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Kaolin

Alternate Names: China Clay

Oxide Analysis Formula
Al2O3 40.21% 1.00
SiO2 47.29% 2.00
LOI12.50%
Oxide Weight 221.96
Formula Weight 253.67

Notes

A wide array of kaolin (also known as China Clay) products are available. These vary in plasticity, crystal and surface chemistry, particle shape and size, flow properties, permeability, etc. However the most common varieties most people will see are two: kaolins intended for plastic bodies or casting ones. Plastic kaolins can rival the workability of a ball clay, casting ones can be so short that it is difficult to even wedge or roll them without the plastic mass falling apart. Strangely, non-plastic kaolins are not necessarily whiter burning.

Pure kaolin is the clay of choice for bodies that need to be clean and white. Many porcelains contain only a kaolin mix as their clay complement. But kaolins have relatively low plasticity when compared to other raw clay types. Thus in non-casting plastic forming bodies it is often not possible to achieve enough plasticity employing kaolin alone. Additions of ball clays, bentonites and other plasticizers are thus common. Where translucency and whiteness are paramount, highly plastic kaolins and white burning ball clays and bentonites can be used .

Because kaolinite mineral has a much larger particle size than ball clay and bentonite materials, blending it with them in bodies can produce a good cross section of ultimate particle sizes (this imparts enhanced working and drying properties). Another advantage of the larger particle size of kaolins is that they are much more permeable to the passage of water. Thus kaolins, especially the larger sized ones, speed up casting rates in slurry bodies and drying rates in all bodies.

Kaolins are employed in glaze recipes to keep the silica, feldspar, frit and other particles from settling out (the surface chemistry of the particles and their interaction with water are responsible for this behavior). At the same time the oxide chemistry of kaolin makes it the primary source of alumina oxide for glazes.

Kaolin is a very refractory aluminum silicate. Kaolin-based bodies are used to make all kinds of refractory parts for industry. Kiln wash is often made from 50:50 mix of kaolin and silica. Cordierite is made mainly from kaolin. High heat duty grogs are made by calcining kaolin.

Kaolin is used in many industries other than ceramics, in fact the ceramics industry uses only a small amount of the total kaolin produced. Kaolin companies tend to be billion-dollar operations and kaolin is used in everything from paper to cosmetics, paint to agricultural products. The spread of pictures across this page at the Ukranian Kaolin Company shows some examples: http://www.ukc-kaolin.com/en/product.html

If you use kaolin in your production there is good reason to be doing routine quality control to make sure it is remaining consistent. Kaolins can sometimes have particulate impurities (can cause firing specks) and exhibit differences in soluble salts content, drying shrinkage, drying performance and behavior in slurries. Clays are often the most variable material that production departments have to deal with.

Kaolin transforms to mullite above 1000C, this is a key factor in the micro structure of porcelain and other types of bodies. This transformation is also exploited in engobes.

Related Information

Two kaolins, one cracks on bisque, the other does not

Both of these are mixed 70:30 kaolin:feldspar. Left is a fine particled kaolin, #6 Tile. Right a coarser particled, less plastic material, EPK. During forming, the larger particles line up concentric to the center better. This causes the body to shrink more along radius lines than along tangent, producing these cracks. Many of these were made and they all cracking like this.

The white one feels smoother, but it is actually far coarser. Why?

Large particle kaolin (left) and small-particle ball clay (right) DFAC tests (for drying performance) demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to add stresses that cause cracking). These materials both feel super-smooth, in fact, the white one feels smoother. But the ultimate particles tell the opposite story. The ball clay particles (grey clay) are far smaller (ten times or more). The particles of the kaolin (white) are flatter and lay down as such, that is why it feels smoother.

What material makes the tiny bubbles? The big bubbles?

These are two 10 gram GBMF test balls of Worthington Clear glaze fired at cone 03 on terra cotta tiles (55 Gerstley Borate, 30 kaolin, 20 silica). On the left it contains raw kaolin, on the right calcined kaolin. The clouds of finer bubbles (on the left) are gone from the glaze on the right. That means the kaolin is generating them and the Gerstley Borate the larger bubbles. These are a bane of the terra cotta process. One secret of getting more transparent glazes is to fire to temperature and soak only long enough to even out the temperature, then drop 100F and soak there (I hold it half an hour).

Bubbles in Terra Cotta transparent glazes. What to do?

Two transparent glazes applied thickly and fired to cone 03 on a terra cotta body. Right: A commercial bottled clear, I had to paint it on in layers. Left: G1916S almost-zero-raw-clay glaze, a mix of Ferro frit 3195, 3110, calcined kaolin and a small amount of VeeGum T. The bubbles you see on the left are from the gas generated by the body. The ones on the right are from body and glaze. How can so many more bubbles be generated within a glaze? Raw kaolin. Kaolin loses 12% of its weight on firing, that turns to gas. Low temperature glazes melt early, while gassing may still be happening. So to get a crystal clear the raw clay content has to be as low as possible. Obviously, a white burning body made from refined materials would be even better. A good compromise: A red slip (or engobe) over a white burning body, it would generate far less gases because of being much thinner and still exhibit the nice red color.

Health warning phrases on a bag of Kaolin

Cleanest kaolin porcelain vs. ball-clay-only porcelain!

These cone 6 clear-glazed porcelains demonstrate just how white you can make a porcelain if you use white burning kaolins and bentonites instead of ball clays. Both contain about 40% clay. The one on the left employs New Zealand kaolin and Veegum plasticizer, the one on the right Kentucky ball clays (among the whitest of ball clays in North America) and standard bentonite. Both are zero porosity. The glaze surface is a little more flawless on the right one (possibly because ball clays have a lower LOI than kaolins).

What happens when you dry and bisque a piece made of pure kaolin?

The way in which the walls of this bisque fired kaolin cup laminate reflect the plately and uniform nature of the kaolin particles. Because they are lining up during the wedging and throwing process, the strength to resist cracks is better along the circumference than perpendicular to it. The bonds are weak enough that it is very easy to break it apart by hand (even though it is bisque fired). The worst laminations were at the bottom where wall thickness was the most variable and therefore the most drying stresses occurred. However, if this kaolin were blended with feldspar and silica, this lamination tendency would completely disappear.

Ball clay and kaolin test bars side-by-side fired from cone 9-11 oxidation and 10 reduction.

Closeup of Halloysite particles

Electron micrograph showing Dragonite Halloysite needle structure. For use in making porcelains, Halloysite has physical properties similar to a kaolin. However it tends to be less plastic, so bodies employing it need more bentonite or other plasticizer added. Compared to a typical kaolin it also has a higher fired shrinkage due to the nature of the way its particles densify during firing. However, Dragonite and New Zealand Halloysites have proven to be the whitest firing materials available, they make excellent porcelains.

We have to fight with the fibreglass industry to get kaolin!

These are bags from three recent truckloads of 880 bags each. Order-delivery delays are getting longer and longer as the fibreglass industry is making more and more demands on kaolin suppliers. This means we have to store this material in larger quantities and for longer periods than in the past. And we must be more diligent in testing for consistency because manufacturers are catering to fibreglass instead of ceramics. When this is coupled with the decline of ceramic manufacturing in North America it means maintaining and documenting the properties important to ceramics are becoming less important to kaolin manufacturers.

The kaolin arrives on a semi of 880 bags. First step: Record the date code.

A shipment EP Kaolin has arrived for use in production of porcelain and stoneware bodies. Of course, this needs to be tested before being put into product. But how? The first step is to create a new recipe record in my Insight-Live account, and find their production date code stamp on the bag. Hmmm. It does not have one! OK, then I need to record the date on which we received it. We need to save a bag on every pallet and sieve 50 grams through 100 mesh (to spot contamination). Then we'll make test bars (of all the samples mixed) to fire across a range of temperatures (to compare fired maturity with past shipments). We do a drying performance disk also, it also shows soluble salts.

Does this terra cotta clay have an LOI higher than kaolin? No.

These two samples demonstrate how different the LOI can be between different clays. The top one is mainly Redart (with a little bentonite and frit), it loses only 4% of its weight when fired to cone 02. The bottom one is New Zealand kaolin, it loses 14% when fired to the same temperature! The top one is vitrified, the bottom one will not vitrify for another 15 cones.

Cone 6 kaolin porcelain verses ball clay porcelain.

Typical porcelains are made using clay (for workability), feldspar (for fired maturity) and silica (for structural integrity and glaze fit). These cone 6 test bars demonstrate the fired color difference between using kaolin (top) and ball clay (bottom). The top one employs #6 Tile super plastic kaolin, but even with this it still needs a 3% bentonite addition for plasticity. The bottom one uses Old Hickory #5 and M23, these are very clean ball clays but still nowhere near the whiteness of kaolins. Plus, 1% bentonite was still needed to get adequate plasticity for throwing. Which is better? For workability and drying, the bottom one is much better. For fired appearance, the top one.

Ball clay vs. Kaolin porcelain at cone 6

Left: A porcelain that is plasticized using only ball clays (Spinx Gleason and Old Hickory #5). Right: Only kaolin (in this case Grolleg). Kaolins are much less plastic so bentonite (e.g. 2-5%) is typically needed to get good plasticity. The color can be alot whiter using a clean kaolin, but there are down sides. Kaolins have double the LOI of ball clays, so there are more gasses that potentially need to bubble up through the glaze (ball clay porcelains can produce brilliantly glassy and clean results in transparent glazes even at fast fire, while pure kaolins can produce tiny dimples in the glaze surface if firings are not soaked long enough). Kaolins plasticized by bentonite often do not dry as well as ball clays even though the drying shrinkage is usually less. Strangely, even though ball clays are so much harder and stronger in the dry state, a porcelain made using only ball clays often still needs some bentonite. If you do not need the very whitest result, it seems that a hibrid using both is still the best general purpose, low cost answer.

Do not rely on material data sheets, do the testing

The cone 6 porcelain on the left uses Grolleg kaolin, the right uses Tile #6 kaolin. The Grolleg body needs 5-10% less feldspar to vitrify it to zero porosity. It thus contains more kaolin, yet it fires significantly whiter. Theoretically this seems simple. Tile #6 contains alot more iron than Grolleg. Wrong! According to the data sheets, Grolleg has the more iron of the two. Why does it always fire whiter? I actually do not know. But the point is, do not rely totally on numbers on data sheets, do the testing yourself.

The difference in fired character between kaolin and ball clay at cone 10R

The top one is EP Kaolin, the bottom one is Old Hickory M23 Ball Clay (these materials are typical of their respective types). These materials have low alkali contents (especially the kaolin), this lack of flux means they are theoretically highly refractory mixes of SiO2 and Al2O3. It is interesting that, although the kaolin has a much larger ultimate particle size, it is shrinking much more (23% total vs. 14%). This is even more unexpected since, given that it has a lower drying shrinkage, and should be more refractory. Further, the kaolin has a porosity of 0.5% vs. the ball clay's 1.5%. The kaolin should theoretically have a much higher porosity? What is more, both of these values are unexpectedly low. This can partly be explained by the particle packing achieved because of the fine particle size. Despite these observations, their refractory nature is ultimately proven by the fact that both of these can be fired much higher and they will only slowly densify toward zero porosity.

Chunks of metal found in contaminated truckload of kaolin

You may not fully appreciate what your clay body manufacturer has to go through to make clean porcelain for you. Every load of material that they receive has to be checked. We now have to check every pallet. This is the third semi-trailer load of material we have had contaminated (ball clays and kaolins are most vulnerable). When we phoned another manufacturer they checked their supply and it was contaminated also! Materials can also be contaminated by larger clay particles that disrupt the fired glaze surface. These chunks of metal were pulled out by magnets in the production line, a thousand boxes of porcelain are now garbage. It is too expensive to return a load, so it just becomes a loss.

Links

Materials Caolín Mantua (Cuba)
Materials Ball Clay
Materials Caolín Blanco (Cuba)
Materials Caolín Gris (Cuba)
Materials Zettlitzer Kaolin
Materials M.G.R. Kaolin
Materials M+M China Kaolin
Materials Richardsons Kaolin
Materials A-1 Kaolin
Materials VC-1 Kaolin
Materials Klondike Kaolin
Materials 38 Kaolin
Materials 1479 Kaolin
Materials 1434 Kaolin
Materials 1433 Kaolin
Materials 1431 Kaolin
Materials Layton Clay
Materials M.W.M. Kaolin
Materials No. 44 Kaolin
Materials Kernick Kaolin
Materials Sterling Kaolin
Materials Peerless 3 Kaolin
Materials Snocal 40
Materials PAR Kaolin
Materials Bilt-Cote Kaolin
Materials Langford Kaolin
Materials Dixie Clay
Materials 372 Kaolin Kaolin
Materials No. 17 Kaolin
Materials 609 Kaolin
Materials Spinks Kaolin Low Residue
Materials Glasurfritten 37.025
Materials Glasurfritten 37.025
Materials Glasurfritten 37.025
Materials Glasurfritten 37.024
Materials Glasurfritten 37.024
Materials Glasurfritten 37.024
Materials Glasurfritten 35.32
Materials Glasurfritten 35.32
Materials Glasurfritten 35.32
Materials Glasurfritten 35.2
Materials Glasurfritten 35.2
Materials Glasurfritten 35.055
Materials Glasurfritten 37.031
Materials Glasurfritten 37.031
Materials SKT-1433 Kaolin
Materials Glasurfritten M1233
Materials Glasurfritten 37.06
Materials Glasurfritten 37.06
Materials Glasurfritten 37.06
Materials Glasurfritten 37.06
Materials Glasurfritten 37.031
Materials Glasurfritten 37.031
Materials Glasurfritten 35.055
Materials Glasurfritten 35.055
Materials Wilco-UPF Kaolin
Materials Eckaglass Kaolin
Materials Caolín Santa Elena (Cuba)
Materials Glasurfritten 35.2
Materials Glasurfritten 35.2
Materials Glasurfritten 35.2
Materials Glasurfritten 34.41
Materials Glasurfritten 34.41
Materials Glasurfritten 34.41
Materials Glasurfritten 33.17
Materials Glasurfritten 33.17
Materials Caolín Damanueco (Cuba)
Materials NSC Kaolin
Materials Kaolin Slurry
Materials Kampaku Kaolin
Materials Kamec Kaolin
Materials K50 Kaolin
Materials K1 Kaolin
Materials JHC White Kaolin
Materials IXL RE
Materials IXL 34E/F 95
Materials IXL 34E
Materials Hong Kong Kaolin
Materials Hillman Kaolin
Materials Wilclay WC Kaolin
Materials Kaopaque 20 Kaolin
Materials No. 50 China Clay
Materials Monarch Kaolin
Materials McNamee Kaolin
Materials Masterfloat Kaolin
Materials Masterfil Kaolin
Materials Mastercast Kaolin
Materials LPC Kaolin
Materials LG Kaolin
Materials KT-Cast Kaolin
Materials Korean Kaolin
Materials KMS Kaolin
Materials Kingsley Kaolin
Materials Helmer Kaolin
Materials Hamilton Kaolin
Materials CC China Clay
Materials Calcined Kaolin
Materials C Kaolin
Materials Puraflo 50 Kaolin
Materials Bell Kaolin
Materials B Kaolin
Materials Avery Kaolin
Materials Albion Sperse
Materials 600 Kaolin
Materials A Kaolin
Materials Ajax Glomax Kaolin
Materials Allen G Kaolin
Materials CC31 China Clay
Materials Ceraclay WTC Kaolin
Materials Gunheath Kaolin
Materials Grolleg Kaolin
Materials Glomax LL Calcined Kaolin
Materials F. C. Kaolin
Materials Ewing Kaolin
Materials EP Kaolin
Materials English Kaolin
Materials E Kaolin
Materials Diamond Kaolin
Materials D Kaolin
Materials CF Kaolin
Materials WTD Kaolin
Materials Ajax P Kaolin
Materials Sparks Kaolin
Materials Supreme Kaolin
Materials Stockalite Kaolin
Materials Super Standard Porcelain
Materials Speswhite Kaolin
Materials Kaolin Natural BP
Materials Sovereign Kaolin
Materials Grade C Kaolin
Materials Snocal 707 Kaolin
Materials Laguna #1 Kaolin
Materials Kaolex D-6 Kaolin
Materials Delta Kaolin
Materials Zedlec Kaolin
Materials Topaz Kaolin
Materials DB Float Kaolin
Materials H-1 Kaolin
Materials D'Arvor Kaolin
Materials Kaolin 171
Materials Kaolin 151
Materials Kaolin 143
Materials Kaolin 115
Materials Kaolin 114
Materials Kaolin 113
Materials Kaolin 111
Materials JB Kaolin
Materials James Bay Stn Kaolin
Materials Allen Kaolin
Materials Lampang Kaolin
Materials KAOLIN-SERPENTINE
Materials Samson Kaolin
Materials SAF Kaolin
Materials SA-1 Kaolin
Materials Rogers Kaolin
Materials Remblend Kaolin
Materials Rampant BB Kaolin
Materials Qc China Clay
Materials Putnam S Kaolin
Materials Putnam Kaolin
Materials Pleyber S Kaolin
Materials Piopot Kaolin
Materials Pioneer Kaolin
Materials Sapphire Kaolin
Materials WS Kaolin
Materials Wilson Kaolin
Materials Wilclay WC Kaolin
Materials Wilclay CR Kaolin
Materials WC-5 Kaolin
Materials Velvacast Kaolin
Materials Plainsman Troy Clay
Materials Treviscoe Kaolin
Materials Tile #6 Kaolin
Materials Stannon Kaolin
Materials Standard Porcelain Kaolin
Materials Snobrite Kaolin
Materials Peerless 2 Kaolin
Tests Shrinkage/Absorption Test
Tests Sieve Analysis 35-325 Wet
Projects Troubles
Projects Materials
Glossary Porcelain
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
Glossary Plasticity
Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.
Glossary Clay
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.
Glossary Permeability
In ceramics, the permeability of clay slurries and plastics determines the rate as which water can move through the matrix
Minerals Kaolinite
URLs http://en.wikipedia.org/wiki/Kaolin
Kaolin at Wikipedia.com
URLs http://kaolin.com/
Information at Kaolin.com
Oxides SiO2 - Silicon Dioxide, Silica
Oxides Al2O3 - Aluminum Oxide, Alumina
Typecodes Kaolin
Pure clay mineral, there are many brand names of varying purity and iron content.
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes Kaolin
Pure clay mineral, there are many brand names of varying purity and iron content.
Hazards Kaolin Toxicity
Articles Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.

Data

Frit Softening Point1770C M
Density (Specific Gravity)2.62

Mechanisms

Body PlasticityIt is possible to make a plastic throwing body using 50% kaolin only, however you must choose one of the highly plastic varieties such as #6 Tile. Even then you will likely need a little bentonite to augment the kaolin. There is a huge range in kaolin plasticities, test for yourself to find out.
Glaze SuspenderKaolin is the most common glaze suspender. Depending on the type of kaolin used, 15-20% should be enough. Many fritted glazes are composed solely of frit and kaolin. Some kaolins make the glaze gel, this is a helpful additional mechanism to keep it suspended.

By Tony Hansen


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