The Caliper

Temperature in a Kiln

What teacher hasn't heard the age-old complaint, "I'm going to be an artist. Why do I need to know science?" Whether the medium used is film, ceramics, paint, or glass, science actually plays an important role in the outcome of the piece. Glass artists, for example, use kilns to bring glass to temperatures exceeding 1300° F. Glass has unusual characteristics at varying temperatures which the artist can utilize to produce beautiful products. The artist is confined by the laws of nature and must be aware of heating and cooling rates, as well as soaking temperatures. Violation of these laws results in shattered glass, frustration, and loss of time and money.

It is easy to record the temperature of a kiln using a thermocouple. A thermocouple is a device that measures temperatures over a wide range. A simple thermocouple has two wires made of different metal alloys: chromium-nickel and aluminum-nickel. They are joined together at two junctions. Whenever these two junctions are at different temperatures, a small voltage is produced between them. This phenomenon is called the Seebeck Effect. The voltage produced is proportional to the difference in the temperatures of the two junctions. By placing one junction inside the kiln and one in an ice bath, the kiln temperature can be easily monitored.

The production of stable glass requires a cooling rate that allows particles to arrange themselves with minimal stress. Since glass is considered to be a supercooled liquid, it has no specific melting point. It makes a transition from being more solid-like to more liquid-like. Soda-lime glass (used for windows and bottles) has a glass transition of around 550°C. Glass used for test tubes and ovenware contains boro-silicates and has a higher glass-transition temperature, making it withstand heat better.

Mike Ferrance, a physics instructor from Rhode Island, is a hobbyist glass artist. He used a LabPro and our Thermocouple (order code TCA-BTA, $37) to produce this graph of the temperatures inside his kiln. The sharp drops in temperature occur during venting —opening the kiln to check the status of the glass. The areas of relatively constant increases and decreases in temperature are called ramps. Notice that the rate of heating is much greater than the rate of cooling. When glass is cooled too rapidly, stresses occur. If the outside cools much faster than the inside, the outside contracts at a different rate, causing stress and possibly breakage. Some pieces take weeks to make and cost thousands of dollars, making it crucial for glass artists to watch the ramps carefully. You can find out more about glass art and its science connections by e-mailing Mike at Michael_Ferrance@brown.edu.