Mold Number on Glass Bottle: How to Read the Marks

The small number moulded into the base of a glass bottle is usually a cavity number: it records which position in the mould set formed that bottle, and nothing else. It does not tell you the factory, the production date, the batch, the order or the customer, and it has no meaning outside the mould register of the plant that made the bottle. Its real value is in fault-finding, where it shows whether a repeating defect follows one piece of tooling or the whole run.

What follows is about reading marks and knowing how far each one can be trusted. Numbering conventions differ from plant to plant and between generations of machinery, so the final authority on any specific mark is always the register held by the producer.

Sort the mark into a family before reading the digits

Nearly every mark on a bottle falls into one of four families, and the same symbol can mean unrelated things depending on which family it belongs to. Classify first, interpret second.

• Tooling marks name the equipment that formed the bottle: the mould or cavity number, and often a machine or section mark. These are the only marks that help pin a repeating fault on a piece of equipment.
• Time and volume marks are the date code (a year, a year and week, or a month and year) and the capacity figure near the base, such as 750 or 330. They say when the bottle was made and which format it is.
• Identity marks are the maker's logo or initials, a plant code, a private or customer-specific mark, and now and then an abbreviation for the neck finish. They say who made the bottle and to which interface.
• Regulatory and compliance marks are symbols demanded by a public standard or a market regulation. They show that a required symbol is on the pack, which is a narrower statement than buyers tend to assume.

Moulded or applied

A moulded mark comes from an engraving in the mould, so it stands proud of the glass or sits recessed in it, and it lasts as long as the bottle does. An applied mark is inkjet printed on the shoulder, laser marked, or carried on a small base label. It can hold much more information, including batch, line and time, but it can be rubbed off. That is why applied marks serve logistics and moulded marks serve tooling.

When a mark is offered as evidence, ask which kind it is. A compliant bottle can have lost an applied mark; it cannot have lost a moulded one.

Where to look and how to light it

Most marks sit on the base, around the rim of the base plate or on the heel just above. Some moulds place a mark on the shoulder next to the parting line, and wide-mouth jars sometimes carry marks on the sidewall because the base is a large diameter. There is no shared convention for position, any more than there is for numbering.

Base engravings are often shallow, and the base may be thick, curved or recessed into a punt. Light from directly overhead flattens the detail. A low, raking light across the surface brings it out. A phone flash aimed straight at the base produces the least useful photograph possible, and it is the one suppliers receive most often.

What a mould or cavity number identifies

American buyers write "mold number", British and European documents write "mould number", and plant records often say "cavity number". All three describe one physical engraving, and "cavity number" is the most accurate of them.

Glass is formed in a mould set made up of several cavities. One set may hold six, eight, twelve, sixteen or more, depending on bottle size, and each cavity is a self-contained set of tooling that produces a whole bottle. The engraved number says which of those positions made the unit in your hand.

Why one delivery carries several numbers

A mould set runs a fixed number of cavities at once. A large order is therefore normally spread over several mould sets, and frequently over more than one forming machine and more than one shift. A pallet showing four or five different numbers is the ordinary result of a volume run. It is not a sign that stock from elsewhere has been mixed in.

Counting the numbers tells you little. The useful check is whether every number in the delivery appears in the mould register the plant has declared for that bottle. A number missing from that register is the case worth investigating.

Using the number to narrow a fault

When a defect repeats (a check-ring chip, a seam line, a distorted base, dimensional drift at the shoulder), note which numbers appear on the faulty bottles and which on good bottles from the same delivery. The pattern divides the possible causes into two groups before anyone leaves the warehouse:

• Faults concentrated on one or two numbers, with the rest of the delivery clean, point to specific tooling: cavity wear, a chipped or over-polished cavity, a damaged plunger on one station, or a neck ring that has drifted out of tolerance.
• Faults spread evenly over every number clear the tooling. Attention moves to the machine setting, the gob, the raw material or the annealing process.

Two limits apply. Some variation from cavity to cavity is expected and is contained by the drawing tolerance: a bottle at the edge of tolerance still conforms, and a bottle outside tolerance is a defect whichever number it carries. And the number shows where to look, not what failed. A supplier who says the cavity number proves a tooling fault is overstating the evidence just as much as a buyer who says it identifies the factory.

Machine and section marks

Many bottles carry a second tooling mark that names the forming station instead of the mould. Modern container glass is made on an individual section machine, where a number of independent sections each turn out a continuous stream of bottles. A section mark ties a bottle to one of those stations; a machine mark ties it to the machine as a whole.

This matters because a substantial share of forming faults belong to a station, not a mould: a plunger set too high or too low, a worn deflector, a section running hot or cold, a gob weight discrepancy on a single stream. If a fault recurs on one section number across several cavity numbers, the mould is effectively ruled out and the machine setting comes first. If it recurs on one cavity number across several sections, the conclusion reverses.

The difficulty is that the two marks look the same. Both are short, usually one or two digits, and often stamped in the same area of the base. No industry-wide rule separates them. Some plants mark the section but not the machine, some do the opposite, and some put a letter prefix on one of the two.

Ask the plant to put in writing which convention applies to your bottle, which mark sits where, and how to tell them apart. A plant that cannot write down its own convention has told you how much weight its marks will bear in a dispute.

Date codes, capacity figures, identity marks and regulatory symbols

Each of the remaining marks was designed to record something narrower than the use buyers commonly make of it.

Date codes

A date code records the production period. Formats range from a year alone to a year with a week number, a month and a year, or a short internal code. Some plants use a single digit for the year, which is only unambiguous if the plant also states the decade or a fuller record travels with the delivery. A single digit works for internal sequencing and is weak in any argument about age.

If the age of a delivery could matter, get the code convention in writing and ask for a code that includes the week. Read properly, a date code gives the order of production within a delivery and can reveal that a shipment was filled from two separate campaigns. That is often the first hint that a quality difference between pallets has a production cause and not a material one.

Capacity figures

The figure near the base is typically the nominal volume in millilitres. It names the format, not the fill. It is not a fill line and it is not the brimful capacity. Brimful, or overflow, capacity is a measured property set out on a drawing with its own tolerance, and a bottle marked 750 does not hold exactly 750 millilitres to the brim.

Treating the figure as a fill specification starts a dispute no supplier can settle, because the mark was never meant to carry that meaning. Use it for a quick format check on a mixed pallet and as a cross-check against the drawing reference.

Identity marks

A maker's logo, initials or plant code is the mark that answers the question of origin. Record it when a delivery has to be divided by source, when stock from two suppliers has ended up in one warehouse, or when a complaint must be assigned to one supply route. Some moulds are also engraved with a private mark for a particular buyer's tooling. That engraving is the visible link between a bottle and the tooling asset behind it.

Regulatory symbols

Where a market requires a symbol on the pack, it is often moulded into the glass or applied to the surface. Its presence shows that the mould or marking station carries the symbol. It does not show that the material, the process or the batch meets the requirement behind it. That proof is documentary, and the paper trail is explained in our guide to the glass bottle quality certificate and the documents that support it.

Taking a moulded symbol as proof of compliance is among the costlier misreadings, because the person who usually discovers it is the buyer's own customer.

Reference table for decoding bottle marks

Misreadings to avoid

Most errors with bottle marks are errors of interpretation made before the supplier is ever contacted. These are the ones that recur.

• The capacity figure read as a batch or date. It is a nominal volume and carries no production reference. An argument about age or sequence built on it wastes a claim cycle.
• A lone digit read as whatever the reader expected. Without the plant's convention, a single digit could be a year, a week, a cavity or a line number. It is the most ambiguous mark on the bottle.
• The cavity number used to name the producer. Two plants can each run a cavity four for completely different bottles. Without the producer's register the number traces back to no one.
• One cavity number treated as one batch. Batch identity comes from a production record, not an engraving. Quality decisions resting on cavity numbers alone will not survive an audit.
• A section number taken for a cavity number. The mistake sends the investigation to the tooling when the cause is a machine setting, or the other way round.
• A regulatory or recycling symbol read as a production code. These symbols hold no production information, and reading them as codes invents batch references that do not exist.
• A missing mark taken as proof of a problem. Plenty of conforming moulds have no identifier. The absence limits what you can investigate and establishes nothing more, and a supplier told otherwise will be right to push back.

When a mark has to be backed by a record

Marks are internal records. As soon as a question has to be answered outside the plant that made the bottle, reading stops and a documentation request begins. Three situations trigger that change.

1. You need to know the tooling behind your order. If you accept cavity-to-cavity variation within a tolerance, it is fair to ask how many cavities that tolerance covers and whether the mould set declared at the start of the project is still the one running. The mould register and the cavity layout drawing answer this.
2. The marks change in a repeat delivery. A number present for a year vanishes, or an unfamiliar one appears. Something has changed: new tooling, a repaired cavity, a re-engraved base plate, or a move to a different plant. The production record and tooling maintenance history show which, and it is better to ask before the change surfaces as a customer complaint.
3. Someone outside asks for traceability. A customer, distributor, regulator or insurer will not accept a cavity number after a complaint. The reply has to be a record.

That third case needs a system, not a single document. The chain from mould register and production log through inspection record and retained samples to the papers that travel with a shipment is laid out in our guide to batch traceability for glass bottles, which is the reference to use once a marking question must stand up to an external audit. Where the buyer paid for the tooling, there is a commercial side too: what the buyer owns and may see, what becomes of the mould when the project ends, and how a private mould is identified so it is not run for another customer. Those points are covered under glass bottle mould ownership.

A worked example with a check-ring chip

Suppose a check-ring chip turns up on a small share of a delivery. Four steps turn the marks into a precise request.

1. From the rejects and from a sample of good bottles, write down the numbers on the base and note whether the rejects share one.
2. Compare those numbers with the mould register declared for the bottle.
3. Read the pattern. Faults on one number suggest a cavity or its neck ring. Faults across all cavity numbers but on one section number suggest the machine. Faults across every number and section point away from tooling, towards the gob, the raw material or annealing.
4. Request the records that belong to the branch the pattern selected, not a general explanation.

Worked in that order, a marking question becomes a targeted request within a day, and the supplier has a defined question to answer.

What to send us when you want marks read

We need two things: the bottle type and the quality issue. For the bottle, give the format, the nominal volume if you know it, the shape family, and the surface as supplied (plain flint, partly frosted, or sprayed and lacquered), since the surface treatment can change where marks sit and how legible they are. For the issue, describe what you see and where: a chip at the check ring, a seam line, a distorted base, weight variation, a poor fit with the closure, or a difference between two pallets of one delivery.

Three photographs do most of the work if they are taken properly:

• the base under low, raking light;
• the whole bottle against a plain background;
• the defect, close enough to show its boundary.

Type the marks you can read into the message even if you are unsure of their meaning, and add the quantity affected and its proportion of the delivery. From that we can say which mark is which, which production records to ask the plant for, how to read the fault pattern across cavity and section numbers, and what wording to add to the next purchase order so that a future question is settled by a document. If the question is really about which bottle format to choose, including how base and heel vary between shapes, start with our glass bottle collections.

Questions buyers ask about mold numbers on glass bottles

What does the mold number on a glass bottle mean?

It is the position in the mould set that formed that bottle. The plant holds it as a tooling reference; it encodes no date, order, customer or factory.

Why does a single order arrive with several mold numbers?

A volume order is usually made on more than one mould set, and often on more than one machine and shift. Several numbers are expected. What matters is that each one appears in the declared register.

Can the mold number tell me which factory made the bottle?

No. Cavity numbering is private to each plant. Origin is indicated by the maker's logo, initials or plant code where the mould has one, and even then the authoritative link to a producer is a document.

How is a mold number different from a machine or section mark?

The first names a tooling position, the second a forming station. A fault that follows the cavity number is a tooling matter; one that follows the section number is a machine setting. Because the marks look alike, neither is usable until the plant has stated its convention in writing.

How should I read a date code?

Only against the convention the plant has declared. Its best uses are ordering production within a delivery and spotting a shipment drawn from more than one campaign. Where age may be argued, ask for a code with the week in it.

What if the mold number is missing or unreadable?

Then the cavity cannot be identified for that delivery, which restricts fault isolation and says nothing about conformity. A shallow engraving is easily lost under a thick or heavily punted base. Agree a batch identification rule for later deliveries.

Can I compare mold numbers from two suppliers?

No. The comparison produces false matches, since each plant numbers its cavities independently. Separate a mixed delivery by identity mark and by document.

How do the marks help when a customer asks for traceability?

Record them first, because they indicate which production records to request. Then reply with the records themselves: the production log, the inspection record and the shipping documents carry the traceability, and the cavity number only narrows the search.