I’ve been experimenting with cheap ways to improve lo-cost wood-fired earthen ovens. How can I make mud denser, harder, and more durable? Without going to bricks and/or spending a lotta dough? Adding sand to mud reduces shrink and increases density. But clay and sand are generally still less dense (hold less heat) than a good, hi-fired dense firebrick. Hmmm…
1. to increase the density and toughness of a clay/sand thermal mix appropriate for building wood-fired ovens (and other wood-fired appliances?), 2. to fabricate a higher quality cast dome (“earth-oven”) style oven. Strategies: 1. adjust the mix of particle sizes to maximize density, and 2. amend/strengthen/improve the clay binder with sodium silicate (aka waterglass, a mineral solution which fuses at typical oven temps). Here’s three cups of packed sand, and a cup of packed dust. Three plus one equals 4, right? Wrong. Add the dust to the sand, mix it well, and you get three cups of mixed stuff.
Each of these resulting three cups was about 15-19% heavier (and denser) than a cup of plain sand. (The dust is soapstone fines from an Oregon outfit that quarries and cuts its own stone; it’s extraordinarily fine, but other stone or brick dust — or super fine sand — will also work. Of course, if the particle size isn’t small enough, you’ll end up with greater volume — and more unfilled space. Engineers calculate surfaces and shapes and mix their aggregates accordingly.)
OK, now I take my three cups of dense mix, and add a cup of wet clay. Three plus one equals four, right? Wrong again. Because clay particles are flat as well as fine, they will fill even smaller, tighter gaps between all those fine particles. And they’ll glue everything into a solid mass — like a dense, heavy brick. (Again, things will vary according to your clay, water content, and mixing, but in general, the increase in volume will be anywhere from negligible, to much less than the single unit of added material.)
Dust, sand, and clay would make a fine mix, but the shortcoming of a mix is just that — it’s a mix; different particles and different compounds behave differently. Over the long term, repeated heating and cooling can cause particles of a mix to separate and fall out — leaving you (or your customer) with bits of grit or sand in their pizza. Not what we want!
While it may be very noticeable in a clay-sand mix, the truth is that it can happen to the best firebrick as well, because outside surfaces get much hotter much faster than interior mass. As a result, surfaces can degrade and crumble — also called “spalling.”
To treat spalling oven surfaces, I often recommend a compound called sodium silicate, or waterglass, which binds everything together, even (especially) at high temps. Mix it with water (about half and half) and either spray or brush on. It soaks in quick and deep, and serves to “fix” a substantial layer of material.
Since sodium silicate is also used in the manufacture of hi-temp mortar, I’ve often wondered whether it would be worth adding to a regular clay-sand mix. So I made a clay:waterglass mix that was approximately 4 parts wet clay to 1 part waterglass. The clay had roughly the consistency of thick peanut butter or bean paste. After mixing, I packed the clay and dusty-sand mix into a brick form (leftovers were made into a ball).
(SIDE NOTE RE: WATER: the clay came out of the bottom of a bucket where it had been soaking for months. From previous experience, I figure it contained about a half gallon of water, so that means the liquid in the mix was roughly a 30% solution of waterglass. I was told by an industry rep that typical refractory mortar is made with a 40% solution, so I figure I’m in the ball park.)
RESULTS: both bricks appear to be roughly the same weight and density (and pretty close to firebrick densities!) But the really interesting result, so far, is shrinkage: the clay brick shrank a good quarter inch out of 7 inches total — almost 4%. In the photo, it’s the bottom brick, in the original mold it came out of. Above it is the waterglass brick, which had negligible shrink!
The surface hardness of both seemed relatively equal; both released grains of sand and grit when I abraded them (pretty hard!) with my thumb. Here, I think, compaction is very important, so all particles are in contact with each other, and you end up with a smooth, hard surface. This requires keeping the mix quite dry, and really whacking the material once the sand form is covered (I use a 2×4).
FOLLOWUP: ADDITIONAL BRICK TESTS:
Around here there are a lot of gravel roads made with local crushed rock. Most of it seems basaltic: black, dense, and heavy, usually about 3/4″ bits and smaller (called “3/4 minus”); it also has plenty of fines so it compacts into a smooth, hard surface. I’ve used it for various mixes, in particular for adding thermal mass to a big oven. After I did the tests above, I added some road base to the sand-and-soapstone-dust mix, in a 3:2 ratio. The resulting brick was even heavier than standard firebrick, and significantly harder than the mix of pure sand and soapstone dust. Given that “road rock” or crushed gravel is probably easier for most folks to come by, it might be just the thing for this purpose. Here’s some pix:
L to R: standard commercial hard firebrick, homemade brick of sand & soapstone dust, homemade brick of roadbase, sand, and dust.
The downside of waterglass is that it’s somewhat caustic, so glove your hands when you work with it.
Waterglass runs about $10-$15/gallon — a pretty cheap addition to an already cheap materials list.
Closing Story: A teacher presents his class with a large glass jar full of golf balls. “Is it full?” he asks. “Of course,” the students reply. He tips in a bag of marbles, shaking the jar as he pours so they fill the spaces between the golf balls. “OK, now it’s full, right?” “Right!” say the students. Then he pours in dry sand, again, shaking lightly to fill the (smaller) spaces. “Now it’s REALLY full, right?” “RIGHT!” yell the students. “The jar is your life,” he says. “The golf balls are the important stuff: family, kids, health, community. The pebbles are pretty important — earning money, keeping your boss happy, keeping your car maintained and your house painted. The sand is the little stuff: email, TV, the newspaper. If you fill up the jar with pebbles and sand, you won’t have room for golf balls, but if you start with the important stuff, there’s always room for the little things.” Then he grabbed his coffee pot, and poured in the whole thing, and a container of milk. The liquid didn’t even come up to the top… “That,” he said, “is just to demonstrate that even when think your life is really REALLY full, there’s always room for coffee with a friend….”