BY TAMRA BOOTH, KEEEPS
Did you know, there are lots of different types of clay?
Yep, clay isn’t just clay! There are many different types of clay available to buy and it is common for beginners to waste their time and money buying the wrong type of clay, which is frustrating not just for the bank balance, but for your overall pottery experience.
Whilst we would always recommend attending a pottery class or taster session to get to know about clay and learn the ins and outs of pottery from an experienced maker, you can also try pottery at home yourself. If your starting from scratch, have a look at our first blog post in our "Pottery for Beginners" series, where we discuss the main techniques and ways of making pottery.
So, if you’re starting your pottery journey solo, knowing a bit about the different types of clay can help you choose the correct clay for your next ceramic project and make the process much more enjoyable.
Generally, the types of clay can be divided into two categories; air dry clay and pottery clay, but what are they, how are they different, and what can you make with them?
What is clay?
Clay is soft and earthy and often looks and feels like playing with mud. However, unlike mud, which is largely soil and water, clay is a mineral substance made up of feldspar. Feldspar is a name given to a group of naturally occurring minerals containing varying amounts of potassium, sodium, calcium and/or lithium. It is the most abundant group of minerals in the earth’s crust making up about 50% of all rocks.[1]
Clay is formed as a result of the weathering and erosion of those rocks over vast periods of time and therefore, the best places to find the different types of clay are where rocks have been in contact with water, air or steam. [2] This can be along floodplains of rivers or streams, on the bottoms of ponds, lakes and seas and clay will remain even when those water sources dried up millions of years ago.[3]
The colour and types of clay will depend on the impurities that the clay has picked up during its creation. For example, earthenware clay is a type of clay which has picked up iron during its creation and therefore is usually terracotta in colour. Porcelain on the other hand, is mostly white as it has picked up no impurities making it a very pure clay.
Air dry clay
What is air-dry clay?
Air dry clay is generally a natural clay with a natural additive like corn starch or cellulose fibres added in to make it harden[4]. Fibres often added to clay to make air-dry clay are cotton, paper or other wood fibres. Glue is also often added to help it keep its structure.
Air dry clay should not be fired or baked because of the natural ingredients which are added to the clay, like paper, which are designed to be dried slowly at room temperature.
Air dry clay is a type of clay that is perfect for those new to making pottery and is great for hand building and making pottery at home as it does not require a kiln. It is an easy type of clay to practice handling and playing with clay and to experiment creatively and is the type of clay that you get if you purchase one of our home pottery kits.
What can you make with air-dry clay?
You can make anything you want with air-dry clay but any bowls, plates or mugs made with air dry clay are for decorative purposes only and are not food-safe, as it is a type of clay designed to be left unsealed and is not watertight or food safe.
Air dry clay, however, is a type of clay that provides a great opportunity to get a feel for making pottery for both kids and adults, and is perfect for making pencil holders, jewellery, key or trinket trays, planters or candle holders, all functional items for your home that you can decorate in any way you wish, all from the comfort of your own home!
Air dry clay also allows you to experiment with colour as it can be used with marker pen, acrylic paint, ink or coloured slip, so it is a fun clay to play with, whether at a pottery class or at home. We painted our pottery creations made with the home pottery kits using nail varnish which worked really well!
Pottery Clay
Unlike airdry clay, pottery clay is a type of clay that needs to be fired in a kiln.
Generally speaking there are three main types of pottery clay, which are differentiated by the following:
- Mineral content;
- Amount of plasticity (the ability of the clay to be bent or stretched);
- The firing temperature;
- Other more science-y things that we won’t get into here!
Whilst there are other, more rare types of clay, these are the three that as a pottery beginner, you are likely to have heard of or will work with in classes.
Choosing the right type of clay is important as the wrong type of clay could make your learning experience difficult and frustrating. So, what are the three types of clay and what should they be used for?
Earthenware Clay
Earthenware is a type of clay that is typically terracotta in colour due to the high iron oxide content. Terracotta, which means quite literally "baked earth", is one of the most popular types of earthenware.[5]
It is the high iron content and impurities in earthenware clay that means it should be fired at a low temperature, or the clay will melt. However, the low temperature means that it is not fully vitrified, which means that the clay particles do not turn to glass in the kiln. If a piece is not vitrified, it is not waterproof and therefore needs to be sealed with a glaze before it can be used as a vessel or as dinnerware.[6] It is very easy to work with and shape earthenware and, therefore, is a great type of clay for those making pottery for the first time It is very versatile, so can be used both on the wheel and to practice hand building.
The downsides are that earthenware is not as strong as stoneware (think of large terracotta plant pots that can be scratched) and it is also a porous clay (it is commonly used for bricks, flowerpots, and construction). Once glazed, earthenware becomes watertight and food-safe, but is hand wash only.
Stoneware Clay
Stoneware clay is a very popular type of clay for making functional pottery, particularly dinnerware and mugs because it is very durable and more chip-resistant than earthenware clay. It is also far less porous and stronger than earthenware so it is usually the clay of choice for water-holding vessels like jugs and vases. This is because the mineral content in stoneware clay allows it to be fired at a much higher temperature in the kiln and the pieces can become fully vitrified.
Stoneware clay comes in a range of different colours from white, sand, brown and grey and it is very popular with those new to making pottery, both on and off the wheel. It can often have a speckled look, which is partly due to the impurities in this type of clay, but also as a result of the “grog” which is added to the clay to improve its performance.
Grog is clay which has been fired, then ground up into small granules which is then added back into the clay to make it easier to work with and less likely to crack when it dries. The granules are different sizes and give the clay a grainy or speckled appearance.[7]
Japanese Mugs by Noriko Nagaoka
Porcelain Clay
Revered as the best quality type of clay, and sometimes referred to as kaolin clay, it is the most expensive and hardest type of clay to work with.
It is smooth as butter, which feels lovely on the hands, but because it absorbs water so quickly it is very soft and can collapse quickly.
For these reasons, it is not recommended that those new to making pottery start their pottery journey with porcelain, rather work towards this once they are familiar with the main techniques and working with the other types of clay.
Porcelain clay, however, has the ability to look very delicate, so as well as being used for detailed sculptural artistic work, it is also very practical and is a popular choice for functional items like dinnerware because of its toughness[8] The reason that porcelain clay is so hard is because of its mineral content – it contains a lot of silica which is the glass-forming ingredient in clay. Its particles are also more densely packed together than in earthenware or stoneware.[9]
Porcelain is also used in industrial and medical fields due to its hardness, being used for the repair and reconstruction of human body parts as it has the ability to bond directly with bone and has very low reactivity in the body.[10] Today, porcelain clay is used for dental fillings, braces, hearing aids and hip prosthetics in modern medicine! [11]
It shouldn’t be forgotten that as well as being practical, porcelain items are stunning, and we have a great collection here at Keeeps made by our experienced and super talented potters.
Pretty Porcelain Bowls by Georgie Gardiner
Colourful porcelain cups by Ali Tomlin
That is just a little taster about the different types of clays and their uses! Keeeps will be starting up pottery taster sessions in Marlow in January 2022, the timetable of which will be released shortly for online bookings.
At Keeeps, our taster lessons will focus on playing with stoneware clay as this is an easy and workable clay for pottery beginners but also because you can make some attractive and functional pieces with it to take home and show off!
Email us at marlow@keeeeps.co.uk and you’ll be the first to know of the launch to our booking app so you can get yourself on a pottery taster session pronto!
Sources:
[2] What is clay? — Science Learning Hub
[4] Sculpting FAQ's (sculpturehouse.com)
[5] The Basics of Pottery Clay (thesprucecrafts.com)
[6] Best Pottery Clay for Beginners | Choose the Right Clay (thepotterywheel.com)
[7] Types of Clay for Pottery – The 5 Main Types of Ceramic Clay (thepotterywheel.com)
[8] Types of Clay for Pottery – The 5 Main Types of Ceramic Clay (thepotterywheel.com)
[9] Best Pottery Clay for Beginners | Choose the Right Clay (thepotterywheel.com)
[10] Ceramics in Biology and Medicine | SpringerLink
[11] Ceramics for Medical Uses (ppceramics.co.uk)
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