Seals on glass bottles keep leaking
We bottle in 500 ml glass Folia bottles, which look great after we nailed the clarity issues. Customers love the bottles and we can charge more than enough to cover the price difference over plastic bottles. But we've been having problems with leakage as the sap cools. We can at 180 degrees and preheat the bottles in a hot water bath. We've always had some issues with leakage, but this year has been especially bad.
I've noticed that if I fill a bottle to the top, when it cools, the syrup has contracted about an inch down the neck of the bottle.
Hypothesis 1: I need to leave more headspace (air gap) rather than less. How much headspace should we leave at the top of the bottle?
Hypothesis 2: Bottle at a cooler temperature to reduce contraction of the syrup so the vacuum isn't strong enough to break the seal.
Hypothesis 3: Plastic screw-on caps suck. Just get used to recanning a lot.
Anything else? How have you beaten the leaking glass bottle problem?
12x40 sugarhouse, 2x6 Leader drop flue wood fired, 514 taps with plans for more