How and Why Slow Firing Porcelain Produces Better Results

Called by some the “diva of clay bodies,” porcelain is notoriously difficult.

Yet, despite its susceptibility to collapse during formation and warp during firing, many ceramic artists are seduced by porcelain’s bright white color, translucency, and ability to beautifully display glazes.

If you find yourself in this category and would like to try your hand at creating porcelain pottery, you’ve come to the right place.

We’ve put together a guide to help ease the process of firing so that you can better understand the properties of this clay body, transformations it undergoes during firing, and dangers to watch out for in order to create beautiful pieces with reliable, successful firing programs.

What is porcelain?

Typically fired between 2381℉ and 2455℉ (1305℃ and 1346℃), porcelain is a high-fire clay body, meaning it matures at a much higher temperature than earthenware, stoneware, or most other ceramic materials.

Porcelain is made of kaolin, the purest form of clay, named after a hill in China from which it was mined for centuries. Because pure kaolin is typically too hard to work with, porcelain is not made exclusively from it; however, the more kaolin in the clay body, the whiter it will fire.

As a clay body, porcelain is known for its hardness, extremely tight density, whiteness, and translucence in thin-walled pieces.

What happens during a bisque firing of porcelain?

The first firing ceramic ware undergoes is known as the bisque firing. In this stage, greenware transforms into a durable, semi-vitrified porous state where it can be handled safely while being glazed and decorated. The other important thing that occurs during this stage is the release of carbonaceous materials and gasses.

Here’s a breakdown of what’s happening inside your kiln when you bisque fire porcelain:

• Around 392℉ (200℃), the last traces of moisture start to leave the clay. Most of the water should already be gone, as you want to fire clay bone dry, but the final traces in the clay’s pores will evaporate at this initial stage.

• At about 450℉ (232℃), organic matter in the clay body begins to burn off. Though this typically doesn’t reduce the weight of the ware, it does produce a lot of smoke, so it is crucial to ensure kiln ventilation at this temperature.

• Between 842℉ and 1022℉ (450-550℃), chemically bound water within the clay minerals evaporates. Known as dehydroxylation, this step reduces the weight of common minerals within the clay.

• At around 1112℉ (600℃), sulfur compounds burn out. Typically taking the form of iron sulfide, sulfur isn’t necessarily found in porcelain, but it is in many ball clays, which are frequently added to kaolin clays to increase workability. Ball clays and the consequences of their incorporation into porcelain bodies will be discussed at length later in this guide.

• Between 1562℉ and 1832℉ (850-1000℃) is the high point of porosity for porcelain bodies, meaning clay pores are farthest apart at this stage, before the clay starts to harden and shrink.

• As the temperature continues to rise, the clay body will stop losing weight and will become stronger and denser as clay crystals bond to each other.

What happens during a glaze firing of porcelain?

The second firing of porcelain occurs after the artist has applied glaze to the piece, hence the name, and is typically faster than bisque firings because most water and and carbonaceous materials have been driven out of the clay.

When fired, porcelain becomes a hard, vitrified, non-absorbent clay body. It develops a body-glaze layer formed between the clay body and the glaze, meaning the two materials fuse at their joined surface to form a thin combined layer of the two by the end of firing.

The importance of a slow fire

For other clay bodies, like earthenware or stoneware, firing slowly is a common practice. Firstly, water expands and is pushed out of the clay as it heats, so if fired too quickly, water can boil inside the clay and cause it to explode.

Firing too fast also doesn’t allow other gases and organic matter to escape as clay shrinks, which can cause the matter to get trapped and sealed between clay layers as the clay shrinks. Much-needed oxygen then doesn’t reach these impurities fast enough to burn them out, causing the clay to crack, become discolored, or even explode.

Because of its kaolin base, porcelain has slightly different properties that might make it at first appear immune to the problems other clay bodies face when fired too quickly. The molecular structure of kaolin is free of any iron, alkalies, or alkaline earths, meaning the risk of trapped organic matter is very low.

Additionally, kaolin causes porcelain to dry much faster than other clays, meaning less time is needed for steam to escape.

Finally, kaolin is a refractory, or strongly heat-resistant, material, which keeps porcelain clay bodies from shrinking very much during firing, again lowering the possibility of trapping impurities.

However, it is important to remember that firing porcelain is not like firing pure kaolin; while porcelain has many of the attributes of kaolin, because it is only partially composed of this material and may be mixed with others, firing too quickly can have consequences just as dire as for other clay bodies.

Here are the important reasons why:

Impurities introduced by other clays

As mentioned earlier, pure kaolin is typically too difficult to work with because of its hardness. To increase the workability of porcelain, as well as lower its firing temperature, artists typically mix other clays with kaolin to create a more plastic, or malleable, clay body.

One such material is ball clay, a highly plastic fine particle clay that makes porcelain much easier to work with. However, ball clays collect iron, coal, and other mineral impurities when they are naturally mulled in water. Though the mulling process is necessary to create a finer clay particle size and therefore create the plasticity so needed in kaolin clays, small traces of organic matter can remain even after machines process and clean the clay.

When mixed with additives like ball clays, porcelain must be treated like all other clay bodies when it comes to timing the bisque firing: amping up temperatures too quickly can introduce the risk of organic matter not having the time to escape properly.

Contamination from human error

Organic matter can also be introduced to porcelain clay bodies accidentally in an artist’s studio. Cross-contamination can cause unintended impurities to be introduced from another clay body into the porcelain being worked with, so be sure to keep your work space immaculate and your different clay bodies separate from each other.

Gum and bentone additions

Porcelain clays now often contain plasticity-increasing substances like gum and bentone. Though bentone in particular is lower in iron, and therefore decreases the risk of firing with impurities, it does cause slower drying.

Increased shrinking from fluxes

Fluxes are substances, usually oxides, that are added to ceramic bodies to lower the melting point, making them essential for high-fire clay bodies like porcelain. However, some fluxes can cause porcelain to become more likely to shrink.

Weakening by silica

Silica is the principle ingredient in ceramic glazes and is also present in clay bodies themselves. Silica becomes most active around the same time the clay body is at the height of the shrinking process, which can cause some potential complications, especially during a glaze firing.

At around 2012℉ (1100℃), sulphates break down and evaporate as gases within the clay body; if enough time is not allowed, the clay and glaze could seal before the sulphate gases are released. Then, since the materials in the clay aren’t allowed the time to mature as a molten mass, your work will be significantly weakened.

What can happen if porcelain isn’t fired slowly?

When faced with any of the risks of higher levels of organic material, shrinkage, and slow drying mentioned above, your porcelain pieces can be seriously damaged if not fired slowly.

If the clay body doesn’t release moisture and dry correctly during firing, it can crack, flake, or even burst.

If oxygen doesn’t reach impurities in time to burn them out, black coring can occur, which results in black spots that can tarnish the beautiful white color of the fired clay. Trapped gases will expand beneath the surface of the clay, bloating and cracking the ware.

Additionally, if all the materials in a final firing don’t melt into a strong, united piece at an even rate, which only occurs when all the ingredients are given the chance to mature at a slower rate, warping may take place.

While saving on electricity may originally seem like a good reason to fire porcelain more quickly, hopefully this article has persuaded you of the importance of a slow fire, no matter the clay body you use. We hope you’ve found this guide helpful, and be sure to contact us if you have any further concerns or questions! Happy firing!

Further Reading: Everything You Need To Know About Porcelain Clay