Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

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11.1 Introduction

This chapter describes some of the common techniques and technology used in the production of ceramic pottery. A complete understanding is not necessary, so it could also be used for subsequent reference when one of the topics above is raised in later chapters.

11.2 General

As a reminder, water is contained at different levels within the clay used to make pottery, from the water loosely binding the clay particles to that chemically locked up within the crystal structure itself – the water of crystallisation. The first phase of pottery production, low-fired earthenware, is achieved when clay is heated to a high enough temperature to drive off this water of crystallization, at between 450 and 700 degrees C.  The clay goes through an irreversible process and its physical properties are changed. It becomes hard and water resistant, and its colour is often altered, depending on the amount of oxidation or reduction.

For higher-fired pottery, the behaviour of potter’s clay during firing and the characteristics of the final product depend on its chemical composition and the firing regime in a very complicated way, which would not have been properly understood until more recent times. As you can imagine there is a vast range of clay compositions possible, and, although the action of the various constituents is similar, a wide range of physical properties results. The reactions caused by firing start at the surface of the body and take some time to reach the interior completely, hence the long firing times. Certain materials can speed up the rate of reaction and are added as fluxes.

Three principal components are needed for the ceramic body, a fine-grained part for plasticity, a refractory crystalline part for mechanical strength and a flux to provide a glassy phase. Typically clay provides the first, silica/quartz the second and feldspar the third. However, some more recent ceramics are made differently. The amount of each ingredient affects the final product; for example, porous earthenware may have only 10% feldspar, but dense floor tiles up to 55%. Remarkably dental “porcelain” can be 100% feldspar.

The plasticity of clay is mainly due to the content of the very fine particles. Kaolinite has small hexagonal plate-like crystals 0.1 to 2 microns across (only visible under an electron microscope). As an example of their characteristics, they are small enough that, when mixed in a liquid such as water, they obey Brownian laws of motion rather than just gravity sedimentation – in other words they float around rather than sink to the bottom. The smaller the particle is, the greater the random movement and disorder. This fineness of particle size improves the plasticity as well as the strength when dry.

To get an idea of the effect of firing, we can consider its effect on kaolinite, a major ingredient of porcelain. The kaolinite crystal lattice starts to break up at 420 degrees C, when the “water of crystallization” starts to be expelled (dehydroxylation), which is mostly gone by 550 degrees C, representing 5% by weight. There are other reactions at 900 when kaolinite enters a liquid phase and at 950 to 1000 when a spinel is formed, and at 1075 degrees C when mullite starts to be formed that completes at 1400 degrees C. So different components of the ceramic melt and then recrystallize as different phases of the material as the temperature and time increases. These changes of phase can cause problems for the potter. For example, when heated, crystalline quartz changes phase at 573, 870, 1470 deg C with rapid changes of volume, so if there is a high level of free quartz in the body it is prone to cracking, especially on cooling.

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