Perlite is a porous, kiln-puffed volcanic glass - think "rock popcorn". It is the little white nubbles you see in commercial potting soil. Superb insulation when dry, good for water-retention and drainage when wet. Theoretically it can melt in refractory furnaces, as it is a glass foam, but in our
experience it happens very slowly if at all: inner layers protect the next ones because it's such a good insulator. We have had good success stabilizing it with ceramic clay-slip or high clay-content local soils. The clay will fuse/harden in place at temperatures that threaten the perlite or steel, creating a stable ceramic foam.
Vermiculite used for similar purposes. It is less insulative, that's why Ernie says you need twice as much. But just from its appearance, slippery with flat scales, I wonder if it has more mica than silica, and might handle higher temperatures? This slipperiness also makes it a little harder to handle in hand-packing cob forms, but it
should bond OK as a poured clay-slip or aggregate material.
Portland Cement is a lime-based bonding agent, DOES NOT handle the very high temperatures in a flame path, so we steer people away from using it in fireboxes and heat risers.
(Refractory cements based on high-temp materials like alumina, or much cheaper fireclay/ceramic clay, are suitable for stabilized-perlite for heat risers.)
Fireclay is two separate things: powdered fireclay, which is unfired, cheap ceramic clay, nice and sticky and no added gunk like talc that makes porcelains softer for hand-work. You can use reclaimed ceramic clay (from a pottery studio's recycle bin) as a substitute in most applications. The other meaning of 'fireclay' as in 'fireclay mortar' is they use this un-fired clay as the binder, and then a pre-fired grog, or ceramic grit, as the aggregate. Fireclay mortars are relatively temperature stable, have almost identical thermal expansion properties as the brick or firebrick they're used with, and if the temperature becomes high enough, they will cast in place as a continuation of the firebrick surface. I often cite these as a reason why clay-based earthen building materials are a better option for low-budget combustion experiments than expensive, and potentially toxic, commercial cements.
Another example of ceramic-foam insulation is kiln brick, highly insulative "light bricks" that are pale and weigh mere ounces. Sold in many industrial masonry suppliers and pottery hobby supply stores. Also online; it's light-weight but brittle, so I wouldn't be afraid to order some by mail and send it back if they didn't pad it well enough.
MATT - In your chimney insulation
project for a fireplace insert, the spendy product would probably be OK. The money is for the guarantee that somebody paid the nice folks at Omni-Labs to make sure it won't catch on fire and kill you, like a DIY insulation could, and rate it for temperature, so your insurance company can relax about their investment in your house remaining intact. The insurance company is out six figures if they let some new kind of stupid through the cracks and your house burns down, so they don't care how much you have to pay (in addition to your premiums) to keep your combustion devices well and truly foolproof. It still bites them in the butt sometimes, like the outside-air thing.
That said, and if you have an uninsured home and a good sense of smell, and don't tend to run your fireplace while asleep: you could make your own. Double-check the spendy stuff to be sure it's not refractor cement that's called for - that's a whole different animal.
If it's Portland cement, and the flame path is in contact with the insulation mix, you may get some powdering off of the lime binder in Portland cement in the inner layers - but I'd guess your flu liner steps down the temperature enough that it protects the cement. Even if it doesn't, the vermiculite and thin layer of compromised cement in the hot zone will likely protect the outer layers of cement by insulating them.
For a heat riser of a
rocket mass heater, which can be directly exposed to flame path (especially if a 'form' or thin metal liner was used, and warps or burns away), temperatures are hotter. Portland cement erodes and blows away as dust.
Perlite that is mixed with a small amount of clay - just enough to stick it together like caramel popcorn, very dry - can be poured into cavities or tamped down to make an insulative, yet strong, masonry material. It does lose some of its insulation value (the clay is denser and conducts some heat through the formerly insulative gaps) but still works as well or better than other materials near the same price range. And tamped or cement-poured insulation is stable over time: it doesn't find any small crack or hole and pour out like beanbag fluff. We feel better doing a stable, durable 3" insulation wall, than making a 1" wall with the same initial insulation value, which then turns into just an air gap as the perlite settles over time. "stable" is a relative term - you can't drop kick it like
concrete, but if it crumbles it will only lose a few pebbles or a chunk too big to fit through a small hole. If the clay in it gets fired into vitreous ceramic, it is like fluffy brick.
Note that when we say 'high temperature' for rocket mass
heaters we are talking near-incandescent, 1000F to 1800F to 2800F depending on fuels and operating cycles.
Most 'high heat' situations you find products for in the hardware stores, like woodstove surfaces or chimney flues, are more likely to be in the 350F to 800F to 1800F range, and there are more options for materials at those temperatures. Always check the temperature rating if buying a commercial product, it should be about 200F higher than what you think you will need. Commercial, UL-approved combustion devices provide very clear instructions about what ratings are needed.
When using natural or re-purposed materials, it's worth digging around and find some industrial tables for properties of materials. There's a lot on the Internet if you know the engineer or science-geek terms for what you are trying to find: "thermal stability," "melting point," "non-elastic expansion" or "non-elastic deformation" etc. Then I just list the materials I want to compare. Materials may deform or crack before they melt, so stay well below the melting points.
Wood heat's theoretical maximum temperature is 3500 F, at which point even industrial refractory furnace designers are thinking about ablative and replaceable surface liners rather than permanent durable materials.
Yours,
Erica
If you have a group of about 20 who want to learn already all you need to do is find a site for the workshop if you need to get more folks you will need to advertise. when we help folks set up work shops we give them a materials list and ask for pictures of the place the stove will be. the other details that we need and what you need to know can be better done on email or over the phone.
