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I have a jug of water which is either made of glass or made of some kind of plastic like acrylic or perhaps a kind of polymer. I want a non-invasive method for identifying the bottle's material. The two methods I find on google are:

  1. Applying heat, and
  2. Scratching the material with a knife.

Both will damage the bottle, therefore they are invasive methods.

Any suggestions?

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    I wouldn't even need a hack for this, it would be obvius from the apparent temperature. Glass would feel cold to the touch whereas plastic wouldn't. Furthermore striking it with an object like a pen would yield a different sound, a dull click for plastic and a ringing sound for glass.
    – Dave
    Jul 21, 2015 at 8:52
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    @Dave That would be a good answer. Would you like to post it as such?
    – Mooseman
    Jul 21, 2015 at 11:32
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    If it's not too thick squeezing the bottle may help. Plastic will give a little whereas generally glass won't.
    – Nigel Ellis
    Jul 22, 2015 at 8:16
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    Sorry the wrong wording. It is not a bottle but a jug, hence not possible to squeeze. Notice that it is not obvious that it is plastic. I'm not that stupid. The sound technique is not enough because it depends on the shape of the recipient, whether it is empty or not... Acrylics can very much feel like glass, hence their popularity...
    – luchonacho
    Jul 22, 2015 at 9:28
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    Sadly, it just broke! At least I could confirm that it was glass. :)
    – luchonacho
    Aug 23, 2016 at 19:53

5 Answers 5

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Glass will be colder than plastic when you touch it. In addition, glass will warm up more slowly from the heat of your hand, whereas plastic will reach the same temperature as your hand within a few seconds.

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  • Is this not inconsistent with their thermal conductivity properties? Acrylic glass apparently has less conductivity properties than glass (see here). Should that be related to the speed at which the energy between two objects is equalized?
    – luchonacho
    Jul 22, 2015 at 9:35
  • @luchonacho Good point. Maybe I'm wrong? Maybe I'm thinking of a non-acrylic plastic. I'm basing this on personal tactile memory, not on science I read, so it could be flawed.
    – BrettFromLA
    Jul 22, 2015 at 19:57
  • The dominant factor here is heat capacity, not thermal conductivity. i.e. it takes more energy to heat up glass than plastic.
    – Hobbes
    Jul 24, 2015 at 9:12
  • @Hobbes, apparently water has a lower heat capacity than acrylic. If I understand heat capacity, that means glass needs less heat in order to raise its temperature.
    – luchonacho
    Jul 27, 2015 at 8:12
  • In any case, I did the experiment and I think that is exactly the case. If I touch a surface made of plastic which is cold (e.g. plastic bowl in the fridge) and a glass, the latter heats up more quickly! In fact, in the glass I can see how the heat is transfered very fast as the surrounding areas around my finger heat too. I am about to conclude that the jug is indeed made of glass.
    – luchonacho
    Jul 27, 2015 at 8:17
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You won't require Scratching it, just tap it with finger nail and you will know by the sound what is it.

A light plastic me produce a pop sound. Glass would be hard and will produce a tunnn sound or no sound at all. Hard plastic will be dull sound unlike glass.

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This is what I have learned from experience.

Glass and stone are cold. Plastic is warm.

Now, obviously; there would be some exceptions like if the plastic were outside when it was quite cold while the glass was inside and warm. So; they will need to be around the same temperature.

As far as exact technique, I typically just hold the object in my hand for a few seconds (or at least have my skin against it); the plastic will warm to my hand almost immediately (and eventually you will probably sweat). However, glass will feel much colder.

I'm planning to find a link to back me up on this. However, for now I will just leave it at the fact that I have been making jewelry for a while now and when I get a rag-tag box of beads from someone, this is how I tell whether they are glass (or stone) or not.

Edit: Here is a link to an E-Bay quick guide on knowing if a bead is glass or plastic.(This will work with larger objects such as what you have as well).

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Try tapping 2 of them against each other or on your front tooth gently. You will get to know what is plastic and what is glass with experience.

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    Welcome to Lifehacks! I'm not a doctor, but I'm not sure it's a good idea to advocate tapping glass on your teeth.
    – Mooseman
    Jan 25, 2019 at 14:16
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Glass is significantly denser than water, and should immediately sink if placed in a sink or bucket and allowed to fill.

Plastic is usually less dense than water, and will float even if completely filled with water.

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  • Sorry, but this is just untrue. There are loads of plastics that don't float in water. Not even exotic ones, but common ones like PET and PVC don't float. Try filling a water bottle to the brim with water and submerge it: it will sink.
    – HenryJekyll1886
    Aug 14, 2018 at 21:12

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