What is fire clay and where to get it
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. 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%.
Where to find & collect fire clay in nature?
Source of fire clay from the nature- OK, maybe no one sells fireclay 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 un stabilized, they would have to be bisc fired in kiln in a slow temperature speed-increase to at least 950deg. Celsius to harden. If you have a chance to fire your fireclay 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 clays 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.
October 16th
Wicked awesome website! You answered my questions about using fireclay, the proportions, etc. We are building a barbecue area and could not find anything on the fire clay based heat resistant mortar recipes and mix ratios. Now I’m thinking we could do a pizza oven, maybe combine it with a smoker….
January 20th
I am looking for a bulk source (supplier) of clay that I can use to make Light Weight Aggreate in a rotary kiln. I live in the South East where there is a lot of clay, however I cannot find anyone who sells a powder clay in bulk quantities.
March 6th
Some Bunnings Stores carry 25Kg bags of fireclay. If your local store doesn’t stock it (usually amongst the sand and cement) they should be able to order it in. The dry fireclay is quite dangerous as it is such a fine dust, so wear a repirator while working with it.
April 16th
I am getting ready to build a firepit to be included in my paver patio. I have purchased a steel fire ring to sit on top. They will be using cement paver material to construct the outer circluar shell and I have purchased fire brick to do the inside and bottom of the firepit, I was told I had to use firebrick because the cement pavers material may explode if they get to hot. I need to know what ratio of the fire clay to use witha 60 pound bag of Quikrete motar mix. They are to start the fire pit in the am on Friday april 17th. I would appreciate a quick response
April 16th
Pat, what you can use to set your firebricks into the pavers enclosure is to use simple 50:50 ratio mix of fireclay:sand which is great refractory. Soak your firebricks in water prior to the work so they don’t soak water out of the mix, they would fast otherwise. This mix will take 2 days to set though. If you needed faster setting time use any of the heat resistant mortar recipes from this building details page (link-s from the text within the page.) In this page you have also described how to use the clay/sand bed mixture. The heat from the fire soaks through firebricks fast, so if needed, you can apply 2″ or 50mm thick insulation layer in between the firebricks and the outer decorative shell. Hard insulation Vermiculite : Portland cement – 5:1 mix will do well – add very little water when mixing. It’ll also absorb the firebricks expansion move protecting the outer deco. Is this answering what you were after?
October 8th
I’m building a oil fired furnace and have been using other ground up material, with one part cement, but I find that it is not hard, it just blasts away the sides of the furnace. As i’m in Essex there is no where to get the fire clay. To buy the clay would mean purchasing it from over 30 miles away!! Can anyone help with info, yours Mr Rumsey.
January 1st
need mixture to parge firebox against old brick’
January 4th
i have large fire brick out of a old water tube boiler they are about 50 years old and i want to cut them down to size to make a furnace to melt brass. what kind of tool should i use? you have a great web site.
Added by admin: Hi Alvin, brick sliding-drop saw or hand held grinder either with good quality diamond cutting wheel. When on a job I use mostly the smallest diameter size cutting wheel hand held grinder.
January 6th
Great site. I’ve been looking around lately for an inexpensive alternative that I think I could handle. I built a backyard tandoori oven using a terra cot pot and works pretty well. Now I made this oven propane fired (for the wood to get going and to sustain heat)
January 23rd
Yes fireclay is as easy to locate as indicated! It’s my understanding that the majority of the mud or clay in the world melts at relatively low temperatures, the type used for low fire kilns and ovens for cooking, and then only a few can withstand temperatures above 2000° F (meaning when the clay content properties aren’t changed commercially by adding or extracting a portion of the ratio from the clay body) which is of course used for productions of fire clay firebricks either dense those used for complete heat absorbing e.g. the brick tyoe we use for cooking and plus those for much higher temp ranges that are very heavy in weight or light in weight insulating fire bricks and high tech ceramics like kiln furniture – shelves – slabs – props, molding shapes, insul. blankets and so on you name it all kinds of items.
February 2nd
I found your oven building website when I was looking for something not related to the fireclay natural clay at all, rather commercial products available, but this page was on the first page of Google, by looking at it your blog must be enormously popular! Continue the good work!
February 12th
many thx for sharing with us i have duild a clay oven which am very happy with using fire clay picked from builder’s digging fountations, overall thickness on the top of the dome is 7 inches first coat was appr 4″ of clay n appr 25-30% sand mixed with straws then vermiculite render n vermiculite render again, fired for an hour i will cook well, took about 2 hour to cook a big goose the outside is bearly warm can i use this oven to fire potery? n can i use the same type of fireclay? what temperature must the oven reach for this? the heat slowly drops down as once it’s fired n sealed. that’s it or do i need constant fire for pottery clay? many thx
February 25th
I am not an expert on the topics regarding fire proof fireclay as a material or products, but after reading whole this page, my understanding on fire clay fire bricks, home made clay adobe or tiles, fireclay powdered body and clay based heat resistant refractory mortar and DIY idea has improved substantially. Wholesome job thank you.
May 5th
I am planning to make a Oven with Dry potters clay, what do you reccomend to add to Clay mix to make stronger so i can transport when moving house, is not going to be to big ie 800 long by 600 deepish?? can i mix Concrete or just reinforce with allitle Wire meshing????
Thanks..
May 17th
Anyone have gas furnace bulding instructions? (for metals)
May 19th
Hi Rado,
I’m going with a cob style oven, but i was a little confused with the comment above about lime, (old ovens last for ever because they replaced cement with lime) added to the refractory mortar. If i wanted to use lime in my clay mix for the dome inner ( 1st layer ) what would be a good mix and would it be worth it? i will use straw in the second layer and then mud plaster on the outside layer. thanks for such a great site, i have referred lots of people to view it.
Added by Rado: try to do a couple of tests with these 2 first to see how the clay type you have hardens. Add the line in one example. In the other replace 1/2 of the lime portion with casting plaster (can buy in large bags like cement, painters use it.) However add only 10-15% of such into the clay. I would work with the powdered clay body, mixing dry first then adding water and mixing (rather than mixing it into the wet clay.) This is for making adobes for the hot face. If you need the plaster not setting fast, mind you it’s only 5-7.5% of the plaster amount in the whole mix so it won’t be fast anyway, you can add just a little cheap vinegar into the water. Vinegar slows plaster hardening time perfectly and it’s always used by people making plaster molds. Then extra mass/dense layer if needed (clay, stone, sand) and after the straw etc. light in weight layer for insulation as you’ve already mentioned. And some weather finish.
May 28th
Dave, for a gas forging furnace there are some good plans on backyardmetalcasting.com . I need temps up around 3200 F (melting brass) so am using a different mix of fire clay, Portland cement, Perlite, and fine silica sand as a refractant.
Anybody know of a supplier of dry fire clay near San Francisco?
July 13th
Can everybody help me find our title project for our science investigatory-project.. this is our title,,, Mussels shells as a substitute for silica in clay tiles … help me because it is in July. Anybody something?…… july is the final!