dense “superduty” brick that are rated to around 3100-3200°F. Typically 8+ pounds each and super tough to take a lot of abuse. You could make your hot face out of those and then insulate around the outside of them. The superduty brick usually say the alumina to silica ratio on the package with the temp rating. The cheapo box stove bricks usually don’t tell you what the composition is.
Certainly not magical, but there’s a lot of complex chemical reactions and mineralogical phase changes that take place when you burn wood in a refractory enclosure at high enough temp to avoid smoke and pollution and capture the heat efficiently. Rocketstoves can create temperatures as high as volcano lava.. like hot enough to melt most natural rock. And refractories are made from the same mineral oxide stuff that rocks are.. only purified to make them resistant to melt, thermal shock, and chemical attack.
Ceramics and refractories are still an emerging field of science. There’s a whole lot of complicated chemical interplay happening inside at very high temp.
Most refractory bricks are made from some combo of alumina, silica, and magnesia. With just those 3 chemical oxides, there’s dozens of different mineral crystal phases that will form, depending on the temperature reached and the % of each of the 3 ingredients. Most of those several dozen different phases aren’t useful for making a good firebrick. Only about 3-4 of them are. So the ingredient ratios and the firing temp are really important.
When you add even more ingredients like calcium in Portland cement, or potassium or sodium in waterglass, plus the 8-12 different trace contaminants in most fireclay (stuff like iron and titanium at 1-3%) ..and also add more silica sand.. then your mineral phase reactions with each other and with the growing contamination of vaporized wood ash.. that causes lower melting temps and cracking problems a lot sooner. Usually not explosions, but it’s not good to have a heater that suddenly crumbles mid-burn in the middle of a heating season. Not just so we can keep using it to heat the space, but because we need to keep the fire and the flue gasses contained for safety.
I think you’ll find that a heater outside doesn’t give you a true picture of the way a thermal mass heater will heat in a building. It’s mostly radiant heat from the surface toward other interior surfaces like the building walls and people in it. It doesn’t put out the blasting kind of convective heat with hot air currents like steel box woodstoves or hot air furnaces do. A radiant heater surface placed outdoors just dissipates and wastes the steady, gentle radiant heat.
If you want to try your hand at making firebrick, I’d recommend reading that thread linked earlier and browsing the one called “why refractories fail: silica plus flux, not heat” There’s a ton of info there about the chemistry, and stuff like a free e-book link that explains the basics of refractory brick making for DIY kilns in undeveloped countries. I think it’s called The Self Reliant Potter.
Firing bricks makes them densify and shrink, and that’s one reason people say to add sand to counteract the shrinkage. But silica sand is acidic and reactive with wood ash alkalis. If you plan on buying fireclay, I’d also pick up some bulk coarse ground kyanite to use instead of the sand. You’ll get much better results without having to fire the bricks as high or having them shrink.
You generally shouldn’t need a binder like waterglass or cement to hold a green brick together.. The fireclay is usually enough to do that and dries hard and solid so you can then fire the brick. You do need to have them bone dry before firing, because THAT can cause explosions from the steam expanding inside the green brick.
Binders are usually only needed in castable refractory mixes where you need them to set up in place.. but castable isn’t a great option for being able to take it all apart later and play with rebuilding new configurations.
Sorry for the text wall of info. I’d just like to see you have success with your experimentation and heater builds by avoiding some common pitfalls in trying to make your own materials.
