A B C D E F G J K L M N O P R S T U V W All
New version of this page available at digitalfire.com: Click here to go now
Words beginning with P
Particle orientation
Clay particles are flat and are either randomly oriented or arranged in some general pattern. The pugging process, for example, orients particles concentric to the center of the clay slug. Likewise, throwing a vessel on the potter's wheel lines up the particles. Rolling, casting, kneading operations affect particle orientation. Particle orientation imposes an influence on a clay's drying shrinkage (a piece will shrink more along one dimension than another) and this needs to be considered when joining pieces to form objects or cracks will result.
One way reveal particle orientation is to freeze an undried. This should reveal the stresses that result from differences in orientation.PCE
Pyrometric Cone Equivalent
A measure of how refractory a material is. The measure is done by making a small cone from the material and firing it till it bends. A typical stoneware clay body, for example, might have a PCE of cone 20.Phase, phase changes
A 'phase' of a material is a physically different molecular or crystal structure induced by a set of conditions (i.e. temperature, pressure). Phases of silica, for example, are chemically the same but have different physical properties. If significant differences are imposed a phase will have its own name (i.e. diamond, graphite are phases of carbon). If differences are not significant an alteration of the same mineral name is used (i.e. alpha quartz, beta quartz). It is important to realize that a phase exists as a recognition of a physical change, not a chemical one. These changes are measurable by instruments such as a microscope or dilatometer. 'Cristobalite' is a phase of silica and has very different properties than quartz, however they are chemically the same. The former can be created by submitting quartz to a high temperature and holding it there.
Catalysts encourage chemical reaction thus they would not be associated with phase changes. However the term 'catalyst' is used to refer to conditional changes that effect phase changes.Porcelain
A clay body which, when fired, becomes very mature and usually translucent. Porcelain is normally quite white and fires to a very smooth pleasant surface. Porcelain clays lack iron impurities and are ground to very fine particle sizes. Plastic porcelain clays tend to be shorter than their stoneware or earthenware counterparts. Porcelain casting slips achieve the whitest and most translucent results.Porosity
The pore space within a fired body. This is measured by weighing a specimen, boiling it in water, weighing it again, and calculating the increase in weight.Primitive firing, pit firing, sawdust firing
Usually refers to a process of firing clay ware under primitive conditions, usually in a fire or a fire pit. It requires a clay that will handle thermal shock well (normally well-grogged). If you burnish your pots you will need to consider whether the grog will mar the finish so it might be better to slip the ware and burnish that.
One challenge is generating enough heat to sinter the pots well. In a typical open wood fire it is difficult to achieve temperatures more than a few hundred degrees above red heat. Use of sawdust, hard wood, and various schemes to contain the heat are all common. Firings may double as a social occasion and take only a few minutes or they may be quite elaborate insulated hole-in-the ground affairs that span several days.
Books are available on sawdust and primitive firing.Propane
Propane fired gas commercial and home-built kilns are quite common. However this process seems to generate more questions than any other, especially on the subject of propane tanks. If the propane tank is not large enough, for example, it will freeze up and be unable to supply the necessary fuel. People often underestimate the number of tanks needed for a firing and the rate at which a given tank can supply gas.
The Clayart discussion group on the Internet has a lot of knowledge people in this area.Pyroceramics
By firing spodumene based bodies a certain way an almost zero-expansion beta spodumene phase can be developed. This the basis for pyroceramic, oven-to-table ware (e.g. corning ware is a good example).
Much more information with complete interlinking to many related Copyright 2003 http://digitalfire.com, All Rights Reserved Please support http://ceramicmaterials.info to improve this library |  |
INSIGHT is ceramic chemistry |