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Bentonite

Alternate Names: Montmorillonite, Bentonite USA

Oxide Analysis Formula
CaO 1.00% 0.14
Na2O 3.00% 0.38
MgO 2.00% 0.39
K2O 1.00% 0.08
Al2O3 20.00% 1.55
SiO2 59.00% 7.76
Fe2O3 3.50% 0.17
LOI 10.00%n/a
Oxide Weight 707.76
Formula Weight 786.40

Notes

Bentonites are classified according to several types (e.g. calcium, sodium, potassium) but in the ceramic industry sodium bentonite is the material of commerce (this is the same material used by the drilling industry). Sodium bentonites have incredibly small particle sizes (and thus surface areas). Combine this with the active particle surface chemistry (which makes them hold onto water) and it is the most plastic and impermeable common clay material used in ceramics. Its contribution to working properties in glazes and clay bodies is balanced by the undesirable properties that are also imparted (so have your eyes open to the advantages and disadvantages).

There is wide variation in the chemistries of bentonites, it is impossible to specify an average (bentonite is not employed in ceramics for its chemistry). Any generic chemical analysis is thus only an attempt to represent the amounts you might find in a common variety. Because of the high iron content, bentonite is considered a dirty material and thus the tug-of-war between the valuable working properties it imparts and the need for whiteness or pure color that it impedes.

Fine particle size: Bentonite is colloidal (particles are so small the action of water molecules is enough to keep them in suspension). It is typically 10 times finer than ball clay. It can have a surface area of almost 1000 square meters per gram (50 times that of kaolin, 5000 times that of silica flour).

Bentonite is usually far too plastic to prepare test specimens (e.g. for drying, strength and shrinkage evaluation). However, a mix of 20-30% virgin material with calcined material can be extruded and formed (test specimens will still shrink to a very small size).

Permeability: Sodium bentonites are impermeable to water. To demonstrate this fill a tall glass cylinder with bentonite, to near the top, and then carefully pour water on top. The water will penetrate down into the clay only a few millimeters and no matter how long you leave it it will not penetrate further. This occurs because the powder swells as the water penetrates and adjacent particles 'hang onto' the water between them. The water thus becomes a glue that holds the mass together and prevents more from entering or passing through. This phenomenon accounts for why glazes and bodies of high bentonite content dry slower. As an example, if you pour a slurry of silty clay onto a plaster surface the water is often pulled out in seconds. However, a bentonite slurry may require days or weeks to pull the water out evenly.

Swelling: Sodium bentonites expand (as much as 15 times) when added to water. This characteristic is valuable in thickening liquids and slurries and is another contributing factor to maintaining suspensions. Bentonite is used in large quantities in the gas and oil drilling industries to suspend high specific gravity slurries which are used as a medium to float out the chunks of rock cut by the drill bit.

Thixotropy: This is a tendency of a suspension to gel after sitting for a time and then re-liquefy when it is agitated. Clay bodies also exhibit this behavior, stiffening on aging but then re-softening when worked. Thixotropy is valuable in clay slurries for this reason, they gel when not being used and thus do not settle out. While typical industrial thixotropic agents employ various mechanisms bentonite works by charge attraction (see above).

Plastic bodies: Because of their active electrolytic behavior and fine particle size, bentonites exhibit extremely high plasticity (and associated high shrinkage). In pottery and porcelain clay bodies additions of only 2% can produce marked improvements in workability and dry strength without much effect on fired color. The use of up to 5% is common, especially where high plasticity is needed in a white-burning body. High amounts of bentonite will dramatically slow down the drying rate. In certain applications, it is practical to use bentonite as the only plasticizer in a mix (e.g. 5-7.5% with Redart to make a terra cotta). The plasticity-producing effects of bentonite depend on the shapes, sizes, surfaces and electrolytics of the particles it is interacting with, equal additions of bentonite to two different host bodies may have very different effects on the plasticities. While bentonites can be called upon to assume the major burden of plasticizing a body, there is a need to assess the practicality of this (the body can become excessively plastic, take too long to dry and be too easily torn). Bentonite makes bodies more plastic and dry harder but this comes at a cost, they shrink more during drying and thus potentially crack more.

Casting bodies: Bentonite is invaluable to tune the balance between casting time and plastic strength (which enables pieces in the mold to shrink and pull themselves away from the plaster without tearing). Adding 0.5-1.0% can transform otherwise normally uncastable non-plastic porcelains. Of course if too much bentonite is added casting time will increase significantly (because bentonite reduces water permeability).

Suspensions: Bentonite is used to keep particulates in suspension in all sorts of consumer and industrial products, and in glazes in ceramics. The mechanism is charge attraction, that is, opposite electrolytic charges develop on the surfaces and edges of dispersed particles and give rise to a stable 'house-of-cards" structure that can be disrupted by shear stress. However, when the stress is removed, the structure re-establishes itself. The amazing thing is that large amounts of other types of particles can be tolerated within this structure, they are kept in suspension as well. Thus maximum suspending benefit can be achieved by blunging bentonite with the water before adding the other dry materials (to ensure that every particle is whetted on all sides). However, this cannot be done without a powerful high-speed propeller mixer. Thus it is normal to blend dry ingredients including bentonite first and then add them to the water. However beware of too much bentonite in glazes, they will dry too slowly and will shrink too much during drying causing cracks that later turn into crawling during firing.

Chemically inert, Inorganic, Non-irritating: Formulations that are not fired are not altered chemically by bentonite additions. Bentonite does not support organic growth. Thus it is suitable as a carrier for personal care products like hand cream and cosmetics.

Trace minerals and toxicity: Like soils and terra cotta clays, bentonite has trace amounts of dozens of elements in the periodic table, including heavy metals. But the percentages are normally very small. Like other clays, crystalline quartz should be regarded as the main hazard to its use.

Binder: Bentonite binds particles together in ceramic bodies to make them stronger in the green or dry state. Its minute particles fill voids between others to produce a more dense mass with more points of contact. Adding bentonite to glazes also imparts better dry strength and a harder and more durable surface. To fully appreciate how plastic, hard and strong bentonite can be, try mixing 25:75 with silica and preparing plastic test bars.

Firing: Standard grades typically vitrify (around Orton cone 6-10) from grey to deep red coloration. However soluble salts can be so high that they form a glaze on pure test specimens. Utility grades often contain granular iron material that causes specking in clay bodies, even materials rated at 325 mesh can contain significant speck-causing particles. For good reason, bentonite is considered a very dirty material. However commercial micro-fine grades (100% minus 325 mesh) are available (these are very expensive, however). Barium carbonate can be added to bodies to precipitate the solubles bentonite brings. Thus the iron content is the only firing issue associated with the visual character. Contrary to what many think, a white body can often tolerate up to 5% bentonite without firing significantly darker.

White-firing bentonites: There are a number of white-firing and highly refined (and highly expensive) bentonites produced for the ceramic industry (and others). However, they can have less plasticity. Testing is needed to determine if the plasticity and plastic character are sufficient and if the extra cost is worth it. Normal microfine bentonite raw bentonite even at 5% does not darken the color of the porcelain as much as you might think. Examining recipes often shows that the kaolin and ball clay are contributing more iron than the bentonite. Even white plasticizers can have up to 0.5% iron. 5% bentonite increases the iron content of the body by .25% without considering the factors above. If you used only 2.5% bentonite the extra iron may not be an issue at all.

Sorptive properties: Granules of this material can be heat-treated to nullify gelling properties but greatly improve liquid absorption, creating hard micro-sponges (e.g. for cat litter and other absorptive products of many types).

Firing cracks, explosions: Bentonite slows down water penetration. Not only does a bentonite-containing clay body dry slower but it does not dry as completely. Although ware might look dry it likely is not, several percent tightly bound water remains. If ware is not temperature-dried before being fired there is a risk that water will not be able to escape fast enough during firing and ware will crack, fracture under steam pressure.

Related Information

Firing shrinkage variation between various clays

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Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and drying shrinkages vary widely. Materials normally acting as fluxes (like dolomite, talc, calcium carbonate) are refractory here because they are fired in the absence of materials they react normally with.

Could these bentonite particles cause specking in a porcelain?

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The stated particle size of a material and fired appearance can both be misleading. For example, these are Volclay 325 bentonite particles fired to cone 8 oxidation. They are from a water washed sieve analysis test, the oversize particles from a 325 mesh screen (left) make up 2% of the total and 1% are from the 200 mesh screen (right). Although the 325 particles appear ominously dark, individually they are likely to small to produce apparent fired specks in a porcelain. However 200 mesh sizes can produce visible fired specks, but that fraction of oversize does not have nearly as high iron or flux content. Still, the finer darker particles could agglomerate, it might be better to use a cleaner bentonite to plasticize a porcelain.

Turbo-charge plasticity using bentonite, hectorite, smectite.

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These are porosity and fired shrinlage test bars, code numbered to have their data recorded in our group account at Insight-live.com. Plainsman P580 (top) has 35% ball clay and 17% American kaolin. H570 (below it) has 10% ball clay and 45% kaolin, so it burns whiter (but has a higher fired shrinkage). P700 (third down) has 50% Grolleg kaolin and no ball clay, it is the whitest and has even more fired shrinkage. Crysanthos porcelain (bottom, from China) also only employs kaolin, but at a much lower percentage, thus is has almost no plasticity (suitable for machine forming only). Do H570 and P700 sacrifice plasticity to be whiter? No, with added bentonite they have better plasticity than P580. Could that bottom one be super-charged? Yes, 3-4% VeeGum or Bentone (smectite, hectorite) would make it the most plastic of all of these (at a high cost of course).

Two plasticizers show vastly different potency

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A comparison of the plasticity of two bentonites was done by diluting them with silica. The top sample is Volcay 325 mixed 25:75 with silica. It is a standard Wyoming sodium bentonite - mined, dried and ground. The bottom specimen is Hectalite 200, a highly refined white hectorite, mixed 50:50 with silica. Notice how much less plastic it is (even though at double the percentage).

An example of a DFAC drying test of a bentonitic clay

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This disk has dried under heat (with the center part protected) for many hours. During that process it curled upward badly (flattening back out later). It is very reluctant to give up its water in the central protected section. Obviously it shrinks alot during drying and forms a network of cracks. When there are this many cracks it is difficult to characterize it, so a picture is best.

A few drops of water on top of a tiny pile of bentonite powder.

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Notice the water just sits there in a little lake. It does not soak in because the bentonite gels in contact with the water and that gel acts as a barrier. This water-barrier property of bentonite is a key to its use in many products but can be a problem in ceramics (because it slows down the drying speed of bodies and glazes that contain it).

Bentonite contents affects ability to absorb water

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An example of how a dried piece of clay having lower bentonite content (left) absorbs a drop of water faster. After 10 seconds (middle picture) the water is gone while the other is still wet. By 30 seconds (bottom) all traces of water are gone.

Fired bars of a bentonite used in porcelains! Is this possible?

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Fired bentonite test bars are dark brown

Perhaps you are shocked that a material this dark and dirty (the bars are fired from cone 1 to 7 oxidation, bottom to top), would be used in porcelains. Why? Bentonites are very difficult to process. This is just raw bentonite (HPM-20), dry ground to -325 mesh (to guarantee no fired specks). That grinding does not reduce the soluble salts (that melt by cone 4) or the iron (which accounts for the dark-burning color). These undesirable properties must be tolerated (as whiteness loss) to get the plasticity supercharge 3-5% of this can impart. Why not use super-white bentonites or smectites instead? They can cost ten or even twenty times more!

What is sintering?

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Bentonite fired to 1950F in a small cast crucible. It is sintering, the particles are bonding even though there is no glass development. The powdered mass is behaving as a unit, the cohesive forces holding it together are enough to shrink the entire mass away from the walls of the container. This sintering process continues slowly, beginning around 1650. Most raw bentonites, this is National Standard 200 mesh, have a fairly low melting point, this will begin to fuse soon.

National Standard Bentonite fired to 1650F in a crucible

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The powder was simply put into this cast crucible and fired. Sintering is just beginning.

What if you just cannot solve a pinhole problem?

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Pinholing on the inside of a cone 6 whiteware bowl. This is glaze G2926B. The cause is likely a combination of thick glaze layer and gas-producing particles in the body. Bodies containing ball clays and bentonites often have particles in the +150 and even +100 mesh sizes. The presence of such particles is often sporadic, thus it is possible to produce defect-free ware for a time. But at some point problems will be encountered. Companies in large production need to have fast firing schedules, so they either have to filter press or wet process these bodies to remove the particles. Or, they need to switch to more expensive bodies containing only kaolins and highly processed plasticizers. But potters have the freedom to use drop-and-hold or slow-cool firing schedules, that single factor can solve even serious pinholing issues.

A body containing excessive non-plastics having a high bentonite addition for plasticity

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This can happen during tooling (I am making a crucible here). While the plasticity is sufficient for throwing, at lower water contents it drops off quickly. This is a mix of 5% bentonite, 10% ball clay and 85% calcined alumina. For better trimming some refractory capability needs to be sacrificed for more ball clay (perhaps 20%).

Cone 6 kaolin porcelain verses ball clay porcelain.

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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.

How fast will Veegum settle in water?

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This is VeeGum T, a processed smectite clay (similar to bentonite, extremely small particle size). I have propeller-mixed enough powder into water that it has begun to gel. How long does it take for them to begin to settle? Never. This sat for a month with no visible change! That means it is colloidal.

Test bar made from 20% bentonite and 80% calcium carbonate

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It fumes a glassy glaze onto nearby test bars at cone 10R. This fumed glaze layer on the other bars is thick enough to craze and is transparent and glossy. Any ideas why this happens? Please let me know.

Bentonite, it dries super slow and shrinks like no other clay

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It took more than a week for this small sample to dry. To make this thick gel I propeller-mixed a slurry as thick as I could make it and then let it sit for two weeks to evaporate off water. It would be very difficult to make it into a plastic material, but if I did the drying shrinkage would likely be 30% (typical plastic forming clays are around 5-8%!).

Natural bentonites fire to a surprisingly dirty surface!

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Pure HPM-20 micro-fine bentonite fired to cone 8 (top) and cone 2 (bottom) oxidation (it is actually a mix of raw and calcined material to make it possible to make the bars). Below that is an 85% silica:15% HPM-20 bentonite mix; they are fired to cone 10 (top) and 6 (bottom); these lower bars tell us the degree of plasticity imparted but also how much the bentonite is staining a normally paper-white burning material. HPM is a very expensive micro-pulverized bentonite, but, like other common bentonites, it still has significant iron. However note that much of the color on the top bars is from the soluble salts on the surface. These salts do not appear to come to the surface in the same way when mixed with the silica. It is very common to put these relatively dirty materials into porcelains to plasticize them. Why? The alternative is a material like VeeGum, it is 10-15 times the price! Still, if only a few percent of this is added, the color is affected less than you might think.

Here is what it takes to make sure P700 has minimal fired specks

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We had to sample every pallet of a 1500 bag bentonite shipment. On testing each one we found dark-coloured particulates. Then we determined which pallets where the worst and did a second round of testing. Then we mixed up 5000 gram P700 test batches from three pallets, made ware and tiles, clear glazed and fired it all to cone 10R (with heavy reduction). We also prepared samples and returned them to the manufacturer for further testing in their lab. As it turned out, the dark particles were not iron-containing and we found only a few tiny specks.

Gum does not work in a glaze if an important ingredient is missing

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These brush-strokes of gummed glaze are painted onto an already-fired glaze (the gum enables adherence and drying without cracking). Notice the brush strokes hold their character. The brown glaze on the left has 1.6 specific gravity (SG) and about 1.5% CMC gum. The white one has the same gum content but an SG of 1.5. Its brush stroke has flattened and it is running downward. Is this because of the lower SG? No - commercial glazes with an SG down to 1.3 still hold in place well. How? Because they also have Veegum to thicken and gel them. That reveals a secret: Gum needs particle surface area to work its magic. A bentonite addition supplies that. The dried strokes on the right demonstrate that - 2% bentonite has been added to that one.

The incredible plasticity of bentonite. It is the secret to win the ThrowDown!

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Two dissected vases showing the comparison in wall thickness

The 20cm vase on the left is thrown from what I thought was a very plastic body, M370. I achieved close to the same thickness top-to-bottom (5mm). The one on the right was the same original height, 20cm. But it has dried down to only 18cm high, it shrinks 14% (vs. 6% for the other). The thinnest part of the wall is near the bottom, only 2mm thick! How is it possible to throw that thin? The body is 50% ball clay and 50% bentonite. Bentonite, by itself, cannot be mixed with water, but dry-blended with fine-particled ball clay it can. That bentonite is what produces this magic plasticity. But it comes at a cost. It took about four days to dewater the slurry on our plaster table. And one month under cloth and plastic to dry it without cracks.

It is impossible to dry this clay. Yet we did it. How?

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Two dried pottery mugs

These are made from a 50:50 mix of bentonite and ball clay! The drying shrinkage is 14%, more than double that of normal pottery clay. These should have cracked into many small pieces. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (admittedly the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the more even wall thickness, the better the joins, the fewer the sharp contour changes, the more even the water content is throughout the piece (during the entire drying cycle) and the damper the climate the more successful drying will be. What did it take to dry these in our arid climate? One month under cloth and plastic, changing the cloth every couple of days. Implementing these same principles on a normal clay body will assure drying success.

The myth of aging clay

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Do you really need to age clay when you make your own? No. In ancient Japan they did not have power blenders and propeller mixers, we do. To illustrate: I just sieved out the +80 mesh and +200 mesh particles from this raw clay (from one of our stockpiles) and then propeller-mixed it as a slurry. That wetted the particles very well and made it easy to sieve. Then I poured the slurry on to a clean plaster table and thirty minutes later it was ready-to-use. Slurry mixing is just as good as deairing in a pugmill. No wait! Particles wet even better. The plasticity of this clay is wonderful, and, it will not get much better with aging. Ancient Japanese potters used non-plastic, coarse particled clays so they needed to squeeze every last bit of plasticity out of them. Today, fine particled plastic clay materials are readily available. And we have bentonite, a few percent of that and any clay can be made super-plastic in minutes.

Soluble salts as a white powder on a commercial bentonite

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A dry lump of bentonite with solubles as a powder

Bentonite is a super-plastic clay. This block of it took months to dry, the material really holds on to its water! It shrunk to about half the size and, of course, broke up into many pieces in the process (because bentonite has such a high drying shrinkage). That white powder is calcium sulphate, it is soluble and comes to the surface with the water as the clay dries. The finer the manufacturer grinds the material, the more salts are liberated. In most ceramic applications for commercial raw bentonites, these soluble salts are not an issue (but the iron content certainly can be). The reason these salts can be tolerated is that bentonite is normally employed in bodies and glazes in the 1-5% range.

Comparison of the swelling of a calcium and sodium bentonite

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Swelling of calcium and sodium bentonites

Left: A small dry lump has been immersed in water (top is a sodium bentonite from Saskatchewan, bottom is a calcium bentonite from Mexico). Right: After ten minutes both have swelled. Although the character of the swelling is a little different, both dewater on a plaster table and exhibit similar plasticity. Both create a gelled slurry having similar feel and character. This seems odd. The lesson appears to be that all sodium bentonites are not created equal. Sodium bentonites normally resist wetting, they literally cannot be stirred into water without the use of high-energy mixing equipment. And tiny amounts can gel a large amount of water or plasticize an otherwise short clay body. However, that is not the case with this sodium bentonite we are getting from Saskatchewan. This is exciting for us because it suggests that this material, although not plastic enough for use in ceramics, can be used for cosmetics, masks and detox.

Links

Minerals Montmorillonite, Bentonite
A clay mineral of extremely small particle size and high plasticity. Raw bentonite is generally a pa
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 Acti-Gel 208
Materials Veegum
A clay of incredibly small particle size. It has the highest plasticity of any known clay and acts as a suspending and gelling in slurries.
Materials Bentone MA
Materials Attapulgite
Materials Hectorite
Materials Bentolite L-3
Materials Bentonite Avonlea
Materials Bentonite CE
Materials Bentonite Tech 5
Materials Bentonite WA
Materials GK 129 Bentonite
Materials Hectalite 200
Materials Ibex Bentonite
Materials Kaopolite K 129
Materials Milwhite Bentonite B
Materials vol331cer
Materials vol351cer
Materials vol364cer
Materials vol446cer
Materials Volclay SPV 200 Bentonite
Materials Polargel
Materials Wyoming K-4
Materials Cape Cross Bentonite
Materials Claytone
Materials National Premium WT Bentonite
Materials HPM-20 Volclay Bentonite
Hazards Bentonite Toxicity
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 Clay Other
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
Typecodes Materials used in Denmark
Typecodes Additives for Ceramic Bodies
Materials that are added to bodies to impart physical working properties and usually burn away during firing. Binders enable bodies with very low or zero clay content to have plasticity and dry hardness, they can give powders flow properties during pressing and impart rheological properties to clay slurries. Among potters however, it is common for bodies to have zero additives.
Typecodes Additives for Ceramic Glazes
Materials that are added to glazes to impart physical working properties and usually burn away during firing. In industry all glazes, inks and engobes have additives, they are considered essential to control of cohesion, adhesion, suspension, dry hardness, surface leveling, rheology, speed-of-drying, etc. Among potters, it is common for glazes to have zero additives.
URLs http://www.volclay.com.au/products.htm#Bentonite.htm
About Bentonite at Volclay.com
URLs http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc03/icsc0384.htm
Bentonite hazards at ilo.org
URLs http://en.wikipedia.org/wiki/Bentonite
Bentonite at Wikipedia
URLs http://www.sorptive.org/
Sorptive Mineral Institute for producers of absorbent clay mineral products
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 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 Permeability
In ceramics, the permeability of clay slurries and plastics determines the rate as which water can move through the matrix
Articles Binders for Ceramic Bodies
An overview of the major types of organic and inorganic binders used in various different ceramic industries.
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.
Recipes L2000 - 25 Porcelain
Base 25x4 porcelain recipe

Mechanisms

Body Plasticity1 part bentonite can plasticize a body as much as 10 parts kaolin. Bentonitic bodies are stronger in the dry form but dry slower, crack more and fire darker with potential iron specks (get a super fine ground grade).
Glaze Suspender1-3% bentonite can greatly improve glaze suspension by geling it. In addition it will harden the dry layer. Coarser varieties can impart some glaze speck. If a glaze already contains more than 15% clay (kaolin, ball clay) you should not need more than 1% bentonite.
By Tony Hansen
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