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In this youtube video about a water cooled desktop PC build log the author uses glass tubing to route the water around the system. Around 4:08, when the first tube break/cut is shown, the author pours a small amount of water on the tube after scoring it. I've heard of "shocking" glass with hot water when cutting it, but the author does not mention that the water is hot. Further the author seems to use the same cup of water over some period of time, so it doesn't seem to be hot.

Why pour water on glass after scoring it to break it?

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    $\begingroup$ There has to be a temperature differential (hot glass/cold water, or cold glass/hot water). It's just to induce a stress in the glass. Glass will often break without being scored, if water of sufficient temperature is applied. $\endgroup$ – Steve Jan 10 '18 at 16:32
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    $\begingroup$ Maybe it is to prevent fine glass dust from getting airborne? $\endgroup$ – fibonatic Jan 10 '18 at 17:17
  • $\begingroup$ I don't know enough to make an answer, but my vote would be @fibonatic's guess. Water would help ensure little bits of glass don't get loose. It doesn't look like it's actually doing any breaking. The break would have occurred a few seconds earlier, before he started using his hands to apply pressure. $\endgroup$ – Cort Ammon Jan 11 '18 at 3:26
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Wheeler,(1958). Scientific Glassblowing, Interscience Publishers, New York & London. Chapter IV Basic Operations, Page 56. "Milligan has found that the strength of glass containing cracks is greatly influenced by a liquid in contact with the surface of the glass. Exposure of the surface to air saturated with alcohol or water greatly reduces the strength. Putting a drop of water on the crack reduces the strength by about 20% whereas moistening the crack with dry paraffin oil strengthens the tube by about 20%. Milligan does not advance an explanation for either of these effects, but many glassblowers can attest to the accuracy of the statement that water in the scratch reduces the strength of the glass tubing".

I can vouch for this, as demonstrated to me by a chemist in a lab while teaching me to cut glass tubing. Also that you can shape glass using a laboratory gauze, provided that it has been heated in the past (so not new), and you can cut glass with scissors underwater (carefully) [but not so much cut, as shape].

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