At a glance
| Feature | Band | What to agree in writing |
|---|---|---|
| Seeds (small bubbles) | Expected | Largest size, number per element and where. |
| Larger bubbles and blisters | Agree a limit | Size; reject any that break the surface. |
| Cords (glassy streaks) | Expected | Whether visible at the agreed distance, lit. |
| Stones (unmelted lumps) | Reject | Or agree case by case with the workshop. |
| Size and shape variation | Expected | A tolerance in mm; which elements must match. |
| Wall thickness variation | Expected | How the workshop checks thin spots at joins. |
| Pontil mark | Agree a limit | Ground smooth, no sharp edge, and where it faces. |
| Scratches and scuffs | Agree a limit | None visible at the agreed distance, lit. |
| Chips, flares, sharp edges | Reject | Remake, or repair if the workshop shows how. |
| Cracks and crazing | Reject | Any length. Intended crazing is a design, so say so. |
| Strain (internal stress) | Agree a limit | Not visible by eye: the workshop states its check. |
| Loose or rocking joints | Reject | Every element seats and holds. |
Recommended project practice, not a standard: no standard was found. Every number in the wording below is an example for you and the workshop to set.
No standard fixes these limits
Hand-blown glass is not machine-made glass with flaws. Our own pages say no two pieces are identical, and our Murano specification sheet asks you to say what that means in practice. This sheet does that. It starts from a plain fact: we found no standard, in the UK, Europe, the US or internationally, that sets limits for bubbles, cords, shape or marks in hand-blown decorative glass.
- Standards for flat building glass exist but are written for a different product. BS EN ISO 12543-6, for example, covers the appearance of laminated glass (the BSI listing says test methods and defect criteria for the visual appearance of laminated glass viewed through it).
- One US supplier of decorative glass says the ASTM visible-defect limits do not apply to hand-made products. It treats random bubbles in mouth-blown glass as characteristic. That is a supplier's summary, not the standard.
- Producers publish their own limits. One lighting retailer publishes a producer's limits for hand-made glass shades, and we show them below as an example of the form, not as numbers to copy.
So the limits that govern your project are the ones you and the workshop write down before production. The table in this sheet is recommended project practice. It is not a standard, and the workshop should add its own limits.
One distinction matters. Some techniques put bubbles in on purpose: our Murano techniques guide describes reticello with a bubble trapped in every diamond of the lattice, and bullicante and pulegoso are bubble effects. Say which bubbles are part of the design, so they are not counted as defects.
The acceptance table, in three bands
Three bands cover most of what you will see. Expected means normal for hand-made glass: record it and move on. Agree a limit means normal in kind but needing a stated limit in size, number or place. Reject means the element is remade, or repaired if the workshop can show how. The list is not complete, and the bands are our recommendation, not a rule.
| Feature | What it is | Band | What to write |
|---|---|---|---|
| Seeds | Tiny gas bubbles trapped in the glass. The museum describes refining as removing the gas bubbles from the melt, which can take several hours. | Expected | The largest size and the number per element, and any place they may not sit (a rim, a joint). Say which bubbles are part of the design. |
| Larger bubbles and blisters | A bigger bubble, which can sit at the surface. | Agree a limit | A size limit. We suggest rejecting any that break the surface or leave a sharp edge. |
| Cords | A thin glassy streak where glass of slightly different make-up has not fully mixed. Common in hand-made glass. | Expected | Whether a cord may be visible at the agreed distance with the fitting lit. |
| Stones | A small hard lump of unmelted or foreign material in the glass. | Reject | We suggest rejecting. A workshop may argue for a limit; if so, write it down. |
| Size and shape variation | Hand-shaped elements differ. Mould-blown elements repeat more closely than free-blown ones. | Expected | A tolerance in millimetres for each family of elements, and which elements must match as a set. |
| Wall thickness variation | A free-blown wall is not uniform, and joins can be thin. | Expected | Ask the workshop how it checks thin spots. We found no figure. |
| Pontil mark | The scar where the glass was held on a rod while it was worked. The museum describes the pontello as the iron rod that supports the glass. | Agree a limit | Ground and polished smooth, no sharp edge, and where it faces on the hung fitting. |
| Scratches and scuffs | Light surface marks from making and handling. | Agree a limit | None visible at the agreed distance with the fitting lit. Reject deep scratches. |
| Chips, flares, sharp edges | Damage to a rim, edge or join. | Reject | No chips. A sharp edge is a safety matter as well as a finish matter. |
| Cracks and crazing | A fracture, or a network of fine ones. Some effects, such as ice glass, are crazed on purpose. | Reject | Any crack in an element is rejected. Describe intended crazing as a design feature. |
| Strain | Stress locked in by uneven cooling. The museum says it can make glass crack, perhaps some time after it has cooled completely. | Agree a limit | The workshop states how it anneals and checks. Reject any element that cracks. |
| Joints and fit | Where glass meets metal or other glass. | Reject | Every element seats, holds and does not rock. No loose parts. |
Writing criteria that can be applied
A limit is only useful if two people could apply it and reach the same answer. That means the criteria sheet has to say seven things.
- Distance. The distance from which a defect is judged, such as 1 m for a surface mark. A chandelier is seen from below and from across a room, so say which view counts.
- Light. Lit, unlit, or both, and at what level. A seed that vanishes unlit can glitter when lit.
- The count basis. Per element, per fitting, or per area of glass. Counts per fitting hide a lot.
- Where. Nothing in a rim, a joint or the pontil area, and no defect on the side that faces the room.
- Sets. Which elements must match each other, and to what tolerance. A set of mould-blown arms is not a set of free-blown flowers.
- Reference samples. A control piece, and if the workshop will make one, a limit sample that shows the worst element you would accept.
- The decision. Who inspects for each party, who signs, and what happens to an element outside the limit.
To show the form a limit takes, here are the limits one lighting retailer publishes for a producer's hand-made glass shades. They are for shades, not chandelier arms, they are undated, and we have not checked them with the producer. Do not use them as defaults. Note that this producer accepts a scratch or crack that is not visible and does not affect strength; for a fitting hung overhead we would recommend rejecting any crack.
| Feature | Limit as published |
|---|---|
| Air bubbles | Less than 4 mm across, and no more than four in any 10 cm squared. |
| Symmetry and oval shape | Acceptable if the balance of the fitting is kept. |
| Chips on edges | Up to 2 mm deep. |
| Scratches | Acceptable if they do not affect strength and are not visible from 1 m. |
| Smudges and trails | Acceptable if not noticeable from 1 m. |
| Wall thickness | Varies by less than 4 mm. |
When, where and how to inspect
Inspect three times, and record each. The workshop sees the element first, when a remake costs least; the contractor sees it when it arrives; and the designer sees it lit in its room.
| Stage | Who | What | Viewing | Record |
|---|---|---|---|---|
| At the workshop, before dispatch | The workshop's inspector, with the client's representative if they wish | Every element against the criteria sheet, and a hold point before crating | At the agreed distance, lit and unlit | A numbered photo set, with the control piece in the frame |
| On arrival | The contractor, with the studio | Unpack and count against the packing list; check each element, lit and unlit; damage in transit | The same distance and light as the criteria sheet | Damage written on the delivery note; photographs |
| After hanging | The designer and the workshop's representative | The fitting as a whole: balance, glitter, joints, the elements that must match | Lit, at full output and dimmed, from the places the room is used | A snag list signed by both parties |
The photo set to ask for before dispatch
A numbered photo set lets you say 'hold' before the glass is crated. Our quality and compliance page says progress photography is shared at agreed milestones; ask the workshop to name the milestones for glass.
- Every element type laid out on a neutral background with a scale, numbered, with the control piece in the frame.
- The same elements lit from behind, which shows seeds and cords.
- Close-ups of pontil areas, rims and joints.
- The assembled fitting on the workshop bench, lit at full output and dimmed.
- The packing before the crate is closed.
- Straight-on and unretouched, with files named by element number.
If an element is outside the limit
Decide the remedy before production, not when the crate is open.
- Compare with the object. A disputed element is compared with the signed control piece or the limit sample, lit, under the conditions in the criteria sheet. See Murano glass colour for colour.
- Name the person. One named decision-maker for the project and one for the workshop, so that acceptance is a signature and not a conversation.
- Remake or repair. Say which, who pays and in how many weeks, and ask the workshop to state what a remake does to the programme. Murano glass is blown to order on the furnace schedule, so a remade element rejoins that schedule.
- Snagging. Items found after hanging follow the snagging process; see our sheet on snagging acceptance criteria.
- Breakage in transit. Agree who is responsible and how it is recorded. Our Murano specification sheet lists it among the questions for the workshop.
Specification wording
Paste into your schedule or specification and replace anything in [square brackets] with your own figures. Wording is a starting point: your specification governs.
Glass elements are hand-made. Variation in size, shape and wall thickness, small bubbles (seeds), glassy streaks (cords) and a ground pontil mark is characteristic of the process and is accepted within the criteria in [schedule ref], agreed before production. The criteria sheet and the reference samples form part of the order.
Seeds not larger than [ ] mm, not more than [ ] per element, none in a rim or joint. Cords accepted where not visible from [1.5] m with the fitting lit. Like elements within [ ] mm of the control piece in [height and width]; mould-blown elements within [ ] mm. Pontil marks ground and polished smooth. Scratches not visible from [1] m lit. Chips, flares, cracks, stones, sharp edges and loose joints not accepted.
Elements judged at [1.5] m, lit at [full output and fully dimmed] with the specified lamp and unlit in daylight, by [named person] in person, side by side with the control piece [and limit sample ref].
The workshop inspects each element against the criteria sheet and sends a numbered photo set [as listed in schedule ref] not less than [ ] working days before dispatch. The client may place a hold before crating. On arrival the contractor unpacks, counts and inspects, and notes damage on the delivery note. After hanging, [designer] and [workshop representative] view the fitting lit and sign a snag list.
An element outside the criteria is remade [or repaired, as the workshop proposes and the designer approves] by [workshop] within [ ] weeks of written notice. The effect on the programme is stated in writing at the same time. Acceptance of an element is recorded on the snag list, signed by both parties.
Terms used on this page
- Seed (bubble)
- A tiny gas bubble trapped in glass. Common in hand-made pieces, and judged by size, number and place against criteria agreed in advance.
- Cord
- A thin glassy streak where glass of slightly different make-up has not fully mixed. Common, and often acceptable.
- Pontil mark
- The scar left where the piece was held on a rod while it was worked, normally ground or polished away.
- Strain
- Stress locked into glass by uneven cooling. The Glass Museum Murano says it can make glass crack, perhaps after it has fully cooled, hence slow cooling in an annealing oven.
- Limit sample
- A sample showing the worst element that would be accepted, kept beside the control piece, so a limit is something you can look at.
Standards and sources
- Fondazione Musei Civici di Venezia, Glass Museum Murano: What is glass? (English leaflet): read 5 Nov 2026: pp. 3 and 4 (annealing; refining), pp. 7 and 8 (the pontello), p. 10 (glossary).
- BS EN ISO 12543-6, Glass in building: laminated glass and laminated safety glass, Appearance: BSI listing read: the 2011 edition, shown as withdrawn on 24 Feb 2022. Named only as an example of a flat-glass standard that does not apply. Text not read.
Questions we are asked
Are bubbles in Murano glass normal?
Small bubbles, called seeds, are common in hand-made glass, and some techniques put bubbles in on purpose. Agree the largest size, the number per element and where they may not sit, and say which bubbles are part of the design. The presence of bubbles is not proof that glass is genuine: for that, see our sheet on origin.
What is a cord in glass?
A thin glassy streak where glass of slightly different make-up has not fully mixed. It is common in hand-made glass and often acceptable. Agree whether a cord may be visible at a stated distance with the fitting lit.
Is there a standard for handmade glass defects?
We found none. Appearance standards exist for flat building glass, such as laminated glass, but they are written for a different product, and one supplier says the US limits for decorative architectural glass do not apply to hand-made products. Producers publish their own limits, so the criteria are agreed by contract: write them down before production.
What is a pontil mark and is it a defect?
It is the scar where the glass was held on a rod while it was worked. It is normal, and is usually ground and polished away. Agree the finish: smooth, no sharp edge, and not facing the room.
Can I reject a hand-made glass element?
Yes, if it is outside criteria you agreed before production. Without agreed criteria the argument is about opinion. Write the limits, keep the control piece and the limit sample, and agree who decides.
Who should inspect hand-made glass, and when?
Three times: the workshop before dispatch, with a numbered photo set and a hold point; the contractor on arrival, unlit and lit; and the designer after hanging, lit and dimmed. Record each, and sign the snag list.
Download the factsheet
A two-page A4 PDF with the acceptance table, the line drawings and paste-in wording for a handmade-glass clause, to keep with your project file.