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The Basics Of Ceramics
(in a nutshell)
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Clay types and firing temperatures can be confusing. Experience is surly the best teacher. However, when you are just beginning to test the waters of ceramics, it is nice to get a few basics and some resources to turn to.
Clay Bodies:
What the clay is made up of. Types of clay include Stoneware, Porcelain (highfire clays), Earthenware and Terra Cotta (lowfire clays).
Every clay manufacturer has their own line of clays. And a clay from one company may be very different than another company’s. Important to note is that clays are not universally compatible with uderglazes and glazes. Even if a clay gets fired at the same temperature it may react differently to the glazes. The glaze may just chip or peel off, a common problem called "Shivering," also it may create a different color affect, or bubble. Luckily most clay manufacturers also distribute paints and glazes that are compatible with their clay bodies so you can avoid these problems. Whoever you get your clay from should be able to recommend which ceramic paints or glazes to use.
* Often people that pour molds or throw pottery on a wheel find clay and glaze combinations they like and stick to it, however experimentation is what gives spice to life.
&Mac183; Modeling clay:
Similar to what you played with in grade school. It can be molded by hand, thrown on a wheel or rolled out to cut out shapes (much like cookie doe). Modeling clay, when purchased is ready to use. Unused clay should be kept in a cool area wrapped in plastic.
&Mac183; Slip:
Clay in a liquid form. It can be compared to thick pancake batter. Slip is used to repair, or connect two pieces of greenware or to pour into cast molds. Slip used in pouring slip cast molds, starts out in a powder form to which Sodium Silicate, Barium Carbonate, Soda Ash and Darvan 7 are added. These are the chemicals that create the right balance for the liquid clay to set up in the molds.
Firing clay:
Depending on what you are using your final piece for it is very important what clay you use and how it gets fired.. Firing ceramics is much like baking food, except ceramics go to higher temperatures. When we bake, we leave food in the oven at a temperature for a certain time. A thermometer may help measure the temperature of our food or we may stick a fork in to test whether it seems right.
It is the same with firing - a combination of temperature and time "cooks" the ware. However, unlike baking we can’t put our ware into a preheated kiln and poke a fork in our pot to test doneness. The next best thing is to place Pyrometric Cones near the ware to measure whether it has received enough heat.
* If you are interested in doing ceramics or pottery without investing in a kiln, talk to your local art supply store for someone that would be willing to fire your pieces for you. Your local college or university may also be a good resource.
Kiln:
Pronounced kiln or kil. A kiln is an oven that can reach temperatures up to ~2,400 degrees Fahrenheit using either gas or electricity. Kilns are constructed of either insulating firebrick or with high purity, high temperature ceramic fiber. If considering buying a kiln please follow all the guideline for kiln installation and ventilation. Carbon monoxide can produce headaches, fatigue, soar throats, and nausea. Testing has shown that carbon monoxide produced during firing can be higher than accepted government (OSHA) standards. [35 ppm (0.0035%) for continual exposure or 200 ppm for short term exposure.] Fumes from kilns may also contain organics, volatile metals, fluorides and sulphur oxides, which should be removed from the work area.
Cones:
When firing clay, the temperature at which you fire is gauged by cones placed in a kiln. The bodies, glazes and decoration products we use are all formulated to be correctly fired when they have received enough heat to properly bend a cone. The companies and individuals who make and test these supplies use Orton Cones. Cones deform when they have received the right amount of heat, not just when the kiln reaches a certain temperature. In other words, cones behave just like your ware. This is why they are such good indicators of whether the ware was properly fired. If the cone indicator is ignored, the piece will be either underfired or overfired which can result in glaze inconsistencies, dripping, bubbles, or colors changing (a beautiful yellow flower may virtually disappear).
Orton pyrometric cones have been the standard of the ceramic industry since1896. They are elongated pyramid shapes made of clay and various fluxes. Each cone number will melt (bend) at a known temperature, when heated at the rate per hour as indicated on the temperature equivalent chart. Cone numbers can be confusing for the beginner. For example: Cone 06 is a lower temperature than Cone 6. Generally Cone 022 through Cone 017 are used for China Paints, Lusters, Metallics and Decals, Cone 07 through 01 for Bisque and Low Fire Glazes, and Cone 1 through 10 for Porcelain and Stoneware.
Other cones include: Large Cones, Small Cones and Bars, Self Supporting Cones and Cone Plaques.
Greenware:
The object you make out of clay is considered greener. It has been shaped to the desired final product and has dried naturally to be "hard" but is very fragile. It is ready to be Bisque fired. At this stage you can not apply glazes. You can however apply underglazes to greenware. This a special type of color treatment that is only applied to Greenware.
Bisque:
A ceramic piece gets fired two times. Earthenware, the most common of clays is a low fired clay that gets fired the first time at cone 04. This is called the Bisque fire. When the ceramic piece comes out of this firing, it is called Bisque or Bisqueware. The piece is now as strong as everyday ceramic pieces. Your piece is now ready to apply ceramic paint and/or glaze.
Glaze fire:
The second firing of a ceramic piece. After a Bisque fired ceramic piece has been decorated with ceramic paints and/or glazes, it is ready to be fired the second and final time. The temperature in this firing is hot enough to melt the glaze and turn it into a hard "glass." Earthenware that was fired at cone 04 in the bisque fire will now be fired at con 06 in the glaze firing.
*(Note: it is possible to do multiple glaze firings on a single piece, however with each firing the chances of cracking increases. The expanding and cooling of the clay during the firing adds stress to the piece each time.)
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