Formulating a Porcelain — Digitalfire

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.

A translucent Polar Ice porcelain mug

Incredible strides in porcelain ware production and firing equipment have occurred in recent decades. Robotics, controllers, inkjets, 3D printers and other equipment have revolutionized the whole ceramics industry. This has put the focus on fiddling with machines rather than the porcelain recipe. Let's try to take at least some of the mystery out of common plastic porcelain formulation (many of the principles also apply to casting and pressing versions).

A traditional functional ware plastic porcelain is actually just a vitrified clay body with low Fe₂O₃ contamination. Unlike glazes, here we are interested in physical properties that can be observed and measured rather than the chemistry.

General purpose functional porcelains contain:

Silica (quartz): It provides an aggregate framework for the fired matrix (like gravel in concrete).
Clay: Imparts plasticity and drying hardness to the wet materials and transforms into a mesh of crystals during firing (which gives porcelain its strength).
Feldspar and Nepheline Syenite: The melting of the feldspar dissolves some of the silica and kaolin grains and fills the voids between them, cementing together a strong mass.

Special purpose porcelains may also contain:

Alumina: Instead of or to augment silica to create a lower expansion and harder fired material.
Organic hardeners: Instead of or in addition to clay (for dust pressing bodies, for example).
Frits: Instead of or in addition to feldspar to achieve a lower temperature maturity.
Low expansion minerals (e.g. pyrophyllite): to reduce co-efficient of thermal expansion (COE), these require glazes also of lower COE also.

The whitest and most translucent porcelains are made using the most expensive and cleanest (low iron) materials.

The "Universal 25 Porcelain" recipe typically produces an inexpensive cone 10 not-very-white porcelain having moderate plasticity and near zero porosity. It is made from 25% each of ball clay, kaolin, feldspar, and silica, or more simply, 50% clay and 25% each of silica and feldspar. Thousands of potters and manufacturers use this "as is" using the brand name materials they have access to. Let's consider some physical properties to look for in a porcelain (not ordered in any specific way).

Translucent vs non-translucent porcelain

Maturity: Water absorption (or porosity) is the most common measure of maturity. Near zero-porosity is normal. Fired shrinkage is also a good comparator of maturity, as bodies mature and densify in the kiln they shrink to a minimum, hold for a while and then begin to expand and bloat as they over-fire. Highly vitreous translucent porcelains can have more than 15-18% total shrinkage (from pugged to fired) whereas whiteware type bodies (0-1% porosity) have around 12% shrinkage. It is wise to target the firing temperature of a body based on a knowledge of its maturity over a range of temperatures below and above.
Fired translucency: This comes at a price - translucent porcelains melt more so they warp more. They require more feldspar and less and whiter clays (typically only kaolin) having a low TiO2 content. White kaolins are less plastic and more expensive.
Consistency. Only quality control over several years will demonstrate this. Look for variations in drying shrinkage, drying performance, fired porosity and shrinkage and fired color and character. Test to connect property changes with a specific raw material. Different kaolin and ball clay companies display diverse commitments and attitudes toward maintaining physical properties of their materials for the ceramics industry.
Plasticity. This can be deduced from drying shrinkage, and judged physically by comparing workability. Highly plastic porcelains can have up to 7% drying shrinkage. Plasticity can be imparted (after firing properties have been achieved) by the addition of bentonite or related mineral products (many different ones are available).
Drying Performance. Porcelains often tend to crack when drying, they do not have the green strength of stoneware bodies (as noted, they can have very high drying shrinkages if significant plasticizers are present). Use an accelerated drying gradient test (like DFAC test) to characterize this property.
Fired and Dry Strength. Porcelains usually have low dry strength but high fired strength. There are a variety of ways you can compare the strengths of different porcelains (use your ingenuity, and lab equipment if you have it).
Speck Development - Observe this in fired ware, especially in reduction burning. Impurities can appear in various raw materials, but especially ball clays and bentonites, identify sources by washing material through a 200 mesh sieve.
Solubles. Ball clays, bentonites and sometimes kaolins have soluble salts, calcium and magnesium sulfates that migrate to the surface with water during drying. These can leave visible surface-scum after drying and firing. Use the SOLU test to determine if this is a problem.
Thermal Shock Resistance. This refers to the fired clay's ability to withstand thermal gradients (sudden changes in temperature to one part of a piece) without cracking. If this property is important to you, compare bodies using a flame or ice water/boiling water immersion test.
Ease of Glaze Fit. Some porcelains craze glazes much more than others, especially if the ware is subjected to sudden cooling after being hot. Porcelains with high silica contents are easier to fit glazes to (provided it is not too fine grained). When additions of pyrophyllite are used the thermal expansion of glazes must be reduced to avoid crazing.
Fired Whiteness. Paper white porcelains are possible. Super white kaolins and iron free plasticizers are amazing materials, but they are much much more expensive.
Firing Volatility. If translucency is needed, for example, it is usually necessary formulate the body so that it softens enough to achieve the desired translucency, but not so much that it warps excessively. The target temperature range can be quite narrow and good firing control is thus needed (consider such “tipping point” bodies).
Surface Character. The most pleasant surfaces are the most vitrified. That means the recipe must contain the most feldspar possible.

Porcelains can be compared in all of the above areas. Testing is needed to find property balances. Most physical properties can be measured or at least compared using simple equipment, methods and observations. Let's look again at each of the materials in the recipe.

Pure #6 Tile kaolin can be thrown like a pottery clay

A true porcelain would normally derive all its plasticity from a kaolin. Since kaolins are often of limited plasticity, this limits the workability of throwing or modeling porcelains made from them. Still, there are some surprisingly plastic kaolins, although the limited range of particle sizes can mean less than ideal drying performance. Do not accept that a kaolin is plastic just because its name includes the word plastic or a supplier says it is plastic, test yourself. For casting porcelains, an all-kaolin approach is quite feasible (using 50% kaolin rather than 25% kaolin and 25% ball clay) since these bodies benefit greatly from the reduced drying shrinkage and increased water permeability associated with the larger particle size of kaolins. However an all-kaolin casting porcelain may not have the strength to pull away from the plaster mold without cracking, add a little plasticizer if needed.

Kaolins can differ widely in maturity. British kaolins require the use of less feldspar because they already have some natural fluxes as part their mineral particle profile. So even though these might be less plastic, less flux is needed, so more kaolin can be used in a recipe. It is an good idea to make up the kaolin complement of a recipe using more than one brand, this provides a better distribution of ultimate particle sizes and minimizes the effect on the body if one kaolin changes.

Since kaolins vary quite widely in their plasticity, maturity, soluble salts, particle size and whiteness, it can be quite a challenge to test and classify them all. Data sheets are often not that helpful because they present information in different ways and seldom does a company explain its materials in terms of other well known alternatives. The burden of picking the best kaolins thus rests on you and your ability to evaluate and compare them using tests that document the appropriate physical properties. We recommend you use the DFAC test, SHAB test, and LDW test to compare kaolins (and ball clays).

About casting bodies: Larger particle size kaolins tend to be dirtier, less plastic, more expensive and have lower dry strength, so their benefits come at a cost. It is possible to make fairly fast-casting zero-ball-clay bodies with ordinary light burning kaolins like EPK. Some companies use large-particle-size kaolin when they also have ball clay in the mix, the latter may be canceling the benefit of the former! Of course large particle ball clays are also available, but remember that they are still considerably finer than standard kaolins. Also, fine-tuning a kaolin mix makes little sense if the body is not deflocculated properly. The best approach is to use a standard white burning kaolin, deflocculate it properly and learn to work with it. Then fine tune it by the substitution of some large particle material to speed casting rate while watching for any deleterious properties introduced.

Following is a sample of a Foresight report on a kaolin (Foresight was the predecessor to insight-live.com). I have tested using DFAC, SHAB, LDW and SIEV tests.

OM4 Ball Clay fired cone 10R and 10 down to 4 oxidation

Ball clay is much finer and thus much more plastic than kaolin. A very wide range of ball clays are available. White burning ball clays should not have near the plasticity of their dirtier counterparts - but often they do! Bentonite is much finer than ball clay (the ultimate particles are much smaller). It is incredibly plastic, adding only 2% to a recipe can drastically improve working properties. However, these materials have a down side. Ball clays can have ten times the amount of brown-firing iron oxide that kaolin has and many have soluble salts that can produce a scum on the burned surface. Many also contain lignite particles that can produce glaze imperfections. Raw bentonites can be downright dirty, burning brown or red with possible specking and soluble salts sometimes so heavy they form a glaze. White firing bentonites come in a wide range of plasticities. Some have so little plasticity one wonders why anyone would use them for ceramics, others are incredibly plastic. But beware, even though only 3-5% might be used, the cost of this one material could be more than the other 95% combined! A study of all the ball clays and bentonites available in your area can be a real education! Again it is important that you have a testing regimen to compare and characterize these materials (e.g. a group account at Insight-Live.com). There is another up-side of ball clay worth mentioning. Ball clays contain free silica, so if there is significant ball clay in a recipe the silica can be reduced leaving room to add more clay for plasticity or feldspar for maturity.

So the easiest way to increase the plasticity of the standard '25 Porcelain' is to add 2%-3% raw bentonite. Even though most inexpensive 200 mesh bentonites can be quite dirty, this small amount may not affect the fired color as much as expected. Do not overlook one detail: Bentonite is difficult to process, one bag of raw bentonite may not contain iron specks while the next does not. Microfine ceramic grades (e.g. 600 mesh) are available, but obviously are much more expensive. If whiteness is not all-important, increase the ball clay at the expense of kaolin to produce a plastic whiteware (feldspar percentage may need reduction also, ball clay is less refractory than kaolin). For a very white porcelain, the challenge will likelybe reducing or entirely eliminating the ball clay. Where super whiteness is secondary to having a good general purpose body, some ball clay can usually be tolerated. If having the most plastic body possible is the most important, then use all ball clay and no kaolin (e.g. B-Mix from Laguna), that brings awesome workability also.

Feldspars (and Nepheline Syenite)

Feldspar vs Nepheline Syenite melt fluidity test

These are the fluxes, or more correctly, contain the fluxes. Fluxes are the oxides that help develop fired maturity by liquefying and slowly dissolving some of the clay and silica. The total flux amount necessary is easily determined by simply firing to a range of temperatures above and below the intended; studying the absorption, strength, and firing shrinkage curves; and adjusting the amount of feldspar to give the desired maturity. The amount of feldspar for a cone 10 body varies from 15%-30%, depending on the other materials in the recipe. For a typical American kaolin, it takes about 25% for cone 10 and up to 50% for cone 6.
Feldspars are not without potential problems. While some brands can be relatively iron free, others fire surprisingly darker. Some can present flocculation problems due to slight solubility. Sodium feldspars are generally cleaner and more potent. Use two or three together if possible (to dampen changes that could occur in one).

When silica is inadequate this happens

Silica tends to be a very consistent and inexpensive material. Quartz grains act primarily as a micro-aggregate or framework structure for the fired matrix. In addition, some of the silica particle and particle edges are dissolved by the fluxes to produce aluminum-silicate glasses. Too much silica in a recipe could mean lower plasticity (since less room is left for clay). However, there is also much discussion about the detrimental effects of crystobalite (i.e. dunting), whose development during high temperature firing is related to available free quartz. Thus there is some merit to lower silica amounts, especially if you have the ability to adjust your glazes to lower their expansion. The use of less silica means more clay can be added resulting in higher plasticity. A finer silica (300 mesh) reacts better with the fluxes and thus less is needed (but thermal expansion drops). Too little silica in a body can mean crazing glazes since the quartz mineral contributes to higher expansion that assists glaze fit. Of course, lack of silica will also mean more of a tendency to warp during firing. For cone 10, many technicians aim at 20-25% for expansion reasons and to provide firing stability over a range of temperatures.

Most people have noted that the '25 Plastic Porcelain' recipe has flaws that can be corrected for individual situations and materials. To improve this recipe for cone 10:

For more whiteness replace some or all of the ball clay with plastic kaolin
Add some bentonite (e.g. 1-3%) if more plasticity if needed (but be prepared for more drying shrinkage)
Diversify the clays and feldspar, using 2 or 3 kinds of each (to make the recipe resilient to change in individual materials)
Test fire and adjust maturity by tuning the feldspar percentage (usually by trade-offs with the clay).

To formulate a white translucent cone 6 plastic porcelain:

Start with 20% silica for glaze fit
Add 40% feldspar for maturity (Nepheline Syenite or Minspar are very white)
Add 40% of the whitest kaolin possible (e.g. New Zealand, UK)
Add 3-5% of the whitest bentonite or hectorite available (Veegum is the best we know of)
Make a trial mix and evaluate the fired porosity, drying performance and workability
Add or reduce plasticizer as needed
Increase feldspar at the expense of kaolin for more maturity, do the opposite if it is over-fired
Test again and repeat.

For a casting porcelain, remember that much lower plasticity is needed. Common consensus in industry is that it is much more important to use kaolins of large particle size, so water can easily be drawn out by the plaster mold. However for a smaller operation that does not need to really optimize mold release time, normal kaolins are fine. The cleanest kaolins and ball clays are also the least plastic, thus casting porcelains (that can tolerate this) can achieve whiter and more translucent effects than their plastic counterparts. It is imperative to understand the principles of deflocculation so that the amount of water in the slurry can be minimized and proper mold release and casting time can be achieved. To cast very thin ware some bentonite May be needed (start with 1%) to give it the strength to pull itself away from the mold during drying.

For plastic porcelain for modelling or throwing pay careful attention to drying properties. Since porcelains are fine-grained, they don't usually dry well, thus plastic porcelains are even worse. Add just enough bentonite to give the plasticity needed (the DFAC test can be handy for this). Some people add molochite grog to porcelains for better drying.

Give careful thought to how mature the porcelain needs to fire. If zero absorption is not necessary, consider reducing the feldspar to get more plasticity from the clays (and be able to reduce the bentonite). Such a body can still be considered functional and vitreous and it will resist warping in the kiln. In addition, material changes that result in more maturity will be less likely to cause trouble in a body that has some 'room to move'. Measure fired absorption at a variety of temperatures (a 'measure-only-at-the-working-temperature' philosophy is a tunnel vision approach that will almost certainly get you into trouble, especially in situations where the working temperature is far above the point at which zero porosity is attained).

Trade-offs to consider when formulating or adjusting a plastic porcelain.

These are fired around 1250C bisque and glazed between 1050 and 1100C. One composition we know of is 45% bone, 30% clay and 25% mix of potash feldspar and quartz. But lower bone versions are used also, it being as low as 10% (the clay is the same but the feldspar obviously has to be much higher). It is not completely clear why companies do not simply add frit to a standard white porcelain to get translucency. If you know this, please email me.

Frit can be used to mature a porcelain, Zero4 porcelain is an example.