🏺 Choosing Clay in Artistic Ceramics
Clay Body: treatment of one or several clays to obtain a homogeneous mixture.
Identifying a clay body is not an easy task… furthermore, it is common to mix several types of clays
Stoneware
It is a sedimentary (secondary) clay but still with a certain degree of purity. After firing, it becomes impermeable. It vitrifies between 1150ºC and 1350ºC.
Sanitary ware is made of this material (sanitary stoneware). It can have various shades: depending on the origin of the clay and the amount of iron oxide, it can acquire tones ranging from white (not as white as porcelain), grey to yellow and brown.
It allows for solid pieces to be made more easily unlike, for example, red clay (normal clay). High plasticity and strength, which is why it is widely used in the making of large artistic ceramic pieces.
It is common for stoneware clays to be mixed with other clays to create bodies that possess specific qualities. They are usually presented with grog (chamotte).
Refractory
These are kaolinitic bodies: obtained through the addition of kaolin. They also contain a lot of grog and aluminum oxide. These are high-temperature bodies, above 1200 degrees (can reach 1600ºC). Used in the production of refractory plates for kiln insulation.
Used in large-scale works or those that will be fired in manual kilns (such as Raku) due to their great resistance (even their shrinkage is reduced).
Earthenware (Faiança)
It is a white clay, which has a ceramic mass less rich in kaolin than porcelain. It is distinguished from porcelain pieces by being thicker, lighter, and less white. It has good plasticity (it can be modeled on a potter's wheel but is usually difficult) and fires at low temperatures (1020 to 1150ºC).
It is not recommended for holding liquids: the higher the temperature, the more compact and dense it becomes, reducing the degree of porosity, but never becoming vitrified like stoneware.
Porcelain
The secret of porcelain is kaolin, which is a primary clay and the whitest (the white color is what differentiates it from stoneware) known clay and is its main ingredient. The other components for its manufacture are: Normal clay 10%, Feldspar 25%, and Quartz 25%. Kaolin makes up 40% of the mixture.
It has low plasticity and undergoes high shrinkage during firing, after which it becomes impermeable and VERY strong. It is bisque fired at 900/1000ºC. This 1st firing is to provide strength and porosity for the application of a glaze in a second firing.
After the bisque fire, the pieces are dipped in glaze and taken back to the kiln (1200/1400ºC). This white glaze is made with the same ingredients as porcelain (except for the clay), but in different quantities. After decoration (hand-painted or decals) it is dipped in glaze and goes back into the kiln.
These white ceramic pieces have a characteristic sound when tapped. They have been made since antiquity in China. The continent learned about the raw material thanks to Marco Polo in the 13th century. From the 16th century, Europeans began to try to produce these pieces and 2 centuries later they succeeded with total success.
Normal Clay / Red Clay
These are low-temperature clays, unlike stoneware and porcelain.
IT IS A SEDIMENTARY/SECONDARY CLAY . They can present various levels of plasticity and can contain grog. Normally they are red, but they can come in other colors.
Almost all cultures that developed a ceramic tradition started with this sedimentary clay because of its abundance and ability to harden with primitive fire techniques.
It is the ideal clay for the wheel due to its ease of throwing and being inexpensive. A great advantage of using a body made of normal clay is its lower energy cost.
During firing, they shrink much less than primary clays because they have less density: high porosity, so it is not recommended for holding liquids.
Terracotta is normal clay fired in the kiln without being vitrified; but they also call red clay terracotta.
CHOOSING CLAY BODY » Type of Work
Work with many details requires a body with little or no grog. Large-scale pieces require bodies very resistant to drying and firing to avoid warping and cracking. Thick pieces require special bodies like stoneware or refractory bodies. Very fragile pieces require bodies with low drying and firing shrinkage to prevent them from cracking or breaking. Clay for the potter's wheel must be very plastic and without any grog.
CHOOSING CLAY BODY » Available Kiln
A HIGH TEMPERATURE kiln (+ 1100/1200 degrees) allows for another range of glazes: High temperature glazes.
We cannot subject a low-temperature body to much higher temperatures because it will melt and we could have serious problems… With a high-temperature kiln we can work with a wider range of bodies. With a low-temperature kiln we can work with high-temperature bodies although they will not achieve the desired solidification (only possible if the pieces are not subjected to stress) and it will interfere with the glazing process. And why?
Because a possible release of gases can interfere with the glaze layer. Also, since it is not well bisque fired, it is more porous: it will absorb more enamel in some parts creating asymmetries in the glaze layer. If we want the glaze color to be accurate, it is better to use a white body…
BASIC RULES
Do not mix different bodies/clays. They may have different shrinkage rates which could cause cracks during drying and/or bisque firing. Dry the pieces completely before putting them in the kiln. The presence of water inside the piece will cause it to explode during the bisque fire. Don't forget that during the summer pieces dry much faster. Very fast drying has the disadvantage of causing warping. Whenever possible, execute pieces directly from the block of clay/body that came from the supplier. This ensures the clay/body is homogeneous and prevents air bubbles that cause the piece to explode during bisque firing. For long-term projects, use plastic to wrap them so they don't lose their moisture.
Avoid adding moisture (for example through spraying): it progressively "breaks down" the body. For tableware going into the oven, we should use a high-temperature body. The glaze should also be high-temperature and appropriate for culinary use.
VERY IMPORTANT: Ask the supplier for information!!