Clay is the fundamental material used in ceramics and pottery. But not all clay is created equal. There are thousands of clay varieties with unique properties suitable for different applications.
In this complete guide to clay types, we’ll cover the characteristics and uses of the three major categories of pottery clay: earthenware, stoneware, and porcelain. We’ll also look at key factors like firing range, absorbency, plasticity, and shrinkage.
Let’s dive in and learn why clay selection matters!
While the list of clay varieties is nearly endless, most fall into one of these three common categories:
Earthenware is the most widely used type of clay for pottery and ceramics. It fires at lower temperatures ranging from 1600-2200°F. Earthenware has high absorbency and shrinkage rates.
It can be found in various colors, including red, brown, grey, and white.
Earthenware clays are easy to work with and fire. Their lower firing range allows use in primitive kilns. Earthenware absorbs water readily, which makes it excellent for functional pieces like mugs but unsuitable for liquid storage. Most clay pottery and ceramic vessels start with earthenware.
Stoneware clays fire at higher temperatures between 2200-2400°F. The increased heat makes stoneware less porous and absorbent than earthenware. Stoneware has better structural strength and resists thermal shock. It comes in grey, tan, and white hues.
Stoneware is ideal for pottery pieces used for cooking and storage. Its nearly non-porous nature allows minimal water penetration. Stoneware has high plasticity, which aids shaping, combined with low shrinkage, which resists cracking. These characteristics make stoneware a versatile studio clay.
Porcelain clays fire at the highest range from 2200-2600°F. It becomes incredibly dense and non-porbose at these temperatures. Porcelain is white in color when pure. It’s highly plastic and extremely strong structurally after firing.
Porcelain is prized for its white color and translucency. It’s more difficult to work with than stoneware or earthenware. Porcelain is ideal for intricate pieces requiring thin walls and precise shaping. Tableware, figurines, dental parts, and technical ceramics often utilize porcelain.
Now that we’ve covered the big three, let’s look more closely at the unique properties of each clay type.
Many characteristics differentiate clay types and influence their best uses. Here are some of the most important:
The firing temperature or heat workability refers to the range of kiln temperatures at which the clay vitrifies or sinters to become ceramic. As we’ve seen, earthenware fires on the low end from 1600-2200°F, while porcelain reaches 2500°F or higher.
Higher firing ranges yield harder, denser clay with lower absorbency. Earthenware remains somewhat porous when fired, allowing things like tableware to “breathe” but limiting liquid-holding ability. Porcelain’s ultra-high firing creates an impermeable glass-like surface.
Plasticity refers to a clay’s malleability and ease of shaping. Plasticity makes clay extremely responsive to molding, rolling, wheel throwing, and sculpting. Porcelain clays tend to be the most plastic, followed by stoneware and earthenware.
Some clay types have “short” plasticity, meaning they dry quickly and become unworkable. Others remain malleable for longer. Plasticity can be affected by storage conditions and the presence of grog or temper additives.
As the clay dries, it becomes fragile in its “greenware” or unfired state - we call this its green strength. Some clays hold their shape better during drying, while others tend to warp or crack.
Porcelain has notoriously poor green strength and requires very careful slow drying to prevent deformation. Stoneware has much better green strength thanks to its lower shrinkage as it dries.
The color of clay in both its dried and fired states depends on the mineral content. Earthenware comes in a variety of earthy hues, from orange and red to grey and white. Stoneware fires tan, grey, or white. Pure porcelain is snowy white when fired.
Clays can be modified with stains, slips, engobes, or pigments to achieve different colors. Glazes also impart color after firing. But the base clay gives the underlying tone.
Fired clay’s absorbency depends on how vitrified clay becomes denser, resulting in low absorbency. Earthenware remains permeable after firing; stoneware has minimal absorbency. Porcelain becomes non-porous at peak temperatures.
Absorbency affects the functions clay is suited for. Permeable earthenware works for tableware, while non-absorbent porcelain is required for liquid storage and containment applications.
As clay dries and fires, it shrinks in size due to loss of water. Shrinkage refers to the degree of reduction from the initial wet size to the finished fired piece. Higher shrinkage increases cracking risks.
Earthenware typically sees 10-15% shrinkage from wet to bisque state and another 5-10% reduction when fired to maturity. Stoneware exhibits less shrinkage, around 8-12% total. Porcelain has the lowest shrinkage of only around 5-7% overall.
Fired strength relates to a clay’s structural integrity under stress and resistance to thermal shock. Strength increases with higher firing temperature. Earthenware has relatively low strength, while stoneware and porcelain have extremely high fired strength.
Durability and hardness also improve with more intense kiln heat. Porcelain becomes remarkably durable due to its high density. Strength allows thinner walls and refined shapes.
With all of these clay characteristics in mind, how do you pick the perfect clay for your pottery or ceramic creation?
Stoneware and porcelain clays are best for functional items like tableware, everyday pottery, and liquid storage. Their low absorbency makes them suitable for items used with foods and beverages. Stoneware is the standard for kitchenware.
For decorative items like planters, figurines, and wall art, earthenware offers a range of colors while being easier to work with. Earthenware's lower firing temperature allows the use of less specialized kilns. Any project where appearance trumps utility is well-suited to earthenware.
If aiming for intricate details, thin walls, transparency, or pure white color, then porcelain is unmatched. It’s also great for objects needing to withstand heat or repeated high-temperature cycling. However, porcelain can be quite difficult to master for beginners.
Oil-based clays like plasticine or polymer clay are often the easiest to shape and modify for sculptural pieces during creation. These clays remain workable without drying out but lack the permanence of fired ceramics.
The possibilities are endless once you understand the unique advantages of different clays! With so many options, you can select just the right clay for every project on your artistic horizon. Let this guide give you confidence in clay choice.