Some bottles break with nothing to blame. No drop, no knock, no visible flaw, just a crack that opens during your filling run or on a shelf. The cause is invisible, and it went into your glass minutes after the bottle took shape.
What Fast Cooling Locks Into Your Glass
Glass conducts heat poorly. When a hot bottle cools quickly, its surface reaches room temperature while the core is still hot and still contracting, and the two layers lock against each other.
That leaves permanent internal stress in your bottle. It does nothing until an external load arrives, then adds to it, and your bottle fails at a level its wall thickness says it should have survived.
The 3 Stages of Your Annealing Lehr
| Stage | What the lehr does | What it means for you |
|---|---|---|
| Reheat | Brings the whole bottle to a temperature where glass relaxes and stress dissipates. | Erases whatever unevenness your forming machine left behind. |
| Slow cool through the annealing range | Lowers temperature gently so surface and core stay in step. | Your stress gets prevented here, not corrected later. |
| Rapid cool to the cold end | Past the annealing range, cools fast for line speed. | No stress forms here, so this time is safe to take back. |
Your bottle spends roughly an hour on that pass. It is one of the few stages in glassmaking nobody can shorten without a cost you discover at your filling line.
How Your Annealing Grade Is Read
Polarised light makes stress visible. Under crossed filters, stressed glass rotates the light and shows coloured fringes where a stress-free bottle stays dark, and the amount of rotation gives you a number.
ASTM C148 sets out the method for containers, expressed as an annealing grade against a reference disc. SGSBOTTLE runs polariscope checks at the cold end on all 14 lines in Shandong and Xuzhou, and your reading goes into the inspection report alongside dimensions and capacity.
SGSBOTTLE grades stress in polarised light on 14 lines, which turns an invisible defect into a number on your report.
Where Bad Annealing Hits Your Line First
Thermal shock is the usual trigger. Hot filling, pasteurisation, a hot-water rinse and a dishwasher cycle all put a temperature gradient across your wall, and residual stress is already using part of the bottle's capacity to absorb it.
Your cold chain does it too. A 500 ml (17 oz) bottle moving from a chilled warehouse into a warm humid port meets the same gradient in reverse, which is why breakage that starts in transit sometimes traces back to a lehr and not to a pallet.
What to Ask Your Supplier
Three things settle it. Your annealing grade from a polariscope reading under ASTM C148, on samples from your own production run and not from a showroom. A thermal-shock result if your fill is hot or your bottle is pasteurised, run at the differential your process actually uses. And confirmation that the lehr is monitored continuously, not checked at shift changes.
A factory that anneals properly answers all three without hesitation, because the data exists whether or not you ask for it.
What Goes on Your Enquiry
Your format in ml and oz, your fill temperature, whether the bottle is pasteurised or washed hot, and your storage and transport conditions after filling. SGSBOTTLE quality engineers across the 14 lines in Shandong and Xuzhou come back with the annealing grade and the thermal-shock result for your design.