Formulating a Porcelain

Section: Clay Bodies, Subsection: Formulation

Description

Understanding the functions of each of the major materials in a high temperature porcelain gives you the ability to tune their amounts and choose brand names to make the porcelain you want.

Article

Incredible strides in porcelain ware production and firing equipment have occurred in the last couple of decades. Robotics and computer controllers have revolutionized the whole ceramics industry. However, clay bodies themselves have tended to resist change. Perhaps it is much easier to understand and troubleshoot a machine than a clay body. Understanding the dynamics of powder, slurry, and wet materials processing, forming, drying, and firing is not easy. Let’s try to take at least some of the mystery out of pottery porcelain formulation.

A pottery porcelain is actually just a vitrified clay body with low Fe2O3 contamination. A general porcelain recipe is fairly easy to derive. The initial thought process goes like this:

The "Universal 25 Porcelain" recipe is a product of this type of reasoning. 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 companies use this as is or alter it to accommodate specific materials or circumstances. Let’s use this recipe as an example and a basis from which to learn how to optimize a porcelain to your own circumstances. Also, I will propose an approach to creating a porcelain from scratch.

Before continuing, let’s define the physical properties to look for in a porcelain (other than price). Here are a few:

Porcelains can be compared in all of the above areas. Logically, you cannot have the best of all of them. There are always trade-offs, compromises.

These physical properties can all be measured using simple equipment, methods and observations as described in the FORESIGHT Ceramic

Database software (and now at www.ceramicmaterials.info). Later, I will show you an example report to demonstrate.

Let’s look at each of the materials in the recipe to understand their functions and how they can be changed.

Kaolin

A true porcelain would normally derive all its plasticity from a kaolin. Since kaolins are normally of limited plasticity, this obviously limits the workability of throwing or modelling porcelains made from them. Still, there are some very plastic kaolins and it is possible to make a fairly plastic body from them, although the limited range of particle sizes can mean less than ideal drying performance. 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.

Kaolins can differ widely in maturity. British kaolins require the use of a lot less feldspar because they already have some natural fluxes as part of the mica mineral they contain. Although these might be less plastic, less flux is needed, so more kaolin can be used in a recipe.
The kaolin complement in the ‘25 Porcelain’ formula can be increased at the expense of ball clay, preferably replacing all of it, but also at the expense of silica and feldspar. Again, it is an excellent idea to make up the kaolin complement using several different types. This provides a better distribution of ultimate particle sizes, minimizes the effect on the body if one kaolin changes, gives you some control over maturity, and even enables you to use neutral kaolins as fillers.

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 often are 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, SHAB, and LDW ceramicmaterials.info (or FORESIGHT) tests to evaluate kaolins and ball clays.

Do not rely too much on particle size numbers on data sheets. Larger particle size kaolins are dirtier, less plastic, more expensive and have lower dry strength so their benefits come at a cost that is often too high. It is possible to make fairly fast casting zero-ball-clay bodies with ordinary white burning kaolins like EPK. Many people use large particle size kaolin when they also have ball clay in the mix so the latter is cancelling 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. I have tested using DFAC, SHAB, LDW and SIEV tests.

TESTDATA REPORT FOR A RUN 
======================================== 
  NUMBER: L2497 
 DESCRIP: K&T DIAMOND KAOLIN 
    DATE: 07/27/93 
LOCATION: BD 725 
======================================== 
This is a montmorillonitic intermediate particle size material. 
I received this sample 7/93 for testing to compare to pioneer kaolin. 

This has 1-2% lower fired shrinkage and 4-5% higher absorption
at the higher temperature than pioneer kaolin. It fired color is similar. 

This kaolin would be valuable to augment pioneer in our bodies
to minimize shift if one material changes. 
 
DRYING FACTOR (ID-DF, ABBR-DFAC) 

DRY_FAC - A000 

LOI/Water Content (ID-LW, ABBR-LDW ) 
    WET-WT  DRY-WT  FIRE-WT  OIL-WT IMM-WT  PERCENT LOI DENSITY 
  +-------+-------+--------+-------+------+ 
1 | 25.99 | 15.03 |  13.10 | 15.53 |      |  42.2% 12.8% 1.00 g/cc 
  +-------+-------+--------+-------+------+ 

SHRINKAGE/ABSORPTION/H2O (ID-SA, ABBR-SAWL) 
    DRY-LEN FIR-LEN FIRE-WT BOIL-WT  CONE   FIRE-SHR  DRY-SHR ABSORP 
   +-------+-------+-------+-------+-----+ 
 6 |96.3   |89.4   |31.68  |36.61  |6.4  |   7.17%    3.7%    15.6% 
 7 |96.17  |88.43  |31.87  |35.98  |7.0  |   8.05%    3.8%    12.9% 
 8 |96.15  |88.16  |32.65  |36.59  |7.4  |   8.31%    3.8%    12.1% 
 9 |96.18  |86.29  |31.09  |33.37  |8.9  |  10.28%    3.8%     7.3% 
11 |96.38  |86.2   |30.96  |33.16  |10.8 |  10.56%    3.6%     7.1% 
12 |96.39  |86.42  |32.74  |35.52  |10.0R|  10.34%    3.6%     8.5% 
   +-------+-------+-------+-------+-----+ 

SOLUBLES (ID-SL, ABBR-SOLU) 
   FIRED-CLAY GLAZ-CLAY DRY-CLAY 
  +----------+---------+-------+ 
1 |       NIL|         |   NIL | 
  +----------+---------+-------+ 

SEIVE ANALYSIS (ID-SV, ABBR-SIEV) 
    TOTAL PLUS-35 PLUS-48 PLUS-65 PLUS-100 PLUS-150 PLUS-200 PLUS-325 
  +------+-------+-------+-------+--------+--------+--------+--------+ 
1 |  100 |       |       |       |        |     .01|    .04 |     .6 | 
  +------+-------+-------+-------+--------+--------+--------+--------+

Ball Clay & Bentonite

Ball clay is much finer and thus much more plastic than kaolin. Bentonite is finer still and gram for gram, 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 or twenty times the amount of brown-firing iron oxide that kaolin has and many have heavy soluble salts that produce a dark colored scum on the burned surface. Many also contain lignite particles that can produce glaze imperfections. 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 are available but they are very expensive and are not nearly as plastic). The fired maturity and plasticity of bentonites and ball clays can vary even more than kaolins. This is so much the case, that a porcelain formulation project often becomes a ball clay/bentonite comparison and testing project (which can be a real education). Again it is very important that you have a well defined testing program to compare these materials.

Following is a FORESIGHT sample report for a ball clay:

========================================
  NUMBER: L2553D
 DESCRIP: GLEASON BALL CLAY
    DATE: 04/29/94
LOCATION: BD 763
========================================
Mixed 50:50 calcine:raw.

This is quite white in the raw state and is much more plastic than 49’r.
Fired bars are the whitest of the ball clays tested this round and are
very clean. There is a little brownish scum at 10r, but this is a very
nice looking ball clay, although it is shrinking much more. This is a very
refractory ball clay.

DRYING FACTOR (ID-DF, ABBR-DFAC)

DRY_FAC - A000 |

LOI/Water Content (ID-LW, ABBR-LDW )
    WET-WT  DRY-WT  FIRE-WT  OIL-WT  IMM-WT PERCENT LOI DENSITY
  +-------+-------+--------+-------+-------+
1 | 33.58 | 24.22 |  22.82 | 24.57 |  8.94 | 27.9% 5.8% 1.57 g/cc
  +-------+-------+--------+-------+-------+

SHRINKAGE/ABSORPTION/H2O (ID-SA, ABBR-SAWL)
   DRY-LEN FIR-LEN FIRE-WT BOIL-WT  CONE FIRE-SHR DRY-SHR ABSORP
   +-------+-------+-------+-------+-----+
 5 | 94.81 | 84.65 | 35.1  | 37.8  | 9.8 | 10.72%  5.2%   7.7%
 6 | 94.81 | 86.05 | 38.70 | 43.15 | 6.3 |  9.24%  5.2%  11.5%
 7 | 94.8  | 85.1  | 37.07 | 40.65 | 6.9 | 10.23%  5.2%   9.7%
 8 | 94.9  | 85.03 | 35.52 | 38.68 | 8.3 | 10.40%  5.1%   8.9%
 9 | 94.94 | 84.18 | 33.47 | 35.99 | 8.8 | 11.33%  5.1%   7.5%
11 | 94.79 | 84.13 | 32.31 | 34.79 |10.7 | 11.25%  5.2%   7.7%
12 | 94.85 | 84.51 | 32.24 | 35.01 | 10R | 10.90%  5.2%   8.6%
   +-------+-------+-------+-------+-----+

SOLUBLES (ID-SL, ABBR-SOLU)
  FIRED-CLAY GLAZ-CLAY DRY-CLAY
  +----------+---------+--------+
1 |      MED |         |    MED |
  +----------+---------+--------+

SEIVE ANALYSIS (ID-SV, ABBR-SIEV)
    TOTAL PLUS-35 PLUS-48 PLUS-65 PLUS-100 PLUS-150 PLUS-200 PLUS-325
  +------+-------+-------+-------+--------+--------+--------+--------+
1 |  100 |       |       |   .01 |    .01 |    .03 |    .28 |    1.1 |
  +------+-------+-------+-------+--------+--------+--------+--------+

The easiest thing you can do to the standard ‘25 Porcelain’ recipe to increase its plasticity is add 2%-3% bentonite. Even though bentonite can be quite dirty (it is important that you get a microfine ceramic grade e.g. 600 mesh), this small amount does not seem to effect the fired color as much as you might expect. If whiteness is not all-important, you can increase the ball clay at the expense of kaolin to produce a plastic whiteware (but remember to reduce the feldspar also, as ball clay is less refractory than kaolin). Reduction firing tends to gray the color of porcelain anyway, so porcelain of this type can tolerate some ball clay. Generally though, if you want a very white porcelain, you have to meet the challenge of reducing or entirely eliminating the ball clay.

Feldspars

These are the fluxes, or more correctly, contain the fluxes. Fluxes are the oxides that help develop fired maturity by liquefying and slowly dissolving both clay and silica. The total flux amount necessary is easily determined by simply firing to a range of temperatures above and below the one you intend to work to; 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 can vary from 15%-35%, depending on the maturity of other materials (especially the kaolin) 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 (i.e. nepheline syenite). Sodium feldspars are generally cleaner and more potent, although they can produce a body with more of a tendency to warp. Use two or three together if possible.

Silica

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 is 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. Too little silica in a body can mean crazing glazes since the quartz mineral contributes to the low expansion that assists glaze fit. For cone 10, many technicians aim at 20-25% for expansion reasons and to provide firing stability over a range of temperatures.

Recipes & Strategies

Most people have noted that the ‘25 Porcelain’ recipe has flaws that can be corrected for individual situations and materials.

Here is an actual starting cone 10 recipe for materials commonly available in North America.

20% Silica
10%   Custer Feldspar
10%   Nepheline Syenite
16%   Tile#6 Kaolin
16%   Pioneer Kaolin
16%   EPK 
 5%   NP Blend Ball Clay
 5%   OM#4 Ball Clay
 2%   Bentonite

The basic method to formulate a porcelain starting from the 25 Porcelain recipe is:

A start-from-scratch method would be:

If you need to formulate a casting porcelain, remember that much lower plasticity is needed and it is much more important to use kaolins of large particle size, so water can easily be drawn out by the plaster mold. The cleanest materials are also the least plastic, thus casting porcelains can achieve much whiter and more translucent effects than their plastic counterparts. Also, if you intend to pour slip, it is imperative that you understand the principles of ‘defloculation’ so that the amount of water in the slurry can be minimized and proper mold release and casting time can be achieved. If you have not witnessed the magic of adding a few drops of a dispersant into a mixer where a hopelessly thick clay-water mix defies agitation, you have not lived!

If you are making a plastic porcelain for use in modeling, throwing or machine forming, pay careful attention to its drying properties. Since porcelains are fine-grained, they don’t usually dry well, thus plastic porcelains are even worse. It is important to have a good test to rate and compare drying performance (FORESIGHT has shipped with such tests predefined and now ceramicmaterials.info continues to improve on them).

No matter what type of porcelain you make, give careful thought to how mature it needs to be. If zero absorption is not necessary to achieve high translucency, consider reducing the feldspar to achieve 0-1.0% values. Such a body can still be considered functional and vitreous and it will resist warping in the kiln associated with firing past the point where porosity reaches zero. In addition, material changes, which result in more maturity, will be less likely to cause trouble in a body that has some ‘room to move’. Also, when you evaluate fired absorption measure the property 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.

True, we have only scratched the surface but this is certainly enough for a good start. In summary, I have prepared a chart which follows. It lists some of the trade-offs you must consider when formulating or adjusting a pottery porcelain.

Material Amount to Use Details
Silica Ideally, use none (use calcined alumina instead).

Better-10%-15%

Normal-20%-25%

 If plasticity or dust hazard is important, cut back as far as possible (to maintain glaze fit) to allow room for more kaolin, and thereby greater workability. Use the finest particle size available to help reduce the amount needed. Try calcined alumina as a safe substitute to produce a stronger (but more expensive) product. Use calculation to lower the expansion of your glazes, and thereby lessen the need for silica. It is possible to produce a fine porcelain using no silica. (Bone China and Parian ware are examples, silica is mostly just a filler)
Feldspar Ideally, use the amount required for vitrification.

Normal-25% cone 10,
40% cone 6-8.

 Measure maturity using FORESIGHT and www.ceramicmaterials.info defined tests (absorption, fired shrinkage and strength over a range of temperatures) and adjust to required amount. Use a mix of feldspars to minimize impact of a change in any one.
Kaolin Ideally, use as much as possible.

Normal-25%-50% cone 10,
20%-40% for cone 6-8.

 Use as much kaolin as possible. Test as many types as you can to find the best combination of whiteness and plasticity. English kaolins are less plastic (and more expensive), but also much more mature, so more can be used. American kaolins are just as white, much more mineralogically pure, but require more flux. Highly processed delaminated kaolins are very clean and white. Use a mix of two or three kaolins to minimize impact of a change in one. Don’t be duped by a supplier’s preshipment sample. It takes several years of use to determine what its consistency will be like.
Ball Clay Ideally, use none.

Normal-10%.

Tolerable-30% for whiteware.

 Provides abundant plasticity but increases drying shrink-age, drying cracks, water smoking cracks, and explosions. If whiteness or translucency is important, use very little, otherwise try to reach a compromise between workability provided by ball clay and whiteness destroyed by it. De-tailed fired and workability comparisons of brand names can really pay off. If necessary, add 0.3% barium carbonate to ball clay to precipitate solubles which cause surface scumming. Use two or three ball clays to minimize the impact of changes in one.
Bentonite Ideally, use none.

Acceptable-1% to 2%.

No more than 5%

 Check drying performance, bloating, specking, solubles and consistency of the brand you will be using. This is a plasticity "miracle material" for throwability, but beware of the possible solubles scumming, fired specking, kiln cracking during drying, and water smoking and bloating in reduction. White firing bentonites might help, but you will find they are much less plastic and very expensive.

Reflection

Keep in mind that this article is intended to impart an understanding based on the 25-Porcelain recipe. However, a simplified recipe could simply be kaolin and feldspar. Ball clay is actually just an impure kaolin anyway, it is used to impart high plasticity and is often not needed (bentonites and other small-percentage plasticizers can be used instead). Silica is mostly just a filler so it can actually be completely eliminated to produce the most translucent possible porcelain (although the silica raises the thermal expansion and prevents crazing). When porcelain is fired it is not just a simple gradual melting that is occuring. The feldpar creates a liquid phase in which some of the kaolin (and quartz) dissolve, but also in which the kaolin crystal transforms into another form of different shape, mineralogy and amazingly, of a higher melting temperature. This crystal matrix bonded by feldspar glass creates the incredible hardness and toughness that porcelain can have.

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