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Porcelain

Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.

Details

Traditional utilitarian porcelains are comparatively white burning and vitreous clay bodies that are normally made from feldspar, clay and quartz. During firing, the feldspar flows and dissolves many of the other particles into an increasingly viscous glassy melt that bonds the quartz particles and, and if temperatures are sufficient, provides the conditions to enable the flat kaolin crystals to convert to long mullite crystals. Porcelains normally have a smooth and pleasant surface (if fired to sufficient temperatures). Those low in titanium and vitrified well can be translucent. Porcelain clay bodies are made from fine grained materials and lack the iron impurities of stonewares. They are usually fired above 1180C, the lower practical limit of feldspar as a vitreous body flux. Porcelains tend to warp during firing because they must be being taken closer to the melting point to achieve the desired properties.

Porcelain bodies are available in a wide range of plasticities. Plastic porcelain bodies have traditionally been much shorter (less plastic) than their stoneware or earthenware counterparts. But in recent years white-burning bentonites have made it possible to make even translucent porcelains plastic. Porcelains used by potters are much more plastic than those used in industry (machine forming can be done using bodies of very low plasticity). Porcelain casting slips achieve the whitest and most translucent results because they do not need to be as plastic (the plastic materials contribute the most iron, which darkens the color, and titanium, which impedes translucency). Typical porcelains are a mix of clays (kaolin, ball clay and plasticizers), feldspar (the melter) and quartz (the low expansion filler and framework). By employing frits instead of feldspar it is possible to achieve vitrification at much lower temperatures. Dental porcelain, for example, is made from very high percentages of frit.

Porcelains contrast with refractories in that the fired matrix of the latter is just particles fused together at their points of contact (with voids between). The feldspars in porcelain form glasses that fill the voids, glue the silica or other refractory particles together and seal the matrix from the penetration of water.

A starting recipe is 50% clay (a mix of kaolin and ball clay) and 25% each of feldspar and silica (the so-called “25 Porcelain”). With most feldspars this will mature at about cone 10. The more kaolin there is in the clay portion the less plastic the body will be (but the whiter firing it will be). Ball clay can be omitted if a plasticizer (like hectorite or bentonite) assists the kaolin (typically 5% or less). By switching to feldspar to nepheline syenite, maturity can be dropped a little. Increasing the feldspar to 35% can drop maturity to cone 6 (35% is the practical limit to allow enough space in the recipe for the needed silica and clay). Substitute a high-soda low-boron frit (e.g. Ferro 3110) for feldspar to drop the temperature even further. Be aware that reducing silica too much (e.g. down to 15%) could mean that glazes will craze. Porcelain recipes that fire very white while still being very plastic will be the most expensive. The most extreme example of a cost-is-no-object super-white porcelain is one that employs a zircon-opacified frit as the flux and a highly processed smectite as the plasticizer.

For maximum density (not always needed) porcelains should be fired to the temperature at which the porosity and fired shrinkage curves reach their minimum and maximum (respectively), or a little beyond that. If they are fired higher than that they become too unstable in the kiln. Development projects to create and maintain a porcelain recipe should do maturity testing at several cones above and below the intended firing temperature to get a complete view of the body's performance (use the SHAB test procedure).

Special purpose porcelains can be made by incorporating a wide range of materials. Alumina for hardness. Pyrophillite for thermal expansion. Zircon for whiteness. Frit and talc for maturity. Stains for color. Wollastonite is also used, although the reasons are not clear. And many more.

Related Information

Why mid-fire Grolleg porcelain is ideal for both throwing and casting

Two purple-outside, white inside thrown Grolleg cone 6 porcelain mugs

The very whitest porcelains are made from New Zealand kaolin. However, while Grolleg kaolin does not fire quite as white, it requires up to 10% less feldspar to produce a vitreous porcelain (it contains natural feldspar). That 10% less spar can be made up in kaolin, imparting better workability and dry strength to the body (and Grolleg is known for its dry strength). Assuming that 25% silica is needed for glaze fit, one only needs to discover what blend of feldspar and kaolin in the remaining 75% achieves the desired degree of vitrification (e.g. we like zero porosity just-reached at cone 6). We found 25% nepheline was too vitreous (pieces warped) and at 20% porosity was not yet zero (do your testing to establish your best percentage). While the Grolleg version fires a little darker, the better workability imparted by the extra kaolin makes up for that. The plasticity needed for good throwing requires the addition of bentonite (4% for NZK and 3% for Grolleg). Both of these can be made into casting bodies by reducing the amount of bentonite (~ 1% for NZK, 0.5% for Grolleg). Do your testing to discover the % of bentonite needed for the leather hard to pull away from a mold without cracking but not take too long to cast.

Twenty six bodies. Porcelains and native. Which do I like best?

I am testing runs of clays we (Plainsman Clays) make for potters. We are doing too many small-run products, bodies that we want to discontinue because others we are really good at making are much better. I am using native bodies and porcelains but the native ones will turn out best. I got lots of s-cracks on the porcelains, the low humidity at this time of the year caught me by surprise. But I got zero cracks on native bodies. And I am using engobes on the natives, these can turn even a dark colored stoneware surface pure white (I am also using a black engobe here). I'll use a mix of base and cover glazes that I make myself (using recipes we publish) for food surfaces and decorate some using bottled commercial glazes.

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.

Could oxidation porcelain look like reduction blue porcelain?

These clear glazed porcelain mugs are fired at cone 6. The one on the right has 0.2% Mason 6336 stain added to to G2926B base glaze. It matters which stain, some agglomerate and causes specking, this one disperses well.

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).

Cross section view of the inside and outside glazed walls of a porcelain vessel

Porcelains look much more glassy and melted than you might expect when viewed close up (this is cone 6 Polar Ice from Planisman Clays). The development of the glassy phase within the body creates a very good bond with the glaze. Actually it is a bonding zone where the glaze has melted into the body enough to create a transition rather than just a point of contact. The degree to which this transition develops determines the integrity of the bond. Of course, with porcelains it is far better developed than with stonewares and terra cottas.

Turbo-charge plasticity using bentonite, hectorite, smectite.

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).

Does Grolleg whiten a glaze the same as it does a body?

Yes. The two specimens are both the same Grolleg-based porcelains. Both of them are glazed with the same glaze: 1947U transparent. But the glaze on the left is using EP Kaolin and the one on the right Grolleg kaolin. The Grolleg glaze is dramatically better, the color has a bluish cast that is more attractive. The Grolleg does not suspend the slurry as well, however it responds well to gelling (using vinegar, for example) more than compensating to create an easy-to-use suspension.

The foot ring on the left is plucking, the right one is not. Why?

These are translucent porcelains, they are vitreous. The firing is to cone 10. The one on the left is a cone 6 body, and, while it survives to cone 10 it does warp. But more important, it is much more vitreous (more melted). The plucking problem makes it quite difficult to get a good foot ring. The other, which has only slight plucking, is also quite vitreous (high in feldspar). The plucking problem on both can be solved by simply using a better kiln wash. What is better? More refractory, and therefore having a powdery, non-stick surface. Spend more money on your kiln wash, base it on calcined alumina or zircon.

A tiny percentage of blue stain in a porcelain has amazing power

The top porcelain bar has only 0.07% Mason 6336 blue stain added (vs. none in the bottom bar). This is a low fire frit-ware body fired at cone 03 in oxidation. At a slightly lower percentage (e.g. 0.05%) this porcelain will have the same color as a cone 10 reduction one (when covered with a transparent glaze). However adequate glass development is needed before the blue color develops.

The blue color in this porcelain develops more as maturity increases

These fritted porcelain bars are fired at cone 06, 04, 03 and 02 oxidation (bottom to top). The body contains 0.2% blue stain. Notice that almost no color develops at the lowest temperature. Glass development is needed.

A Grolleg based cone 10R porcelain (left) vs. 25-Porcelain

25 Porcelain refers to a recipe of about 25% each of ball clay, kaolin, feldspar and silica. In this case the 25-porcelain employs Tile #6 kaolin and a Kentucky ball clay. Both of them contain raw bentonite to augment the plasticity and both have about 50% clay in the recipe. The Grolleg body requires more bentonite to achieve the same plasticity. In spite of the fact that the raw bentonite has a high iron content and it darkens the color, the Grolleg porcelain is still much whiter firing.

Translucent Porcelain Lithophane by Stephanie Osser

Made using porcelain fired to cone 11+. This demonstrates translucency.

Two reasons why porcelain recipes need silica

A porcelain cup with serious crazing and base crack concentric to the center

This is 70% kaolin and 30% feldspar. Fired at cone 6 with glaze G2926B. The fired body has a nice porcelaneous surface. But, right out of the kiln, it crazes like this! The dense craze pattern indicates a very serious fit problem. The thermal expansion of the kaolin:feldspar mix is much too low. Adding 25% low-expansion silica will solve the problem. The other issue is with the flat particle shape of kaolin. The throwing process has lined up the predominant kaolin particles concentric to the centre. During drying, and especially firing, more shrinkage occurs across them than along them. All ten of the cups made cracked like this! The solution is adding a filler, one with rounded particles to separate the kaolin plates. Silica is perfect, using the same 25% addition. The grains act like aggregate in concrete, strengthening the matrix and separating the clay particles, forcing them to orient more randomly.

Mother Nature's porcelain with no glaze!

Fired at cone 6. It is impossibly vitreous, the surface is smooth like a glaze. And it has not warped. In fact, other pieces made from it having walls as thin as 2mm did not warp either!. This comes from a two-foot-thick section of the 3B layer from a Plainsman Clays quarry near Ravenscrag Saskatchewan, Canada. A cretaceous dust storm! It is plastic and feels impossibly smooth. Smoother than any commercial porcelain. It does not fire white because mother nature did include a little iron oxide. It accepts glaze like a porcelain.

A problem with super-white porcelain mugs

These are made from Plainsman Polar Ice translucent NZ porcelain. The one on the right was used in the coffee room of the plant and washed between uses in a common manner (which is: not very much!). The stains are obviously not nearly as visible on a stoneware mug.

With porcelains, poor plasticity gets worse at the leather hard stage

This porcelain becomes quite brittle as it gets stiffer making it difficult to make these cuts in the foot ring. This creates extra sponging work when it is dry. It also means that dry strength will be low. Porcelains do not need to be this way, plenty of white burning bentonites are available (although they increase cost).

This is how much iron is in a box of the cleanest porcelain you can make!

The recipe: 50% New Zealand kaolin, 21% G200 Feldspar, 25% silica and 3% VeeGum (for cone 10R). These are the cleanest materials available. Yet it contains 0.15% iron (mainly from the 0.25% in the New Zealand kaolin, the VeeGum chemistry is not known, I am assuming it contributes zero iron). A 50 lb a box of pugged would contain about 18,000 grams of dry clay (assuming 20% water). 0.15% of 18,000 is the 27 grams of iron you see here! This mug is a typical Grolleg-based porcelain using a standard raw bentonite. A box of it contains four times as much iron. Enough to fill that cup half full!

Working with Polar Ice translucent porcelain requires impeccable cleanliness

Using stonewares it is easy to get pretty sloppy in the studio because a particle of iron or cobalt in a glaze or body is no big deal. But on a ice white, translucent, transparent-glazed piece it is a really big deal. These specks are particles of cobalt that were trapped in my 80 mesh glaze screen from previous use. I use a soft brush to coax the glaze through the screen faster, but even that was enough to dislodge some of the cobalt particles. The lesson: I need a dedicated glaze screen for use with this transparent glaze, it gets used for nothing else.

Why is this glaze so different on these two different porcelains?

Why the difference? The one on the right (Plainsman M370) is made from commodity American kaolins, ball clays, feldspars and bentonite. It looks pretty white-firing until you put it beside the Polar Ice on the left (made from NZ kaolin, VeeGum plasticizer and Nepheline Syenite as the flux). These are extremely low iron content materials. M370 contains low iron compared to a stoneware (less than 0.5%) that iron interacts with this glaze to really bring out the color (although it is a little thicker application that comes nowhere near explaining this huge difference). Many glazes do not look good on super-white porcelains for this reason.

Translucency of Polar Ice compared to another porcelain at cone 6

On the top you can see the color difference. The other porcelain is made from a low TiO2 mix of typical North American kaolins, feldspars and bentonites. Bottom with a light inside: Polar ice on the left is far more translucent. Yet it is not overly mature, it resists fired warping remarkably well. And it is also more plastic (which seems impossible). There is a secret to the translucency that goes beyond the fact that it employs New Zealand kaolin and the percentage of feldspar it has. But I cannot tell you. But if you read this site carefully you will discover it in the most unlikely place!

A novel way to compare degree of porcelain vitrification

These two unglazed porcelain tiles appear to have a similar degree of vitrification, but do they? I have stained both with a black marker pen and then cleaned it off using acetone. Clearly the one on the right has removed better, that means the surface is more dense, it is more vitreous. In industry (e.g. porcelain insulators) it is common to observe the depth of penetration of dye or ink into the matrix as an indication of fired maturity.

Want to make a cone 10R super translucent porcelain? Think again.

On the right is a porcelain used in China, renowned for its whiteness and translucency. On the left is a body made from Grolleg kaolin, this is commonly used by potters. They were fired in reduction. The tiny iron specks that potters do not even notice are enemy number for the blue-white porcelain like this. Although they might be small the reduction atmosphere makes them blossom out in full glory to ruin the piece. These specks come as contaminants in the materials (especially the silica) and they are easily picked up during fabrication. For very white bodies like this, it is incredibly difficult to prevent the specks. For a perfectly white flawless result, the entire factory must be dedicated to this one body; they use wet processing, magnets, filter pressing, stainless steel equipment and impeccable procedures.

The same liner glaze crazes on the porcelain but not the stoneware

The stoneware has a higher silica content and is not vitreous. This means there are more quartz particles to impose their high expansion because fewer are taken into solution by the feldspar.

Reduction and oxidation porcelains

Left: Cone 10R (reduction) Plainsman P700 porcelain (made using Grolleg and G200 Feldspar). Right: Plainsman Cone 6 Plainsman Polar Ice porcelain (made using New Zealand kaolin and Nepheline Syenite). Both are zero porosity. The Polar Ice is very translucent, the P700 much less. The blue coloration of the P700 is mostly a product of the suspended micro-bubbles in the feldspar clear glaze (G1947U). The cone 6 glaze is fritted and much more transparent, but it could be stained to match the blue. These are high quality combinations of glaze and body.

Links

Tests Shrinkage/Absorption Test
Glossary Maturity
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
Glossary Stoneware
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
Glossary Translucency
A highly sought after property in porcelain, they are fired close enough to melting to pass considerable light. It can be very difficult to fire translucent ware without it warping.
Glossary Earthenware
What is the difference between earthenware and a regular stoneware body? Earthenwares lack the glass development to fill voids and glue particles.
Glossary Vitrification
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
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 Interface
In ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.
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.
Recipes L2000 - 25 Porcelain
Base 25x4 porcelain recipe
Media A 3-minute Mug with Plainsman Polar Ice
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.
Materials Kaolin
Materials Bentonite
Minerals Smectite

By Tony Hansen


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