Manufacturing earthenware, stoneware, and porcelains area an extremely technical kind of manufacturing. To fire all these kinds of materials, you need to really understand a lot about the kiln and the correct firing temperature.
If a ceramic piece is not fired at the correct temperature, it will make a huge difference in the final product’s outcome. It is also important that the correct kiln is used for the production process. This is because many things must be considered when using a kiln to fire ceramics. During the firing process, many things can cause the final product to fail or crack.
All About A Ceramic Kiln
To understand why the firing temperature will affect the final product, we must first understand what a ceramic kiln is. The best way to describe a ceramic kiln is a giant oven that will bake your ceramics just like you would bake a cake in your home oven.
The technical definition of a ceramic kiln is:
Some form of the ceramic kiln has been used for 1,000s of years. These kilns have turned various objects that were made by clay into pottery, tiles, or bricks.
Some forms of manufacturing may still use kilns that are heated by a wood source or fire. Usually, I have seen wood or fire kilns in factories that need to have the wood or fire to get the finish they desire or due to the clay they are using. But most kilns today use gas or electricity as their heat source. The reason is that this is usually safer and more economical.
Today there are so many ways kilns are built. That is why just the kiln itself to fire your ceramics can be extremely technical.
Whatever the kiln’s heat source, a kiln must be able to high-fire the clay at temperatures sufficient enough and even enough so that a clay reaction will occur and the body of the clay will be permanently altered into a hardened state.
But not all kilns are created equal, and some kilns will work better than others. Here are a few things to remember about a kiln:
- Temperatures – The kiln must be able to fire extremely high temperatures safely. For example, porcelain should fire at 1200C (2190F) to 1400 C (2550F). It is due to these very high temperatures why porcelain is scratch resistant and extremely hard.
- Unequal heat in kiln- Not all the kiln heat is the same unless you have an extremely high-tech kiln where they have worked to get the kiln as equal as possible.
- Placement in kiln- Since the heat in most kilns can be a bit unequal, even just a few degrees or less for some finishes can affect the color finished or hardness of the product. This is why we say that ceramic production is very complicated.
All about Firing Ceramic Products
Just forming the shape in the clay is not enough. Once that shape has been formed, then the clay must be fired. The firing of the clay can change the clay in a lot of profound ways. Essentially during firing, the clay is going from a soft, fragile substance to one that is rock-hard and solid.
Here are some things to understand about clay and the kiln firing process:
Bone-Drying in the Atmosphere
After the clay is formed, it is almost always dried to become what is known as bone-dried. Bone-dry is defined as:
I can tell you from first-hand experience that at this stage, the pottery is still very fragile. If you do not pick the pottery up correctly, it will break into pieces. The clay may be dried, but the pottery has not yet gone through the process to be changed through the firing process.
Making the clay bone-dry is just the first step in the firing process. The clay is now shaped and bone-dried. Bone-dry the clay by leaving the clay out for at least a few days or, in some cases, longer. The idea is to get the clay to be as dry as possible.
Here are a few things about bone-dried clay:
- Bone-dry – Completely dried out unfired clay.
- Brittle and fragile – In this state, the clay is still very brittle and fragile. It needs to be handled as little as possible.
- Usually lighter – The color will change, and the clay is usually lighter than it was in the moist state.
- Can not add anything – Once the clay is bone-dry, nothing can be really added to it except some very careful scratching or polishing.
- Difficult to re-wet – At this point is difficult to re-wet and to re-work the piece. But if you carefully rewet it evenly over the entire piece, sometimes you can rework it a bit. This is why it is best to make any changes before the clay is bone-dry. If you made a mistake, it is usually easier to start over than to re-wet a bone-dried piece.
Once the clay is fully bone-fried, it can be put in the kiln to be fired. Once it is in the kiln, several changes take place.
Fire to Burn Off the Carbon and Sulfur
You must fire the clay to burn off the carbon and sulfur content in the clay. All clay will have some carbon and sulfur that will need to burn off in the firing process. If this is not able to burn off properly, then carbon coring or black coring can occur. This is generally not desirable as it will weaken the structure of the clay.
Carbon coring or black coring happens when:
Carbon coring can happen if the firing is too fast or the kiln is poorly ventilated, so there is not enough oxygen in the kiln. Black or grey coring happens when the carbon and sulfur will strip away any red iron from the clay body, turning it into a black color.
Water Needs to Be Taken Out – Quartz Inversion.
Clay can still have close to 14% of water after it is considered to be bone-dried. When the clay is fired, the water needs to be able to escape from the clay. But if the water is taken out too quickly, it can instead cause steam to happen inside the clay. This will essentially break the clay pieces in the kiln. Quartz Inversion is when the water is taken out too quickly.
Expansion and Contraction of Clay -Dunting
When the clay is being heated up, the clay can temporarily expand in size to about 1- 2% during the heating process, and then it will contract to about 1-2% during the cooling process. If the ceramic is heated or cooled too quickly, it will cause cracking of the pottery. This kind of cracking is called dunting. You can identify dunting cracks as the ceramic piece will usually be warped, and the cracked edges will not fit together.
Clay Partials Sticking Together – Sintering
Another thing that happens in the kiln is that the clay particles will start to stick to each other or fuse together. They call this process of the clay fusing sintering. During the sintering process, the clay stops becoming clay but becomes a ceramic or porcelain material.
Fired to Mature or Vitrify Structure
Mature and vitrify structure is when the clay is fired so that it is hard enough that it can be used. Mature means that you have fired the clay to a tight, hard, and serviceable kind of structure.
On the other hand, Vitrify is similar to mature but not the same, for a lump of clay to vitrify means that it is fired to the point of glassification or turned more into a glass-like substance. Porcelain is fired at such a high temperature, so it is considered vitrified.
There are many ways that the kiln firing process will affect the production of earthenware, stoneware, and porcelain materials that are used in ceramic production. The kind of kiln you use and how the piece is actually fired can make a huge difference in the final ceramic product’s outcome.
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