Thursday, September 13, 2012
Old is gold...
I rarely talk about materials science on this blog, but today is your lucky day if you get excited about the physical properties of matter. As it turns out, all glass baking items are not created equal, because all types of glass are not created equal. A report in the Bulletin of the American Ceramic Society (also covered in this Scientific American Podcast) reports on changes in the material that Pyrex glassware is made from that affects its performance.
Prior to 1994, all Pyrex cookware was made of borosilicate glass (which is also what most laboratory glassware is made from). The advantage of this type of glass over normal glass is that it has a low coefficient of thermal expansion. This means that when the glass is heated, it expands less than normal glass. This might seem insignificant, but it can be important if the glass is going to be used in a way that results in it being rapidly headed or cooled, such as going from an oven to a refrigerator. Rapid changes in temperature can cause glass to expand unevenly, causing stress within the glass that leads to cracks or even breakage. Glass with a lower coefficient of thermal expansion is less likely to crack or shatter after exposure to rapid changes in temperature, making it ideal for kitchen (or laboratory) use.
However, since 1994, Corning has been licensing the Pyrex name to companies that produce products made of soda lime silicate glass, which is the type of glass found in most common glass items in your home. This glass is less likely to break when dropped (although this is not tested in the above bulletin), but has a coefficient of thermal expansion that is about three times that of borosilicate glass, making it more likely to shatter when exposed to thermal stress. In fact, the report says that a temperature change of 100 degrees F is enough to break the new Pyrex products, while a change of 330 degrees F was required to break the old borosilicate products. To put this in perspective, the difference between a raw, room-temperature roast and a hot oven is about 275 degrees F.
The take home message is that if you want the old, shatter-resistant formulation of Pyrex, look for older pieces (or, presumably, look for items that are labeled as being made from borosilicate glass). That casserole dish from the thrift store might be an even better deal than you previously thought. And, if you're a fan of pictures of glass shattering, definitely check out the full report from the ACS bulletin above!
(Image: IMG_5201, a Creative Commons 2.0 licensed image from gruntusk's photosream)