Is there a laser beam hidden in my dishwasher?

September 15, 2006 8:39 AM

Several times over the past few months, glasses have come out of my dishwasher sliced into two pieces. The cut is always smooth, and I don't find any glass shards left over. This only happens with glasses--mugs, plates, etc, are entirely unharmed. What might be causing this?

From left to right: an unharmed glass for comparison; the bottom part of a sliced glass; the sliced-off top part.

posted by yankeefog to science & nature (15 comments total)

I'd say it's heat. Is it a new dishwasher, or was there maintenance it sometime in the past few months? Or, are these new glasses? I've noticed similar things happen simply hand-washing glasses (or leaving them to soak) in extremely hot water.

Admittedly, none of them have been quite as cool as that cut though.
posted by purephase at 8:42 AM on September 15, 2006

Fracturing caused by thermal expansion. It is more prevalent in glasses with thick bottoms like the ones you show.

I'd say your dishwasher is running too hot - if it has a "water-heat" setting that you are using, it may be that the thermostat that determines when the water is hot enough is broken.
posted by MonkeySaltedNuts at 8:47 AM on September 15, 2006

Stress lines left in the glass from molding process + heat and physical stress from the dishwasher mechanism = broken glass.
posted by Blazecock Pileon at 8:47 AM on September 15, 2006

No mention of whether they're in the top rack or not, but a guess might be the glass is on it's side and tapping against something just enough to make it split? My neigbor makes stained glass windows as a hobby, and the only time I've ever seen a clean break like your glasses is with a glass cutter and a very gentle tap to seperate the pieces. Any more force would cause shattering. Maybe experiment with putting some on the bottom rack and see if it happens again.
posted by Carnage Asada at 8:48 AM on September 15, 2006

It's not an uncommon mode of failure of glasses - for example see here.

I doubt it's your dishwasher. But I've not seen a good explanation of this.
posted by edd at 8:49 AM on September 15, 2006

(I should clarify that I think it's the glasses that are somehow faulty rather than the dishwasher that is faulty, although the act of dishwashing is clearly provoking the failure)
posted by edd at 8:52 AM on September 15, 2006

There was an AskMefi question awhile ago about someone with something similar happening to their wine glasses. It happened that they had placed the hot, wet glasses on a flat surface.
posted by JJ86 at 9:00 AM on September 15, 2006

There was this previous question on a similar subject.

I think there are a couple of related culprits in mystery glass breakage:

a) air pressure. Put a hot, wet glass upside down on the counter, it forms an air tight seal, and when it cools, the outside pressure is much greater than the inside and it shatters. Same as the well known aluminum can crushing experiment.

b) internal stresses. Glass is an odd material. When formed, it can have substantial unrelieved material stresses. In effect the outer surface of the glass can be stretched tight over the inner glass material. Apply hot, cold, or other stresses, and the material can suddenly fracture in odd ways to relieve the stress. Automobile glass is specifically made to fracture in just such a way, so as to produce non-sharp shards. While drinking glasses aren't normally made that way, they might be, accidentally.

I suspect the culprit is something like this: Dishwasher starts spraying hot water. The thin lip of your glass heats quickly and expands, while the thick bottom takes much longer to heat up. The disparity causes stress and the glass fractures nicely.
posted by jellicle at 9:03 AM on September 15, 2006

Ha! I beat jellicle to the link to the previous post. The best solution is to turn down the temperature of your water heater a few degrees. You must have the setting much too hot.
posted by JJ86 at 9:10 AM on September 15, 2006

Do you stack your glasses? It looks to me that the point that the cut occurs might be exactly where the two glasses may rub when stacked, forming an ideal score line for the glass to break.
posted by davey_darling at 9:16 AM on September 15, 2006

Just looking at the unbroken glass, I can see a line running around it at nearly exactly the point the other one is breaking. I think davey_darling's on the right track - something about how you use or store your glasses is causing a stress line and the heat of the dishwasher is then causing it to crack.
posted by jacquilynne at 9:33 AM on September 15, 2006

Thanks for the answers, everybody.

I think you all figured it out. As it happens, I turned up the temperature on our hot water heater a while ago for not-very-interesting reasons, and forgot to turn it back down. It was VERY hot, and I bet that was the cause. I've just turned it down to a more reasonable level.

So, not only have you saved my glasses, you've saved the Earth.

Thanks.
posted by yankeefog at 9:49 AM on September 15, 2006

If you have angered a local dab (mischevious Hmong spirit), he may take vengeance on your crystal, stemware, and various other transparent vessels. These attacks will unfortunately continue until you placate him with a sacrifice. A chicken should suffice, but if not, a pig definitely will. Try to be a bit more careful in your local environs, and be respectful to your elders.
posted by kookoobirdz at 12:46 PM on September 15, 2006 [6 favorites]

Hmmm... What might I have done to anger a dab? Would a dab have been angry about my wasting energy by running water too hot?

Also, I keep loosely kosher, so I would rather not sacrifice a pig. Plus, I live in a somewhat Jewish part of London, so it's possible that the dab also keeps kosher and would be angered by a pork sacrifice rather than placated. Is turkey bacon an acceptable substitute?
posted by yankeefog at 2:45 AM on September 16, 2006 [2 favorites]

Actually, I think I've found a solution. I can make a cash donation to the local dab through this convenient website, and he will signal its acceptance through the delivery of computer supplies or other home electronics.
posted by yankeefog at 2:27 AM on September 17, 2006