Annealing, and why a jar is not a candle jar
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On this page5
The most useful page in this section, and the one that separates a container catalog from a candle supplier.
The standard that exists for this
ASTM F2179 is the Standard Specification for Annealed Soda-Lime-Silicate Glass Containers That Are Produced for Use as Candle Containers.
The title is doing a lot of work. It is not a standard for glass containers; it is a standard for glass containers produced for use as candle containers, which is a category distinct from the jars a packaging catalog mostly sells.
What annealing is
Formed glass cools unevenly, and uneven cooling leaves internal stress. Annealing is a controlled cooling cycle that relieves it.
Stressed glass looks identical to unstressed glass. It behaves differently the moment a temperature gradient is applied, which is exactly what a candle does: hot at the base and the pool, cooler at the top and outside.
Which is why this is invisible until it is a failure.
What the standard requires
A scratch test, a temper specification and thermal shock requirements.
Temper here means the degree of residual stress, measured optically. Thermal shock means the container survives a specified temperature differential.
The standard is voluntary. A supplier who specifies to it has made a decision about what the container is for; a supplier who has not, has not.
Asking the question
Ask directly: is this container intended and specified for candle use, and is it annealed to ASTM F2179?
It is a question a container supplier can answer, and a vague answer tells you what you need to know. "People use it for candles" is not the same answer as "it is specified for candle use".
Then test with your own wax and wick, all the way to the base, with a hand on the glass. The standard describes the container; only the burn describes your candle in it.