All refractory are based on fire clay, what it contains, alumina and silica. In fact all high heat resistant firebricks are made of fire clay. Like heat resistant mortars, insulation, pottery, ceramics, ceramic blankets or ceramic tiles on space shuttle, origins of these start from the fire clay, its melting starts at 1600 Celsius °C or 2912 Fahrenheit °F point. Only special manufacturing technologies of those expensive materials change their properties and usage applications. But we are not going to space, at least not for now.
What is Fire Clay?
Fire clay in detail photo. Fire clay is a normal mud, simple as that, but a mud with higher Alumina (AL) content. Has usually whiter-lighter color. Whitish to yellowish, pinkish, light brownish. It’s also cheap as mud. Refractory or pottery suppliers sell it. Even if it comes in dry powdered form in bags, fire clay is still very heavy (physical properties calculators for various refractory materials.) You only need one bag per dome if you buy the clay in dry powdered form. Commonly Alumina content of fire clay ranges between 24% – 34% Al and Silica from 50% to 60 percent – percentage calculator
In mass sense, even when in a dry powdered form with density of 1.303 gram/cm3 or correspondingly 0.753 ounce/cu-in, fire clay is still a heavy product; when compared with cement for instance. Every time I lift the bag I notice that. With this online fire_clay volume vs. weight tool, measuring units can be easily converted/calculated.
Where to find & collect fire clay in nature?
Source of fire clay from the nature- OK, maybe no one sells fire clay in your location. No worries you wouldn’t be the only person in this situation. Close your eyes and think about this…, give it a go a several times. Think of a place outside where you saw mud of a lighter color. I mean a place with water soil erosion or excavation work going on. When wet, mud is soft and sticky without any organic matter in it. Not like a top soil, don’t confuse these two (organic material would burn off.) Mud can be found usually deeper below top soil. It’s everywhere around us but can not be seen until it’s uncovered. You have to find a spot where it is not mixed with sand or rocks, and remember the light color. You need to uncover a mud which has similar structure to the play dough (when wet.) Sounds like interesting outing doesn’t it(?), I wouldn’t mind to go out with a good friend (if I had one;) and do this now. Just dig it out.
Fire clay shrinks about 10 to 15% after water dries out so take home an extra bucket more. You can make a test if you like;
- Stretch and flatten piece of the clay into 13cm or 5 inches long strip
- With a sharp object make 2 marks in it - EXACT 10cm or 4″ between the marks
- Leave the clay to dry
- Re measure when dry to see the shrinkage difference
Some Potters still keep their own clay source spots as the world’s best kept secrets. Own sourcing was very common years ago but for sure it’s many artist’s hobby and pride not buying commercially packed modern clay bodies. It’s different however with quantity producing potters. Lot’s of enthusiasts dig for the clay. Mix fire clay with sieved fine sand (Loam is great) to make the top refractory mortar! Mix it with coarse river or creek sand and make clay adobe tiles out of the stuff. Mixing send into a wet clay by walking in it, or mixing sand with clay both in dry form and then adding water in, second example requires less effort and is much more faster. As these adobes dry, they shrink a bit and should be covered with plastic so the drying speed is slower (otherwise you create propeller instead), that prevents banding and cracking. More grog is added into the clay, for adobe tiles, less shrinkage and cracking.
Never add straw or wood saw into body of adobe or mud bricks for creating wood fired oven dome, because it burn off just as organic material does. It’s being added into heat insulation only to create honeycomb like airy lightweight effect (air is the best insulation and such material doesn’t absorb much heat!) House building bricks are different to refractory application. Clay adobe and bricks must be dense and less porous, solid and heavy. These adobes are joined by sand clay mortar – 50:50 sand:fireclay ratio. Although inner face of the dome made of adobe or mud bricks is quite fragile it cooks very nicely; if you touch it harder with e.g. a pizza paddle it drops off some clay because mud bricks and adobe are unstabilized, they would have to be clay bisque fired in kiln in a slow temperature speed-increase to at least 950 degrees Celsius to harden. If you have a chance to fire your fire clay in an electric or gas kiln do a test firing with one adobe.
When making arches of the dome by using wooden template: If you can support dome side wall bricks to prevent the dome arch pushing on them and possibly collapsing, then the best mortar you could use is 50:50 fire clay:sand mix. Fine sieved sand that is or pebbles can give hard time in places where tiny gap between bricks is desired. Cement is used only to make the mortar to dry faster, to set as you work to progress fast. Also experts in manufacturing and selling firebricks tell you that, no need for refractory cements everywhere, these are expensive and used by industries for theirs hi temps kilns and furnaces or for castables.
Fire clay sand mortar mix dries very slowly, but if you support the side walls by pouring concrete cladding behind them, leave it cure and then form arched ceiling using the 50:50 fireclay : sand mortar is the best option. When I go fast I use a little of Portland cement in the mortar so it’s setting in 2 hours. Portland cement is already a bit refractory but it’s wise to replace one half of the amount with lime. Lime is calcium and it takes over the cement when it burns out in heat. Old ovens were built using only fire clay with lime not cement. These ovens work forever, many of them are being fired daily for decades and never cool down.
Where else can I find Fire Clay?
Soft pottery clay body – usually 10 – 15Kg per packaging. Ask for clay with more alumina content, stoneware clay, and it can be coarse not too fine. Normally it is whitish color. You don’t need to go for expensive clay, just pick one type for higher temperature and light in color. REMEMBER: don’t be confused, when you see the soft-wet clay in bags it can be darker but when it dries it’ll change to lighter color. Then there are also clay’s which are white when wet. Usually the shop has dry samples. Don’t forget to ask for clay which they stored for a long time, it’ll be harder in plastic bags, not too fresh in pottery terms, potters don’t want dry clay to work with when throwing on potters wheels so you can get a better price to pay for it too. Often they run discounted sales of these nearly dry clays to get rid of it. So you brought your clay home now … but what to do next you may ask? Take the clay out of plastics bags. Use wire or stronger fishing line to cut it in thin slices. Leave it to dry.
After about a week in an airy place the clay will be dry. Sit on some low comfortable soft seat on the driveway and use a hammer to brake it into almost powder. It’s not hard at all but it takes some time. You will need about a bucket to mix the mortar, and half a bucket for the sand : fire clay (50:50) bed to lay hearth floor bricks onto and level them up nicely into one smooth surface.