Glass bottle quality is controlled at five points: the specification before any mould runs, the first articles off the line, process tests during the run, an AQL-based pre-shipment inspection, and a check on arrival against a written breakage threshold. Most failures trace back to the first point, because glass defects are fixed at the furnace and the lehr (the annealing oven) and cannot be reworked afterwards. The checklist below gives each stage its checks, test methods and owner.
What the specification has to fix before production
A quality problem is usually a specification gap that nobody noticed until the goods were made. "Same as last time" is not a reference anyone can inspect against, so four things need to be on paper first.
- A controlled drawing or an approved sample. It states nominal and brimful capacity, height, diameter, weight, the neck finish with its gauge numbers, the points where wall thickness is measured, the label panel and the base push-up.
- The material. Standard or extra flint, and for colours a pair of approved limit samples marking the lightest and darkest acceptable shade. Add the food-contact or cosmetic compliance rules of the destination market.
- The closure as an assembly. Cap, liner, pump or cork are specified together, with a torque or pull-force window and an agreed seal-test method.
- The packing. Carton construction (5-ply corrugated), divider style, case count, pallet pattern and wrap. Settling this at the loading dock is too late.
First-article inspection sets the reference for the run
The first bottles produced are measured, recorded and approved, and everything after is judged against them. A plant that cannot hand over a completed first-article sheet is not running controlled production, and that sheet is the best single predictor of how the run will go. It should record:
- brimful capacity in ml, which underpins the volume you declare on the label;
- weight in g, the practical indicator of how the glass is distributed;
- height, diameter and verticality;
- go/no-go gauge results on every neck finish;
- wall thickness at several body points, to show consistency and not only a minimum;
- the visual standard, with limit samples for seeds, blisters, stones and cords.
Tests during production and when each applies
Once the line is running, control is a mix of sampling and process checks. Which laboratory tests you need depends on what the bottle will go through after it leaves the plant.
| Check | Method | Needed for |
|---|---|---|
| Annealing (residual stress) | ASTM C148, plus lehr temperature logs | Every programme |
| Thermal shock | ISO 7459 | Hot-fill, canning, candle and pasteurised products |
| Internal pressure | ISO 7458 | Sparkling wine and carbonated drinks |
| Migration | EU 1935/2004 and FDA 21 CFR expectations | Food contact |
| Closure leak test | Filled or surrogate-filled assemblies stored inverted at 40 °C for 24 hours | Every bottle-and-closure assembly |
Annealing deserves the most attention because its failures are delayed. Badly annealed glass can crack weeks afterwards, during decoration firing, on a hot-fill line or in a consumer's hand. Ask also for the online inspection records: the camera and gauge rejection logs show how the defect rate moved during your run, which a final count hides. Reports from a third party such as SGS or an equivalent lab make each result defensible in an importer's file.
Pre-shipment inspection and defect classes
On a first order or with a new supplier, an independent pre-shipment inspection (PSI) costs little compared with the problems it prevents. It works to an AQL sampling plan, with defects sorted into three classes.
| Class | Examples | Common acceptance level |
|---|---|---|
| Critical | Cracks, leakers, finish damage that prevents a seal, contamination, wrong capacity | 0 |
| Major | Finishes out of gauge, visual defects beyond the limit samples, wrong colour, packing that does not match the spec | AQL 1.0-2.5 |
| Minor | Cosmetic imperfections inside the agreed limits | Looser than major |
The inspector also checks carton quality, dividers and palletising, and photographs the loading before the container is sealed. Tighter sampling suits first orders; a programme with a clean record can move towards skip-lot inspection over time.
Arrival check and claims
Inspect before the goods go into stock. Count breakage, spot-check seal integrity and note the condition of the packing. Compare the result with the claims threshold written into the order, typically 1-2% breakage, photograph everything and file inside the agreed window.
Each claim should then change something: either the spec sheet or the inspection plan gets tighter for the next run.
The document set to keep for each programme
Five documents hold the whole chain together, and they are what we assemble when we coordinate a project with a plant chosen from our network of glass bottle manufacturers:
- the controlled drawing or spec sheet;
- photos of the approved sample with its limit samples;
- the first-article inspection record;
- test reports for annealing, thermal shock, pressure and migration, as applicable;
- the packing spec with loading photos.
When a conversation with a supplier turns vague, these pages bring it back to something measurable. The same set is a useful yardstick when you are still comparing vendors; our guide to choosing a glass bottle supplier covers that step. To see which documents your own product family and market would need, send us the project details.
Frequently asked questions
Which defects turn up most often in glass bottle shipments?
In the programmes we coordinate, the order of frequency is: seeds and blisters beyond the limit samples, finish gauge failures that affect the seal, capacity drift caused by variation in gob weight, delayed cracks from poor annealing, and transit damage from under-specified cartons. Each one has a checkpoint above that prevents it.
Can quality be controlled without visiting the plant?
Yes, through documents and a third-party inspector: first-article records, process test reports, an AQL pre-shipment inspection with photos, and loading documentation. This often catches more than a visit does, since a visitor is shown the showroom while an inspector looks at the actual run.
What breakage rate is acceptable after sea freight?
Bottles packed in 5-ply divider cartons, palletised and shrink-wrapped normally arrive with well under 1% breakage. Put the threshold in writing before the purchase order, typically 1-2% with a credit or replacement remedy, and insist on loading photos so responsibility is clear.
Who runs the checklist, the buyer or the supplier?
Both, at different stages. The supplier owns process control: first articles, online inspection and leak testing. The buyer owns the specification and the verification: spec sheet, PSI, arrival check and claims. Programmes fail when each side assumes the other is checking, so every checkpoint needs a named owner.