Interesting colors and decorative patterns can be attained on pottery without glazes. Saggar firing is one method where pottery is exposed to vaporized minerals during the firing process allowing the pot to absorb colorants in random patterns.
A saggar is a protective chamber used to contain a delicate piece of pottery. The concept of firing within a saggar originated with the ancient Chinese in an effort to protect a glazed pot from the ash and grit that is normally dispersed in a wood kiln. Today the purpose of the saggar is quite different. The bisqued pot is placed in the saggar along with combustible materials to be exposed to the pot during the firing; no glaze is used. Typically the pot is placed on a base of fine sawdust sprinkled with some copper carbonate, red iron oxide, or copper sulfate mixed with salt. Other items may be draped directly on the pot, such as steel wool or copper wire. Some (more adventurous)potters have experimented with vegetable or fruit peels, dog food, and seaweed. The fuming that results from the materials added to the saggar produce irregular color patterns on the pot.
Saggar fired pots are purely decorative; they are not food safe and will not hold water.
Potters use various types of saggars, such as bricks and kiln shelves, thrown clay, and aluminum foil. At Roberson we have used clay saggars thrown from a highly grogged clay.
To make the saggar, throw two deep bowls. The walls of the bowls should be relatively thick (approx 3/8" to 1/2"). The bowls do not have to have the same overall dimensions, however, their diameters must match because one bowl is inverted on top of the other to make the closed container. Bisque fire the saggar, and you're ready to go.
A saggar should survive several firings. Furnace cement can be used to repair fine line cracks in a clay saggar.
The smoother the surface of your pot, the more receptive it is to the fuming effects within the saggar. A smooth pot surface can be achieved through burnishing the greenware with a smooth flat stone or the back of a spoon with a little mineral oil, or you can apply several thin layers of terra sigillata.
Loading the Saggar
Like most firing techniques, there are many variables that can affect the results of a saggar firing. The materials you place in the saggar obviously contribute to the results, but also the shape of your saggar, the amount of space between your pot and the wall of the saggar (you want at least an inch), and the amount of open space above the pot in the saggar. If your saggar is big enough, then multiple pots can be placed in a single saggar.
To load the saggar, place a bed of sawdust at the bottom. Mix equal parts of salt and copper carbonate into the sawdust. Drape organic materials (seaweed, banana peels, etc) and/or metals (such as steel wool or copper wire) over your pot. Invert the second bowl as the lid of the saggar. Some air intake is desirable. If the lid of the saggar is tight against the bottom, you can roll up some steel wool to place along the rim of the base before covering.
We fire in a gas kiln. Since you cannot view the pots during the firing process, it is imperative that you have a pyrometer for monitoring. Our recipe for firing was adapted from a workshop given by two well known potters, Charlie and Linda Riggs.
The temperature is gradually brought up to 1600-1700 degrees. In our kiln this takes about 45 minutes. The kiln is then held at that temperature for approximately 20 minutes, giving an overall firing cycle of about an hour. The kiln is then left closed for about an hour followed by another hour of cooling with the kiln opened.
If you are unhappy with the result, you can refire the pot.