A few days ago I was going to rack 4 different beers. To do so, I needed to clean and sanitize an empty carboy. The empty carboy was sitting on my basement floor. The outside temperatures are currently around 0°F here in Minnesota and anything on my basement floor is around 50°F, including the aforementioned carboy. I placed the carboy in my utility sink and turned the water on. I only turned the hot water on. My intent was to turn the hot water on, get the sanitizer, turn the cold water on and start sanitizin'. Before I got a chance to turn the cold water on, I heard the most terrible and unmistakable sound. I can't really describe what it sounded like. It was more of a pop that a shatter sound, but I knew exactly what had happened. My carboy was dead.
The hot water on the cold carboy shattered the bottom of my 6.5 gallon glass carboy. It was irreparable. Thankfully, the whole thing remained intact enough for me to lift the whole thing into the garbage can and get it outside without getting glass everywhere.
I should have known better.
Glass primer. So, what is glass? Why did the carboy shatter?
Glass is an amorphous solid composed of silica (SiO2). That's right. I said "solid," Contrary to popular belief, glass is not a "highly viscous" liquid. It is an amorphous solid meaning it is a solid that does not have an organized crystal structure.
The primary glass making material is SiO2, but this silica is not that great for making glass. It has a high melting point (1723°C) and is highly viscous when molten. That makes it difficult to work with. To make the silica more useable, chemicals known as fluxing oxides are added. These fluxes can affect the properties significantly and are responsible for the different types of glass.
Soda-lime glass is the most common type of glass and the type that was used in my carboy. This glass is also found in windows. It is made by mixing sodium oxide (soda, Na2O) and calcium oxide (lime, CaO) with the silica. This does two things. First it lowers the melting point of the glass to around 1300°C. That makes it much more workable. The soda and lime also make the glass more robust and resistant to corrosion. However, soda-lime glass has a high coefficient of thermal expansion. That means it expands significantly when it gets hot. Inconsistent expansion can create stress points and result in cracks. This can be demonstrated very well using the cold carboy in hot water trick!!!
Borosilicate glass is much more thermally robust. It is made by adding boron oxide (B2O3), among other things, to the silica. The boron oxide also reduces the melting point, but more importantly,it also reduces the thermal expansion significantly and makes the glass highly resistant to cracking as a result of temperature changes. Any glass you use for cooking is borosilicate glass. Pyrex and Kimax are two brand names. Science glassware is almost always borosilicate glass. My carboy was NOT made of this type of glass.
There are other types of glass, but these are the most common.