How to Label Glass and Mirror Dimensions for Export

How to Label Glass and Mirror Dimensions for Export Without a Claim

If you label glass and mirror dimensions for export the way most factories do — one length, one width, one thickness number — you are shipping a claim, not a spec. An overseas buyer orders "6 mm clear tempered, 1200 × 800." The lites arrive, their incoming QC drops a caliper on the edge, reads 5.9 mm, and opens a discrepancy. Nothing is wrong with the glass: 5.9 mm sits dead center of the range ASTM C1036 permits for a 6 mm lite. But nobody wrote the tolerance on the paperwork, the buyer assumed 6.00 mm, and now your balance payment is stuck behind a dispute.

Labeling glass and mirror dimensions for export means putting the measured cut size, the nominal-versus-actual thickness with its tolerance, the edgework, and the coating or backing callouts directly on the spec sheet, so a buyer in another time zone can confirm fit before the container is sealed. The standards already define every number you need. Your job is to transfer them onto the drawing instead of leaving them implied.

A reality check first: pre-sale spec questions and size-driven rejections are pure margin leak. Before you argue a claim, run the landed cost of one rejected pallet through a return cost calculator — it almost always costs less to over-document the spec than to eat one container of "wrong" glass.

Glass Thickness: Nominal vs Actual

Nominal thickness is the label on the order (6 mm); actual thickness is the range the standard legally permits around it. They are not the same number, and the gap is wider than most buyers expect.

Under ASTM C1036, "6 mm" is a designation, not a measurement. These are the actual thickness ranges the standard assigns to each common designation (Table 2, Type 1 transparent flat glass):

A "6 mm" lite can legally land anywhere from 5.56 to 6.20 mm. A "10 mm" lite can be 9.02 mm. If your buyer is designing a glazing pocket or an insulated-glass spacer around an exact 10.00 mm, that 9.02 mm surprises them — and it is your paperwork's fault, not the furnace's.

The European route is tighter and expressed as a plus/minus. EN 572-2, the float-glass standard, sets thickness tolerances of ±0.2 mm for 4 mm and 6 mm, ±0.3 mm for 8, 10, and 12 mm, and ±0.5 mm for 15 mm, per NSG/Pilkington's published tolerances. EN 572 also defines how "actual" is measured: the average of four caliper readings taken to 0.01 mm, one at the center of each edge — worth quoting on the spec so the buyer's QC uses the same method you do.

On an export spec, the tolerance is the product: a 6 mm lite that measures 5.9 mm is compliant, and a 6 mm lite you labeled "6.00 mm exact" is a claim waiting to happen. If a buyer keeps confusing the label with the measurement, send them a short primer on nominal versus actual dimensions instead of re-explaining it deal by deal.

Dimensions Buyers Actually Need on a Glass/Mirror Spec

Length, width, and thickness are the start, not the spec. These are the fields a buyer's QC actually checks against, and the tolerance each one carries:

The cut-size and squareness figures come straight from ASTM C1036 Table 2 and the fabricator tolerance sheets built on it; the uniformity limit is ASTM C1036 clause 5.1.4.

Here is the fix that ends most of these arguments. Instead of typing dimensions into a side table the buyer has to cross-reference, lock the measured numbers — thickness, cut size, each tolerance — onto the product image itself, right where the buyer's eye lands: snap-to-edge measurement on the photo or drawing, then one-click export at each platform's spec size. That is deterministic geometry pinning the actual size, the opposite of an AI-generated "spec image" that invents a plausible-but-wrong number and can't survive a caliper. Tools like a dedicated spec-annotation editor let you put the tolerance next to the exact edge it belongs to, so nothing is left implied.

Tempered, Annealed, Laminated: What Changes the Spec

The process changes which numbers are fixed and which the buyer can still touch.

Annealed glass is the base float product under ASTM C1036 / EN 572. It can still be cut, drilled, and edge-worked by the buyer, so your spec mainly needs the size, thickness, and edge condition as shipped.

Tempered (fully tempered) and heat-strengthened glass are governed by ASTM C1048. Fully tempered glass carries a minimum surface compression of 10,000 psi (69 MPa) and edge compression of at least 9,700 psi (67 MPa); heat-strengthened glass runs 3,500–7,500 psi (24–52 MPa) with no edge-compression requirement. The detail that burns exporters: neither heat-strengthened nor tempered glass can be safely cut after heat treatment. Every hole, notch, cutout, and edge finish must be final before the glass enters the furnace. A tempered spec therefore has to be complete — hole positions, diameters, cutout coordinates — because the buyer cannot fix a missed dimension on their end.

Laminated glass adds an interlayer, and the interlayer adds thickness you must declare. The shorthand encodes it: a "6.38 mm" laminate is two 3 mm lites plus a 0.38 mm PVB interlayer. Standard PVB interlayers step 0.38, 0.76, 1.14, and 1.52 mm; 0.38 mm suits glass up to 6 mm, while 0.76 mm and thicker go with 10 mm lites or larger panels. Write the full make-up ("3 + 0.38 PVB + 3"), not just "6.38 mm," so the buyer knows both the glass and the interlayer. For safety-rated laminates, add the impact class: CPSC 16 CFR 1201 Category I is the 150 ft-lb test, Category II the 400 ft-lb test — the number the buyer's building code will ask for.

Cut-Size and Edge Tolerances

Cut-size tolerance scales with thickness. Under ASTM C1036 Table 2, the length-and-width tolerance for a cut lite is ±1.6 mm (±1/16 in) for glass 6 mm and thinner, opening to about ±2.4 mm (±3/32 in) at 9–10 mm and ±3.2 mm (±1/8 in) at 12 mm. Squareness — the difference between the two diagonals — runs roughly 2 mm per 1 m of edge (±1/16 in per 39 in). Put both on the drawing; a buyer who dry-fits into a metal frame is measuring diagonals whether you mention it or not.

Edge finish is a dimension too. A flat-polished edge is a 45° chamfer (arris) on both faces, and the chamfer width itself carries a tolerance of about ±0.8 mm (±1/32 in). For laminated or insulated units, edge alignment between the two lites is held to about ±3 mm (±1/8 in). And for heat-treated glass, overall bow and warp are size- and thickness-dependent per ASTM C1048 — a long, thin tempered lite bows more than a small thick one, so if flatness matters to the buyer's frame, state the maximum bow you will hold rather than leaving it open.

Mirror-Specific Callouts (backing, safety film, edge)

A mirror is glass plus a coating stack, and the back of the mirror needs its own callouts. Silvered float-glass mirrors are covered by EN 1036-1, JIS R3220, and ASTM C1503; EN 1036 applies to mirrors from 2 mm to 10 mm for internal use.

Working from a representative manufacturer datasheet (Guardian UltraMirror Plus, a copper-free silver mirror), a mirror spec should name:

• Backing type. Copper-free silver, with a double-coat backing paint at least 40 micron (1.6 mil) thick over roughly 65–75 mg/ft² of silver. Copper-free versus copper changes corrosion behavior, so label it.
• Thickness and its tolerance. Mirror thickness follows the glass standard: ±0.2 mm for 2–6 mm (per EN 572-1), with cut width and length at +1 / −2 mm on that datasheet.
• Edge condition. Mirror edges chip and corrode more readily than plain glass; durability standards cap edge corrosion at 1.5 mm or under after salt-fog testing (EN 1036 / JIS R3220). Specify seamed or polished edges, not raw cut.
• Safety backing. For a bathroom, gym, or public area, call out a safety backing film that holds fragments if the glass breaks, and state the impact class the buyer's market expects (CPSC 16 CFR 1201 Category I or II).

Reflectance belongs on a mirror spec too — clear mirror runs from >93% at 2 mm down to >88% at 6 mm on that datasheet — but for fit and returns, it is the backing, thickness tolerance, and edge condition that buyers reject over.

Pre-Export Spec Checklist

Run this before the drawing leaves your desk:

• Nominal thickness AND actual range stated (e.g., "6 mm, 5.56–6.20 mm per ASTM C1036" or "±0.2 mm per EN 572-2")
• Cut width × height with tolerance (±1.6 mm for ≤6 mm glass, more for thicker)
• Squareness / diagonal tolerance noted (≈2 mm per 1 m)
• Edge finish named (seamed / ground / flat-polished) with arris width tolerance if polished
• All holes, notches, and cutouts dimensioned — mandatory for tempered, since it can't be cut later
• Process stated: annealed / heat-strengthened / fully tempered (with ASTM C1048 class) / laminated
• Laminated make-up spelled out ("3 + 0.38 PVB + 3", not just "6.38 mm")
• Safety/impact class where required (CPSC 16 CFR 1201 Category I or II)
• Mirror only: backing type (copper-free), safety film, edge-corrosion limit, reflectance
• Every number placed on the image next to the edge it describes, not buried in a separate table

FAQ

How do I label glass dimensions for export so buyers don't reject the shipment?

State both the nominal thickness and the actual range the standard allows, not a single number. Write "6 mm (5.56–6.20 mm per ASTM C1036)" or "6 mm ±0.2 mm per EN 572-2," add cut-size and squareness tolerances, name the edge finish, and put every figure on the drawing next to the edge it belongs to. Most rejections come from a buyer assuming an exact number you never actually promised.

What is the thickness tolerance for 6 mm glass?

A 6 mm lite is allowed to measure 5.56 to 6.20 mm under ASTM C1036, or ±0.2 mm under EN 572-2. Both are compliant. A caliper reading of 5.9 mm on "6 mm" glass is normal, so label the tolerance up front rather than defending it after the container lands.

Can tempered glass be cut or drilled after tempering?

No. Neither fully tempered nor heat-strengthened glass can be safely cut, drilled, or edge-worked after heat treatment (per Vitro's TD-138 and ASTM C1048). Every hole, cutout, and edge finish must be final before the glass is tempered, so a tempered export spec has to carry complete dimensions — the buyer cannot correct a missing one.

What does 6.38 mm mean on a laminated glass spec?

6.38 mm is a laminated make-up: two 3 mm glass lites plus a 0.38 mm PVB interlayer. Standard PVB interlayers are 0.38, 0.76, 1.14, and 1.52 mm. Always write the full build-up ("3 + 0.38 PVB + 3") so the buyer knows both the glass thickness and the interlayer, which changes how the unit fits the frame.

What extra callouts does a mirror spec need versus plain glass?

A mirror spec needs the backing stack (copper-free versus copper silver, backing-paint thickness), the edge-corrosion limit (≤1.5 mm under EN 1036 / JIS R3220), a safety backing film with its impact class for wet or public areas, and the usual thickness tolerance (±0.2 mm for 2–6 mm). Mirror edges chip and corrode more than plain glass, so specify seamed or polished edges rather than raw cut.

For the wider building-materials context — coverage, packaging, and how these callouts tie into a full product sheet — see how to label building material dimensions.

Sources & References

• C1036 Standard Specification for Flat Glass — thickness designations, actual thickness ranges (Table 2), cut-size and squareness tolerances, and the ≤0.1 mm/100 mm uniformity limit.
• What are the thickness tolerances for float glass? - Pilkington — EN 572-2 float-glass thickness tolerances and measurement method.
• Glass Technical Document TD-138 Heat Treated Glass for Architectural Glazing — ASTM C1048 surface/edge compression values and the no-cutting-after-tempering rule.
• Technical Data Sheet UltraMirror Plus — mirror thickness tolerance (±0.2 mm), size tolerance (+1/−2 mm), silver/paint backing, reflectance, and edge-corrosion durability.
• EN 1036:1999 - Glass in building - Mirrors from silver-coated float glass for internal use — scope and quality requirements for internal mirrors (2–10 mm).
• 16 CFR Part 1201 -- Safety Standard for Architectural Glazing Materials — Category I (150 ft-lb) and Category II (400 ft-lb) safety-glazing impact classes.
• How To Choose The Most Suitable PVB Interlayer Thickness - Elite Safety Glass — standard PVB interlayer thicknesses (0.38, 0.76, 1.14, 1.52 mm) and glass-thickness pairing.
• Tolerance Standards - A3 Glass Fabricator — cut-size, squareness (2 mm/1 m), arris width (±1/32 in), and edge-alignment tolerances built on ASTM C1036 / C1048.