What Is The Difference Between Bone Firing And Open Pit Firing?

What Is The Difference Between Bone Firing And Open Pit Firing?

How do you fire your ceramics? Is it the same way you’ve always done it? Have you ever tried using different materials to fire your creations? Achieving the right firing temperature can be a challenge when starting out, so you may want to take advantage of more advanced firing methods, like bone firing or open pit firing. These two techniques have their own strengths and weaknesses, and they work best with different kinds of ceramics, so you’ll need to decide which one works best for your creations.

Mixing Of Clay

Mixing clay is typically done by hand, but there are different ways to do it. Some clays will blend together easily and others will need to be broken down into finer particles before mixing. When mixing clay, it's important to keep your tools clean and free of debris, or else you might contaminate your clay. You'll also want to make sure you don't add water until after you've mixed all of your ingredients together; otherwise, it can actually cause problems with how well your final product fires. Once everything is mixed, it's ready for pressing!

Pigmill

To Fire Pottery, Shape It With Clay: Pigmilling is a technique used to fire ceramics in high-temperature kilns. This technique was originally developed by early Native Americans who did not have access to specialized materials or equipment. Since pigmilling does not require complex equipment or expensive clay, many potters still use it today for both functional and decorative pieces.

Shrinkage Of Clay

When we talk about firing clay, we mean that when you take your pottery out of a kiln, it has changed in some fundamental way. All of your pots have been fired to some degree before, right? So let’s call that firing. What happens during firing is what makes all the difference. Two very important parts of pottery making are shrinkage and fusing. These are controlled by your temperature choices and play very different roles in how your finished piece will look and feel. Let’s break down both types of firing into their individual components: bone or open pit firing, shrinkage of clay and each has its own purpose.

Bisque Firing

In bisque firing, pieces are fired at temperatures of 1,400 degrees Fahrenheit (or higher) in a kiln for about six hours. After a piece is bisque fired, it is allowed to cool overnight before glaze firing. It’s important to note that bisque-fired pottery isn’t food safe. Additionally, if your piece has small openings or areas that allow air flow, you need to wait an additional 24 hours after bisque firing before firing with glazes (this process takes longer than it sounds). If you wait too long between firings and skip glaze firing altogether, colors on your ceramic piece will look dull when fired because unglazed clay absorbs oxygen.

Plasticity Test

Mixing clay is easy, but it's important to know how much water to add. After mixing your clay, place a pinch of it in a glass of water. If it sinks like a stone and doesn't float back up even after hours of soaking, you've mixed too much water into your clay. If it floats on top and stays there without moving, you need more water in your mix. When making pieces for ceramic art forms such as bowls or vases, most people aim for somewhere between 10% and 20% moisture content; if you're using your clay to create 3D figures or other sculptural pieces that will dry out completely once fired (often called bone or open pit firing), then anywhere from 20% to 35% is good enough.

Firing Temperature

How Hot Does Your Pottery Have To Be To Fire Successfully? It is well known that firing temperature is a big factor in pottery success. But what are appropriate firing temperatures for both bone and open pit firing? Many potters do not know where to start when it comes to deciding what is a good firing temperature. Here is a quick overview of bone versus open pit firing temperatures, along with a few basic tips to help you determine what are best-suited for your needs.

What Is A Bone Fire?

Clay pots fire in kilns at temperatures between 1,000 and 1,400 degrees Fahrenheit. This process is known as bone firing because it leaves some moisture (or bone) in the pot. Potters often use a smaller version of a kiln to fire their individual pieces before reassembling them into larger units. It's also common for potters to mix clay bodies with extra ingredients—like volcanic ash or silica—that help keep their work from cracking during firing and make certain pieces better suited for bone-firing techniques.

What Is An Open Pit Fire?

When an open pit fire is used, hot coals are shoveled into a pit in which clay pieces have been placed. They are covered with sand and then left to fire overnight. As you can imagine, one of these fires creates quite a lot of smoke. An alternate way to do an open pit firing is to build two fires instead of one: one fire to burn out and another for firings. While not as smoky as an open pit firing, you will still want to be outside when doing an open pit firing.

When To Use Each Method

There are multiple ways to fire ceramics and understanding when to use each is helpful. This guide explains why open pit firing and bone firing are commonly used for their respective industries. It also describes how shrinkage of clay can affect your fired pieces. 

What Is Kiln

Not all clay can withstand direct firing—this is particularly true for plastic clays and stoneware, which tend to have more plasticity than other types of clay. When fired directly in a kiln, these clays can deform, crack or develop blowholes. For that reason, they are often subjected to what is called a pre-fire before being placed in a standard kiln. A pre-fire is essentially an indirect firing that fires with less heat over a longer period of time (typically at 1120 degrees F / 610 degrees C over 12 hours). This way, plastic clay becomes more rigid before it's placed in a full-force flame.

Updraft Kiln

It’s hard to understand how two firing methods could be fundamentally different, but they are. Let me take you back to ancient times: pit-firing pottery was first practiced in Western Asia and it quickly spread across Europe and into Africa. In pit-firing, pieces are placed inside a sealed oven that is partially excavated from the ground. Air is circulated around these underground chambers and ash or other items are added for insulation. Pottery is fired between 1,300 °C (2,372 °F) and 1,400 °C (2,552 °F), but most pits don’t reach full temperature until a few days after they have been lit.

Downdraft Kiln

When you’re firing pots in a downdraft kiln, it’s important to keep in mind that you’re essentially burning fuel and air. This process isn’t as straightforward as just pushing go and waiting for your work to cook. If you want everything to turn out right, then you have to control those two essential elements—fuel and air—at all times.