Top Types Of Clay And Glazes
What is Pottery?
Pottery is one of our oldest handicrafts, and it is still used just as much today as in prehistoric times when men and women first discovered that wet clay left out in the sun dried and became hard.
This discovery led to a boom in pottery production, and by around 400BC, earthenware pots, bowls, jugs and decorative objects were being produced on a huge scale.
It was the enigmatic Egyptians who invented kilns to fire their clay pots, and this method was adopted all over the world. They also created the first glaze, a blue-green mix of minerals that gave the pots a hard, colorful coating after firing.
Skip forward to more modern times and we are still using very similar processes to create ceramics.
As well as being eminently practical, pottery has become a form of high art, with traditional and contemporary artists creating incredible works which inspire millions of budding potters all over the world.
And now, you can do pottery at home — no studio required!
Types of Pottery Clay
Clay is at the heart of the pottery process.
It comes from the ground, usually in areas where streams or rivers once flowed. It is made from minerals, plant life, and animals — all the ingredients of soil.
Made up from decomposing rocks, and rich with minerals and organic remains of plants and animals that have been broken down by water pressure into super fine particles over time, it is slick and cool to the touch.
There are several different clays used in pottery making, each one containing a different mix of minerals and other elements that give them different properties and make them suitable for different applications.
Often, different clays are mixed together, or other things are added to make the clay stronger, or finer, to change its color, or to give it more plasticity.
When clay is mixed like this and used to form objects, it is called a ‘clay body’.
Different clay bodies ‘mature’ (meaning the point at which the clay is formulated to go hard) at different temperatures.
As a general rule the higher the temperature the clay matures at, the less absorbent it is once firing is completed. This is important as if clay absorbs water after firing and then freezes, the force of the water as it swells can break apart the clay.
Here’s our rundown of the main clay types…
This is the clay which was used by the earliest potters to create pots to carry water.
Earthenware typically contains a large amount of minerals such as iron which give it a rich red, orange or brown color. This distinctive color has led to it also being called by the familiar name ‘terracotta’, which translates as ‘baked earth’.
It’s a sticky clay which is easy to work with and holds its shape well.
Earthenware is fired at low temperatures (between 1745°F and 2012°F), and can be more porous and fragile than other clays.
This makes it a good choice when used as a container for things that need small amounts of air to circulate — like potted plants.
However, less durable earthenware clay bodies are also susceptible to freezing and cracking in cold climates, so may need to be brought indoors over the winter.
Equally, due to its fragility, earthenware may not be the best choice for other applications where a more resilient clay is required.
The name ‘stoneware’ makes reference to the dense, rock like nature of stoneware clay bodies after firing.
It’s extremely tough with practically no absorption properties, but is known to be a little brittle, hence it is often mixed with more plastic clays.
Colors of stoneware clays vary from a sandy brown or dark brown color, to shades of gray.
There are two different categories of stoneware clays: mid range (those that mature at a firing temperature of 2150°F to 2260°F), and high range (those that mature at a higher temperature of 2200°F to 2336°F).
Depending on which group they fall into will affect the resulting color after firing, and those fired at a higher temperature will be much more robust — ideal for utility ware.
Porcelain ceramics are well known for their fine quality and delicacy, and white color.
This is due to their extremely fine grained nature, and also their high kaolin content (a white colored mineral with a low expansion when wet, and low retraction when dry) and relative lack of other impurities which could cause discoloration.
However this means they are also quite inelastic, and they have the reputation of being tricky to work with, so porcelain clay bodies are rarely pure porcelain, instead they often contain a mix of other clays to stabilize them and make them more workable.
When moist, they will be light grey and shades of white after firing range from a delicate light gray to an almost pure white.
Porcelain clays are always high fired (around 3272°F) and as well as being beautiful, this makes them durable, hard and gives them a glossy sheen.
These sedimentary clays have highly variable compositions, and are significantly more rare than earthenware or stoneware clays.
They are grained with ultra fine particles, and have a high level of plasticity.
If used on their own, ball clays are not a practical choice as they shrink excessively, but when mixed with other types of clay, they act as a binder that makes the clay body more elastic, easier to work with, and much stronger (they are often combined with porcelain clays).
Another plus which makes them a popular choice with potters is that despite their dark grey color when moist, when fired (they fire to their mature hardness at about 2336°F) ball clays take on a pleasing, high quality white color.
They are commonly used to make white ware and sanitary ware.
Types of Pottery Glaze
Glazes are applied to the surface of a ceramic object with a brush, or a spray, or by dipping, before your creation gets fired in the kiln.
Available in a whole host of colors and textures, glazes can enhance not only the beauty of your finished object, but also the durability.
They can be applied in many different ways and fired at a range of temperatures to give a wide range of exciting results.
When a glaze is heated at high temperatures it melts and adheres to the clay surface, creating a hard, glass like surface which seals the object to protect it from breakage, and making it stain resistant.
In their unfired state, glazes have a liquid yet creamy consistency, and the color of the glaze at this point is often very different to the final color after firing.
The different colors are the result of a mixture of minerals and inorganic compounds within the glaze, for example iron oxides (which give red tones), and cobalt oxide (which gives a blue color).
Glazes are either made from natural and synthetic compounds such as quartz and clays which oxidize when fired, or from pieces of glass that have been previously melted down.
Just like different types of clay have their own ideal temperature at which they need to be fired, different glazes melt at different temperatures too. The heat at which a glaze is fired determines many of the final characteristics of the ceramic glaze.
As a general rule, glazes that fire well at low temperatures produce very vivid colors, and high fire glazes are mostly delicate in color and may require repeat applications and firings to get the desired result.
Glazing can be a fun — but messy — business, so if possible you’ll need to dedicate a space just for this purpose.
Glazes often start life as a powder which needs to be mixed, and you’ll need large buckets with lids in which to mix them. You’ll need a sturdy surface area at a comfortable height for you to work on to minimize lifting the heavy buckets.
Here’s a quick introduction to the different basic types of glazes you might like to try…
Glazes that have a transparent appearance after firing.
They can be clear; letting the original surface color shine through but giving it a darkened tint, or colored; which add an additional semi-transparent color over the top of the original surface color.
These are glazes which give an object a solid color thanks to either tiny particles or trapped air bubbles contained within the glaze itself. In addition, different minerals can sometimes cause opacity.
Glossy glazes are super smooth, and reflect light enough that you can see your own reflection in them once they are fired. Glossy glazes are often bright and showy.
The opposite to gloss glazes, matte glazes nevertheless have their own charm, giving either an understated elegant simplicity or rustic appeal to an object. A matte glaze on a surface has no shine and absorbs light with no reflection.
When a flowing glaze is fired and begins to melt it becomes very runny and bleeds into any glaze next to it.
This can produce some interesting and unexpected finished glazes.
Likewise a stiff glaze does not move from where it has been applied during the firing process, making it ideal for ‘painted’ glaze designs.