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During the process of producing a piece of pottery, the clay goes through different stages, and in one of those stages, the clay is called bisqueware. So, what is bisque in pottery? And why is it important?
Bisque pottery has been fired once but has not been glazed. Usually, pottery is fired twice. The first firing is the bisque fire and the second one for glazing. Bisque pottery is hard and insoluble. Its porosity depends on the type of clay used and how it has been fired.
What Is Bisque?
Bisque is a word that can be used to describe a piece of pottery, i.e. bisqueware. Or, it can be used to refer to a way of firing clay, i.e. a bisque firing.
Bisque is sometimes referred to as ‘pre-firing’ pottery ware before it is fired for glazing. Prior to the bisque fire, the clay is air-dried to get rid of its water content. The unfired pottery goes through the stages of being leather hard (when there is still a little bit of moisture evident in the clay).
It then dries out to the point of being referred to as ‘bone dry’. Unfired pottery is called ‘greenware’.
Bisque ware has a number of properties. Firstly, once it has been bisque fired, it is said to have become ceramic. Unlike greenware, it cannot be dissolved in water anymore. It is hard, and generally has some level of porosity.
The degree of porosity depends upon the type of clay that has been used and the way that the bisque has been fired. For example, an unglazed earthenware like terracotta is very porous and will absorb water. (Think of plant pots in the garden).
By contrast, some clays, such as porcelain can become non-porous after a bisque fire and do not need to be glazed1
The Difference Between Greenware and Bisque in Pottery?
Greenware is clay that has been shaped into an object but that has not yet been fired. Bisque is greenware that has gone through the first bisque firing.
Regardless of how dry greenware becomes, it can still be transformed back into soft clay by introducing it back to water. It is absorbent and will readily lose its fashioned shape if it is made wet again.
By contrast, during a bisque firing, the clay undergoes chemical changes that alter its composition forever. Once clay has been fired and become ceramic, it is hard and no longer workable.
Greenware may look and feel completely dry, once it has reached the bone dry stage. However, even when bone dry, it will still contain some moisture and there is still some water content that is chemically bonded to the clay particles.
During firing, the greenware goes through a number of phases, which are:
- As the kiln starts to heat up, the residual moisture in the clay evaporates.
- At around 450 F (232 C), organic matter (that is carbon-based compounds) in the clay starts to burn away.
- Then, the remaining water that is chemically bonded to the clay particles is driven out.
- Next, various chemical and mineral compounds are burnt off.
- ‘Sintering’ occurs, when the surfaces of the particles become more chemically active and bond with one another because of the heat.
- The next stage is called vitrification, during this process, the various ingredients in the clay body melt to form a liquid glass. Clay particles bond to the glass. If all the spaces between the clay particles are filled with glass the body becomes non-porous. However, most bisque ware is referred to a semi-vitreous, meaning that whilst some glass has formed, the ware is still porous.
Because the clay is undergoing such big changes, the bisque fire has to take a long time, otherwise, the clay might be damaged during the fire.
You may be wondering how long it takes, and that is what I will cover in the next section….
How Long Does a Bisque Fire Take?
Bisque firing needs to be done according to a ‘firing schedule’. This means that the temperature of the kiln is closely monitored and controlled to avoid the pottery from exploding or cracking in the kiln.
It is unavoidable that there is some moisture and chemically bonded water left in the clay. The water needs to evaporate and be driven out, but it needs to happen gradually. If the water expands and turns to steam too quickly it can damage the pottery.
Small air bubbles trapped in the clay, can also cause the pottery to explode in the same way during firing. This is why it is recommended that clay is kneaded and slapped before it is used.
This process is called ‘wedging’ and it is designed to get rid of the air bubbles from a piece of clay.
The firing schedule refers to the stages that the firing process goes through and the different temperatures that the kiln will reach over a certain period of time. The term ‘firing ramp’ refers to the rate at which the temperature in the kiln increases.
The Three Different Ramps in a Bisque Fire:
The Low Ramp
The low ramp is the first ramp. After the kiln has been warmed up, it needs to stay at a low temperature for two hours.
If the pieces of clay have thick walls, the length of the low ramp needs to be increased to either four or six hours, depending on how thick the clay is.
The Second Ramp
The second ramp is the medium ramp which also lasts for around two hours. As above, if the clay is thick, this stage may need to be doubled or tripled in length to six hours long.
Once the kiln is ‘red hot’ at the end of the medium ramp, then the final high ramp stage begins.
The High Ramp
During the high ramp, the kiln’s temperature, now at a high heat, continues to increase until ‘bisque temperature’ is achieved. This is the temperature at which the clay will have turned to bisque.
It varies according to the type of clay being used. Getting to bisque temperature can take between three and eight hours.
So, a bisque fire can be anything between seven and twenty hours long depending on the thickness of the clay and the type of clay used.
In addition to this, the kiln needs to be warmed up before use. Sometimes kilns are left to warm up overnight at a very low temperature. The kiln also needs to be given plenty of time after the firing (again often overnight) to cool down. Opening the kiln up before it is cool enough can cause damage to the pottery ware.
Given that it takes so long to bisque fire pottery, you may be wondering if it is absolutely necessary to do it. This leads me on to the next section…
Do You Have To Bisque Fire Before Glazing Pottery?
As you will have seen from the above article, bisque firing gives the pottery ware important properties. For example, it makes the clay harder and more resilient and it also makes it water-resistant
Another important reason to do a bisque fire is that it prepares the pottery ware for glazing. Not all bisque fired clay needs to be glazed. For some purposes, being porous is a useful property. An example of this is terracotta plant pots. Likewise, fired porcelain may have reached the point of being 100% non-porous during the first firing, so that a glaze is not necessary.
However, some clays do need to be glazed. The porous quality of some bisque fired clay makes it perfect for the application of glaze. Because bisque is porous, it absorbs water from a liquid glaze on application. The glaze then sticks to the clay.
Water from a glaze mix is absorbed quickly by bisque pottery. In fact, the glaze may feel dry to touch after a few seconds.
Different bisque ware will have different levels of porosity. The higher the temperature in a bisque fire, the less porous the bisque ware will be. This is sometimes done because it makes the bisque ware harder and less resistant to cracks.
However, different levels of porosity will require different kinds of glaze to be applied. For example, less porous bisque ware will need a glaze with a lower water content as less water will be absorbed into the pottery on application.
Single Fire Glazing
There is such a thing as single fire glazing, where the glaze is applied to unfired clay or greenware. The topic of glazing greenware is an article all of its own.
However, in short, it is possible to apply glazes to unfired clay if the clay is bone dry when the glaze is applied and if the glaze has a high clay content. Having a high clay content in the glaze makes it less likely that the glaze will flake off during the firing process.
One of the disadvantages to single fire glazing is that the pottery ware has a higher chance of cracking or exploding in the kiln. Whilst bisque firing does require patience, you do stand a greater chance of your pottery remaining intact at the end of the process.