Why glass, and when plastic is not an option
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The temperature argument
This is the reason that settles it and the others are secondary. A hot fill goes in well above the point where common plastics begin to distort, and a container that deforms as it fills does not seal as it cools. The argument is about a temperature ceiling rather than about glass being better.
| Material | Holds to about | Usable for a processed jam |
|---|---|---|
| PET | 120F | No |
| HDPE | 190F | No, a boiling-water process exceeds it |
| PP | 200F | Marginal, and not with a canning lid |
| Glass | 300F | Yes |
A boiling-water canner is at the boiling point for your altitude, which is above every plastic in that table except at the margin. The container is the limit here, not the product.
The other reasons, which matter less
Several other arguments get made for glass and most of them are true without being decisive. Worth knowing so you can tell which one is actually load-bearing when somebody offers you a cheaper alternative.
- Glass shows the product, and this is a product people buy by looking at it.
- Glass takes a two-piece canning lid, which is how a vacuum seal is made.
- Glass does not pick up fruit acid or fragrance over a year on a shelf.
Where plastic does appear
There are places in this craft where plastic is correct, and knowing them stops the rule hardening into a superstition.
In a refrigerated or freezer product, which is not processed and is not shelf stable. That is a different product with a different regulatory position, and it is usually outside a cottage food exemption: see cottage food and jam.
What glass costs you
Glass is not free of downsides and the honest version includes them, because two of them show up in freight and breakage rather than at the shelf. Both are real costs and neither changes the conclusion, which is worth saying plainly rather than pretending the choice is free.
Weight in shipping and breakage in transit. Both are real and both are on shipping it by the case.