- Clay body - Capp's Potts are made from a unique blend of mined clays, each adding a desirable quality to the clay body. For example: how elastic it is for wheel throwing, how does the glaze fit on the fired body, or how well it can take thermal shock. The various clays are purchased in fifty or one hundred pound bags and mixed dry according to a recipe. This dry mixture is then mixed with water until it's the correct consistency. It is compacted some by wedging and set aside to age. The clay ages for at least one month, hopefully much more. After it's aged it is kneaded, in a similar fashion as a baker kneads bread, and is then ready to use.
The Clay Body recipes were arrived at by test firing various recipes from books by other potters and then altering them from there to produce the desired product. It will probably change over time to improve quality, adjust for clay types becoming unavailable, or aesthetic concerns.
My Two Primary Clay Body Recipes are:
| Zam-TEC-A |
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ST-40/TEC-A |
| Hawthorn Bond Fire Clay |
25% |
|
Hawthorn Fire Clay |
60% |
| Goldart |
40% |
|
OM4 Ball Clay |
20% |
| Old Mine Number 4 (OM4) Ball Clay |
15% |
|
Redart |
20% |
| Silica (240 mesh I think) |
7% |
|
Custer Feldspar |
5% |
| Custer Feldspar |
10% |
|
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| Redart |
3% |
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| Grog (medium) |
8% |
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- Forming Method - Most of Capp's Potts are wheel thrown, that is, a potters wheel is used to shape the clay into the base form. Some pots come off the wheel head as a finished product, such as some vases or a mortar and pestal. Usually some alteration is needed, like handles added or sides pushed or pulled. Some pots require multiple parts to be thrown like lids and spouts.
Throwing is fairly straight forward to explain, harder to master. A lump of clay of the appropriate size is plopped down close to the center of the wheel. The wheel is spun and the potter uses his hands to gently guide the clay into a centered state. From there, the potter opens it up by pushing down the center axis and pulling it out depending on the final shape he wants. Then the sides, or walls, are guided up using alternate pressures from one hand on the inside of the pot and the other on the outside. It usually takes one pull to even out the walls, one or two pulls to gain the height and basic form and one final pull to finish off the shape. One tries not to overwork the clay. A pot can usually be thrown in a few minutes. When the pot is finished, it is cut from the wheel head using a wire or string. This can leave decorative marks in a ring pattern on the bottom of an untrimmed pot. The pot is carefully lifted off the wheel and placed on a board to dry. Drywall is great to set a wet pot on to help keep the bottom from cracking from uneven drying.
Once the pot has dried a bit, to a stage known as leather hard, anything that needs to be completed, like handles or trimming, is done.
Trimming is performed by turning the pot upside down on the wheel head and centering it again. Then the excess clay can be cut away using a tool.
After the pot is finished being formed, it is set aside to dry or it is glazed if it is to be once fired.
Two types of wheels are used for Capp's Potts. Primarly, an electric Pacifica and a Brent kick wheel.
- Glazes - All the glazes are hand mixed from purchased raw materials. Most contain Feldspar, Calcium Carbonate, clay of some sort, Silica, and then various colorants. The recipe is mixed first by dry weight and then water is added until the glaze is a heavy cream like consistency. This is then applied to the pot any number of ways, from dipping to pouring to brushing it on. Many times, multiple glazes are used together for artistic reasons. New glazes are constantly being tested. There is as much room for artistic expression in the quality of the glaze as there is in the form itself.
- Firing - When the pots are all glazed and dry and ready to go, they are loaded into a downdraft gas kiln (fiber insulated and approximately 25 cubic feet with six burners entering from the bottom). Special high temperature shelves are coated with a mixture of silica and kaolin to prevent any wayward glaze drip from sticking to them. These shelves are set on bricks, the pots are placed on them, and then a new layer is set. It is built up until the kiln is full. Glazed pots cannot touch each other or they will stick together. Considerations also have to be made about heat flow in and around the pots, shelves, and bricks.
Once the kiln is loaded, it is lit. It is fired slowly in an oxidized atmosphere to about cone 08, (955 degrees centigrade or 1750 Fahrenheit). It is held at this temperature and atmosphere for about an hour to burn off any organic compounds that may be trapped in the clay. After this, it is put into a body reduction for about 30 minutes and the temperature is increased slowly. After the body reduction a slight reduction atmosphere is held until cone 10 is reached, (1305 degrees Centigrade or 2380 degrees Fahrenheit). The temperature is held at cone 10 in an oxidized atmosphere for 15 to 30 minutes and then it is shut down to cool. The firing process usually takes about 13 hours. The pots can be removed once they are cool to the touch.
Unloading a kiln is like Christmas each time because there are so many surprises.
Below are some before and after images.



The Kiln in reduction.
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