Pottery Crash Course for Beginners
Section: General, Subsection: Pottery
Description
Where do you start to teach children about pottery if you have no background in it?
Article
Are you a teacher who has just inherited an art room equipped with ceramic equipment and clays and don't know where to start? Children love to work with clay, but their first experience can be a very disappointing one unless you have planned things properly.
Here is a quick crash course:
Theory:
You might want to explain to the children that clays have a property called 'plasticity', that is, they can be transformed into a new shape and will hold it (vs. 'elastic' materials that want to go back to the old shape). Plasticity enables you to roll (or stretch the clay by hand) into slabs, roll it into coils, pinch it into bowl shapes, shape and model it, bend it, etc. Microscopic clay particles are electrically charged and hold onto water, thus the water is both the glue and the lubricant that makes plasticity possible.
Clays:
Pottery clays are available in dark and light colors and classified as low, medium and high temperature. Low temperature clays are fired at around 1000C. 'Low fire' clays save energy and there are lots of brightly colored commercial glazes (in jars) for use on them. But the fired product is not as strong and glazes are quite expensive. At the other extreme, high temperature stoneware clays and porcelains fire around 1250C in gas or electric kilns. People using these usually make their own glazes (high fire glaze materials are quite inexpensive). Medium temperature is a compromise between earthenware and stoneware, and most hobbyists and schools work with this type of clay. Medium temperature clays are still classified as stoneware and commercial glazes are available (you can also make your own quite inexpensively).
Projects:
What can you make? Try visiting Axner.com and search for 'students' in the books section. There are lots of books directed at teachers in your situation. Look for the ones that have projects you can do with the children. Kids love to make dinosaurs, for example. If you can show them how to make eyes and legs and then stick them on it will help them alot. Pre-making slabs and then having them wrap these to make vessel shapes works well also.
Forming, Techniques:
Rolling coils and making slabs, for example, might seem simple, but can be surprisingly difficult for beginners. You should education yourself on some of the basic techniques so you can convey these to the children well.
Things to Watch Out For:
Strangely, when water hits the wet clay it can start a split. Thus it is best to use 'slip' to glue pieces together. Slip is simply clay that has enough water in it to be a thick paintable liquid. You can make slip by mixing powdered clay and water (store it in cottage cheese containers). Children can tend to overwork clay aimlessly, thus it is best to have a clear project in mind so they know exactly what to do at each stage and when to stop. It is not usually possible to regiment a whole class to do each stage at the same time, each student needs to work at this own speed. Also, help them avoid making pieces that are overly delicate and have a lot of fragile appendages.
Drying:
Clays shrink when they dry. In most climates, simply leaving a piece out to dry invites disaster because some parts dry first, and then cracks emerge to relieve stress when the remaining parts dry and try to shrink. It is important for drying to occur evenly over the cross section of the piece. Thus you should impress on the children the importance of avoiding very thick sections. The best way to dry is to put all the ware on a ware rack with shelves and then use plastic to cover and enclose the entire rack. This will slow drying down and therefore make it more even. You can cover individual pieces, but it is best to use a rag and then plastic on top of that (covering with plastic only causes droplets of condensate to fall on the ware). Help the children to have a patient attitude, pieces should be allowed to dry for several days (or longer if they are thick).
Glazes:
Glazes are suspensions, not solutions. They consist of powdered clay to form a gel mixed with powdered minerals like feldspar, quartz, dolomite, calcium carbonate. Glazes typically settle out if left to stand. The minerals are blended to melt at a specific temperature and form either a matte or glossy surface. Metal oxides are added for color (eg. cobalt produces blue) and opacifiers are adding to make glazes opaque (rather than transparent). The chemistry of glazes relates directly to the fired properties (eg. matteness, color, hardness), many experienced potters like to study this.
Glazes are applied by painting, dipping or spraying. Dipping glaze (usually made by the potter) is normally applied to ware that has been prefired to a low temperature, and the porosity of the ware absorbs water from the glaze leaving a layer. The clay component in the glaze hardens it (although it can still feel quite powdery when dry). Commercial glazes also have binders added (like glue) to make them more paintable and dry harder, thus they can be applied to ware that has not been prefired.
Glazes are mixed by weight. Since some of the ingredients can be required in less than 1%, it is important to have a scale that can weigh to the nearest gram. Inexpensive electronic scales are now available for this. While people get glaze recipes from books and pass them around, we recommend settling on one good transparent glaze recipe that is hard, does not settle in the bucket too much, works well with the glaze, does not leach in acid, etc. Then you can make various colors by simply adding metal oxides or stains. Visit http: //www.digitalfire.com/education/wheredoistart.htm for more information on starting base recipes.
Glaze safety is an issue today, so it is best to test glazes by leaving vinegar in a vessel for a couple of days to see if it leaches out any color or changes surface character. Glazes with a lot of stain or colorant or underfired glazes can be leachable.
Glazing:
For an economical approach we recommend using commercial underglaze colors (or adding stain powders to a clear glaze (typically 75% glaze, 25%) to make your own). Then have the children paint designs on the bisque ware (not too many layers and not too thick). Then use dipping tongs to completely immerse each piece in a bucket of transparent glaze. Then fire to reveal the living color.
Firing:
When you fire a clay you are creating a metamorphic rock. Clays are composed of tiny particles of all kinds of minerals. On firing some of them melt into a glass (e.g. feldspar) and others do not (i.e. quartz). The effect is thus a framework of unmelted particles cemented together by glass. The higher a clay is fired the more particle types melt, the result is overall shrinkage of up to 10%. A clay that has been fired high enough to purge all air pockets between particles is said to be 'vitrified'. Typical pottery clays are designed to fire short of the density and strength of complete vitrification in order to prevent warping and melting of ware. It is usually important that a firing goes to the correct temperature (otherwise glazes will not melt properly or overmelt and run off the ware).
When you fire a glaze you are creating a glass. All of the particles melt into a liquid which freezes into a smooth surface when the kiln is cooled. If the kiln is fired too high the glaze melts too much and runs down off the ware. Firing temperature is generally determined as the temperature needed to get the glaze to melt to the desired degree. A clay body that achieves good density at that temperature is then used.
Thermometers for high temperatures are called pyrometers. Most kilns have one. However the inexpensive ones on pottery kilns are not very reliable. Historically, firing temperature is measured by observing the bending of a 'cone' through the peephole. These 'cones' are numbered 1,2,3, etc and are stood erect at a slight angle and they bend over at predefined temperatures when heated. While you can fire a kiln by periodically checking for bending, most kilns have an automatic device called a 'sitter' in which you place a smaller bar or cone; the bending of this cone triggers the device to shut the kiln off. Most people also place the standard large cones in the kiln as a double check that it went to the right temperature.
Most pottery is fired twice. The first firing is called the 'bisque' and is done so that the ware has enough strength for handling and can be dipped into a glaze without turning back into mud. The bisque firing is typically done to cone 06, this is hot enough to turn the ware partly into a rock, but low enough that there is still a high porosity (the porosity is needed to get the glaze to dry quickly and stick). The second firing is called the 'glaze firing', it goes high enough to melt the glaze and mature the body. Glaze firings can generally be fired quickly (150-200C per hour) but need to be cooled slowly to prevent hairline cracks in the ware. However bisque firings need to be taken slowly in early stages to give remaining water in the clay an opportunity to escape (most people leave the kiln on low overnight so that it is a little above boiling point by morning).
The Kiln:
Although 1200C does not sound a lot hotter than 900C, once you get up into these ranges it takes a lot of energy to go just a little higher. Electric kilns do not stand up to 1250C or hotter very well, thus most people use them in the 1150-1180 range (middle temperature). Firing a kiln is not that dissimilar to cooking a roast, you need to watch it and go to the right temperature (although you do not usually need to hold the final temperature when you reach it). One big difference is that you need to allow a kiln to heat slowly through the boiling temperature of water (80-100C). This is because clay, although it might appear dry, still has about 5% water content, if heated too fast this turns to steam and the pressure fractures the ware. Typically people put a full kiln on low overnight (with the peepholes out and lid slightly ajar to vent out the water vapor) and then turn it up to full speed in the morning to complete the firing during the afternoon.
Most kilns have a shut-off, but these are not foolproof (they can jam or shut off too early or late). Most kilns also have a shut-off timer, but these are not that useful unless you have a enough experience on when a kiln should finish to be able to set the timer about 1/2 hour later. You really should be present during the time the kiln will shut off so you can watch it. If a kiln does not shut off and over fires too much it can easily be ruined (however it will not melt) . Many kilns have programmable controllers now, they are connected to a thermocouple (a probe that senses temperature). Although they look a little daunting, the manual explains how to program them (if you can't find it you can look at the brand name and visit their website and download one). Typically you program them by specifying how many segments in the firing and the temperature, rate-of-rise and hold time for each. Usually we use three segments, the first to 100C and hold for a few hours, the next as fast as possible up to 100C below the final temperature, and the last goes at about 100C per hour until the final temperature is reached.
Kilns produce some fumes that are not terribly healthy, (i.e. SO4, CO), they should be vented or be in a separate room with good ventilation. Use a dark colored glass to peer into the kiln to avoid damaging your eyes.
Kilns have posts and shelves made from materials that do not warp at typical firing temperatures (these are called refractories). To load a kiln you simply stack the ware carefully and use posts and a second or third layer as needed. Typically kiln wash (a paintable liquid that dries to a hard powder) is applied to kiln shelves incase any glaze that either over melts or was applied too thick runs down onto the shelf (this wash prevents it from sticking).
The Potter's Wheel:
Working on the potters wheel is called 'throwing'. Students really need to see it done properly before attempting it themselves, otherwise it is very frustrating and messy. We recommend getting a video, try visiting http: //axner.com to find one.
Books and Videos:
As noted, visit axner.com to find hundreds of books on pottery.
Young Children:
They want to touch the clay as soon as possible, they are not going to be patient enough for you to talk too much. Tell them about shrinkage and drying, not getting glaze on too thick, not over working the clay. Sometimes it is good to get the whole class to make one thing (i.e. a drape molded plate). If you can premake the slabs for them, this will really help. Everyone can paint using colored versions of your base glaze, then you can dip all the pieces in a transparent.
Dust:
Clay dust should be kept to a minimum. There are many things you can do, please visit http: //www.digitalfire.com/education/toxicity/dealingwithdust.php for more information.
Tools:
You should have a small board for each child to work on. We recommend some sort of pressed board that has some absorbency. Children should build pieces on a piece of newspaper so it does not stick to the board. You will need containers and old brushes for slip. You will also need a wire tool to cut the clay, rags to cover pieces (t-shirt type material works well). You will also need a good sink with lots of sponges and buckets for clean up.
To learn more, please visit Digitalfire.com and click on education.
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Authors
- Tony Hansen (Owner)
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