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Particle orientation
Clay particles are flat and are either randomly oriented or arranged in some general pattern (in bodies containing other non-clay ingredients, they arrange within the matrix of other particles with which they are combined). 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 piece of clay. This should reveal the stresses that result from differences in orientation.
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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.
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- (Tests)
PCE - Pyrometric Cone Equivalent
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Phase Diagram
A triangular chart showing graphically the development of different phases across different tempertatures for mixtures of three oxides or oxide blends.
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Pictures
Phase diagram and stull chart showing the SiO2-Al2O3-(0.7CaO+0.3KNaO) system
Phase Separation
This phenomenon is responsible for some of the most interesting glazes used in ceramics. A glaze without any visible phase separation can be seen on a sink or toilet, it can be considered a homogeneous glass. Phase separation occurs when a glass melt separates into two or more liquids of slightly different chemistry (and therefore potentially different firing appearance). This phenomena usually happens on the millimeter scale and where there is a catalyst (for example, the formation of crystals, the movement of a melt, the sudden melting of particles in the glaze melt into a glass of different fluidity, changes in reactivity around particles in the melt or released from the body below). Oxides that influence color and surface gloss or other visual characteristic can preferentially gather in one of the phases. Silky surfaced dolomite matte glazes are example of phase separation at many small sites, macro crystalline glazes are an example of it at many fewer much larger sites.
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- (Glossary)
Crystalline glazes
Crystals can form during cooling and solidificatio... - (Glossary)
Matte Glaze
A glaze that is not glossy. Of course, unmelted gl... - (Glossary)
Variegation, Reactive Glazes
Variegated or mottled glazes are those that do not... - (Recipes)
G2571A - Cone 10 Silky Magnesia Dolomite Matte
The beautifully silky surface and reliable glaze
2003-12-18 - A standard Plainsman Clays dolomite matte glaze us...
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Pictures
Micrograph of phase separation in a glaze
Example of variegation and phase separation with about 5% rutile added to a dolomite matte cone 10R glaze.
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.
Pinholing
A glaze fault where tiny pinholes appear in the glaze surface.
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Pictures
Pinholes in a cone 6 glossy white glaze
Pinholing
A glaze defect where tiny holes are present in the fired glaze surface. These holes normally go down to the body surface below. Pinholing is a plague in industry, the tiniest hole in the glaze surface of a tile or utilitarian item can make it a reject. Industry goes to great pains to get materials of very fine particle size for their bodies and glazes to reduce the occurrence of glaze defects. Glazes which melt and flow well often still have pinholes if gas producing particles are present in the body (these expel gases up through the glaze melt thereby disturbing its surface). Blisters, dimples and pinholes often occur together.
Pictures
Pinholes in a cone 6 glossy white glaze
Testing for pinholes and dimples is often best done using a transparent glaze over a large surface and looking at the surface in the light.
Plasticity
This term is used in reference to clays (or more often bodies which are blends of clay, feldspar and silica particles) and their ability to assume a new shape without any tendency to return to the old (elasticity). In industry plasticity is gauged by the way a clay behaves in forming machines and by its stickiness. However potters find that simply throwing two samples of clay on the potters wheel is an excellent way to compare their plasticities. Plastic clays are responsive, large thin pieces can be made, a piece can be made faster, wet pieces can be moved without excessive deformation and plastic clays center more easily. Non plastic clays tend to split at edges during wedging and rolling, they generate alot of slip, they are more difficult to center, they are more flabby and unresponsive and require more finicky refining work in the latter stages of the process.
Plasticity is a function of particle size (normally clay of finer ultimate particle size is more plastic), surface chemistry and charge of particles, particle shape and water content. Bentonites are the most plastic common clay and kaolins the least. Clays of different plasticities exhibit vastly different properties. For example, ball clays are very plastic but they shrink so much on drying that cracks cannot be prevented. Bentonites have such a high affinity for water that it can take a week to dry a specimen and it can shrink to half the size. Kaolins can dry in a short time and have little shrinkage, but they can have very little dry strength (some plastic kaolins are available but their plasticity is usually because they contain bentonite or have a mineralogy that is bordering on ball clay). Thus a typical white-burning clay body might employ as much kaolin as possible for whiteness, enough ball clay to achieve the needed plasticity, and possibly a small addition of bentonite if plasticity cannot be achieved any other way. A white stoneware pottery clay might have as much ball clay as possible to achieve lots of plasticity but some kaolin to reduce firing shrinkage and get better drying properties. A white casting porcelain can be made using only kaolin.
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- (Glossary)
Clay
Clays occur when parent clay-making rocks (there a... - (Materials)
Kaolin - Al2O3.2SiO2 or Al2Si2O5(OH)4 - Hydrated alumina silicate, Pure clay mineral
China Clay
- (Materials)
Ball Clay - Highly Plastic Fine Particle Clay
- (Materials)
Bentonite
Montmorillonite
- (Glossary)
Ultimate Particles
Physical particles of materials are those we can m... - (Glossary)
Splitting
Refers to a phenomenon where a plastic clay develo...
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- (Articles)
Simple Physical Testing to Compare Clay Materials Some of the key tests needed to really understand what a clay is and what it can be used for can be ... - (Glossary)
Bone China
True bone china is a special type of porcelain tha... - (Glossary)
Porcelain
A comparatively white burning clay body (unless st... - (Project)
Ceramic Minerals Overview
The materials we use are powders and we assess the... - (Project)
Ceramic Properties
A property in this context is a created physical p...
Pictures
Albany Slip DFAC dried disk showing the soluble salts and characteristic cracking pattern and cut edge of a low plasticity clay.
Porcelain
A comparatively white burning clay body (unless stained e.g. blue) that, when fired, becomes very vitreous (close to melting) and usually translucent with a smooth pleasant surface. Porcelain clays lack the iron impurities of stonewares and are made from materials of fine particle sizes. They are usually fired above 1180C. Porcelains tend to warp during firing because they must be being taken closer to the melting point to achieve the desired properties.
Porcelain bodies in a wide range of plasticities are available. Plastic porcelain clays tend to be much shorter (less plastic) than their stoneware or earthenware counterparts. Porcelains used by potters are much more plastic than those used in industry. Porcelain casting slips achieve the whitest and most translucent results because they do not need to be as plastic (the plastic materials contribute the most iron which darkens the color). Typical porcelains are a mix of kaolin (for plasticity), ball clay and bentonite (for extra plasticity but some contamination), feldspar (the melter) and silica (the low expansion filler).
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- (Glossary)
Stoneware
A high fired (about 1150C+) ceramic clay that is s... - (Materials)
Kaolin - Al2O3.2SiO2 or Al2Si2O5(OH)4 - Hydrated alumina silicate, Pure clay mineral
China Clay
- (Glossary)
Vitrification
'Vitrification' is a process. As clay is fired hot... - (Glossary)
Earthenware
A clay fired at low temperatures (cone 010-02) whe...
Pictures
Brown and buff stoneware clays compared to a porcelain at 1300C in reduction. Courtesy of Plainsman Clays.
This is what about 8% iron can do in a transparent base glaze with slow cooling at cone 10R on a refined porcelain.
Plainsman P580 (35:17:1.5 ball clay:kaolin:bentonite), H570 (10:46:2.5), P700 (50:5 Grolleg:bentonite) and Crysanthos Porcelain (China) fired in oxidation at cone 10.
Porosity
In ceramic testing this term generally refers to the pore space within a fired clay body, as such it is also referred to as absorption. It is measured by weighing a specimen, boiling it in water, weighing it again, and calculating the increase in weight. As ceramic clay bodies vitrify in a kiln they densify and shrink (thus reducing pore space). The % porosity of a body is thus an indicator of its degree of vitrification. Porosity also implies strength (in comparison to specimens fired at different temperatures that have greater or lesser porosities). Porcelains normally can be fired to a point of zero-porosity but doing so brings them close enough to melting that ware tends to warp in the kiln. Stonewares and earthenwares reach a minimum porosity that can be well above zero (as much as 3%), firing beyond that bloats or melts the body. If porosities are measured over a range of temperatures for a body it is possible to create a graph to get a visual representation of the body's maturing range. The porosities plotted against temperature produce a line that decreases to a minimum, levels out then drops quickly rises.
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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).
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