How Cups Are Made From Clay?

Creating a cup out of clay is an intricate process that requires several steps. From obtaining the clay and preparing it, to forming the actual cup shape, applying glazes, and firing at high temperatures, cup making involves many phases of production. In this article, we will provide an overview of the complete manufacturing process, from start to finish.

Obtaining Clay

Clay is a naturally occurring earthy material that can be found in many locations around the world. There are two main types of clay that are used in pottery – primary and secondary clays.

Primary clays remain in the place where they were first formed. They are usually found in beds or seams, often fairly near the surface of the earth. Primary clay deposits can be found across the world, including in the central and southeastern United States, in Britain and other areas across Europe and Asia. To access primary clay, shallow pits are dug and the clay is extracted.

Secondary clays are those that have been moved over time by water, ice or wind. As natural forces erode and weather rock formations, clay particles are transported and then settled elsewhere. Large deposits of secondary clay can be found in river beds, at the bottom of lakes, and in shoreline areas. Secondary clay is extracted by dredging it from river and lake bottoms.

Whether mining primary or secondary clay deposits, the basic techniques involve digging and excavating the clay. Power equipment like bulldozers and backhoes are used to remove layers above the clay beds. The clay is then extracted and loaded into trucks to be transported for refining and processing before it is ready to be used by potters.

Preparing the Clay

Before clay can be shaped into a cup, it must be properly prepared. This involves cleaning and purifying the clay, as well as mixing it with water and wedging it. Here are the key steps in preparing clay for cup making:

Cleaning and Purifying: Raw clay directly from the ground contains impurities like sand, grit, and organic matter. These must be removed to prevent structural defects and cracking when firing. Clay is suspended in water in a blunging machine to dissolve and separate out impurities. The purified clay slurry is then filtered and dried into a reusable clay body.

Mixing with Water: Once purified, the clay powder is mixed with water to reach the optimal consistency for shaping. Enough water must be added so the clay becomes plastic and malleable. Too much water makes the clay sticky and difficult to handle. Clay manufacturers precisely calculate water content to achieve ideal plasticity.

Wedging: After mixing in water, the clay is wedged by hand on a wedging table or by machine. Wedging compresses the clay to remove air pockets and blend particles for uniform moisture and plasticity. It also aligns the clay particles in the direction of pressure, improving its workability.

Forming the Cup

There are several techniques that can be used to form a cup from clay. The most common methods are using a pottery wheel, molds, and handbuilding with clay coils or slabs.

On a pottery wheel, the clay is centered on the wheel and opened up. The clay is pushed up from the bottom and gently pulled up at the sides using fingers and various tools to form the basic cup shape. More clay can be added and refined to create the cup walls and smoothed out. The potter uses their hands, sponges, and scraping tools to refine the shape and thickness of the spinning cup.

Molds can be used to form the outside shape of the cup. The clay is pressed into a cup-shaped mold by hand to capture the form. The clay is then removed from the mold and touched up as needed. The unfinished walls are still quite soft and pliable at this stage.

Handbuilding involves forming the cup shape without the use of a pottery wheel or mold. Clay coils are rolled out and then stacked on top of each other, gradually building up the cup walls. The coils are blended together from both sides to bond them into one solid piece. Slabs of clay can also be cut out and joined to form the cup shape and walls.

Drying the Clay

Once the cups have been formed, they must be thoroughly dried before firing. There are a couple ways to dry the clay:

Letting cups air dry: One method is to simply leave the finished cups out on racks or shelves at room temperature to air dry. The cups are left for 1-2 weeks to allow all moisture to evaporate. This is a slower but very effective drying technique. The waiting time lets moisture escape slowly to minimize cracking and warping.

Using dryers: Many studios also use special dryers to speed up drying. These operate at low temperatures around 100-150°F to gently remove moisture from the clay. The air circulation ensures cups dry evenly. Using dryers reduces the drying time down to just 1-2 days. However dryers do run the risk of rapid uneven drying, which can cause defects.

Once the cups feel completely dry to the touch, they are ready to be fired in a kiln. Careful controlled drying is crucial to avoid cracks and damage before firing. Allowing sufficient drying time ensures quality finished results.

Bisque Firing

Bisque firing is the first firing step in producing ceramic ware like cups. It involves heating the dried, unfired clay pieces to a high temperature in a kiln. This hardens the clay body and converts the clay minerals into a ceramic material.

During bisque firing, the temperature is slowly raised to around 1200-1300°F. This gradual increase is necessary to allow any remaining water and organic matter to evaporate from the clay without cracking it. As the temperature rises, chemical changes begin to occur in the clay body. The clay particles bond together in the heat and the piece becomes hard and durable.

When the peak temperature is reached, it is held for a period of time to ensure the clay has become fully vitrified or glass-like. This transforms the soft clay into a strong, permanent ceramic material ready for glazing and further decoration. After sufficient heat exposure, the kiln temperature is gradually decreased to avoid shocking and cracking the ware.

The bisque firing changes the physical properties of the clay but does not make it impermeable. At this stage, the clay object is still porous and able to absorb water, so glazes can adhere to it. The bisque fired clay is also lighter in color, changing from its original brownish tones to a buff or light gray color. After inspecting for cracks or flaws, the bisque ware is ready to be glazed and decorated.

Glazing

After the cups have been bisque fired, they are ready for the glazing process. Glazing involves applying a thin colored coating to the bisqueware cups before they are fired a second time. Glazes serve two main purposes – decoration and creating a glassy finish.

Glazes come in many bright, vibrant colors that allow potters to decorate the cups in beautiful patterns and designs. The colors and textures of the glaze enhance the aesthetic appeal of the final ceramic cup. Glazes with metallic oxides can create iridescent and shimmering effects. Colorful images and motifs can be painted onto the bisqueware before glazing to create one-of-a-kind works of art.

In addition to decorative purposes, glazes provide a protective glassy coating and finish to the porous bisqueware. The glaze melts during the firing process and fuses to the clay body, making the cup non-porous, water resistant, and durable for holding liquids. The glaze finish also makes the surface smooth to the touch. Proper glazing technique ensures the glaze adheres evenly across the entire piece.

Glaze Firing

Once the bisqueware cups have been glazed, they are ready for the second and final firing process. This firing is done at an even higher temperature, usually between 2200°F and 2300°F. The intense heat melts the glaze, causing it to fuse with the clay body.

Glazes are formulated to melt at specific temperatures. As the kiln heats up, the glaze ingredients transition from a solid to a liquid state. The glaze melts and spreads evenly across the surface of the bisqueware. As it liquefies, the glaze is absorbed into the porous clay body. When the maximum temperature is reached and then cooled, the glaze hardens, bonding securely to the clay.

It is imperative that the kiln reach the optimal heat during glaze firing. Temperatures that are too low will result in underfired glazes that do not properly fuse with the clay. If the kiln gets too hot, the glaze can become overfired, changing its appearance and compromising the bond with the clay. Careful monitoring and controlling of the kiln during glaze firing ensures that the glaze will mature correctly and create a strong, waterproof coating on the finished ceramic cups.

Inspection

Once the cups have been fired with glaze, they undergo a thorough inspection process to check for any defects before being approved for sale or use. Experienced inspectors carefully examine each cup, looking for imperfections in the clay, issues with the glaze application, and firing flaws.

Common defects that inspectors look for include cracks, chips, uneven glazing, pinholes, blisters, and discoloration. Cracks are often caused by improper drying or firing, while chipping can happen during demolding or packaging. Blisters in the glaze are from trapped gases, and pinholes result when bubbles burst. Any of these flaws can compromise the cup’s durability or appearance.

Inspectors also check that the cup has maintained its proper shape without warping and that the foot or base is flat so it sits evenly on a surface. The glaze should have a uniform color and surface texture without any crawling or peeling.

For quality control purposes, sample cups from each batch are stress tested by holding hot liquids for a period of time. The cups must be able to withstand sudden temperature changes without cracking. Drop tests may also be performed by letting sample cups fall from a standard height onto a hard surface to ensure they do not shatter easily.

Any rejected cups are either discarded or recycled back into clay body for reuse. Only cups meeting quality standards are approved for sale or further use after thorough inspection.

Conclusion

The process of making a cup from clay is an intricate craft that has been practiced for thousands of years. From preparing the clay and forming the initial shape, to multiple phases of drying, firing, and finishing, many careful steps go into creating a finished ceramic cup. The end result is an item of function, beauty, and legacy.

Pottery making connects us to ancient cultures across the globe that shaped their daily lives around clay. Simple yet fundamental vessels like cups, bowls, and plates were vital for storing, serving, and consuming food and drink. Even today, handmade pottery retains an organic, traditional feel in our modern world. While factories can mass produce cups cheaply out of glass and plastics, artists continue working clay into cups, mugs, and goblets using traditional techniques. The personal handiwork invokes a sense of care and craftsmanship.

Whether making pottery as a profession or hobby, the process of forming cups from clay is rewarding. We shape a lump of earth into something solid yet delicate that can be decorated and enjoyed. With each step done mindfully, from wedging the clay to glaze firing the finished form, we imbue the cup with our creativity and care. The finished product reflects our attention and becomes a lasting testament to the ancient pottery craft.