What is Bisqueware in Ceramics?
Most pottery made from potters' clay (rather than air-dried clay) is fired twice. The first time in what's known as a bisque firing, in temperatures of between 950 - 1160 degrees Celsius (1742 - 2120 degrees Fahrenheit). Pottery is then dipped or painted with glaze and put into a second firing, known as a glaze firing.
In this blog post, we’ll discover more about what bisqueware is, how it is made, and whether or not bisque-firing is an essential part of the pottery-making process.
What is bisqueware or ‘bisque-fired’ pottery?
Essentially, bisqueware in ceramics refers to pottery that has undergone the first firing to bisque temperatures. Confusingly bisque can also be known as biscuit! Bisque and biscuit mean the same thing and can be used interchangeably in pottery. After the first firing, you may hear pottery being referred to as biscuitware or bisqueware. After a pot has been biscuit fired and a glaze applied, it is ready for the second stage of firing; the glaze firing.
What is the purpose of bisque firing?
Biscuit-firing pottery has become standard practice in the handmade pottery industry today. There are two main reasons why Potters bisque-fire their work:
Reason 1: Bisque-fired pottery is more robust than raw dry clay.
Biscuit-firing pottery permanently changes the molecular structure of the clay, making it into a much more durable state than raw dry clay. Bisque-fired clay can no longer be recycled or reclaimed back into wet clay. This is very useful because pottery glaze application is much easier on biscuit-fired pottery. Pots can be dipped into a glaze without them disintegrating.
Reason 2: Bisque-fired pottery is very porous allowing easy and even application of pottery glazes.
Biscuit firing increases the degree of porosity. This creates a perfect clay surface for the application of pottery glazes. For example, pottery only needs to be held in a glaze bucket for a few seconds to create a beautiful even layer of glaze across the surface of the pot ready for firing into the finished piece.
What happens to clay during the bisque firing process?
Clay undergoes chemical changes during a bisque firing where chemically bonded water molecules are released from the clay. In the first bisque firing stage when clay is heated from between 90 - 300 degrees Celsius (194 - 572 degrees Fahrenheit) any organic matter in the clay is burned away.
Further heating at the bisque stage to 600 degrees Celsius (1112 degrees Fahrenheit) causes the water molecules in the clay to be permanently driven out of the molecular structure of the clay. Between 220 - 573 degrees Celsius (428 - 1063 degrees Fahrenheit) Silica crystals in the clay are converted to Crystobalite and mullite.
At 700 degrees Celsius (1292 degrees Fahrenheit) the clay particles start to fuse in a process known as sintering. These changes have permanently altered the chemical structure of the clay and it can no longer return to wet clay. Further heating of clay causes it to vitrify where it becomes impervious to water in its final state. This is why pottery can be used for mugs, vases and jugs which can contain liquid without it leaking through.
The main ways to identify bisqueware or biscuitware pottery.
You can often identify bisque ware because it will be harder and more robust than raw clay that has simply dried. If you flick it with your finger it will have a ring to it rather than a dull noise like dry pottery does. Often the colour has changed too. Bisque-fired pottery looks whiter than it did before going into the kiln. Some types of clay also go a slight pinkish colour in the bisque firing rather than a grey colour like dry pottery.
Bisque-fired pots will also be much harder than dry clay, they can no longer be sponged or smoothed. This is why it is important to smooth any rough edges on dry pottery before it is bisque-fired because these edges can become extremely sharp once the pottery has had its first firing.
If you practice pottery at a community pottery studio, or in an educational setting, the pottery studio is likely to separate unfired pottery and bisque-fired pottery into separate places in the pottery studio so it is clear which is which. This could be done on separate shelves marked ‘greenware’ or ‘drying shelves’ (unfired pottery) and ‘fired pottery’ ‘biscuitware’ or ‘bisqueware’ for pottery which has undergone its first firing. In the adult evening classes where I learned pottery, bisqueware was kept in a separate room where the kilns were to save the Pottery Technician the effort of moving all of the bisqueware too far from the kiln, where it has just been fired.
A bisque-fired pot being lifted out of an electric pottery kiln
Some pottery suppliers actually sell already fired bisque pottery shapes and unpainted ceramics that you can buy and glaze yourself. If you have ever gone to contemporary ceramic cafes for example, where you can paint-your-own pottery, you will be painting glaze onto unglazed earthenware pottery blanks such as bisque tiles mugs and plates. Your painted bisque piece is then fired at the cafe and you return at a later date to collect your finished ceramic piece after it has undergone a final firing.
How is pottery bisque-fired?
Step one: Drying the clay.
The first stage is to dry the newly made pottery, very thoroughly. This can take several days or even weeks for large pieces! Pottery needs to dry slowly and evenly in order not to crack during the drying process. Pots need to be dried in areas that are warm (but not too warm!), that have even temperatures where there are no extremes of hot or cold, and that have good ventilation.
In the adult education ceramic studio where I learned pottery, there were specific drying shelves to put finished work on. The drying shelves were in the middle of the studio for easy access and away from the kiln room as it was too hot in there, and would have dried the work too quickly. The shelves were also built with slats of wood rather than a solid shelf, to encourage ventilation around the drying pottery.
During the drying process clay shrinks as water is released. The shrinkage rate is usually around 5 - 10% from wet clay to dry clay. Once the clay has become completely dry it is often referred to as bone-dry clay or greenware. It is only at this stage that clay can be successfully bisque-fired. If the clay is wetter than this, there is a chance it could explode in the kiln as it is heated because there will be too much water being released from the clay body during the firing. So the clay must be completely dry before being packed in the kiln.
Step two: Packing the kiln.
The next stage is to pack the pottery inside the kiln ready for firing. The great thing about packing a kiln for a bisque firing is that the pots can touch each other (unlike in a glaze firing). In a glaze firing pots must not touch because the glaze melts at the very high temperatures. If the pots are touching they will fuse together. However, greenware pots in a bisque firing can be packed very closely together, stacked on top of each other, and even inside one another in the kiln. The only thing to remember when packing a bisque kiln is that greenware is extremely breakable, so you don’t want to stack heavy items on top of more fragile ones.
You also don't want to stack too many things in one stack as this can put a lot of pressure on items at the bottom of the stack. It is a good idea to separate pots with at least some kiln shelves supported by kiln props.
Step three: Firing the Kiln
The difference between low fire bisque and high fire bisque temperature kiln firings.
Pots can be fired in what is known as a low bisque firing or a high bisque firing. Low-bisque firings and high-bisque firings are done at slightly different firing temperatures. Generally, low bisque is used for stoneware clay because it will be fired much hotter later in the making process in the stoneware glaze firing. Low-fire bisque is fired between 960 - 1000 degrees Celsius or 1760 - 1832 degrees Fahrenheit.
High bisque is fired to between 1120 - 1160 degrees Celsius or 2040 - 2120 degrees Fahrenheit. Although a high-fire bisque firing creates a less porous ceramic surface because the clay is more vitrified, high-fired pottery can still be successfully glazed. High bisque can be used for earthenware ceramic ware that will be fired with low-fire glazes in an earthenware glaze firing.
The purpose of a high bisque firing is to make the final pottery as strong as possible. This is because earthenware glaze firings are often actually at lower temperatures than the high bisque firing. Clay fired at a lower temperature (such as earthenware ceramics) is generally weaker than clay fired to higher temperatures.
Glaze firings for stoneware clay are hotter than bisque firings. A stoneware glaze firing is done in temperatures of between 1180 - 1300 degrees Celsius (2156 - 2372 degrees Fahrenheit), whereas an earthenware glaze firing is done in temperatures of between 1020 - 1180 degrees Celsius (1868 - 2156 degrees Fahrenheit).
The difference between bisque firing standard pottery and thicker pottery.
Another variation in bisque firing is when the work being fired is particularly thick. Thicker pottery needs to be fired at a very low temperature at a slow speed, for a long time. This is to stop water from being released too fast during the firing which can cause pottery to explode in the kiln.
Both of these types of bisque firings actually reach the same ideal range of temperature in the kiln of around 950 - 980 degrees Celsius or 1742 - 1796 degrees Fahrenheit. The difference between a thick work bisque firing and a standard bisque firing is the speed and the bisque firing duration. Kiln firings are organised in sections of time which are also known as segments. Each section or segment tells the kiln how high to heat the kiln chamber, how quickly and for how long. The term set point is used to describe the highest temperature that the kiln gets to during that section or segment. The term soak refers to the time spent holding the kiln at a certain temperature.
Electric kilns can be programmed using an electronic controller that usually sits alongside the kiln in a special holder. Each section or segment is programmed into the controller.
All the sections or segments that make up one complete firing is called a kiln schedule or a kiln programme. Electric kiln controllers usually allow many different kiln schedules to be programmed into it. For example programme 1 could be a standard biscuit firing, programme 2 a thick work biscuit firing, and programme 3 a stoneware glaze firing.
Here is a typical kiln schedule for a bisque firing of thicker work:
Section / Segment 1:
50 degrees Celsius per hour (122 degrees Fahrenheit) up to a set point (the hottest temperature) of 90 degrees celsius or (194 degrees Fahrenheit) and then a soak time of 4 hours.
Section / Segment 2:
100 degrees Celcius per hour (212 degrees Fahrenheit) up to a set point of 600 degrees Celsius or (1112 degrees Fahrenheit) and then a soak time of 30 minutes.
Section / Segment 3:
150 degrees Celcius per hour (302 degrees Fahrenheit) up to a set point of 980 degrees Celsius or (1796 degrees Fahrenheit) and then a soak time of 30 minutes.
The kiln is then left to cool down naturally to room temperature.
Here is a typical kiln schedule for a standard bisque firing:
Section / Segment 1:
80 degrees Celsius per hour (176 degrees Fahrenheit) up to a set point of 200 degrees Celsius or (392 degrees Fahrenheit).
Section / Segment 2:
120 degrees Celsius per hour (248 degrees Fahrenheit) up to a set point of 950 degrees Celsius or (1742 degrees Fahrenheit), with a 10-minute soak at 950 degrees Celsius or (1742 degrees Fahrenheit). The kiln is then left to cool down naturally to room temperature.
Do Potters always need to bisque-fire?
Actually no, whilst bisque firing is now common practice when making pottery it is not an essential part of the pottery-making process. The technology to control kiln temperatures and create the right conditions for bisque firing has only fairly recently been developed. Before this, and for thousands of years, pottery was simply fired in one single firing and any ceramic colour was added in this single firing stage.
Some Potters continue this tradition today in a practice known as ‘raw glazing’. Raw glazing is when a pot is glazed at the bone-dry or leather-hard stage and then fired straight up to glaze temperatures. Most Potters today bisque fire their work before glazing because raw glazing can be technically quite tricky due to the fragile state of unfired clay. Re-wetting an unfired pot with glaze can cause it to disintegrate. However, the good news is that there are many different ways Potters can make raw glazing easier!
The first way is to add paper fibres to their work to make the ceramic material stronger, allowing the application of glazes. Whilst ceramic bisque pieces have a higher porosity than unfired clay, bone-dry clay still has a porous surface that can be successfully glazed.
Using ceramic glazes suitable for raw glazing which often have Ball Clays and China Clay as ingredients, is also essential for raw glazing. Another thing Potters can do if they want to try raw glazing is to only glaze parts of their pots or try painting on glaze rather than dipping whole clay pieces in the glaze.
Raw glazing can be a great option for Potters because it can save time, energy and money.
If you are interested in trying raw glazing there are 10 high-fire (or stoneware) raw glaze recipes and a kiln firing schedule in the book Eco-conscious Pottery Glazing: Make your own pottery glazes with minimal harm to humans, animals and the environment, for you to try!
