I’ve gotten the pleasure to do more than a few tours of the building projects here at the farm over the past while. And most everyone leaves with at least an inkling of wanting one of these beasts at their place. If I can offer a few tiny pieces of advise on the process. This is in no way a definitive list and I am almost as far from an expert as it is possible for one to get. Just sayin. At risk of over explaining things, I’ve decided my thoughts into sections. Skip them if you know about the subject.
Obviously you will be needing a pile of bricks for the core of your rocket. If you have read any of my postings about building them you will be familiar with what I mean. All bricks are not equal in material or form. Regardless of what they’re made of, you should have a pile of bricks that are all the same size. Generally speaking, the bricks I have used are 2.5×4.5×9″ in dimension. This is one of the most common sizes I have found when dealing with higher temperature bricks. But the actual dimensions of brick you find is fairly immaterial. Just so long as they are the consistent. This will make your design simpler and more predictable. Especially when you are working to keep your cross sections consistent and to achieve the heights you need for proper flow/draft. It will also make stacking them easier if they all lay flat and in a single plane with each layer/course. If you get more than one size, just be sure to separate them before you start laying them up.
Next, you need to conceder exactly what the brick is made of. Stone should never be used in a fire environment (inside a rocket chamber) as it simply is too unpredictable and could crack/explode from the thermal shock. Cement/cinder block is not to be used. Paving block fall into this category. The cement simply can’t take the temperatures and will break down fairly quickly as the materials tend to absorb water more redily than fired clay. Cement block tend to be grey in color.
You need to get bricks that are made of clay, or better yet refractory materials. Most old bricks are made of clay. It was cheaper and easier to just use the clay from nearby to make them. Clay brick usually has a reddish color, depending on what was found close by. Think earthenware. Most road brick and solid house brick are made of low temperature material.
Sometimes there will be a name stamped in the large face. This is the name of the brickyard they came from. They will work if that is all you can get, but you should go for solid brick rather than bricks with holes in them. The bricks with holes are what are most commonly used in newer construction. The holes are meant to save on weight and material and to facilitate their being cut down in regular intervals.
Fire/hard brick, on the other hand, are more towards the stoneware end of the spectrum. These bricks are what you will find in wood stoves and blast furnaces and boilers and pizza/bread ovens. These brick can, often, be found as “reclaimed” and they will be noted as having carbon black smudges on one face. These brick tend towards yellow/buff rather than red. They are always solid (without holes). These are what I used through most of rockets I’ve built so far. They tend to be fairly common in large late/end-stage capitalist post industrial cities. Especially in areas of “urban renewal” where those old factories are being torn down. Mind you, not the walls of the buildings, rather they are used in the internal fire environments.
Now, that said, when it comes to making that actual riser, I will now and always go for “soft” brick. This is where we enter the 20th century of refractory materials. The most obvious difference between soft and hard brick is their weight. Soft brick will be surprisingly light as they are mostly made of air. As I’m not a ceramic engineer, I can’t really explain their manufacture. They tend towards white when new. Unlike hard/fire brick, which needs a diamond wheel to cut them, soft brick can be cut with a hand saw, even a fairly dull one. These brick are easy to carve into whatever shape/size you want them to be. They can also break with not a lot of effort as well.
In my experience, the single best source of soft brick is an old busted electric kiln. These things can be found on Craigslist and other people’s basements fairly easily and cheaply. Don’t pay more than $50 for a large kiln, especially if the kiln doesn’t work.
The thing about soft brick in rocket stoves is that they don’t need any time to heat up before they begin to facilitate the “rocket” in the riser. The fire surface heats up almost instantly with no need for the heat to soak out the cold of the middle of the brick. Their amazing insulative/refractory properties are the stuff of space shuttle heat shielding. The weight savings can mean the difference of needing to do extensive shoring up of the floor in your house as well.