Alternate Names: Montmorillonite, Bentonite USA
A comparison of the plasticity of Volclay 325 Bentonite:Silica 25:75 (top) and Hectalite 200:Silica 50:50. Both are mixed with silica powder. The latter (a highly refined bentonite) is much less plastic even though it is double the percentage in the recipe.
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.
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.
Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and diring 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.
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).
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).
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%!).
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.
These brush-strokes of gummed glaze are painted onto an already-fired glaze. Gummed glazes can do this, they will adhere and dry without cracking. And dry hard and resist washing off. Brush strokes hold their character. The brown glaze 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. It's brush stroke has flowed flat and it is running downward. Is it because of the lower SG? No. Commercial glazes with an SG down to 1.3 perform well also. The secret: Gum needs particle surface area to work its magic. We can get that with a bentonite addition. The dried strokes on the right were much better, that glaze adds 2% bentonite (and we raised the SG to 1.6). That made all the difference, it painted beautifully.
The 20cm vase on the left is thrown from what I thought was a very plastic body, 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. It took about 4 days to dewater the slurry on my plaster table. This is the poorest drying body one could possibly use. But there is the lesson here: Even this can be dried crack-free. How? One month under cloth and plastic to assure water content throughout! This means that pretty well any other body can be dried without cracks if done sufficiently evenly.
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. It should be impossible to dry them, the most bentonite bodies can normally tolerate is 5%. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (but the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the better the joins, the more even the water content is throughout the piece during the entire drying cycle and the more damp of a climate you live in the better your drying success will be. What did it take to dry these: 1 month under cloth and plastic! I changed the cloth every couple of days. So by implementing these same principles you will have better drying success.
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.
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.
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.
HPM-20 micro-fine bentonite fired from cone 1 to 7 in oxidation. This bentonite is expensive compared to others and it used for the guarantee that there are no speck producing particles. However it is still high in soluble salts (that melt by cone 4) and is very dark burning in color. It is not unusual to put 3-5% of this (and other dirtier bentonites) into Grolleg porcelain bodies (where whiteness is supposedly important).
Bentonite fired to 1950F in a small 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.
The powder was simply put into it and fired. Sintering is just beginning.
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 production 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.
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%).
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.
This is VeeGum T, a processed Hectorite 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.
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.
|Materials||HPM-20 Volclay Bentonite|
|Materials||National Premium WT Bentonite|
|Materials||GK 129 Bentonite|
|Materials||Kaopolite K 129|
|Materials||Milwhite Bentonite B|
|Materials||Bentonite Tech 5|
|Materials||Volclay SPV 200 Bentonite|
|Materials||Big Horn CE 200 Bentonite|
|Materials||Cape Cross Bentonite|
|Suppliers||Best Industrial Corporation Ltd.|
|Suppliers||American Colloid Company|
|Suppliers||BPM Minerals LLC|
|Typecodes||Ceramic body and glaze binders, plasticizers
|Typecodes||Materials used in Denmark
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.
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.
|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.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
In ceramics, the permeability of clay slurries and plastics determines the rate as which water can move through the matrix
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.
|Recipes||L2000 - 25 Porcelain
Base 25x4 porcelain recipe
Sorptive Mineral Institute for producers of absorbent clay mineral products
Bentonite hazards at ilo.org
Bentonite at Wikipedia
About Bentonite at Volclay.com
|Hazards||Quartz, Crystalline Silica Toxicity|
|Body Plasticity||1 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 Suspender||1-3% 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.|