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Pottery Tips and TechniquesThermal Shock And CrackingAvoid sudden temperature changes. |
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Cracks that show up in fired pottery or ceramics which were not caused by casting or drying problems may be the result of thermal shock. Why Does Cracking Occur?Some of the causes that makes a piece of pottery or ceramic more apt to be damaged by thermal shock are the strength of the piece and the thermal expansion of the material.Thermal shock can result when changes in temperature occur in the kiln during heating and cooling. When temperature changes rapidly, the outside of the piece and the kiln furniture become much hotter or cooler than the inside. This causes stresses which may result in cracking or breaking. What Causes Thermal Shock?A fast heating rate or rapid cooling.A sudden surge of cool air such as opening the kiln lid when the kiln has not finished cooling. In a gas kiln, turning off the gas and allowing cool air from the burners to enter the kiln. Thermal shock can also occur when a piece is stressed in use, such as a casserole or dish that is taken from the freezer or refrigerator and put into a hot oven. The hotter the ware is, the better able it is to resist cracks due to thermal shocking. Weak pieces will be more likely to break when stressed. A piece that is porous will also be weaker, making it easier to crack. Water or condensation that enters pores in the ware can turn into steam and expand and this can cause cracking when heated. The hotter a piece is fired, the less porous it will be. A piece of pottery or ceramic that expands and shrinks a great deal during heating and cooling is also more likely to be affected by thermal shock. Most kiln shelves are made of cordierite because this material has a lower expansion rate than most of our pieces and so is less affected by thermal shock. What Happens During Firing?During heating and cooling, the body and glaze undergo many physical and chemical changes.Moisture is driven out of the pottery and ceramic, and if this occurs too rapidly, cracking can occur. Organic materials are oxidized and released from the material. The glaze softens, melts and flows during heating and may trap gas. The body expands as it is heated and contracts during cooling. The glaze solidifies and contracts during cooling. If the body or glaze contains silica, it will expand rapidly at 1063° F when heated and contract during cooling. If the heating or cooling is rapid near this temperature, this change can lead to cracking of the piece. Control of heating and cooling is especially critical when firing thick walled pieces or pieces with an irregular wall thickness. Reducing Thermal ShockUse a smooth, moderate heating rate.Let the kiln cool naturally with the lid closed. Use a controller to slow down the cooling time. Do whatever is necessary to avoid sudden temperature changes. A programmable controller is the best solution to control the heating and cooling rates and to get a smooth temperature rise. If you do not have a programmable controller, heat loss during cooling can be controlled to some extent by keeping the kiln closed until well below red heat, which is about 900° F. To be sure that the pottery or ceramic is properly matured, be sure to use witness cones. Under fired bisque will continue to shrink during the glaze firing and this can result in a poor glaze fit.
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All About Clay The best way of starting out is to take a few lessons. You will probably waste quite a bit in materials when you first get started. Figuring out how to truly make handmade pottery correctly and shape into what you want it to be can be quite an ordeal. The different tools that a normal shop will have can be fun to try. You will soon see which ones you like to use the most and then when you are ready you will know which ones to buy. When you get all set up, just enjoy the
hobby and have fun at it. Some people get pretty
serious and start selling their creations at craft
fairs and small stores, but others just like
to create items for themselves, relatives, and
friends. Whichever kind of handmade pottery you
desire to endeavor, enjoy the hobby and have
fun doing it. |
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Patriotic and Christian
Gifts, Russ Berrie,
Teddy Bears
from the Past Collection. Wonder mugs (Amazing how the colors change when adding hot liquid). Winter Hat and Scarf Sets, fleece warm and cozy. Holy Bears inspirational bears with a message for every occasion. Neckties of Silk by Eagle Wings, 100% Silk, with Patriotic and Christian Designs. Travel Blankets the perfect take-along for any outdoor activity. Outback Hats available in all canvas or canvas with navy under brim. |
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