Clay must be fired in order to create goods that are long-lasting. The more you understand about the process of firing ceramics, the more control and success you will have with the pots and pieces you make. Here is a helpful explanation of what occurs inside the kiln when pottery is being fired.
Firing clay turns raw clay into ceramic through high-temperature heating. This occurs in a kiln. Clay is fired in two stages: bisque firing and glaze firing. The first fire is a bisque firing, which is slower and reaches temperatures up to 1830 F (1000 C). If the bisque is glazed, it goes through a second fire to transform the glaze into a coating for ceramic objects.
To achieve long-term durability, ceramics need to be fired. Potters need to be familiar with the procedures that are taking place in order to have control over the final product. In addition to the firing of the clay, the glaze must also be matured by being fired.
Firing Clay Video
What Happens When You Fire Clay?
When clay is fired, it undergoes several physical modifications. The first thing that happens is water trapped between clay particles evaporates. Firing clay turns raw clay into ceramic through high-temperature heating.
Water that has chemically reacted with the clay (the water embedded in clay) is removed during firing. Removing water from the clay enables you to proceed to the next step, which is bisque firing.
Pots must be completely dry before firing; otherwise, the steam that escapes from within may cause them to explode. The kiln temperature should be brought up to operating temperate slowly (This is called ramp up the temperature) at first to allow all the water to evaporate before increasing the temperature.
The drying process can be sped up by preheating the pottery in the kiln for a few hours to 176 degrees Fahrenheit (80 degrees Celsius).
Notice I said, “can be”. Many potters let their clay air dry. In this stage of the firing, temperatures may reach 212 degrees Fahrenheit (or 100 degrees Celsius). This process is known as “water smoking” because it causes the water trapped between the clay crystal pores to evaporate.
Why Do You Need To Fire Clay?
Clay must be fired to create long-lasting goods. To achieve long-term durability, ceramics need to be fired.
I encourage potters and artists to be familiar with the procedures that are taking place to have control over the final product. Besides the firing of the clay, the glaze must also be matured or hardened by being fired.
What Is Clay Called After It Is First Fired?
The initial firing is called bisque. The process is more gradual and takes place at a lower temperature. Although it may still become as hot as 1,000 degrees Celsius (1,830 degrees Fahrenheit), temperatures are not obtainable in a kitchen oven, therefore a kiln is needed.
A bisque piece is still porous even after it has been hardened, which allows it to hold glaze. On the other hand, it can be painted, polished, or left in its natural state.
If the bisque is to be glazed, the piece will need to undergo a second fire (or a next stage firing) in the kiln, which is referred to as the glaze firing.
What Is A Bisque?
A bisque is a clay body fired once.
Bisque firing can occur at temperatures as high as 1,022 degrees Fahrenheit (550 degrees Celsius). After firing at these temperatures, a chemical transition occurs. The process cannot be reversed, and the clay cannot be returned to its original plastic state.
Bisque Fire Clay Tips Video
When heated to 1,063 degrees Fahrenheit (573 degrees Celsius), crystalline quartz in the clay body expands. If the temperature within the kiln is increased too rapidly, the material may break. Your clay can explode in extreme cases.
The clay body begins to vitrify and shrink when heated over 1,652 degrees Fahrenheit (900 degrees Celsius). As the silica begins to melt, it fills the spaces between the clay particles and bonds them together. Did you know at 1,832 degrees Fahrenheit, clay crystals begin to dissolve and melt (at 1000 degrees Celsius)?
In the fired clay, needle-shaped crystals of mullite begin to form. These crystals are responsible for the strength and hardness of the clay.
When the kiln is cooled to 439 degrees Fahrenheit (226 degrees Celsius), the cristobalite in the clay contracts abruptly. This can cause cracking in the clay body if the kiln is opened too rapidly, causing the temperature in some portions of the clay body to drop too quickly, causing stress, cracks, and even breakage in the clay body.
Glaze Firing, Stains, and Oxides
One reason to apply a glaze would be for decoration.
You might also apply a stain for decoration.
After the clay body has been fired, a transparent glaze is can be applied on top of the stains to intensify the color and protect it.
Glazed and Fired Ceramic Art
Ava’s ceramic artwork called Finely, Captain and Bubbles is a perfect example of clay after it has gone through a glaze firing.
The artwork went through a bisque firing and then glazed, fired again and signed by the artist.
Note the brilliant, rich deep colors resulting from the glaze firing.
Oxides are frequently used as a kind of decoration and are mainly used in in educational institutions like schools and colleges. They are very efficient, simple in their operation, and inexpensive. They are available in the form of powder and come in a variety of colors.
They can be brushed on bisque-fired ceramics after being mixed with water.
Because the color pigment is so strong, it is best to make a test piece so that the result can be accurately predicted.
Oxides work best when they are put on textured surfaces because they can settle into the texture of the clay surface.
What Are The Three Steps For Firing Clay?
Before you start firing up your kiln, it is essential to have some background knowledge regarding the drying process, otherwise you could damage your sculpture or pottery.
Step 1: Leaving Your Pottery to Air-Dry
Because wet clay might produce explosions in the kiln, you need to make sure that it is completely dry before you fire it. This is because moisture transforms into steam when it exceeds the boiling point of water, which is 212 degrees Fahrenheit (100 degrees Celsius). When water transforms into steam, its volume quickly increases.
The term “bone dry clay” refers to clay that has been allowed to dry out.
Step 2: The Bisque Firing Stage of the Pottery Making Process
In most cases, a second firing is required for the pottery once it has reached the bone-dry stage. The initial firing in the kiln, is known as the bisque fire.
This process is also known as “biscuit firing” in some circles.
Pottery made from clay is transformed into the ceramic with the use of a process called “bisque firing.”
Artists, will often bisque fire their clay to a temperature that falls between 1,823 and 1,940 degrees Fahrenheit (F) (995–1,060 degrees Celsius).
This range of temperatures is usually used for the bisque firing of pottery, no matter what kind of clay is being used.
Potters can slightly alter the characteristics of the bisque ceramics they create by making slight adjustments to the temperature at which the bisque is fired.
However, the firing process for bisque is usually commonly referred to as a “low fire.”
Step 3: Glaze Firing Ceramics
Glazing pottery is done primarily for two different reasons.
The first one is only for looks. Glazing enables potters to achieve a wide variety of colors, textures, and finishes in their work.
The second one has a purpose. A glaze is put on pottery to give it a glassy coating that protects it from water and, in some cases, makes it completely waterproof.
When the glaze on the pot dries, a layer of the ingredients that make up the glaze will attach to the surface. A piece of bisque ware often requires several coats of glaze before it is fired. The number of layers varies depending on how the glaze is applied and the type of glaze that is used.
The second fire can begin on the pot as soon as the glaze has had sufficient time to harden. This process is referred to as glaze firing.
My Most Important Money Saving Tip When Firing Clay
Here is my money saving tip, or I should say here is my clay saving tip: I strongly advise you to wait until your kiln has cooled to a temperature, which is usually less than 212 degrees Fahrenheit (or 100 degrees Celsius) before opening.
I usually wait almost a full day before opening the kiln. And even then, only crack the kiln door open, leaving it that way for a few hours before fully opening the door. This process has saved me money and clay. I cracked more clay when not using this method because I was so eager to see the results I open the kiln door too soon causing my wares to crack.
Each potter usually has a particular procedure that they fully trust, and this is one of mine.
References
Image by juliansch from Pixabay – pixabay.com/photos/ashes-bakery-bolivia-branch-bread-1543382/
Reber, E. A., Kerr, M. T., Whelton, H. L., & Evershed, R. P. (2019). Lipid residues from low‐fired pottery. Archaeometry, 61(1), 131-144. onlinelibrary.wiley.com/doi/abs/10.1111/arcm.12403
Johnson, J. S., Clark, J., Miller-Antonio, S., Robins, D., Schiffer, M. B., & Skibo, J. M. (1988). Effects of firing temperature on the fate of naturally occurring organic matter in clays. Journal of Archaeological Science, 15(4), 403-414. sciencedirect.com/science/article/abs/pii/0305440388900386
Sparkes, B. A. (1991). Greek pottery: an introduction. Manchester University Press. https://books.google.com/books?hl=en&lr=&id=ntlRAQAAIAAJ&oi=fnd&pg=PR11&dq=Firing+Clay+pottery+introduction&ots=tI50O-xkmc&sig=DODdCgO3WYAWZcDJvTDq2fjPF6Q#v=onepage&q=Firing%20Clay%20pottery%20introduction&f=false
