Grandma's canning.... is it safe?

It's OK to reuse actual canning jars (e.g. the Ball jars, etc) as long as you use new lids and rings. It is not OK to use jars not intended for home canning (e.g. salsa and mustard jars) because you will have to reuse the lids since the traditional canning lids and rings will not fit. The USDA has guidelines for canning safety based on research and history. It's hard to get our grandparents to understand this b/c they've been canning forever and haven't had any problems. (It's like trying to get someone to understand that they should wear a seatbelt even though they PERSONALLY haven't ever been in a car accident.) Even though our grandmas may not have accidentally given someone botulism (yet!) the guidelines have been created b/c it has happened often enough that it is worth adhering to certain safety standards so that it can be prevented. That said....I can def understand not wanting to hurt her feelings or waste her food. My guess is that even if the canning police showed up at her door and showed her the rule book she would still insist that there's nothing to worry about so you're probably not going to change her mind. As for keeping yourself safe...from what I was told in a canning class I took if you store the jar in the fridge and eat the contents within a week it's OK. (This applied to jars that didn't seal properly so I'd imagine it would apply to mustard jars since the problem really is that you can't guarantee a proper seal b/c of the re-used lids.) Even though you don't want to waste the food it's better to throw it out than deal with a case of botulism.

From the National Center for Home Canning

"With careful use and handling, Mason jars may be reused many times, requiring only new lids each time."

You CAN reuse canning jars, just make sure they are the real canning jars from reputable makers. If they are, then you don't have any problems. Something like a mayo jar is not acceptable for canning,

I don't reuse product jars. Glass is made more cheaply these days. And a loss in the canner kettle is not worth being cheap. O use product jars for dry goods storage instead.