Clay 101: How to be Safe and Successful with the Rock Star of the Art Room

Students of all ages love the versatility of clay and how tactile it is. The possibilities are endless when it comes to what students can create with clay! From mugs to figurines to maracas, clay brings magic to any art lesson. However, clay is not always as much fun for the art teacher. Clay brings lots of prep and dust! Plus, because it involves a kiln, there’s a lot of complex chemistry behind the process. Whether you’re new to clay or a clay expert, break down the medium and learn how manageable it truly is with answers to the top clay questions. 

Keep reading to learn about different kinds of clay, the stages of clay, and answers to frequently asked clay questions!

Where does clay come from?

Clay is a natural material that comes from the ground. It consists of minerals such as silica, magnesia, and alumina mixed with water. You can find clay deposits and dig them up. Of course, naturally found clay can have any number of other elements mixed into it. Once the clay is harvested from the ground, it goes through a labor-intensive process of drying, filtering, and rehydrating to make the consistency perfect. However, most of us order clay online and get it shipped straight to our classrooms or studios.

What are the different kinds of clay?

There are many different kinds of clay on the market. This is exciting because there are so many options to choose from! However, if you’re new, too many options can be intimidating. Below is a short list of the most common types of clay, categorized by firing needs.

Here are four types of clay:

1. Air-Dry
   Air-dry clay can sit out and harden without any baking or firing. Unfortunately, air-dried pieces remain porous and tend to be more fragile. These are good for quick activities where you need a tight turnaround or need to create something small like a pendant, ornament, or beads.
2. Low-Fire
   Earthenware, also referred to as low-fire clay, is fired to a lower temperature in the kiln. Fire low-fire clays in the kiln at cone 06 to cone 04 to make it hardened and non-absorbent. This equals around 1,600-2,000 degrees Fahrenheit! Low-fire clays are popular in the classroom because they are forgiving and easy to work with. They are great for beginners and hand-building.
3. Mid-Fire
   Stoneware, also referred to as mid-fire clay, is typically fired around cone 5 or cone 6. Stoneware tends to be smoother and more pure than earthenware clays. It’s a popular choice for industrial production because it’s very durable and versatile. Mid-fire clays are suitable for hand-building, wheel throwing, and molding. 
4. High-Fire
   Porcelain is the most common type of high-fire clay and tends to be dense. Because of that density, they can be harder to work with but result in thin yet strong and durable pieces. High-fired clays are less absorbent and vitrified, meaning they fire at a temperature high enough for glass to form and act as a glue. This type of clay is suitable for very advanced courses. 

What are the different stages of clay?

While different types of clay have characteristics based on mineral composition, the stages are the same. Each stage is based on how water affects the clay and, therefore, its plasticity, or how moldable it is.

Here are the six most common stages of clay:

1. Slip
   Liquid clay is referred to as slip. Create slip by crushing dry clay and mixing it with water until it has the consistency of a milkshake. You can use slip for casting, but it is most commonly used as a “glue” when hand-building and attaching pieces.
2. Plastic
   When you open a new box of clay it should be in a wet or moist state. Wet clay is soft and easy to shape. This is the stage clay should be for hand-building or wheel-throwing.
3. Leather-Hard
   As clay partially dries, it becomes leather-hard. Clay, at this stage, is strong but still somewhat flexible. It’s still dark in color and will feel slightly cool from the remaining moisture. It’s good for carving, trimming, or attaching pieces. 
4. Bone-Dry or Greenware
   This is clay that has completely dried out and is ready for the kiln. It will be light in color and dusty. It’s brittle, so handle it gently! 
5. Bisqueware
   The first firing is referred to as a bisque firing. This firing will harden or even vitrify a piece, making it impervious to water. You can’t rehydrate or attach wet clay to a bisqueware piece.
6. Glazeware
   Glaze bisqueware to add color or a finish. Glazes have a combination of minerals, but essentially, they coat a ceramic piece in a layer of glass and pigment during a glaze firing. Different glazes melt at different temperatures, so it’s important to use glazes that fire at the same temperature as the clay. Other alternatives to glaze include paint or oil pastels.

For a student-facing resource on these key clay stages, download The 6 Stages of Clay anchor chart in FLEX Curriculum. This resource has each stage of clay with a short description and visual. Make copies for students to put in their sketchbooks or post in your room near your clay station.

What makes clay “blow up?”

Most of us experienced a mini explosion in the kiln or have seen someone else post about one in an art teacher group. What causes this to happen? The short answer is water. If you fire clay before it’s completely dried out, moisture trapped inside the clay body will turn to steam. As water molecules turn to steam, they expand. The steam gets trapped in the clay and builds up pressure, resulting in cracks or a shattered piece.

Air bubbles are a hotly debated cause of explosions in the kiln. An air bubble itself is unlikely to cause an explosion, but a pocket of air sealed in the clay may hold moisture and lead to the problems outlined above. If there is a hollow cavity in your clay, it is best to poke a vent hole for the steam!

How can I avoid explosions in the kiln?

Avoid problems by allowing ample dry time. While the exact number of days a clay piece needs to dry will depend on several factors, including the size of the piece and the humidity in the room, give all clay at least a few days to dry before firing. Pay attention to the clay’s color and feel to determine if it’s bone-dry. Greenware will often appear lighter in color. If it looks darker and feels cooler to the touch, it’s still at the leather-hard stage. An easy test is to place the item on a piece of scrap paper. Note if the paper warps or leaves a damp mark.

Clay is a medium that requires patience. Waiting before firing is critical, but a longer time in the kiln can help too. Most electric kilns will allow you to choose a firing speed, so opting for a slower bisque fire is always a good idea. A strategy called candling heats the pieces up at 180-200 degrees Fahrenheit for a number of hours to ensure the clay is dry before firing. 

What other logistics do I need to consider?

Ceramics can be a complicated artform—it has a lot of chemistry elements and stages in the process. Don’t let the science intimidate you! Just like with any medium, learn the foundation, such as the stages of clay, basic hand-building techniques, and how to operate your kiln. Then, add in new elements to experiment with every now and then! Before you know it, you’ll be an expert.

Here are a few quick tips to be safe and successful in the art room:

• Stay clean.
  Silica particles in clay can become harmful over time if you inhale too much dust. Wipe surfaces with a wet cloth and ask custodians to wet mop your floors more often. 
• Get organized.
  Labeling shelves by days of the week can help you track how long pieces have been drying before bisque firing.
• Take pictures.
  If you teach younger students who may not always recognize their work (or carve legible initials in the bottom of their pieces), take a picture of each child with their sculpture before they drop it off on the shelf. 
• Keep clay out of the pipes.
  Clay going down the drain will leave sediment that can build up and clog your pipes. Wipe off excess clay before students wash their hands. Use a bucket to “pre-rinse” hands so students can remove the clay then go to the sink and wash with soap. Keep another bucket under the faucet to catch any lingering clay, like in the video below. The clay will sink to the bottom of the bucket and the “clean” water will flow over the top and down the drain.

How can I reclaim and rehydrate clay?

At some point, every art teacher encounters a box of clay that has sat on the shelf too long and it’s stiff and dry. Save money and rehydrate old clay into brand-new clay!

Here is an easy strategy to rehydrate your old dry clay:

1. Place the block in a bag.
   Put the dry block of clay into a plastic bag. If there are no holes, the one it came in will work perfectly.
2. Add water.
   Pour about a cup of water into the bag, around the clay.
3. Seal the bag.
   Place the sealed bag into a bucket. 
4. Submerge in water.
   Fill the bucket with water until the bag is submerged.
5. Wait.
   Let the clay soak for at least 24 hours. The outside water will create a pressure that will force the inside cup of water back into the clay.
6. Wedge.
   Once rehydrated, remove the clay from the bag and wedge it to evenly distribute the moisture. Store in an airtight container or bag to prevent it from drying out again.

Another way to rehydrate your clay is to add water, let it sit, and then wedge it, like in the video below. This method creates a super fun activity for early finishers of all ages! Plus, it’s a great way to recycle clay scraps or incomplete projects and build hand muscles. 

Clay is one of the oldest and most enduring art materials. Students love to get their hands dirty and build something they can cherish for years to come. Keep the experience positive by allowing plenty of dry time so every sculpture can make its way through the kiln safely. Remember student safety and wipe up as much clay dust off the tables and floors daily as possible. Stay mindful of the science and stages of clay to keep intimidation at bay and provide a magical experience for your students! 

What clay lessons do you love to teach year after year?

What was your biggest blunder with clay?

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