Mad about science: Ceramics

By Brenden Bobby
Reader Columnist

I know what you’re thinking: “Oh great, an article about pots. I guess I’ll turn the page.”

Or maybe you’re thinking: “Oh great! An article about pots! I guess I’ll wait to turn the page!”

Whether you align with the former of the latter, there is something cool for everyone in the world of ceramics.

Ceramics are among the most ancient and pivotal of human technologies, developed right alongside heavy hitters like the wheel and the archer’s bow. Creating leak-proof containers allowed humans of 29,000 B.C.E. to transport fluids like water, milk and even honey. Pots that could be sealed were also great for keeping pests like mice and insects out of human food. 

The ability to easily transport lots of loose items inside of an easily manufactured ceramic container cannot be overstated. Do you really want to carry your food around in a leather pouch or the stomach lining of the former-family calf, or would you rather rinse out a big pot and call it a day?

Beyond that, you may be wondering: What is a ceramic? A ceramic is a nonmetallic, inorganic chemical structure that becomes denser when exposed to high heat. In ancient times, this was achieved by mixing mineral-rich clays with water and forming them into pottery before baking them in a kiln. Today, there are countless applications for ceramics, ranging from the mundane to the incredible.

Considering just how long humans have known how to manipulate clay to make ceramics, there is a shocking amount of chemistry that’s applied to the process, which are natural processes that just happened to coincide with the technology that was available throughout antiquity. It’s easy to ruin a piece of pottery, especially with modern technology.

There are often a large amount of silicates trapped inside clay, giving the substance a crystalline structure at a microscopic level. Adding water allows this crystalline structure to be viscous and malleable, so that we can form it into shapes. The easiest and most uniform way for us to manipulate clay into something useful is to apply centrifugal force, spinning the clay on an axis and applying pressure on its surface to form it into cylindrical or spherical shapes. This is the exact process that makes planets spherical, as they are spinning on an axis while gravity is pulling loose mass toward the center.

The crystalline structure of the clay is the key to its hardening process. Once the piece is sufficiently dry, it can be placed into a kiln where it is slowly heated. Flash heating pottery is extremely dangerous, as there is still water trapped inside of the clay and rapid heating leads to sudden and violent expansion of the water into steam. This is great for powering a locomotive, but it’s bad news for your belated Mother’s Day gift.

Slow heating allows water to evaporate in a less explosive manner, yet extremely high temperatures are required for the real magic to happen.

Carbon, sulfur and other organic compounds need to burn off from the clay, which happens in a temperature range of 572 and 1470 degrees Fahrenheit. If these materials aren’t burned off, they can compromise the finished structure.

The silica in the clay is a form of quartz; and, once the temperature reaches 1,060 F, a process called quartz inversion occurs. You’d have to check out a book from the library or talk to a chemist to get the nitty-gritty details about this process, but essentially this causes the clay to expand by up to 2% during the heating process as the quartz molecules change. 

By the time the kiln has reached 1,650 F, the clay begins to fuse and transform into a ceramic.  This is called sintering, and it’s the critical moment of transformation from earth to earthenware.

Once the quartz molecules have transformed, other chemical compounds trapped in the clay have melted to fill the porous gaps in the structure. Aluminum silicate within the clay is transformed by the heat into long, needle-like structures that act as a binder for the ceramic. In some cases, red clay with an abundance of iron acts as a flux, which helps to bind everything together.

The cooling process needs to be gradual in order to avoid catastrophic shrinkage, which can create cracks and other structural damages to the ceramic.

This process may vary slightly depending on the type of ceramic being produced. Ceramics used for things like stopping ballistics or cooling space vehicles during reentry will be subjected to different levels of heat than that botched clay cat your grandmother uses as an ashtray.

Ceramic plates placed into body armor for soldiers or security personnel are designed with their destruction in mind. Rather than completely halting a ballistic projectile, the ceramic is designed to fracture and deform the nose of the bullet while simultaneously spreading the energy of the projectile throughout a larger surface, which is further reduced by rippling it throughout multiple fractured pieces. 

These plates are extremely durable, but they’re not intended to be reused — especially in the case of private or civilian body armor that’s intended to be lightweight.

Ceramics are also used extensively in the space industry. Ceramic structures are lighter than metals while also having a spectacularly insulatory effect. Some structures, called ablative plating, are actually designed to absorb heat and flake off from the body of a spacecraft, carrying the heat away with the shed material.

I bet that thousands of years ago, when someone first wedged a brick into a fire, they had no idea they were laying the foundation for humans to safely explore the stars.

Stay curious, 7B.