Quality is not a final inspection event. It is determined by decisions made at four stages, and a problem detected at the first stage costs a fraction of what it costs at the last. Understanding the stages tells you where to focus your attention and your inspection budget.
Stage 1: raw material and batch preparation
Glass is made from silica, soda ash, limestone and other ingredients. Problems here create problems everywhere downstream.
What can go wrong:
- Batch colour variation from inconsistent raw material ratios
- Bubbles from moisture in the batch or insufficient melting
- Contamination from furnace wear or refractory damage
- Temperature instability leading to inconsistent viscosity
What to check:
- Batch colour standards approved against a physical reference
- Melting temperature logged and stable
- Refractory condition inspected on a defined schedule
- Batch records retained so a problematic batch can be traced
Why it matters: a colour problem identified at the batch stage is a conversation. The same problem discovered after decoration means scrapped finished goods.
Stage 2: forming
Molten glass is formed into the jar. This is where geometry and weight are set, and where most dimensional problems originate.
What can go wrong:
- Mould wear causing dimensional drift across the batch
- Temperature variation producing wall thickness variation
- Drips or flash at the parting line
- Chips at rim or base from ejection
- Distortion in the base causing rocking
- Sealed bubbles
What to check:
- First-article measurement against specification at batch start
- Dimensional checks at defined intervals, not only at the end
- Mould condition and cycle count recorded
- Mould changeover managed with an approved first-off sample
Why it matters: dimensional problems detected mid-production can often be corrected by adjusting temperature or replacing a worn mould. Detected after firing, they mean scrapping the batch.
Stage 3: decoration and finishing
Spraying, silk-screen printing, decal application, frosting, plating and engraving all happen here. This stage has the most defects and the most opportunities to catch them early.
What can go wrong:
- Registration errors in multi-colour printing
- Uneven spray coverage, thin spots, runs
- Overspray onto untreated surfaces
- Frosting inconsistency
- Plating discolouration or flaking
- Wrong colour batch used
- Incorrect artwork version
What to check:
- Artwork proof approved in writing before the run
- First decorated pieces inspected before continuing
- Colour checked against the approved physical reference under standardised lighting
- Screen or bath condition checked on a schedule
- Batch sequencing controlled so a colour change does not contaminate the next batch
Why it matters: decoration defects are highly visible and directly affect sell-through. This is the stage where early checking has the highest return.
Stage 4: firing, annealing and packing
Firing sets the decoration permanently. Annealing relieves internal stress. Packing determines what happens in transit.
What can go wrong:
- Under-firing, causing decoration to wear off in use
- Inadequate annealing, causing later spontaneous failure
- Fire or burn marks from uneven temperature
- Insufficient internal dividers causing transit damage
- Carton strength inadequate for piece weight
- Miscounts at packing
What to check:
- Firing temperature and duration logged per batch
- Annealing cycle verified, with records
- Thermal shock test on samples from the batch
- Packing specification verified against the agreed standard
- Carton count, weight and piece count reconciled against the packing list
Why it matters: annealing problems are invisible at the factory and appear weeks later in customers\’ homes. Packing problems destroy value in transit. Neither is visible without a deliberate check.
Where inspection spending pays
| Stage | Cost of catching a problem here | Return on inspection effort |
|---|---|---|
| Raw material | Lowest | High, but requires trusting supplier records |
| Forming | Low | High |
| Decoration | Medium | Highest |
| Firing and packing | Medium to high | High |
| At the customer\’s warehouse | Highest | None |
For a new supplier relationship, ask for pre-production and pre-shipment inspection. During production, focus on the first decorated output. Everything else is verification of what those checks should already have caught.
How this works at our own factory
Every stage described above exists because a specific defect originated there. We control the whole chain in-house, which means when something is out of specification the correction happens on our floor rather than being passed between companies.
That control is the practical argument for buying direct from a manufacturer. Annealing records, firing logs and mould condition are not documentation you have to request and wait for. They are generated on the line and we can show you the record for your batch, not a policy document describing what we would do.
Ask us for the production records behind a specific batch and you will get them.
What to specify in your purchase order
State the inspection points you require and require records at each: first-article measurements, in-process dimensional checks, decorated first-off approval, firing and annealing records, and a final pre-shipment report against the agreed defect limits. Then define what happens if a stage fails.
Ask us how our production is controlled at each of these stages and we will send you the detail. Request our QC documentation.