Selecting a coffee cup is a very personal choice. While you may have a variety of mugs, you probably have one that you favor above all the rest. Typically, a favorite mug is an aesthetic matter, but there are other factors that can come into play when settling on one go-to cup. If you’re looking for a new favorite coffee mug, here’s a look at glass and ceramic choices from a thermodynamic and environmental point of view.
Ceramic Retains Heat Better Than Glass
As far as keeping your coffee warm goes, ceramic outperforms glass. In addition to heat lost through evaporation, which depends on the mug’s shape and the surface area of the coffee exposed to the air, your coffee will cool off as the mug, itself, draws heat through conduction and loses it through convection. In both of these areas, ceramic loses heat at a slower rate than glass.
Conduction is loss of heat through the direct contact of two materials, one of which is a colder than the other. Because ceramic is more porous than glass, conduction occurs slower in ceramic mugs. The little pockets of air that are trapped inside the ceramic act as insulators and slow the process of conduction.
Convection is the loss of heat through contact with air. Once the mug initially warms up, it will lose heat through convection. In turn, this causes it to draw more heat from the coffee via conduction. Ceramic has a higher specific heat (~900 J/kg.K) than glass (~800 J/kj.K), which means ceramic will lose heat through convection at a slightly slower pace than glass. With a ceramic mug, heat loss through convection should occur about 11.1 percent slower (100 – (800 J/kj.K / 900 J/kj.K)) than it would through a glass mug of the same shape and size.
In short, in case you aren’t a science person, a ceramic mug will keep your coffee a little warmer than a glass one.
Locally Recycled Glass is Most Environmentally Friendly
Deciding whether glass or ceramic has an edge from an environmental point of view is not as straightforward as comparing their heat loss. Both have advantages and disadvantages.
On the one hand, glass can be recycled, but ceramic generally cannot. (Technically, some ceramics can be “downcycled” and incorporated into other ceramic products, but downcycling produces a poor quality ceramic that isn’t typically used in mugs). This gives glass a distinct edge over ceramic in a comparison of their environmental impacts.
On the other hand, ceramic doesn’t weigh as much as class, and, therefore, requires less energy to be shipped. While the difference in weight between one glass mug and one ceramic mug may seem negligible, even a fraction of an ounce adds up when shipping hundreds or thousands of mugs.
The best environmental choice would be to purchase a mug made with glass locally sourced recycled glass. This takes advantage of recycling glass without the tradeoff of using more fuel to ship it long distances. If you can’t find a mug that is made from locally recycled glass, you’ll have to decide whether recycled glass or lighter ceramic is better for the environment.
The next time you come across a mug you like, don’t just think about the aesthetics of a mug. Consider how the mug will impact your coffee drinking and the environment. A mug says a lot about a person. Yours should reflect well on you in all three areas: aesthetics, thermodynamics and environmental impact.