The following is the process for our glaze fired work. We also do some fast fired pieces. The process is the same through the bisque firing but rather than glazing and firing in an oxidation kiln, the pieces may or may not be glazed and are fired in a propane powered kiln. There are to many techniques for these fast fired pieces to describe here. One of the differences is that they are not water tight or food safe. Any piece fired in this technique will be noted.
We use various cones of clay (cone relates to the temperature the clay must be fired to allowing the components of the clay to melt and fuse together and become vitreous which means it does not absorb water.) depending on what we are making. Most of our stoneware glazes are fired to cone 6 which is a temperature of 2165-2269 degrees Fahrenheit. For these pieces we use cone 6 clay that we have purchased. The amount of iron in clay can make a big difference in how the finished glaze looks. Sometimes we use a body of clay high in iron that will give a darker earthier color and sometimes we use a clay body without iron that will produce a brighter glaze color. We choose the clay body based on what we desire for a finished product. When we want to use glazes that are fired to a higher temperature we use purchased cone 10 clay.
The Form Construction
Our primary method of forming is by centering clay on a potter's wheel and throwing. We also do some hand-building using slabs rolled on a slab roller, coils both solid and hollow made with an extruder or pieces made totally by hand. Sometimes we combine wheel thrown and hand built together to form a piece. After forming they are set aside to dry. When it becomes medium to leather hard the piece can be trimmed of excess clay and a foot trimmed on the piece. Trimming is a process where the pot is turned upside down on the wheel and the excess clay is trimmed off. The clay can be removed or trimmed off in a manner that will give the piece a foot (or a ring of clay) the piece sits on. The foot will give visual lift to the completed piece. At this stage handles and other attachments can be made. The piece is covered and allowed to dry slowly to bone dry. Now it is ready to bisque fire.
Bisque firing is performed in an electric kiln that reaches a temperature of 1,850 degrees F (cone 06). Firing takes about 9 hours to heat up and 12 hours to cool down. These fired, or "bisqued", pieces are now hard, but still very fragile.
The bisqueware is coated with glazes that we either made from our recipes or commercially made and purchased that we have found to fit our clay body. There are many methods of applying glaze. We find that the glaze sprayed on gives the best flow between colors on the pieces. In some instances we do dip, pour or brush glazes if there is an effect we are trying to achieve. Prior to applying the glaze we wax or coat the bottom of the pot or anyplace we don’t want the glaze to be. This allows any glaze that is not wanted to be easily cleaned off. This is essential on the bottom of the pot to prevent the pot from sticking to the kiln shelf and ruining the expensive shelf. When the glaze is fully dried it is ready for the final firing.
This final firing is done in our electric kiln. This is called an oxidation firing. This firing takes between 9-12 hours to reach our desired cone and 12 hours to cool down to room temperature. It is like Christmas when we open the kiln. There are some beautiful presents and some you would rather not have. This opening of the kiln anticipation is one of the biggest joys of doing pottery.
Fused glass is a term used to describe glass that has been fired in a kiln at a range of high temperatures from 593º C (1100ºF) to 816º C (1500ºF). There are 3 main distinctions for temperature application and the resulting effect on the glass.
* Firing in the lower ranges of these temperatures 593º-677ºC (1100º 1250º F) is called slumping.
* Firing in the middle ranges of these temperatures 677ºC- 732ºC (1250º-1350ºF) is considered "tack fusing".
* Firing the glass at the higher spectrum of this range 732ºC -816ºC (1350º-1500ºF) is a "full fuse".
All of these techniques can be applied to one glass work in separate firings to add depth, relief and shape or can also be limited to one technique.
Most fusing methods today involve stacking, or layering thin sheets of glass, sometimes using different shapes and colors to create patterns.
The stack is then placed inside the kiln and then heated through a program of rapid heating cycles called ramps and then soaking which is holding the temperature at a specific point until the separate pieces are bonded together. The longer the kiln is held at the highest temperature the more thoroughly the stack will melt together. The longest hold at the highest temperature will result in a flat piece of glass. A lower highest temperature being lower will result in a piece bonded together but with the shapes that were placed on top being raised from the bottom piece with rounded edges but retaining the original shape.
Once the desired effect has been achieved at the maximum desired temperature, the kiln temperature will be brought down with ramps and holds (which are cooling the kiln at set degrees per hour and hold for a set length of time) to avoid devitrification. It is then allowed to cool slowly over a specified time, soaking at specified temperature ranges which are essential to the annealing process. Annealing is the the process of slowly cooling glass so that the internal temperature matches the external temperature. This allows the glass to reach a stress-relief point, which relieves internal stress. After the glass is fused it can be left as a flat piece of glass or can be slumped to create a functional or art piece such as a bowl, vase, or platter. This is done in a separate firing by placing the fused piece of glass on a prepared mold and firing to a lower temperature than the fuse firing. This will result in the piece slumping into the mold and retaining the shape of the mold.