Digitalfire Ceramics Technical Articles

Understanding Ceramic Materials

Section: Materials, Subsection: General

Description

Ceramic materials are not just powders, they have a physical presence that make each unique and amazing. We cannot adequately describe the properties using just numbers, thinking in terms of generic materials is a key.

Article

When you first started in ceramics, you were probably amazed at the array of exotically named raw materials. At that time, they were just white powders that were weighed out to match the recipes on hand. There were so many materials that it seemed impossible to ever understand the function of all of them. Our instructors often ignored their individuality, focusing only on the fired visual and tactile properties of 'mysterious' mixes of them.

You probably noticed that textbooks sometimes contain a separate section dedicated to describing major raw materials. They invariably classified materials according to major groups and this simplifies understanding the individuals. Like many people, you may have hurriedly skipped these parts for the more interesting 'how-to' chapters. It didn't take long to find out that some textbooks on ceramics did not even describe basic materials like kaolin; others were narrow in their treatment of it. Some like Hamer's "Dictionary of Materials" are much better.

Now that you are learning the oxide viewpoint, materials will take on a whole new life. They become "sources of oxides". However, as we have seen, materials have a chemistry, a mineralogy, and a host of physical properties to recon with.

Consider the following as typical of what you would often get from a supplier if you asked for information on a material like Barnard Slip:

CaO 0.57, K2O 1.14%, MgO 0.68%, Na2O 0.57%, TiO2 0.23%,
Al2O3 7.60%, SiO2 47.01%, Fe2O3 33.90%, LOI 8.31%

They can give you plenty of other numbers also (numbers that are often inaccurate and out-of-date). What do these number this tell us? To be honest, very little! Is it really a clay? Is it plastic? Is the SiO2 combined with alumina and others or is it in the form of quartz particles? When does it melt? What is the particle size? What is the raw color? How is it possible for a clay to have such high iron? What really is this stuff mineralogically? Is it synthetic? Is it similar to yellow ochre? Is it practical for use in clay bodies? Where can I get it? Is it available in different grades and from multiple manufacturers?

If you are like me, you want practical information; you want to know what a material is. We need to know enough to be able to ask some intelligent questions. The materials area on this site database is addressing these very issues. Efforts are under way to accumulate the kind of information people want from manufacturers and to standardize the way it is presented.

Barnard Slip is an interesting example. Since I have not received a data sheet from a supplier I simply tested it myself. The results of this testing resided in the FORESIGHT material database for some years, now they are in the materials area of this site.

    Barnard clay has long been used by potters as a source of iron in dark
    firing glazes. It offers price advantages over using iron oxide and being a
    clay aids in suspending the materials in the slurry. Barnard has proven
    valuable for iron slip glazes requiring high clay content.

    However, it is a silty material with very low plasticity; so low that it is
    difficult to form test specimens from it in the plastic state. The material
    is extremely messy to work with an stains containers and everything it
    touches. There is some variation in the color and thus fired results of
    glazes and slips employing it.

    This is undoubtedly one of the most stained clay materials available.
    Fired bars are very dark brown at cone 02 proceeding to black at cone 4.
    At cone 6 it is beginning to melt.

    SHRINKAGE/ABSORPTION @ 24% water
    --------------------------------
    Cone          04     2      3      4       6

    Shrinkage   11.5%  15.7   16.2   15.5    15.6
    Absorption  10.4%   2.5    1.3    0.6     n/a

    Drying Shrinkage: 3.8-4.2%

This is the kind of information we need about materials, isn't it?

Let's consider kaolin. This is quite different. There are hundreds available. In this case, we need a textbook description of what generic kaolin is, what it is used for, and why it is used in preference to other materials. It is much more than a simple source of Al 2O3 in glazes. Kaolin is the only practical source that does not bring fluxes also and that it is ideal because it is inexpensive, consistent, and imparts suspension and raw hardness properties. It is a popular body constituent because of its whiteness in comparison with ball clay and bentonite. Kaolin has a much larger particle size than either and its use improves drying performance by reducing drying shrinkage, enhances water penetration, and reduces fired shrinkage. Kaolins vary widely in their plasticity and they do not introduce soluble salts and coal the way ball clays do.

The key to wrapping your mind around the dizzying host of raw materials available is to learn them generically so that you can better evaluate the strengths and weaknesses specific name brands. Try to build first hand experience with a benchmark material of each type then relate the properties of new materials to those of the benchmark. In this way, you will develop flexibility and be able to become increasingly independent and capable.

In the materials area of this site we have adopted this concept of generic materials. Look up a material like kaolin, dolomite, feldspar, etc. to see an example. In time we will assign each generic description to an expert (or group of experts) in the field for maintenance. All name-brand materials will then be described in terms of the generics.

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