Glass Sink vs. Shaving Man


This is a story about how even tough glass sinks have a nemesis. Glass is actually a quite strong and durable material, but one thing it?s susceptible to is thermal shock. Unfortunately, the way we men sometimes shave, well, this can create a thermal shock situation.

Thermal Shock happens when a big change in temperature causes different areas of a glass sink to expand or contract by different amounts. If one part of the sink is expanding while another part is not, that will cause internal stress between those two areas. The stress can cause a crack to form in the glass. The two things that need to happen to cause this is a big swing in temperature from cold to hot or vica-versa, and for that to happen in a short period of time.

When we first heard of one of our sinks forming a crack emanating from the drain hole, our immediate thought was that the drain may have been overtightened because this is something that would be applying stress directly to the glass. When it became clear that the sink was installed correctly, we were stumped. The next morning while I was shaving, it hit me what was going on.

Like a lot of men who shave with a razor, I like to have the blade hot and wet when I scrape my face, so I turn the hot water on and let it run a little while shaving (this was pre-drought). After every stroke, I place the blade under the running water to clean out the whiskers and reheat it. Since I?m not putting my hand in the running water, it can be as hot as it goes. Then after the last stroke, I turn off the hot water and crank on the cold so that I can splash cold water on my face as the finishing touch to a good shave.

Well here?s what?s happening to the sink. The water is beating down on one spot, usually pretty close to the drain. After a few minutes, that spot of the sink has been heated up nicely. Even though we can?t see it, at a microscopic level, the glass has expanded just a bit from where it was at room temperature. At this point the glass isn?t really stressed because the heat emanates into the glass fairly evenly. The problem comes when immediately after the glass has reached it?s hottest temperature, water that could be 60°-80° cooler comes splashing down on the surface, in a sense freezing it. The surface of the glass is now contracting, while the inside of the glass may still even be expanding. The difference in temperature between the surface and the inside of the glass creates a stress line which can (but doesn?t always) create a fracture in the glass.

Our glass being as thick as it is, is much stronger than normal 1/4" thick window glass and it actually takes a lot of stress to cause it to fracture. Think of glass as being like a person. You can handle a lot stress, but sometimes if you hold it in, it builds and you?re on edge. You may not look any different, but then the slightest little thing can set you off, and you crack. Well glass can actually hold stress too, so sometimes the fracture may not happen exactly at the time of the main stress. If a sink cracks when you weren?t even shaving, it still might be a previous shave that?s responsible.

Now that I?ve made mortal enemies out of the shaver and the glass sink, let me propose the solution. All you really have to do is take a minute between turning off the hot water and turning on the cold. What I do is turn the cold tap on while the hot is still running, cooling the temperature of the running water to warm. I wash out the blade, and then turn the hot off after a minute or so, leaving the water to cool down to cold. At that point I get my splash and everyone?s happy. Notice that you can still use the very hot water?the sink can handle that just fine (freezing too).

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