Glenn Herbert

2" is a good minimum top gap between riser and barrel; for a 6.5" system, 1 1/2" may be enough, but 2" is safe.

Side gap would preferably be at least 1 1/2". If the riser is not centered in the barrel, the side with more space is likely to get hotter due to more airflow.

Did you build the riser with firebrick laid flat or on edge? Full or split (half thickness) brick? The less mass in the riser, the quicker it will come up to full operating temperature. Around a firebrick core, I think 1 1/2" of perlite-clay should be okay. More is better if you have the space for it.

A few years ago now, I saw a craigslist ad for free bricks. A mason was working on a renovation of a century+ building that involved cutting new windows into the old brick bearing walls, and couldn't bear the waste of good brick. He said there were dumpsters full of brick that went to the landfill before he placed the ad. He would have taken them except that he couldn't store them. I picked through the dumpster at night (in a back alley behind the building), and filled my van with a full load of a couple hundred bricks several times, and ended up with probably enough bricks for the non-core parts of two RMHs each for me and my best friend.

Another craigslist ad netted two overfull van loads of firebrick, unused but slightly dirty and mossy, for about $1.25 each. You can see some of the spoils in my RMH build thread.

You would have to measure the specific blocks available to you, to see if they have at least the same area as your riser. If so, they may work fine as the channel for your mass. As mentioned, the heat levels in the mass are not likely to be high enough to damage the blocks. Be aware that rough inner surfaces, or the steps you get from the slight difference in dimensions from one side of a block to the other, will increase turbulence and friction.

I might use both cores of a block as parallel flue paths, depending on how much space you have. As long as the paths end up being essentially identical, you would probably get reasonably equal flow through them; minor differences would not matter that much in this application, especially if you let the channels recombine at each turn.

Cast iron is actually not pure iron; wrought iron is nearly pure and quite soft, while cast iron has about 2% to 4% carbon and is very rigid and more or less brittle. Carbon steel has more than 0.25% and less than 2% carbon and can be hardened. Mild steel has less than 0.25% carbon and cannot be hardened.

Reading up a bit, cast iron can have as many trace alloying elements as any other variety, to improve its qualities in various ways.

There is such a thing as too much airflow; more than is needed for full combustion, counting excess to compensate for inevitable less than perfect mixing, will only cool the fire. A rule of thumb is that you need around three times theoretical stoichiometric volume for best results. Batch box designs have been calibrated to use about 15-20% of system size for primary plus secondary air. I find my J-tube burns best with the top about 2/3 to 3/4 covered. This creates a jet of air blowing on the fire as well as limiting the volume to what is needed.

A rocket stove can be called so because it has an L or J shaped combustion core, with a vertical riser to increase draft and allow combustion to complete. Small or metal ones may not have as good efficiency as bigger ones, but if the core design gives proper turbulence and dwell time, combustion will still be good. In any case, I don't think that a well-configured rocket stove would actually have more combustion in a surrounding barrel where the temperature is much lower.

When you get to the little tin can sized units, they can't have enough heat concentration or dwell time to achieve full combustion. It used to be that DIY designs for small units still emphasized insulation around the core; if ones are being sold with no insulation, I would hesitate to call them rocket stoves with or without a barrel.

There is one professional emissions testing unit I know of, the Testo 320 I believe, that Peter van den Berg uses for his stove development. It can give levels of oxygen, carbon dioxide, carbon monoxide, efficiency, temperature (usually of the exhaust to test how much heat is being lost in spent gases), and a filter that gives a relative gauge of particulate emission by how clean or dirty it looks at the end of a run. Unfortunately it costs a couple-few thousand dollars.

A core designed according to published proportions and sizes should have no volatiles left after leaving the riser. The only real way to test this, aside from gross differences in output, would be with a Testo. You might be able to get a decent relative approximation by actually sniffing the (cooled) exhaust; this would only be practical among relatively clean designs. I once made a firebrick mockup of a 4" batch box core, and could stand a few feet above the riser with my face fully in the exhaust and comfortably breathe. The flames in that little free-flowing core hardly reached more than halfway up the riser, but obviously burned clean.

Willow is certainly a heavy feeder, but I would advise black willow (native to the northeast at least, not sure of its whole range) rather than weeping willow which is a Chinese import. Black willow is not graceful in the same way, but can be majestic. It does grow very large, and you might want a shrub-type like pussy willow for restricted areas.

I have a couple of red pine plantations, one that my father planted before I was born and another I helped plant at around 6-7 years old. Although they grew well, with clearing low limbs to make good timber, we were told 15 years ago when the plantations were 40-45 years old that red pine was susceptible to borers and no longer used for log building, and the only value of the trees was for pulp. The price we would get for that was not worth the scarring of the land that commercial logging would cause.

"Too much mass" depends on your climate. If you are in a place where it gets cold and stays cold, a big mass will even out the heating load. If you can see large swings in temperature from day to day in the heating season, a smaller mass will be more responsive to burning or not, and reduce the chance of having too much heat for a warm day, or taking too long to build up heat for a cold day.

Brick generally has faster heat transmission than cob, and I would expect 8" total of brick to be reasonably responsive. Much more than that would probably only be suitable for deep freeze conditions. For variable weather areas, I might use bricks laid on edge to give a thinner skin while keeping the safety of a double shell.

Yep, insulation, no matter how good, does not prevent heat transmission, just slows it down. If you do not have an air gap to let heat dissipate before it gets to combustible material, the combustible material will get hot over time. Any combustion core on a solid base over wood will eventually cause a fire and is a fatal mistake. I hope the OP will consider rebuilding with safe clearances and air gaps and not let this experience frighten him away from an effective, efficient heat source.