We've had a bumper crop of plums from our two plum trees. Way more than we can eat our way through in the short plum season. It appears that we aren't the only ones - the last couple of weeks have seen bucket loads of free plums turn up in the tea-room here. (And yet they are several dollars a kilo at the supermarket.) So, yesterday, Karen had a go making plum sauce, to add to the plum jam and frozen plums we already possess. As far as I can work out, the difference between plum sauce and plum jam is merely that plum sauce is more saucey. The mechanism appears to be the same - everything gets cooked up in a large pot and the hot sauce gets added to sterilized preserving jars. The lids go on and should seal shut as the temperature inside the jar drops and the small amount of air inside loses pressure.
A lid that's popped downwards is a good sign that there is a decent seal between the lid and the jar. If there weren't, then air could get in and equalize the pressure between inside and outside, and up would pop the lid. Conversely, if there were, for some reason, some multiplying nasties in the contents, producing carbon-dioxide, the pressure would build up inside and the lid will buldge upwards. That's a sure sign that what's inside isn't edible. The trick is to spot it before it explodes at the back of the pantry and splatters jam and glass everywhere.
That's why there's a warning on shop-bought jam - if the 'button' isn't down, don't eat the contents.
However, all this only works if the jar is up to scratch. I came home from work yesterday to discover that parts of the kitchen had been painted in plum sauce. This wasn't the work of the baby - it was down to a jar that had failed. When the sauce went in, the lid went on, and, sure enough, the lid popped downwards. But it wasn't the only thing that 'popped.' The jar did as well, leaving it with no bottom.
There are a couple of reasons I can think of why this might have happened. First, it could simply be down to rapid thermal expansion of the glass. When the hot sauce goes in, the inside of the glass jar gets hot much more quickly than the outside, and so there is stress built up as a result of different amounts of thermal expansion on the inside and outside. This is what happens pouring boiling water into a cold glass.
Or, perhaps the bottom of the jar acted as a popping lid. If there were some air trapped at the bottom, it would reduce its pressure as it cooled, and create a force on the bottom of the jar. If that force were great enough, it could break the glass. An implosion, rather than an explosion.
Now, neither should have happened because the jar concerned was a proper preserving jar, designed for the purpose of having hot stuff poured into it and being sealed. But, for some reason, it did. I was tempted to bring in the jar to work today and get one of our materials engineers to examine it to determine the mechanism of failure. It would have been interesting, but I thought they had better things to do with their time, so we may never know exactly what happened here. But the good news is that there are still several intact jars of sauce, so we will be well supplied for the coming months.