temperature limit of clay/sand mix

[iau461]

I am nearing completion of my 8" DSR.

I am doing the inner bell similar to the Mallorca build:

    - below the level of the DSR3 exit port, red brick mortared with clay/sand mix

    - above the level of the port, an outer skin of red brick turned on edge, an insulating layer of Superwool, and an inner lining of dense firebrick.

Can I use the clay/sand mix as mortar for the dense firebrick? I'm not 100% sure what was used in Mallorca (or elsewhere). Seems like it would withstand the environment, but I sure don't want a failure. I could of course use pricier refractory mortar.

[peterberg]

In Mallorca, refractory mortar was used.

[masonryrocketstove]

Clay/sand should work for all those places, but you’ll want to be sure it’s not a high-iron clay or a bentonite (like kitty litter is.)

High iron clay fires hard at very low temps and will go crumbly.

Bentonite clays like Kitty liter swell too much with water, then they shrink too much when they dry and that makes them crack out of the mortar joints. You can test your clay for this by forming a disc and then letting it dry to see how much it shrinks and cracks during drying.

If you’re buying clay, just get a fireclay from the supplier that has a decent amount of plasticity.

[iau461]

Thanks for the inputs, gents. I decided to proceed with the clay/sand mix I have. I don't think it is high iron, at least it isn't reddish in color. No idea if it is bentonite, it does crack when laid on thick e.g. 4" walls, but none of the mortar joints at the bottom of the bell, which have completely dried out, have shown any cracking. That's what gave me enough confidence to proceed.

Even if it cracks a bit, I'm guessing the fire bricks will stay in place, and the Superwool will still protect the red brick well enough. Assuming no disaster this winter, I plan to take the top off the bell come spring, to peek at all the innards. If needed I can pull out the firebrick and put it back in with refractory mortar.

[masonryrocketstove]

Sounds like your clay is good to go since the bottom mortar layers didn’t crack when they dried.

Nov 8, 2023 19:32:07 GMT -8 iau461 said:

I don't think it is high iron, at least it isn't reddish in color.

Just a general note for anybody looking to use local clay they dig themselves: High-iron clay can be reddish *or* dark brown to black. A lot of iron salts are a deep inky black, so high amounts of iron can sometimes pigment the whole clay to a dark brown or blackish color before it’s fired.

Even if it cracks a bit, I'm guessing the fire bricks will stay in place, and the Superwool will still protect the red brick well enough. Assuming no disaster this winter, I plan to take the top off the bell come spring, to peek at all the innards. If needed I can pull out the firebrick and put it back in with refractory mortar.

I think you have very little to worry about disaster-wise, particularly because it’s a double skin bell. Even if there is a tiny amount of cracking in the clay/sand joints, the double skin should be more than capable of containing the flue gasses.

Using clay instead of refractory mortar will be easier to dismantle and inspect inside at the end of the season, too.