How full to fill it
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Why there is a figure at all
Headspace is specified rather than suggested because it is doing three jobs at once, and the gap is not space left over after filling. It is the volume the jar needs in order to seal itself as it cools.
Headspace is doing three things at once: leaving room for expansion during the process, leaving little enough air that a proper vacuum forms as the jar cools, and keeping product off the sealing surface.
Both failures
It fails in opposite directions and only one is visible on a shelf, which is what makes the other dangerous. Too much gives an underfilled jar and a weak vacuum; too little forces product past the lid during processing.
| What happens | When you find out | |
|---|---|---|
| Too little | Product forced past the lid in the canner, seal compromised | At cooling, if you check |
| Too much | Weak vacuum, and a jar that reads underfilled on a shelf | At the point of sale |
Getting it right at volume
Accuracy drifts over a run and it drifts in one direction: as the pan empties and the pace picks up, jars get fuller. A gauge is indifferent to how tired you are, which is the argument for one. Check the fill as you go rather than looking along the row at the end, because a jar can be corrected before the canner and not after.
A headspace tool removes the judgment and costs almost nothing. It matters least on the first jar of a run and most on the fortieth, which is the one filled while you are thinking about something else.
The interaction with a stiff set
A set that is too stiff will not level in the jar, so the headspace is uneven and reads as short measure through the glass. That is a texture fault presenting as a filling fault, and it is on it set too hard.