muller@market.alliant.com (Jim Muller) (02/22/90)
>Herr Doktor Muller, help me out here! Whew! You raise a lot of questions here. But since you asked... >I can't buy the analogy about how water cracks rocks and using >Terry's hot water scraping method. Isn't it the alternate >*freezing* and thawing process that splits the rocks? Why should >putting hot/warm water over a pit on a window do any harm? This was prompted by someone's posting about a Sci.Am. study. I am not familiar with it, but here are a few thoughts. Certainly the *fastest* way temperature might crack rocks is by freezing and thawing, but your question seems to imply that this is not the case here. I suggest that it certainly could be the case here. Hot water on a windshield will raise the temperature of some of the glass and ice, and provide enough heat of fusion to melt some of the ice (and maybe all of it in some spots), but we might expect that some of the glass can freeze again too. The edges of the area where the water is applied will not warm as much as the center, so they will act as a heat-sink. Also the air may be cold enough to refreeze any residual water, especially if it is such a cold morning that these drastic measures are required to remove the ice. If any water enters a crack and then refreezes, it certainly will increase the crack's size, and it seems possible, if not always a certainty, that some of the water could refreeze. Hence, that mechanism is at least a possibility. With auto glass being tempered and hence internally-stressed, the result also may be more than just a slight crack size increase. >I can see that perhaps eventually the water I pour on might >freeze again, but in practice it blow dries as you drive... The key question is how fast any refreezing or the "blow-drying" happens. It obviously depends on the temperature and humidity, and on how quickly you hop in the car and drive away. I'd guess that you don't get in the car and drive that soon after pouring the water, and even that if you did, refreezing would be likely to occur more quickly anyway. As mentioned above, the air and parts of the window or frame will still be cold, and the same wind convection that dries the window will also provide better cooling. > and besides it shouldn't be any worse that the water coming >from off the road or from the sky, right? On that cold a morning, I would not expect "water coming from off the...sky". Water from the road will contain salt or will be slightly warmer, or else it would not be water at all. If frozen, it will melt eventually, but by then, the windshield will have warmed from the defroster, quite some time after you started driving. In any case, environmentally-sourced water will be more or less at the same temperature as the windshield, and except for rain, will not be in great, concentrated-in-one-place quantities that pouring would provide. >The danger I was worried about was a too fast transition from >very cold to very hot. After all the previous discussion, this is probably closer to our interest! The Sci.Am. article may have been concerned more with how a crack grows over time, i.e. why it is bigger today than it was last week. In our case, we are interested in one thermal cycle rather than the possible effects of many in a natural environment. So maybe all that freezing/thawing stuff doesn't apply very much after all. For example, we don't know how much water, if any, can actually *enter* a crack in such a short time. What does happen though is that thermal expansion will occur around the point of pouring. Differential expansion can, of course, lead to stresses which would concentrate around the crack tip. Slow crack propagation, enhanced by any pre-stressed nature of the glass, would be a likely result. The faster the transition from cold to hot, then the larger any temperature gradients must be, and so the larger any potential stresses will be. Your fears are well-founded! One conclusion should be: *Always pour as slowly as possible.* >But, since we know there is a latent heat to melt which must be satisfied >before the surface temperature begins to rise, I've about convinced myself >we're all right -- provided we stop pouring when the ice is gone or to be >safe when we're down to the last coat which should be easy to scrape manually. Ah! You have your own built-in caveats to protect you. They look reasonable too. Pour slowly. Stop when the ice is gone or even before. Don't use HOT water. Don't concentrate it too much in one place, since you are still raising the temperature locally even before melting-at-constant-tempteraure begins, while other places on the glass are still COLD. And avoid any known cracks. Jim Muller Obligatory British-car comment: My British car never sees ice!