How To Make Fireclay

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Cooking wares, oven wares, garden kilns, pizza stones, wood stove isolation. Refractory clay are used everywhere, but here I search for a low-cost, flame-safe, clay recipe, with high modeling qualities.

Refractory clay – Mortar clay – Fire clay – Flame safe – Flameware – Ovenware

There are many refractory categories.

(If you want to jump to the recipes, scroll down in the article)

Fireclay for ceramic artist is about mixing a flame-safe clay with high modeling quality: One that after it is dryed and fired, can withstand direct flames, uneven heating, and rapid temperature changes. Flameware / Fireclay are simply ceramics with low-temperature expansion, a type that withstand high-temperature changes without cracking.

What is Fireclay?

How To Make Fireclay with modeling qualities? A recipe for a good, flame-safe ceramic clay with plastisety and high modeling qualities is nowhere to be found.
From time to time, I stumble upon a recipe, but when testing them, they are just not as flame-safe as they claim to be; the recipe can be incomplete, not refractory at all, or made with unrealisticly expensive components. Commercial fireclay does not have modeling qualities.

Here I share with you what I have learned so far in my research for a better recipe. I’m not a refractory expert, I’m a ceramist. If you disagree with my conclusions or have a recipe that outperforms mine. I would like to hear from you. (email me at: hello@woodfireceramic.com)

refractory clay mortarclay fireclay flamesafe flameware

How To Make Fireclay

All flame-safe clay has something in common: they are all made of:

Grog, to reduce shrinking
Woodchips or small objects burn away to reduce weight
Refractory materials
Component to increase plasticity

Fireclay is made of clay and aluminium oxides
www.brunner.com

Worn-out Saggars and material stress:

I have fired my best Fireclay recipes 20 + times, but small cracks on the inside tell me that direct flames are tough and that my firepot is not going to live forever. Something I also read about Saggars (a Saggar is a box used inside the kiln to protect wares from direct flames). Fireclay saggars were said to be used 30-40 times before they were worn out.

cracks in the refractory saggar firepot — Small cracks in my old firepot

A material unknown to me; the T-material is a pure type of clay used in the steel industry to prevent steel slag formation. It’s resistant to shrinking and cracking, and therefore highly valued by potters making large objects or raku. But it’s pricy.

How To Make Fireclay, Refractory Materials:

Five oxides with special importance in refractory materials:

Al₂O₃     Aluminium oxide
MgO      Magnesium oxide
BeO       Beryllium oxide
ZrO₂      Zirconium dioxideThO₂
ThO₂     Hydrogen peroxide

Many refractories are (expensive) metal oxides; they are not the answer to how to make fireclay for potters. Making refractories for small metalwork kilns, yes.

Firebricks are made of high-temperature clay, sold in four grades:

Superduty
High Heat Duty
Medium Heat Duty
Low Heat Duty

“Fireclays are often just the crude form of ball clays”
Kilns. Design, Combustion, and Operation – Daniel Rhodes

Soft bricks are made of diatomaceous earth, or Fuller’s earth. Diatomaceous earth is a natural deposit of diatom. This material is porous, soft, and light. It largely contains Silicon Dioxide.

List of some refractory materials:

Alumina Hydrate
Bentonite
Ball clay
Cordierite
Clay stoneware
Dolomite
Fireclay
Grounded soft refractory bricks
Kaolin
Kyanite
Mullite
Molochite
Perlite
Spodumene (expensive)
Sodium silicate (improves the green strength)
Talc
Vermiculite
Volcanic ash or pumice
Wood chips
Zircon

How To Make Fireclay:

“Tapron 11” Fireclay – My best homemade Fireclay:

fireclay, refractory clay, flame safe, mortar clay

I fired it to a strong heat inside the ring, while the outside did not receive enough heat to transform to ceramic. No cracks yet, a good sign.

fireclay grounding soft refractory bricks

Picture 1 – Grounded soft refractory bricks
Picture 2 – Fine wood chips
Picture 3 – The fireclay mix is quite plastic but only for rough modeling with few details.

recipe:

2 part Craft Crank 2,4 kg (wet from the bag, this clay contains 40% fine chamotte)
1 part / 400g Grounded soft refractory bricks
1 part / 340g Talc
2 part / 110g Fine wood chips (I prefer wood chips over Perlite since it makes longer voids)
1/6 part 80g Bentonite (makes it both plastic & sticky, adjust it, mix dry with the rest)
1 part / 560g Water (the mix ended up a bit too wet).

Craft-Crank is a fantastic strong clay; handle almost anything, the best construction clay around. Big sculptures keep their shape without cracking or misshaping even when burned at 1300 degrees. But it doesn’t handle a bar flame; hope this recipe improves it.

“Tapron 7” Fireclay – My most tested homemade Fireclay, first version:

This has been my best so far, it does the job but is quite heavy

Works quite well, is plastic, and withstands direct flames and quick temperature changes, but after 20 firings, it starts to show some fine cracks on the surface.

Recipe in volume, not in weight:

2 part Crank (wet from the bag), this clay has 40% fine chamotte
1 part Ballclay (makes it quite sticky, try to reduce the amount?)
1 part Grog
2 part Perlite
1 part fine wood chips

Also, see the Clay Kiln built with refractory clay:
woodfireceramic.com/coil-building-the-clay-kiln/

“Pippa” White Fireclay (untested):

Recipe in volume:

2 part Kaolin
2 part Talc
½ part Ballclay, just enough to make it stick together
2 part Molochite
2 part wood chips

Grog is high-burned and crushed ceramic; it helps against shrinking, but does the Grog itself have low-temperature expansion? If not, it’s not a refractory solution. I try Molochite instead. Kaolin is very refractory but shrinks a lot the first time fired, test it, but if it cracks, replace Kaolin with Crank.

“Darbin” Fireclay – Porous Fireclay with Talc:

In volume:

1 part talc
1 part ball clay
2 part grog
2 part perlite

Promising!

Coarse, porous, low modeling ability, too sticky when wet, too crumbly when dry, low green-strength. It can, with difficulties, be modeled in simple shapes, can be “hammered” into plates, and pressed into forms. It’s really hard after it’s burned to 1100 degrees Celsius; it handles uneven heating and rapid temperature changes really well. When wet, it soaks up water quickly thanks to the clay body’s open structure.

One of the most interesting Fireclays.

homemade fireclay, homemade refractory material, recipe

“Tussby” Fireclay – Porous Clay Body:

1 part talc (refractory)
1 part Molochite (refractory fine particle grog)
1 part Dolomite (refractory, makes the Fireclay porous)
1 part fine wood chips
1/2 part Ball Clay (for plasticity)
Add some Sodium silicate to improve the green strength

First test:
400g Talc
300g Molokite
300g Dolomite
25g Bentonite
25g Ballclay

Crumble and unflexible, it feels like making a fine particle sandcake; joining the clay is difficult without extra water, needs more plasticity when wet, and is all too fragile when dry. But it has some interesting qualities; Dolomite works almost like paperclay, distributes moisture equally in the clay, så joining bone dry clay is done by just adding water to the joints, no cracking!

Second test:
1000g Talc
1000g Molokite
1000g Dolomite
1000g Ball clay
500g Kaolin
200g Bentonite
500g fine wood chips
2000g coarse shammot

According to this source: https://digitalfire.com/
A recipe with:
65% Dolomite
35% Ball Clay
5% Ferro frit 3110 (so the fired body does not rehydrate).
Gives a clay body porosity of 35%.

I believe porosity is beneficial for Fireclay to spread the heat evenly, and for Fast Fire Clay to quickly get rid of moisture so it does not crack.

New test:

Base receipt:
30% talc (refractory)
30% Molochite (refractory fine particle grog)
20% Dolomite (refractory, makes the Fireclay porous)
10% Kaolin (refractory clay, shrinks a lot)
5% Ballclay (plastisity)
5% Bentonite (plastisity)
100% TOT

Additivs:
100% Perlite (burns off, makes small voids for isolation)
5% Sodium silicate (improves the green strength)

Test: Mullite synthetic mix

(for extreme temperatures)

40% kaolin
40% molochite
20% ball clay

Makes Mullitt with high temperatures

Cordierite-“claybody” mix

50% Kaolin
30% Silika
20% Talc

Cordierite-slip

Same recipe, but this time as a thin slip to apply with a brush inside the kiln as a protective layer. First, it must be fired over 1200°C to transform into real cordierite, but it will still function as a protective layer if fired to a lower temperature.

Refractory matrix / Refractory filler

If you load a clay body with 40% refractory powder or grog, you just added a refractory filler, but what you need is a refractory matrix. Adding high refractory material to a low refractory matrix is not what we want; we want to make a high refractory matrix. It’s the matrix that determines when things melt, including its thermal shock resistance and mechanical strength.

Industrial-produced firebricks are made under heavy pressure and almost no water. Homemade fireclay can not be molded under such conditions, but with a flat hammer, it’s possible to “hammer” the clay into the molds, to make it more compact and reduce voids.

Some refractory recipes:

I have not tested these:

“A high-quality refractory material can be made using a three-part mix of kaolin, silica, and ball clay. It can support 25% grog without problem and fires white and very refractory”. This source says: https://digitalfire.com/glossary/fireclay

Raku clay

Raku clay needs to be refractory, though it only needs shock resistance once:
Raku clay often has an open claybody structure and can contain:
spodumene (expensive), Kyanite, talc, flint, feldspar, and fireclay.

Refractory clay:

30 parts stoneware clay
1 part Alumina Hydrate
1 part Zircon
Add perlite and grog

Saggar formulas:

Var 1:
50% grog
40% China clay
10% Bounding clay( ball clay?)

Var 2:
40% grog
20% China clay
20% Ball clay
20% Talc

Var 3:
2 part Fireclay
1 part Ball clay
1 part Grounded soft bricks
2 part Coarse sawdust

Homemade refractory bricks:

Var 1:
70 parts vermiculite
15 parts ball clay
Add some Sodium silicate to make it stronger

Var 2:
3 part sawdust
1 part fireclay
1 part ball clay
Add some Sodium silicate to make it stronger

The coil-built Egg kiln certainly cracks! But it won’t fall apart. This is 5 time it’s fired to 1100 C. Read more here:

/the-egg-kiln-low-fired-pottery/

/coil-building-the-clay-kiln/

Since Fireclay for ceramic artists is a lot about plasticity, ceramic materials like Ball clay and Bentonite also make the Fireclay sticky. To receive plasticity with organic materials looks promising (though it will start to rot and smell bad after just a few days, so work fast):

See “The alchemist clay”:
https://www.youtube.com/embed/HbLdNKM9y68

And his updated process: “Making Alchemist’s Clay: My Updated Process”
https://www.youtube.com/embed/vBAzZHIFb0o

Also, see the material list:

https://www.woodfireceramic.com/ceramic-materials-and-glaze-ingredients

And read about the basics about raw-material ceramics fluxes:

https://www.woodfireceramic.com/9-important-fluxes-for-ceramics

How To Make Fireclay: