Monthly Tech-Tip from Tony Hansen SignUp	No tracking! No ads!

• Materials for Ceramics

Kaolin

Alternate Names: China Clay, Generic Kaolin

Description: Hydrated alumina silicate, Pure clay mineral

Oxide	Analysis	Formula
Al2O3	40.21%	1.00
SiO2	47.29%	2.00
LOI	12.50%	n/a
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

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

Tap picture for full size and resolution

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.

Do not rely on material data sheets, do the testing

Tap picture for full size and resolution

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.

Ball clay vs. Kaolin porcelain at cone 6

Tap picture for full size and resolution

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.

Cone 6 kaolin porcelain verses ball clay porcelain.

Tap picture for full size and resolution

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.

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

Tap picture for full size and resolution

These two samples demonstrate how different the LOI can be between different clay minerals. 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.

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

Tap picture for full size and resolution

A shipment EP Kaolin has arrived for use in some of our production 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 to assess soluble salts.

Closeup of Halloysite particles

Tap picture for full size and resolution

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.

How a kaolin and ball clay compare in a dry performance test

Tap picture for full size and resolution

These are DFAC drying performance disks of a large-particle kaolin (Opticast) and a ball clay (Plainsman A2). The DFAC test displays a plastic clay's response to very uneven drying (these disks are dried with the center portion covered to set up a water content differential). These materials both feel super-smooth, in fact, the kaolin feels smoother. But the ultimate particles are ten to one hundred times smaller than kaolin, thus it shrinks much more. The ball clay also has much lower water permeability, being unable to channel water from the center protected portion fast enough. When the inner section finally dried the outer was already rigid so it split the disk in two and pulled all the edge cracks. Most ball clays shrink more and crack worse than this (cracks concentric to the center also appear). So why use ball clay? This kaolin is so lacking in plasticity it was barely possible to even make this disk. And it is so weak that it can easily break just by handling it. Still, it is useful to make casting bodies. But the ball clay, when used as a percentage of a body mix, can produce highly plastic bodies than can be dried without trouble if done evenly.

Tap picture for full size and resolution

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

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

Tap picture for full size and resolution

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).

Chunks of metal found in contaminated truckload of kaolin

Tap picture for full size and resolution

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.

How plastic is a pure kaolin? Could one use it pure for pottery?

Tap picture for full size and resolution

Pure kaolins are clay. It seems logical that "pure clay" is plastic. However most kaolins are not plastic (compared to a typical clay for throwing or modelling). This is because they have a comparatively large particle size (compared to ball clays, bentonites, etc). This small bowl was thrown from #6 Tile kaolin. It is, by far, the most plastic kaolin available in North America. It's throwing properties are so good that one might be misled into thinking it should be possible to make pottery from it. Unfortunately, if it was survive drying without cracks, it would not make it through firing without this happening. This was fired, unglazed, to cone 6. Pure kaolin particles are flat and the throwing process lines them up concentric to centre. So shrinkage is greater across than along them. A filler is needed to separate the kaolin particles. All pure kaolins are also refractory, so even if this bowl had not cracked, the porosity of this piece is very high, completely impractical for functional ware (it needs a flux like feldspar to develop fired maturity).

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

Tap picture for full size and resolution

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.

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

Tap picture for full size and resolution

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

Tap picture for full size and resolution

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.

Health warning phrases on a bag of Kaolin

Tap picture for full size and resolution

Bubbles in Terra Cotta transparent glazes. What to do?

Tap picture for full size and resolution

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, I ended up getting it on pretty thick. Left: G1916S, a mix of Ferro frits, nepheline syenite and kaolin - one dip for 2 seconds and it was glazed. And it went on more evenly. Bubbles are of course generated by the body during firing. But also in the glaze. Raw kaolin loses 12% of its weight on firing, that produces gas. Low temperature glazes melt early, while gassing may still be happening. Keeping raw clay content in a glaze as low as possible is good, but at least 15% is normally needed for working properties. Improvements? Both of these could have been applied thinner. And I could have fired them using a drop-and-hold and a slow-cool schedule.

What material makes the tiny bubbles? The big bubbles?

Tap picture for full size and resolution

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).

Links

Materials	Caolín Damanueco (Cuba)
Materials	Ball Clay A fine particled highly plastic secondary clay used mainly to impart plasticity to clay and porcelain bodies and to suspend glaze, slips and engobe slurries.
Materials	Caolín Mantua (Cuba)
Materials	Caolín Blanco (Cuba)
Materials	Caolín Gris (Cuba)
Materials	Zettlitzer Kaolin
Materials	M.W.M. 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	No. 44 Kaolin
Materials	No. 17 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
Materials	Layton Clay
Materials	Wilco-UPF Kaolin
Materials	Eckaglass Kaolin
Materials	Spinks Kaolin Low Residue
Materials	SKT-1433 Kaolin
Materials	Glasurfritten 33.17
Materials	Caolín Santa Elena (Cuba)
Materials	Peerless 2 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	Kaopaque 20 Kaolin
Materials	Kingsley Kaolin
Materials	NSC 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	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	Glomax LL Kaolin
Materials	Allen G Kaolin
Materials	CC31 China Clay
Materials	Ceraclay WTC Kaolin
Materials	Gunheath Kaolin
Materials	Grolleg Kaolin A white burning kaolin from the UK, commonly used in porcelain bodies and as a glaze suspender. Sticky when wet, low plasticity.
Materials	Glomax LL Calcined Kaolin
Materials	F. C. Kaolin
Materials	Ewing Kaolin
Materials	EP Kaolin A kaolin that gels slurries (thus handy to suspend ceramic glazes). It is plastic and fires white enough that it is also valuable in porcelain bodies.
Materials	English Kaolin
Materials	E Kaolin
Materials	Diamond Kaolin
Materials	D Kaolin
Materials	CF Kaolin
Materials	WTD Kaolin
Materials	Ajax P Kaolin
Materials	609 Kaolin
Materials	Topaz 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	DB Float Kaolin
Materials	Allen Kaolin
Materials	Sparks 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	Zedlec Kaolin
Materials	Lampang Kaolin
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	Sapphire Kaolin
Materials	Snobrite Kaolin
Materials	KAOLIN-SERPENTINE
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	Pioneer Kaolin
Tests	Sieve Analysis 35-325 Wet SIEV particle size distribution test to evaluate and compare the particle size profiles of ceramic material powders
Tests	Shrinkage/Absorption Test SHAB Shrinkage and absorption test procedure for plastic clay bodies and materials
Projects	Troubles
Projects	Materials
Glossary	Porcelain How do you make porcelain? There is a surprisingly simple logic to formulating them and to adjusting their working, drying, glazing and firing properties for different purposes.
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 than 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 The most fundamental clay mineral. This mineral is found in nature in its purest form as kaolin. How
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	Clay 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 Point	1770C M
Density (Specific Gravity)	2.62

Mechanisms

Body Plasticity	It 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 Suspender	Kaolin 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 Follow me on

Got a Question?

Buy me a coffee and we can talk

---

https://digitalfire.com, All Rights Reserved
Privacy Policy