Normally we use the oven, but didn't do enough and needed more in a hurry. We put the jars in the nuke machine wet from the sink, but part filled would have been better. I think the moisture trapped in the gap between the jars was what did the damage.

BTW, it's lime maralade. We have lime tree that is ejecting about 5 limes a day at the moment. Not the little green jobs, these look like smallish lemons, but with green flesh. Also on the home-preserve plan at the moment is lime curd plus lots of Thai curries and salads.

We have about 6 citrus trees and do some grapefruit marmalade each year but give that away as I can't eat grapefruit - it counteracts the cholesterol medication I am on.