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I recently got a glass tea mug. It's has double wall construction with a vacuum insulation (advertised as such). I noticed on the label, it specifically mentioned it's made of borosilicate glass, and to "discard if dropped".

I understand the purpose of making it out of borosilicate glass (thermal expansion when adding hot tea). But what would be the reason for the directions to link the material to needing to discard the mug if dropped (and not cracked or broken)?

Does the fact that it's made of borosilicate mean it's structural or thermal properties might be compromised if it's dropped?

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    $\begingroup$ Chances are the glass is perfectly fine. The inner space in the double wall is evacuated, so it's going to implode if it breaks. The evacuated volume is actually very small, so this thing is actually not going to send glass shards all over your room. But, if you have it if front of your eyes when it breaks, you might get injured. So, to be sure you don't sue them, they put this sentence in the label text. $\endgroup$
    – Karl
    Jan 19, 2018 at 23:25
  • $\begingroup$ @Karl That would make sense, but the way they worded the warnings, it made it seem the two were linked (the material and the drop warning) $\endgroup$
    – CDspace
    Jan 19, 2018 at 23:28
  • $\begingroup$ I absolutely agree with @Karl; also, it might be helpful if there were a photo of the mug of a link to manufacturer. Borosilicate glass is a sturdy material. $\endgroup$
    – andselisk ♦
    Jan 19, 2018 at 23:29
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    $\begingroup$ I think not, you just cannot make dewar mugs out of anything than borosilicate (except quartz, if you have the money). That sentence is from a lawyer, not a chemist or physicist. ;-) $\endgroup$
    – Karl
    Jan 19, 2018 at 23:30
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    $\begingroup$ @andselisk I can't upload any photos right now, and even if I could I don't have the warning label anymore. The mug looks like any other double walled thermos, just made of glass. $\endgroup$
    – CDspace
    Jan 19, 2018 at 23:33

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Pyrex glass labware can be dropped, scratched, even used if cracked (I've started filling a beaker, then noticing that there was a crack - but it still seemed OK). Borosilicate glass for commercial (non-lab) use is often tempered, so that a scratch is a potential hazard - the item might fail unexpectedly. The issue is not so much the borosilicate but the tempering.

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