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I have a few glass candle jars that I'm thinking about cleaning and recycling. But I've read that candle jars might be made from different kinds of glass, and some glass should not be recycled.

Your main concern when recycling glass candle jars is the type of glass used. Candle jars are safe for curbside recycling if they’re made from standard container (soda-lime) glass. However, other types of glass, such as borosilicate, may be used for these jars to help withstand the heat of the burning candle, in which case there could be a problem. This is because mixing different kinds of glass in a recycling batch is a recipe for disaster. Unfortunately, the different melting points of the different glass types can lead to low-quality recycled glass and even broken machinery. [1]

The labels stuck to the glass don't identify the kind of glass, and the company that made them has gone out of business so I can't email them to ask what they used.

Is there some way to identify whether a given piece of glass is soda-lime or borosilicate, and therefore know whether it's safe to return to the glass recycling centre?

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    The first thing to do is to see how your local area recycles glass. If glass is crushed and used as construction aggregate, it doesn't matter what sort it is.
    – Mark
    Sep 3 at 20:08
  • Borosilicate is more expensive, and unlikely to be used in a disposable container, especially where it's not needed - and it's really not for a candle holder, that doesn't even get all that hot. If the label says it's glass for recycling purposes, then the best you can do is trust the manufacturer. The exception may be if there's glass that's in contact with the flame, like in mini-fondue sets or oil evaporators
    – Chris H
    Sep 6 at 9:42
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If the glass is uncolored, you can tell which one it is by looking at a thick layer of it (e.g. along the sides): soda-lime has a green-blue tint, while borosilicate is more yellow:

Borosilicate:

enter image description here

Soda-lime:

enter image description here

If you don't mind shattering the jar, you can always test it: put some ice cubes inside, wait until the jar is cold to the touch, and then pour boiling water on the outside (or put the jar in a pot with just boiled water). Borosilicate glass will not crack, soda-lime will.

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