Closures, and why the flat is single use

How the seal is actually made

Heat softens the compound, the lid is held down by the band while the jar is processed, and as the contents cool they contract and pull the lid down. The compound has by then taken the shape of that rim. Re-using it means asking a compound already formed to one rim, and already heated once, to do it again.

Where a thick sauce makes this harder

Product on the rim does not run off. Any trace of sauce between the rim and the compound is a path for air, and the thicker the product the more likely it is to sit there rather than clear itself. The rim wipe does more work on this property than on most.

The band is not the seal

Once the jar has cooled the vacuum holds the lid, not the band. Bands left on in storage trap moisture against the lid and hide a failed seal by holding a loose flat in place, which is the opposite of what you want from stored stock.

What the catalog holds for this

The closures on this shelf are matched to the two neck finishes it carries, and every container page lists the closures verified against its own neck rather than leaving the match to you. That verification is the half of the catalog nobody else publishes, and it is worth using rather than guessing from a diameter.

If a closure is not listed against a jar, treat that as information rather than an omission.

Buy flats in proportion to what you will make

Because the flat is single use and the band is not, the two are consumed at completely different rates, and ordering them as if they were a set leaves you with a shelf of bands and no lids at the worst moment. Work out the ratio from a real season rather than buying them in pairs.

References