Glass glossary: the words of the trade, and the page that uses each one

Kinds of glass

Float glass

Ordinary flat soda-lime glass, the base of every other kind on this list. It weighs 2500 kg/m³ under EN 572-1: 2.5 kg/m² for every millimetre of thickness. See: Glass density and unit weight

Annealed glass

Float glass as it comes from the line, not toughened: the only kind that can still be cut, drilled or edged at the table. Through polarised sunglasses it shows nothing, where toughened glass shows faint dark spots or stripes. See: Is this glass toughened or annealed

Toughened (tempered) glass

Glass heated and quenched after it has been cut. It cannot be cut, drilled or notched afterwards, so every cut, hole and edge is done first, and it breaks into small dice. Under EN 12150-1 it carries a permanent mark, almost always in a corner. See: Is this glass toughened or annealed

Heat-strengthened glass

Quenched more gently than toughened glass, so it is weaker and breaks into large pieces rather than dice. Design strength in the EN 16612 form, for a 10-minute load: 39.4 MPa, against 81.0 for toughened and 18.5 for annealed. See: Glass load calculator

Laminated glass

Two or more plies of glass bonded with an interlayer, usually PVB. It weighs the glass plus the interlayer: 2.5 kg/m² for every millimetre of glass and about 1 kg/m² for every millimetre of PVB. See: Laminated glass weight

PVB interlayer

Polyvinyl butyral, the film between the plies of laminated glass, made in 0.38, 0.76, 1.14, 1.52 and 2.28 mm. It weighs about 1.07 g/cm³: a 0.76 mm interlayer is 0.81 kg/m². See: PVB interlayer weight

Low-iron glass

Float glass with less iron in it, almost colourless. The edge gives it away: ordinary float is green there. See: Is this low iron glass, or ordinary clear float

Mirror

Float glass with a reflective coating on the back. It weighs what its glass weighs, 2.5 kg/m² for every millimetre: a 4 mm mirror is 10 kg/m². See: Mirror weight calculator

Safety glass

Toughened or laminated glass where a person can fall against it or through it. In the US the hazardous locations are those of IRC R308.4: doors, next to doors, large low windows, baths and showers, stairs and guards. See: Where is safety glass required

Nickel sulphide inclusion

A speck in toughened glass that can make it break on its own, with no impact, years after it was installed. The break leaves a pair of wings around a point in the body of the glass. See: Why did this glass break

Heat soak test

Holding toughened panes hot for hours, to EN 14179-1, so that most of those with a nickel sulphide inclusion break in the oven instead of on the building. See: Why did this glass break

Sheets and cutting

Jumbo sheet (PLF)

6000 × 3210 mm, 19.26 m²: the sheet the float line produces. At 6 mm it weighs 288.9 kg. See: Jumbo glass sheet (PLF)

DLF sheet

3210 × 2250 mm, 7.22 m², cut from the jumbo: the sheet most merchants stock and deliver. At 6 mm it weighs 108.3 kg. See: DLF glass sheet

Edge trim

The strip taken off every edge of a delivered sheet before the panes are cut: typically 10 mm per side, 10 to 15 mm on a delivered sheet, and none on an offcut from your own rack, whose edges are already clean. See: Glass weight and stock sizes, on one page

Breakout

The glass lost between two neighbouring pieces at each cut: typically 3 to 5 mm. See: Glass weight and stock sizes, on one page

Score and snap

How glass is cut: a line is scored on the surface and the glass is snapped along it. The snap runs the full length of the piece, so every cut goes from one edge to the other. See: How to cut glass

Guillotine cut

A straight cut from one edge of a piece to the other, the only kind a glass table can make. A layout with a piece tucked into a corner, fine on a panel saw, cannot be cut in glass. See: How to cut glass

Three-stage guillotine

The cut pattern of a glass layout: strips snapped off the sheet, columns cut out of each strip, panes cut out of each column. See: Glass cutting optimiser

Strip

A band snapped off the full width of the sheet, as tall as the tallest piece in it: the first stage of a glass layout. Each strip is finished before the next one is cut. See: How to cut glass

Cut sequence

The order the cuts are made in: trim the edges, cut the strip across, rip it into columns, cut each column across into panes, then the next strip. See: How to cut glass

Cutting optimisation (nesting)

Laying a cutting list out on stock sheets so that as few sheets as possible are bought. The optimiser runs the list through six orderings and keeps the best. See: Glass cutting optimiser

Yield

The area of the finished pieces divided by the area of the whole sheets put on the table. See: Glass cutting optimiser

Waste

Everything on the table that does not become a piece, offcuts included: (1 − yield) × sheets × sheet area. See: Glass cutting optimiser

Scrap

Waste minus offcuts: the glass that goes in the bin. The optimiser judges a layout on its scrap, not on its yield. See: Glass cutting optimiser

Offcut

A leftover rectangle big enough to rack and use again. A common cut-off is 150 mm on the shorter side; below that it is a strip nobody racks. See: Glass weight and stock sizes, on one page

Offcut rack

The rack against the wall where the offcuts are kept, already bought and paid for. The optimiser cuts from it first, but only when a piece saves a new sheet. See: Glass offcut inventory

Cutting plan

The paper for the cutting table: a labelled diagram of each sheet, the cut sequence in order with the measurement of each cut, and every offcut with its size. See: The plan that goes to the table

Cutting labels

One label per pane, in cutting order, with its code, size and reference and a QR code that holds the text. See: Cutting labels

Cut to size

Glass ordered at a finished width and height, with its thickness, the type of glass, the edge finish and a sketch of any holes or notches. See: Glass cut to size

Edges and fitting

Polished edge

The edge to ask for where it can be seen or touched: a freshly cut edge is sharp. See: Glass cut to size

Arrised edge

An edge lightly ground to take the sharpness off: enough where it disappears into a frame or a channel. See: Glass cut to size

Rebate

The recess in the frame the glass sits in. An opening is measured between the rebates, and across both diagonals. See: How to measure an opening for glass

Setting blocks

The blocks a pane sits on in its frame. Missing or wrong ones, or glass touching metal, leave the pane nowhere to move, which is one of the causes of breakage. See: Why did this glass break

Silicone joint

The sealant bead around the glass. A joint 6 mm wide and 6 mm deep takes 36 ml per metre, so a 310 ml cartridge does about 8.6 metres. See: Silicone sealant calculator

Structural silicone

A silicone joint that holds the glass instead of only sealing it. Its bite and thickness are sized from the wind and the movement. See: Silicone sealant calculator

Insulating glass and windows

Double glazing

Two panes and a sealed cavity, written outside first: 4-16-4 is 4 mm glass, a 16 mm cavity and 4 mm glass. With argon and a soft low-E coating (ε 0.03) it reaches Ug 1.1 W/(m²·K). See: Ug value table for double and triple glazing

Triple glazing

Three panes and two cavities: a 4-16-4-16-4 with argon and two low-E coatings reaches Ug 0.6 W/(m²·K). See: Ug value table for double and triple glazing

Insulating glass unit

The factory-sealed unit of double or triple glazing. When its edge seal fails, moist air gets in and the mist forms on surfaces 2 or 3, inside the cavity, where no cloth reaches. See: Misted double glazing

Cavity

The space between two panes of a unit, filled with dry air or gas. Its width is the middle figure of the make-up: 16 mm in 4-16-4. See: Ug value table for double and triple glazing

Spacer bar

The bar round the edge of a unit that holds the panes apart, with the desiccant inside it. It does not change the centre-pane Ug; it counts in the window’s U-value. See: Glazing configurator

Warm edge spacer

A spacer that conducts less heat than aluminium. The default values of EN ISO 10077-1 give Ψg 0.05 W/(m·K) in a PVC or timber frame, against 0.06 for an aluminium spacer. See: Window U-value calculator

Desiccant

The silica or molecular sieve inside the spacer that keeps the cavity dry. Once it is spent it does not dry out again. See: Misted double glazing

Argon

The gas that usually fills the cavity, at 90 % fill. A 4-16-4 unit with a soft low-E coating goes from Ug 1.35 with air to 1.10 with argon. See: U-value calculator for insulating glass

Low-E coating

A thin coating on one face inside the cavity that cuts the heat lost by radiation. A hard coat has an emissivity of 0.16, a soft coat 0.03, the best available 0.013. See: Ug value table for double and triple glazing

Emissivity (ε)

How much heat a surface gives off by radiation: 0.837 for uncoated glass, 0.03 for a soft low-E coating. See: Ug value table for double and triple glazing

Face numbering

The faces of a unit are numbered from outside, every glass surface counted: in a double unit, faces 2 and 3 are the ones inside the cavity. See: Glazing configurator

Ug value

The thermal transmittance of the centre of the glass, in W/(m²·K), worked out to EN 673: the lower, the less heat it lets through. See: U-value calculator for insulating glass

Uf value

The thermal transmittance of the frame: about 3.2 W/(m²·K) for aluminium with a thermal break, 1.3 for PVC with 5 or 6 chambers. Use the manufacturer’s declared value when you have it. See: Window U-value calculator

Uw value

The thermal transmittance of the whole window: the area-weighted average of glass and frame, plus the edge of the glass, Uw = (Ag·Ug + Af·Uf + lg·Ψg) / Aw. See: Window U-value calculator

Ψg (linear thermal transmittance)

The heat lost along the edge of the glass, where the spacer bridges the panes, in W/(m·K). It is added to the window’s U-value for every metre of glass edge. See: Window U-value calculator

Dew point

The temperature at which the moisture in the room air turns to water. A window shows water where its glass, its glazing edge or its frame is colder than that. See: Window condensation calculator

Rw and Ctr

Rw is the single figure acoustic glass is sold on; Ctr is the correction for low-frequency noise such as road traffic, always negative. Against traffic, compare Rw + Ctr. See: Why the two panes in an acoustic unit must not match

Loads and safety

Wind pressure

The peak velocity pressure times a shape coefficient, under EN 1991-1-4: the load a facade pane is designed for. See: Wind pressure calculator for glazing

Deflection

How far a pane bends at its centre under load. It goes with the cube of the thickness: a 1000 × 1500 mm pane under 1 kPa bends 5.8 mm in 6 mm glass, 2.45 mm in 8 mm and 1.25 mm in 10 mm. See: Glass deflection calculator

Design strength (EN 16612)

The bending stress a pane may carry in the calculation. For a 10-minute load: 18.5 MPa annealed, 39.4 heat-strengthened, 81.0 toughened. Glass is weaker under a load that lasts. See: Glass load calculator

Glass balustrade

A free-standing glass balustrade is a cantilever, so the interlayer decides everything: one that can drop a person if it breaks has no business being monolithic. See: Glass balustrade thickness calculator

Glazing schedule

The list of every pane of a job: reference, width, height and quantity. Checked at once against a design wind pressure, it gives the thinnest glass that passes for each pane. See: Glazing schedule checker

Only terms the site deals with are here. Solar factor and light transmittance, for example, are not: the glazing configurator does not calculate them.