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I can't find a single explanation online, and image search engines did not help either. One theory is that this refers to a pressure cooker, but I am not sure. My intention is to vacuum seal the glasses by boiling them, so I am a bit wary that the high temperature might damage them.

Weck glass label with a highlight on the symbol to query

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    Regardless of what the symbol means, Weck glasses are intended for canning. They are absolutely suitable for water bath canning, without getting damaged. I have also used them for pressure canning, and also for that they work well, although I was somewhat unhappy with the performance of the rubber rings in that case. No glass breaking though.
    – rumtscho ♦
    Commented Nov 17 at 11:13

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I am reasonably sure but can’t yet find a source/citation for this, but it appears to refer to the “thermal shock” resistance. The symbol looks a lot like the delta used for “delta T”, or a change in temperature. That would fit with the symbol on the left which looks like the maximum temperature.

Taken together, it’s saying that it’s rated to a maximum temperature of 200°C and up to a 50°C temperature change, both which are typical of soda-lime glass.

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    That’s correct - and the reason why Weck states that for canning jars with cold contents go in a cool water bath and jars with hot contents in a warm or hot water bath. (And yes, I’d have to dig through my library to find that reference, if I even still have it.)
    – Stephie ♦
    Commented Nov 16 at 21:11
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    Interesting: Once I understand it it seems quite clear! I think it's the interpretation of the triangle as a delta which makes the difference. I first thought it might be a roof or something.
    – Peter - Reinstate Monica
    Commented Nov 17 at 6:18
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    @Peter-ReinstateMonica I noticed that the triangle is not the sane line thickness all the way around - it’s subtle, but that’s what convinced me to write an answer (plus when I looked up the thermal shock resistance and temp limit of soda-lime glass, it matched the numbers in the image).
    – fyrepenguin
    Commented Nov 17 at 8:28
  • This sounds about right, thanks a lot!
    – martinarroyo
    Commented Nov 17 at 14:07
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    Weck's FAQ confirms this; "How do I avoid breakage? — In order to avoid thermal shock the temperature between the glass jar and the operating ambient temperature do not exceed max. 50 degrees C."
    – TylerW
    Commented Nov 17 at 21:38

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