Every glass factory produces some level of inherent defect. The commercial question is never whether defects exist, but which ones are acceptable for your specific product and what limits you set in advance. This article covers the ones you will actually encounter.
Defects that affect function
These make the product unsellable or unsafe. Zero tolerance.
Cracks and chips
Any crack is a rejection. Chips at the rim are the most common form because that is the edge most exposed to handling. Chips at the base are less visible but still reject the piece since they compromise stability.
Sharp edges
Deburred but still abrasive edges cause injury and will generate complaints. Check by running a finger carefully around the rim, or visually with a loupe.
Fire and burn marks
Cloudy or opaque patches caused by improper annealing. These indicate internal stress and predict failure in use, often in a customer\’s home with a lit flame. Reject all of them.
Sealed foreign bodies
Bubbles of gas, or unmelted batch material trapped inside the wall. A large bubble in a wall is a stress concentrator. Reject bubbles visible in the wall area; accept tiny bubbles within stated size limits in the base or upper area where they carry no structural risk.
Rocking base
A base that does not sit flat. Reject. The piece is unstable and looks defective on shelf.
Defects that affect appearance
Bubbles
The most common defect and the most contested. Acceptable levels depend on where the bubble sits and how large it is.
- Bubbles in the base — commonly accepted up to a stated diameter, since they are barely visible with product inside
- Bubbles in the wall — lower tolerance, since they are visible and act as stress concentrators
- Bubbles near the rim or neck — reject, as this is the most visible area
- Long drawn bubbles — reject, these indicate poor process control
Agree a numeric diameter limit in writing rather than using words like \”few\” or \”minor\”.
Parting line and mould marks
The seam where the mould halves meet. This is a normal feature of pressed glass, not a defect. Any quality standard that rejects it is wrong and will cause disputes.
Mould scratches are different. Fine surface scratches inherent to mould wear can be accepted within a limit. Deep scratches that catch a fingernail should be rejected.
Wall thickness variation
Caused by mould wear over the mould\’s life. New moulds run to the upper side of tolerance, older moulds to the lower. Beyond tolerance it affects capacity consistency and thermal performance. Specify a tolerance band and check actual wall thickness with a caliper on a sample.
Pontil marks and cold marks
Surface irregularities from the forming or handling process. Small ones below a stated threshold are normal in pressed glass. Large or badly placed pontil marks on a visible face are rejected.
Colour variation
Between pieces in a batch and between batches. This is where most complaints originate, and it is the fault of insufficiently precise colour specification.
Define your standard by a physical reference, not a colour name. \”Ecru\” means nothing without a physical approved sample. Ask for a batch reference and require continuity between orders, which is achievable and should be part of the specification.
Defects introduced by decoration
- Printing misalignment or off-register
- Incomplete ink coverage or patchiness
- Print bleeding or feathering
- Spray unevenness, thin spots, runs or overspray on adjacent surfaces
- Frosting inconsistency or overspray onto untreated areas
- Plating discolouration, flaking or pinholes
Decoration defects are a different category from glass defects, because they are caused by a different process step. Approve an artwork proof and a decorated physical sample before production rather than relying on technical drawings.
Setting sensible acceptance levels
A workable standard for pressed glass candle jars:
| Category | Suggested limit |
|---|---|
| Critical defects | 0% |
| Major defects | under 3% |
| Minor defects | under 8% |
| Measurement tolerance, dimensions | ± specified value |
| Measurement tolerance, piece weight | ± specified value |
| Capacity tolerance | ± specified value |
These are starting points. A premium positioning justifies tighter limits; a budget line can work with looser ones provided the selling price reflects it.
The rule that prevents most disputes
Agree defects and limits in writing before production, attach them to the purchase order, and hold a signed approved sample. Then the question at inspection is never whether a defect is acceptable in principle, but whether this shipment matches the agreed standard.
Send us the defect categories that matter to your customers and we will confirm what our process delivers to that standard. Discuss your quality requirements.