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What is fire clay and where to get it in nature

Fire Clay packed in bag 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 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?

fire clay location 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;

  1. Stretch and flatten piece of the clay into 13cm or 5 inches long strip
  2. With a sharp object make 2 marks in it – EXACT 10cm or 4″ between the marks
  3. Leave the clay to dry
  4. 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.

Respond to the What is fire clay and where to get it in nature article:


  1. […] bricks just because they are made of simple fireclay (which is actually the most ordinary mud.) Fire clay can be easily located out in the nature but it must containing the right refractory properties, […]

    Pingback Firebricks are actually fire clay bricks. Thermal conductivity. Cutting with diamond wheel.

  2. 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….

    By Edwina

  3. 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.

    By Ronald Mehl

  4. 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.

    By Stephen

  5. 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

    By Pat Schirtzinger

  6. 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?

    By Rado

  7. 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.

    By Keith rumsey

  8. need mixture to parge firebox against old brick’

    By mike

  9. 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.

    By alvin cronk

  10. 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)

    By Burr grinders

  11. 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.

    By Jim

  12. 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!

    By Guadalupe Luce

  13. 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

    By takis

  14. 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.

    By carl

  15. 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????

    By Kieran

  16. Anyone have gas furnace bulding instructions? (for metals)

    By Mark Swanson

  17. 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.

    By DAVE

  18. Dave, for a gas forging furnace there are some good plans on . 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?

    By mike

  19. […] spend 4-5 hours in the evening treading and mixing the sand with clay mud collected in my local nature. As with collecting and using a fire clay from the ground outdoors to build cob ovens. So far we have cooked pizza’s, bread and roast potatoes and Mediterranean vegetables. It’s […]

    Pingback Cob earth clay wood fired oven built by the sea

  20. 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!

    By sittie

  21. I searched for how to make a homemade fire brick and this article provides absolutely excellent information! Firebricks are made of fire-clay which makes a lots of sense. I want them to be heavy and dense so they absorb and retain as much heat as possible. Lightweight material for insulation goes then on the top at the end.

    By How do i make homemade fire brick?

  22. I think that I have found fire clay. I’ve put a brick into form and I’m letting it dry in the house. I’m running a sand to clay test on the material now. I am in hopes that I don’t have to mix anthing other than water. This stuff is quite tacky. I am trying to build a version of a masonry heater to heat my house. I was in hopes of doing it in forms rather than brick in a pouring method. But, it looks like I will be working it into mold forms in this tacky form. I was wondering if adding plaster of paris to the mixture to make it set faster and limit shrinkage is a good idea for a masonry heater? Have you ever tried pouring a clay sand mixture into forms like you would concrete?

    Tim, Poolville, TX

    * Added by Rado: Hi Timothy,
    Yes we tried to mix into powdered clay plaster, lime, also liquid glass (within the water for mixing) and the resistance properties improved. That was the reason for adding these additions. Don’t stress about the drying water out goes in a slow speed, it is better if it’s in slow drying process!! Even cover the exposed drying clay with a plastic cover to slow down the drying time further! Why? Because the clay will deform a lot less, less bending and shrinkage cracks develop especially with bricks, or a larger bulky mass of made clay shapes. If you are concern specifically about how many you can make if the clay dries slow, to remove the clay from a form simply make the form out of plaster (nice and heavy and add metal rods in to make it physically stronger), then push the clay in and when full, turn the form upside-down and press on the ground onto a clean metal sheet. Turning movements while pressing the clay down onto the surface so it sticks on well. This is very easy no worries. Stand on the edges of the metal sheet and pull the form upwards. The clay gets released easily from the day plaster mold. You can make several per hour this way. When you plaster mold needs drying again you will see it will be harder to remove the clay from. Simply dry it on a sun or in a wind. Then you can continue. Do you have a photos of the mold and clay how you go? Email us a few pictures, large photos for detail.

    Don’t make your clay bricks / adobe too large in volume or dimensions. How is your house heating system that you work on going to operate? Describe it, interesting. We must help to ourselves, people in general ourselves, to keep energy bills small and equaly to save our environment is very important! If you have a drawings, images of what you are making, email them to me and I will comment on what I see. Water circulation based heating system or only a hot air usage? Also water heating exchanger in it or more?

    By Tim on masonry fire-clay heater to heat my house

  23. Mike, Check out Laguna Clay’s web site, laguna clay. They have two distributors in Richmond; in San Francisco: Ceramics & Crafts Supply Co. (415-982-9231).

    By mac

  24. The best knowledge page for fireclay matters, details, working with fireclay, applications, all in one information. Thank you.

    By fireclay info

  25. Awesome site. I know its a little off-topic, but…
    I came here looking for a way to upgrade a ‘coffee-can’ type foundry, thinking I might use heatproof cement or something. Thanks for posting all this, using what I learned I’ve developed a thermal concrete recipe using clays mixed up like plaster to bond Perlite granules, making a pourable mix to cast with. I’m experimenting with fireclay and powdered Perlite to make crucibles as well; without much of the information about shrinking and drying times it would have been a nightmare. I’m hoping I can get the new ones up to melt copper, aluminium was no trouble.

    And I also rediscovered a childhood love for mud. Fantastic! ;)

    added by Rado: Hi Jez,
    Thanks for the comment, it’s suitable for this topic!

    Have you tried adding plaster and Sodium silicate to test your mixes – Na2SiO3 (also called water glass or liquid glass). It’s not toxic, it is being used in refractories and for fire protection and also in food industries. You can mix it in a mix, also paint it on with brush or dip your fired items in it. If they are still porous clay, like firebricks and other low fired clay bricks (for example chimney and fire place bricks) for instance, the liquid gets absorbed into the body and protects then the hot face. Sodium silicate can be purchased in liquid form or in white powder form for mixing with the clay or cement, Na2SiO3 dissolves in water, stays clear like a clear lacquer. Sodium silicate will nicely bind solids such as powdered perlite you use and withstands the high heat. Email me some pictures of what you do! send to: pizzapaddle at gmail dot com

    By Jez

  26. Can any one help me. I have started a cob oven and got all the supplies I thought I needed. After making the foundation, I have noticed what I thought was fire clay is mortar clay which is a mixture of clay / sand / concrete, a product from a company called Muddock. Can anyone tell me if this product is OK and if not a brand name of a clay product available. The concerns are the concrete and the large ratio of sand to clay (6:2). Unfortunately there is little clay available naturally. Thanks much for any help!

    added by admin: When you say sand with clay and concrete, the concrete; what do they mean, is a cement added into the mix or actual real crushed concrete like a sharp coarse or finer grog? What is the intended use for this clay based product, if the clay is mixed in, is there anything mentioned on the bag? What type of cement it contains? If that is a cement. Maybe a heat resistant cement? Otherwise it does seem there is only small amount of clay to reheat it for cooking many times.

    I haven’t seen any brand for fireclay as such, it’s always called simply “fireclay” or “fire clay“. Pottery shops will have it as well, or they can show you a cheapest version of clay, clay body with coarse sand in it usually used for low fired pots, modeling or sculpturing. Ask them also for an old clay like that, when it’s packed in plastic for a long time it gets much drier so potters don’t want it anymore, usually these shops put it on sale to, literally, get rid of it.

    By Cob Oven Clay

  27. Thank you for your reply. The cement is added to the mix in the form of a fine powder. The bag does not comment on the type of cement but does say the function is a brick layer’s mortar. After looking at the bag closer there is actually 1 part clay, 6 parts sand and 2 parts cement. Thanks for the suggestion of pottery clay. I have been looking at some redart clays, I have also heard to stay away from added bentonite as it tends to crack? Appreciate your time.

    added by admin: Ah, thanks for the information. In this case the clay is added to the cement with sand for the mix holds in more water. If the work is slow etc., bricklayers add lime for the same purpose. The mortar sets in a lot slower time rate. I reckon about 3 times more slowly, lime can be quickly remixed once too in a wheelbarrow if needed. It will be the same with the clay. The Bentonite will stand the heat or a much higher heat but you don’t need to add that. In work with clays it’s used more for glazing, or for example also in mold making, molds from packed sand for casting metals where it helps the mold to face the heat, protecting the hot face. I think the clay adobes would become unnecessarily brittle, less porous and don’t know about its properties under continuous refiring and cooling down where you can get the cracks. Maybe a separated fire box made of clay would find a use for Bentonite, but that’s completely different oven firing concept not too suitable for small family sized cooking ovens whose are build out of clay or put together from clay adobe tiles. Try to lean towards putting your dome together from many adobe tiles in opposed to a large block segment or segments. It’s described more in this fireclay text above on the page.

    By Cob Oven Clay

  28. I’ve poured an outer ring of concrete for a fire pit & want a 4′ dia. inner area for the fire. I want to make this circle a solid pour 4″ thick & strong enough to take some abuse. what type mix should I use? I’m not planning on using any fire brick.

    By ricko

  29. This site is amazing. I want to make a wood firing oven; I found lots of information in other sites, BUT NONE SO MUCH EXPLICIT AND EASY TO UNDERSTAND about natural clay and how to work with the clay. Thank you very much for the time you spent telling us all what we need to know.
    Keep up the awesome work.

    By rosa

  30. I am building a small backyard wood fired kiln for low fire pottery/Raku. I live in Oregon, we have LOTS of native clay here. I will probably purchase commercial firebrick for the interior. Can I cover the exterior with a Cob style mud, or should there be no organics in the exterior as well? Your site and comments have been very helpful. Thank you!

    added by Rado: Sepha, if you want to create the kiln out of clay and use your kiln for many years, it is a wise idea to build the inner walls of this kiln, the hot face lining, out of firebricks. Because these bricks last. Otherwise if it was just a test kiln you could also make it out of an old metal barrel for instance. You can use standard firebricks for raku and you don’t necessary need a large volume space inside. Raku pottery is fired in a much lower temperature than other pots,(some blue” Egyptian glazes require low temp. as well), you can achieve absolutely amazing glazes on pots in raku kilns. Look on web for Japanese raku pots images.

    Are you planing to do the kiln outside with some arty like clay finish? Or mosaic? If that was the case, then leave a gap between the inner firebricks and the outer decorative skin. The inner part will be expanding as it heats up. If there wasn’t some gap space in between these two, the inside will push onto the outer part and, the movement would crack it. You can gradually fill the gap with the ash from firing. It will work as a cushion and if the gap was 2″ – 5cm plus wide then also as a heat insulation preventing the outside clay from getting too hot. I cannot wait to see images of some of your pots.

    By sepha

  31. […] What are the pretty rocks that can be lit on fire and go into firepits called? Also, where can they be bought? Perfect knowledge for where to collect fireclay in nature and how or for buying a different types of fire clays. […]

    Pingback Portable Firepits & Portable Fireplaces

  32. […] Fire Clay, the full article about fireclay, and what is fireclay plus how to use it and how to work with fire clays and where in what projects, is at Traditional Ovens Web Site […]

    Pingback Geopolymer House Blog

  33. I want to build very big kiln with a width: 3m, length: 5m and height: 7m for a charcoal plant.

    But I do not have sufficient funds for this work, and I should use very cheap materials to build my vertical charcoal kiln.

    And I realized, that I can make main body shell of the furnace with 20 cm thickens from concrete.
    But I do not know what materials I should use for the internal body of the furnace, which is stable against heat 1200 ° C.
    Please suggest something that may be resistant to 1200 degrees Celsius temperature, doesn’t crack, don’t separate from the body of the furnace casing, won’t burn and oxidize and Most importantly, it will be very cheap to build.
    Thank you very much in advance for your information.

    By M.Khosravi

  34. Good information thank you. Only one question, what is Vermiculite and where can I get it from?

    added by admin: Hi Debbie, please fine all needed information about Vermiculite insulation information (in a way it can be replaced with ash for instance).

    By Debbie

  35. The firebricks I laid for the hearth have some gaps. I was wondering if I could simply fill the gaps (no more than 5mm or 6mm) with the 50/50 fire clay/sand mixture. Thanks for your thoughts.

    added by Rado: Hi Mick, email me a photo of the firebrick floor I will have a look at it. If you progressed in work further, I suggest to leave the floor as is because ash will pack in and fills the gaps between bricks.

    By Mick

  36. MUD Cob Clay High power Kiln
    In the recorded building progress below, Jon made a cob clay mud oven and 2 cooker kitchen top which is fired with cob/clay/mud rocket stove design below it. He successfully used the cob oven to reach 1200 degrees Fahrenheit. He as able to use it as a kiln to fire bricks and ceramics and oven to cook pizza, roasts – turkey, breads etc. I think their experiment proves that inexpensive materials – 50% organic matter and 50% clay – seeds, sugar, potash, water, clay, (“no sand necessary as high silica causes cracks, high lithium (spodumene) clays are more durable” (Crispin Pemberton) – cob mixture is sufficient to build durable kiln. So if finding fireclay is difficult I think this recipe would work as well.

    By MUD Cob Clay High power Kiln

  37. Hi can we make furnace by using fireclay (Al:18% & silica: 50-60%) and Masonry only?

    By Zelalem

  38. Thanks for the info on mixing fireclay with other materials. In your heat resistant refractory mortars page.
    I’m planing on building a rocket stove and was unsure of the cement ratio to fire clay,lime, perlite, portland cement and sand. I appreciate your work.

    Robert from California

    added by Rado:
    Hi Robert,
    Thanks for the note. I am not sure exactly what the Rocket Stove is like, but something is telling me that you plan to cast perhaps section-s from that heat resistant mix. It’s mortar for firebricks, where the hot face is the clay firebrick. For casting a hot face try to replace the Portland cement part with plaster – plaster or paris. Add less water, and pack it near semi-dry into a form or hand build. There is a way to slow down plaster hardening – setting, this can be achieved by adding small amount of vinegar in the water you use for mixing the mortar or simply just plaster. You can experiment with this, but plaster hold well even when red hot. Another thing is water glass, if you add it in your mix it will be very strong in these heat conditions.

    Water Glass = Liquid Glass = Na2SiO3 = Sodium Silicate

    They are few common names for the compound Sodium Metasilicate. Just different names for the same thing.

    If you look for other loose particles that won’t burn, to experiment with to make a heat resistant castable (for small segments), dip wood ash in water, then mix it, then remove all waste which floats on top. The sediment is actually quite refractory. You can sieve it with a fine sieve too. Mix the clean fine ash with water glass and then the mixture can be painted on or over inner hot face walls of stoves, inside of fireplaces, brick ovens, et cetera to protect the bricks or to fix some bricks if they show signs of pealing down. How is this for help?

    How does the rocket stove look? I want it too! Write me about it, how it looks and what it does, why you want it!?

    By Robert

  39. I’m in Suriname and i want to build a brick oven to make charcoal. Can you tell me which materials and in what ratio I need to use them? Thank you for these loads of good information for builders owners!

    By Harvey Bijnaar

  40. I bought too much fire clay (two bags of 55 lb each) and I wonder what I will do with the extra besides making fire resistant mortar to make the oven more efficient. The composition of the fire clay I bought is below.

    Chemical Analysis: Approximation and calcined basis –
    Natural Alumina ( Al2O3 ) 35.5%
    Natural Silica ( SiO2 ) 58.7%
    Natural Iron Oxide ( Fe2O3 ) 2.0%
    Natural Titania ( TiO2 ) 1.9%
    Natural Magnesia ( MgO ) 0.4%
    Natural Lime ( CaO ) 0.3%
    Natural Alkalis ( Na2O and K2O ) 1.2%

    Thanks for your plans,

    By fireclay properties

  41. When we built a pizza oven we had trouble finding clay, didn’t know u could buy clay powder but in the end found that Foulton Hogan had stock piles of clay at a reasonable price here in Christchurch, New Zealand (had heaps left over). Now just halfway through building our second pizza oven and like the idea of clay powder but haven’t found any yet.

    added by Rado:
    You can make your own powdered fire clay. Simply cut wet clay you mentioned into little chunks, smaller pieces the faster they dry of water, leave them fully dry and then crush them. It was dome manually x-many times.

    By Pizza Oven

  42. I really battled to find fire clay, or as you call it Vermiculite, when I built my pizza oven. If I used regular cement/sand mix and fire bricks, would that be okay? What the worst that could happen?

    By Build A Pizza Oven

  43. I found fireclay available at Marjon Ceramics in Phoenix, AZ – 35th Ave & Elliot (n. of Thomas). They have 100 lb Missouri (Hawethorn) (30% alumina) fire clay for $34. They also have 50 lb bags of Lincoln fire clay (29% alumina) for $9. I grabbed 3 bags recently and am looking forward to making my firebrick oven, as well I’m looking at making a Forge and a Kiln …. whoa! Hope this helps anyone in the Phoenix Valley in Arizona :)

    By Joel

  44. I am building another oven but this time I want to build a cob style oven. But I have a question regarding powdered clay. I am building a 38″ dia oven, and am trying to figure out how much powdered fire clay I need, and would play sand be better than regular sand?
    Also I have heard that its a 5:1 ratio of sand to clay?
    Another question is could I make a solid floor by mixing up the sand a clay mixture? Instead of using fire bricks.
    Thank you

    By Pfranklin

  45. […] wood fire heat.) But, at the same time, it does not shrink down in the heat as clay would. E.g. fireclay or pottery clays have a lots higher shrinkage ratio. Metal casting molds can be carved from this […]

    Pingback Soapstone properties converter | Volume | Weight | Technical Data

  46. What is the common substance that will be added to mold clay to make it very strong. How will you coat a clay mould and what will you use.

    By Godwin Iwara Ekuta

  47. I assume you would like to achieve a strong mould into which a soft clay is being pressed, or poured if casting with a clay slip.

    For making a one piece mold: First create a model out of soft clay. Then make a frame around it, from pieces of wood or built up clay (this frame should be ~ two times higher than the original model.) Then pour plaster into the frame. Leave the plaster set well before lifting it and removing the soft clay. Then leave the plaster to dry completely.

    After that you can press soft clay in. Place the clay side onto a smooth surface (metal, plastic), press the mold down onto the surface so the clay sticks to it. Wait a bit to pull it up again, so the clay gets released from the mold. Then with a wire cut the soft clay from the surface. Let it gradually dry from water. It depends what type of subject it is, sometimes it’s beneficial to cover the molded clay with plastic bags to slow down the drying process (it can start to twist doe to various shapes.)

    The mold strength depends on several factors;
    1. how much mass is in the plaster mold walls (how thick are the mold walls.)
    2. the mold can be reinforced with wire or some metal rods.
    3. If it’s some rough molding, like adobe tiles for instance, a little bit of sand can be added into the plaster. But not too much.

    * Do not mix the plaster in water by turning the runny matter with a stick, That would create air bubbles in the soft/fresh plaster. Instead, have water in a bucket and then gradually/slowly pour the plaster into the water until the water basically disappears, gets covered with the plaster. Then without mixing use it for making the mold. You can slow down the plaster setting time by adding a little amount of vinegar into the water, before plaster goes in.

    By Rado

  48. Hi Rado: Finally spring is here and hope everything is A ok with you. Its time to restart with my oven and very anxious. I have gone to Alphatherm dealer to by the clay for the leveling of the oven floor. They sold me fireclay in a large plastic pail. Now the question to you. Do I use it alone as is or do I need to mix the 50% sand? I mean half clay with half sand. Thank you and have a good day since here its 12 noon Sunday.

    By Bart

  49. Hi Bart,
    I find work with the clay mixed with sand much easier. The clay has also lower shrinkage range with sand present in the clay body. You will not need too much of sand. Needs to be sieved to fine, to remove all pebbles first. First mix all dry, then add water gradually in.

    Some coarse clay types already contain sand in the clay body, you would notice the sand grains volume in the clay when it’s wet.

    Are you putting any sections from the new MTo design into your build? In it, for example the 4″ thick insulation under the hearth slab is performing really well, it’s very effective design in all aspects.

    By Rado

  50. I have a clay looking container with lid that sits in a metal frame. How can I identify it?


    Added in by Admin: Bill, can you email us photos of this item which would show it in details?

    By Bill Black

  51. We are wanting to build both a foundry and an oven. We would like to make our own firebrick and have an idea we would like to run by you.

    We live in an area where a steel manufacturing company has their slag crushed into various product sizes for driveways (ours is slag, nice for that), drainage, etc..

    We think it may be a good idea to screen for the fines out of the 3/4″ minus crushed slag to add to the mix rather than Perlite, etc.., for both firebrick construction and the refractory mixes that will be used in conjunction with overall construction.

    We are also thinking of using the larger slag pieces in the bottom of the forge for the diffusion layer/air-fuel chamber.

    Interested in your, plus others, comments or ideas on this project. Please advice.


    By Shaudi

  52. Great fire clay info, I’ve learned alot but am new and a novice and still confused. So my question is, what’s the difference between fire clay and regular clay? Forgive my not knowing but I can’t find the answer anywhere. Can I use clay that I can harvest from a stream bank in my back yard to create a refractory mortar to handle high temperatures?

    By Jal okech Ogwaro

  53. Jal,
    Hi high temperature/s are you after? If it’s just a cooking mark or low fire pottery etc., then if it is clay, not soil, then most probably the clay can be harvested easily. You should make a test first by firing a small samples.

    There is not a great difference between most clay types. Most will be low to mid duty refractoriness/heat. Content 23% Alumina and 73% Silica for medium duty or for low duty refractory 22% to 27% Alumina and 67% to 71% silica amount. Intermediate duty fireclay refractory 24% Alumina to 70% Silica for instance. A good source found for high duty fireclay, 33% to 40% Alumina and 55% to 63% Silica content, is not difficult to locate also, obviously in suitable locations. Each of these could be used for cooking temperatures. Potters clay is fine too for certain things, potters normally dig their clay, some still do have and make their own clay.

    *Just make some tests, so it’s for sure that it is clay (not soil or something else that burns away.)

    By Rado

  54. Some raw materials for making these bricks can be easily bought. Nature is also providing the quality and also low cost.

    By Thaara

  55. I’m building a cob dome oven and using fire clay and silica sand from a local fire brick distributor. What do you recommend for clay/sand rate ratio?

    By Joel

  56. How fine is the silica sand? The more coarse the better. It will be at 20% added, evenly distributed, sand into fire clay. It depend what mesh (how coarse) the original clay is. Or 4:1 ratio. Try to make 2 small samples, 1 time 5:1 and 1 time 4:1 small adobe pieces. Dry them of water for a week, then fire both samples in red hot embers or a kiln.

    Cool these down slowly – not to be removed from under the ash blanket, or from a still closed kiln, until below 150 degrees Celsius or 302 degrees Fahrenheit or around that level (so the surfaces of both blocks do not create material’s heat differences cracks just when the silica shrinks-cools down – “heat differences in material” = hot/expanded inside the body center while cooler/shrunken just on the surface.

    Can you take photos of what you do and of your test results results. We would add that to this page, to help others, if you are interested with your personal analyses description would be present too of course. Thanks.

    By Rado

  57. What about the “earth” that is very red. I was always under the impression that it was the clay. Would that be effective mixed with Portland cement and sand?

    By Carol

  58. Oklahoma has a lot of red clay and I was wondering if this is good to use for anything even like a furnace. I live on a crap ton of it I can dig for days have truck loads of it and I can use it mold things but like for use for blast furnaces or anything do you know if Oklahoma red clay is good for it.

    added by Rado: Brandon thanks, sound quite fine but it would be the best if you try to make those tests as mentioned in this article. Making the tests does not take a long time nor too much effort, etc.. You never know, it might be all right, but I would make the tests in the same conditions which you plan to use the clay for. Even if it didn’t last for a very long time, you can always reline your furnace if you can get this clay on your property. Let us know how you go with this please. You might even find out you have a valuable clay there around and under where you live!

    By Brandon Lindeman

  59. Hello, amazing info.

    I want to make cooking pots out of fire clay. I’m wondering if that is possible?

    And that also I don’t have any means to bake this clay cooking pot other than to use my conventional oven and perhaps later season the pot to give it strength.

    I’d love some help. There’s not much info around on how to make your own baked clay cooking pots.

    I wonder what type of other clay I could use to achieve my goal.

    By Ab

  60. By all means clay pots are amazing for cooking. You could be able to use fireclay for making the pot as there are many fireclay varieties. By this clay would need to be tested first to find out its suitability. It has to fire well in stoneware-clay temperature condition. This assures the finished ware will be holding liquids 100% well.

    To fire at stoneware temperature you will need to ask a potter around the area for instance because a pottery kiln could be used for this. Any conventional kitchen oven will not reach this temperature not even close. The clay actually needs to be in the high temperature and only after that it will contain the properties a cooking pot should have (a gradual consecutive re-heating won’t replace it no matter how many times).

    If your fireclay type will not be suitable, just buy a cheaper coarse pottery clay for hand building to suit the project … which is the option I would go for firstly.

    By Rado

  61. I was thinking of using some clay and making half and half dice molds to cast some aluminum and copper dice. Would this work? If not, do you know of a way that I can do it as a man of very limited income?

    By Kenny

  62. How much lime should be added to the mix?

    By lisa whitmore

  63. Can you fire terra cotta clay (air dry) in a low temp wood kiln heat?

    By Gil

  64. Good evening Rado,
    Regards from the S.E England.
    Thanks for the CD; the photos really have helped.
    I am building the 3G MTO and currently about the pour the concrete hearth slab on the vermiculite slab.
    My conundrum is as follows; I purchased 5 tubs of WASC (Wet Air Setting Cement) when getting the firebricks.
    Can I use it as is for laying the first layer of firebricks on the concrete hearth slab or should I mix it with 50% sand or source fireclay to and to 50% sand?
    Many thanks

    By B Daley

  65. BD if your firebricks are accurately cut, same dimensions, then you could use the air set in a thin layer application. To make a very smooth firebrick surface/hearth by setting the firebricks in. Firebricks will be bonded harder into the slab surface, compared to fireclay based mortar. But if they differ e.g. >2-3mm then I suggest using sand as it lowers the shrinkage rate.

    By Rado

  66. I have wet clay. Do I put any cement in with the wet clay and sand thanks.

    By Ken demott

  67. Thank you for your extremely informative page. I will think of you fondly when I build my brick kiln.

    By Suzi W

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