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Warping
Normally refers to a body problem where vessels warp out-of-shape during firing. Warping is often exhibited on tops of vessels that have pulled out-of-round or overhung bowls that have sagged. Any clay that is being fired to a vitreous condition is subject to warping since clays soften as they approach the melting point. Porcelains are the most susceptible. Normally a balancing act must be done, firing as high as possible to get the most dense and strongest possible body while using ware shapes that are resistant to warping. When using a body or process subject to this problem it is important to have kilns that can fire consistently and have a consistent body. Even earthenware, although not fired anywhere near vitrification, can warp if ware is extremely overhung or ware is very thin.
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Water
There is a need to discuss water in ceramic production from a number of viewpoints:
Drying ceramics: Water must be removed in such a way that articles do not crack during drying or explode during firing (due to steam pressure). Even when a piece appears dry there is still likely 5% by weight or more physical water that only drying above its boiling point will remove.
Pugged bodies: Water not only makes the clay plastic, but electrolytes in the water can greatly influence the stability (stiffness changes over time, thixotropy). Thus, in industry, water is filtered and processed before being put into bodies.
Glazes: To maintain consistency in glaze slurries it is important to understand the rheological properties in relation to water and water quality.
Casting slips: The principles of deflocculation, thixotropy depend on understanding the role of water in relation to slurry rheology.
Decomposition: Understanding the role of chemical water in the crystal structure of many minerals is important to understanding when, how and why they generate water vapor during decomposition during firing and how this impacts firing results.
Mineralogy: Many minerals are hydrates and understanding them and their properties involves understanding how water incorporates into their structure during hydration. In some cases water molecules insert into crystals, in other cases oxides are converted to hydroxides.
Solubility: Generally in traditional ceramics, insoluble materials are needed to make body and glaze slurries. It is important to understand which materials to use to source oxides for the chemistry based on solubility, and if solubility can be tolerated, how is the process configured to do so.
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Water Solubility
A host of water soluble materials are available to source most of the important oxides needed in ceramic glazes. However such materials cannot normally be used in glazes because they are suspensions of particulate materials (not solutions of soluble materials). Such suspensions have a far lower water content than a solution could ever have, this is necessary to create a slurry that will deposit an adequate glaze thickness when applied to ware. Because ceramic powders do not react with water, a clay or glaze slurries can be stored and the rheological properties do not change. However if a soluble material is added to such a mix the usual result is a destabilizing of the slurry properties (viscosity, thixotropy). Worse, if a glaze slurry containing a soluble is applied to ware, especially bisque ware, as the water is absorbed into the body the solute goes with it. This changes the firing behavior of the body and leaves the glaze devoid of the oxide the solution was intended to supply.
This being said, many materials used in ceramics are slightly soluble over time. This includes nepheline syenite, lithium carbonate, gerstley borate, colemanite, ulexite and even frits. Thus, if glazes or slurries are stored for long periods precipitates can form.
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Pictures
An example of two types of crystals that have formed on the surface of a fritted glaze after a long period of storage (Frit 3249 in this case).
Wedging, kneading
Wedging clay is similar to kneading bread dough. Clay tends to set up over time and the process of wedging it loosens it up. It is not uncommon for the clay to soften dramatically on wedging, this is thought to occur because of of mobilization of water between the flat particles of clay and the disruption of a stable electrical charge pattern between water and clay that develops over time.
Wedging also performs the function of lining up the flat clay particles concentric to the center of the mass allowing them to slip over each other more easily in that direction.
Wood Firing
A firing technique used by necessity in many countries and by choice in others. In a properly designed kiln wood is capable of delivering high temperatures so it is possible to make stoneware and porcelain. The kiln chamber in a wood kiln subjects the ware to alot of ash and smoke and this profoundly affects its appearance. It is possible to fire pieces without glaze and the products of combustion of the wood will deliver enough fluxes to fuse the surface of the clay in a glaze-like manner.
Pictures
Wood fired test samples. Made in the Medalta kiln in Medicine Hat, Alberta, Canada.
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