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.
Conclusion To Firing Clay
Ceramics must be heated in order to be durable. Despite the fact that there are a great many distinct kinds of kilns, the phases involved in firing clay are the same. It is extremely important, the clay is allowed to completely dry out, then it is given a bisque firing, and finally, it is given a glaze firing.
Pit Firing – The practice of firing clay in a pit was previously referred to as “baking” clay. Its history dates back around 30,000 years. Pots are frequently placed in a hole in the ground, sometimes known as a pit, and then fired. The colors and patterns produced by pit firing are solely the consequence of the method and the materials burned in the fire. Pit firing is a natural process.
Firing Schedule – When the kiln is turned on for the first time, it goes through a sequence of phases that are prescribed by the firing plan. These processes are repeated until the kiln is either turned off or has cooled sufficiently to be emptied. These alterations are distinguished by changes in the kiln’s surroundings or the firing ramp.
Firing Temperature – For mid-range materials, kiln temperatures should be between 2124 and 2264 degrees Fahrenheit (116 and 240 degrees Celsius). This temperature range is the most commonly utilized for industrial ceramics. Cone temperatures.
Cone 04 – Cone 04 – Orton Cones Final temp in degrees F at ramp rate of 27 degrees F/hr 1915 and Orton Cones Final temp in degrees F at ramp rate of 108 degrees F/hr 1945
Cone 06 – Orton Cones Final temp in degrees F at ramp rate of 27 degrees F/hr 1798 and Orton Cones Final temp in degrees F at ramp rate of 108 degrees F/hr 1828. 2200F is Cone 6 (also known as “middle temperature” by potters) is the temperature at which most hobby and pottery stoneware and porcelains are fired.
Cone 10 – Cones are numbered, and each number corresponds to a heating rate / temperature combination that will cause that cone to distort. The hottest is cone 10, which can reach 2381F. (read more about firing to a cone and see a cone chart with temperatures).
Raku Firing – Raku is a low-fire technique with a maximum temperature of roughly 1830F (1000C). Raku pottery is taken out of the kiln when it is red hot. It is cooled swiftly, generally in flammable material like sawdust or paper. Raku firing is often carried out in a fuel-burning kiln rather than an electric kiln.
Carbon Dioxide – Is a chemical substance composed of molecules that are covalently doubly linked to two oxygen atoms. At room temperature, it exists as a gas. Carbon dioxide in the atmosphere is invisible to visible light but absorbs infrared radiation, functioning as a greenhouse gas.
Molecular Structure – The three-dimensional arrangement of the atoms that make up a molecule is known as molecular geometry. It contains the molecule’s overall form as well as bond lengths, bond angles, torsional angles, and any other geometrical characteristics that dictate each atom’s location.
Thermal Shock – Is a sort of abrupt mechanical force. It is, by definition, a mechanical load induced by a fast change in temperature at a specific place. It may also be used to the scenario of a thermal gradient, which causes various portions of an item to expand differently. Ceramic Firing Techniques
Mullite Crystals – Mullite, also known as porcelainite, is a rare silicate mineral that forms during the contact metamorphism of clay minerals.
Quartz Inversion – This term refers to the abrupt change in volume that crystalline quartz particles go through as a result of passing through a temperature of 573 degrees Celsius in the ceramics industry.
Organic Matter – The term “organic matter,” which may also be referred to as “organic material” or “natural organic matter,” describes the abundant supply of carbon-based chemicals that can be found in both natural and manmade, terrestrial and aquatic settings. It is a type of stuff that is made up of organic compounds that has originated from the excrement and dead bodies of creatures like plants and animals.
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