Glass Glossary

Specification language for glass is precise, and the same word often means different things to a supplier and a specifier. These are the terms as TPRS uses them across this site. Where a term has a common alternative spelling or trade name, it is listed alongside.

Glass types

Annealed glass also float glass: Ordinary float glass that has been cooled slowly after forming to relieve internal stress. It is the base product all processed glass starts from. It can be cut, drilled and edge-worked, and it breaks into large sharp shards, which is why it is not a safety glass.
Toughened glass also tempered glass, tuffen glass, tafan glass: Glass heated to around 620 degrees Celsius and then rapidly cooled, putting the surfaces into compression and the core into tension. The result is several times stronger than annealed glass and breaks into small blunt granules rather than shards. It cannot be cut, drilled or notched after toughening.
Heat strengthened glass: Glass cooled more slowly than toughened glass, giving roughly twice the strength of annealed glass and much better resistance to thermal stress. It is not a safety glass: it breaks into large pieces like annealed glass, so it is normally used laminated where fragment behaviour matters.
Laminated glass also laminated safety glass: Two or more panes bonded to a plastic interlayer under heat and pressure. When it breaks the fragments are retained on the interlayer, so the pane stays in the opening. This is why balustrades, overhead glazing and security positions specify laminated rather than toughened glass.
Insulated glazing unit also DGU, double glazed unit, IGU: Two or more panes separated by a sealed cavity containing dry air or argon, used to reduce heat transfer. The make-up is written as outer pane, cavity, inner pane in millimetres, so a 6-12-6 unit is 6mm glass, a 12mm cavity and 6mm glass.
Curved glass also bent glass: Flat glass heated until it softens and formed over a mould to a specified radius, then either toughened in the same cycle or laminated with a second bent pane. All cutting and edge work must be completed before bending.
Switchable glass also smart glass, privacy glass: Laminated glass with an electrically controlled layer that changes it from transparent to opaque. PDLC types switch instantly between clear and a frosted white; electrochromic types tint gradually and hold their state without continuous power.

Coatings and finishes

Low-emissivity coating also low-E: A microscopically thin metallic coating applied to a cavity face of an insulated unit. It reflects long-wave heat radiation while passing visible light, and it is the single largest lever on the U-value of a double glazed unit.
Solar control glass: Coated or tinted glass that reduces the solar heat entering a building while still admitting daylight. Coated types reflect infrared energy; tinted types absorb it, which heats the pane and re-radiates part of that heat inward.
Body tinted glass also tinted glass: Glass coloured through its thickness by adding metal oxides to the melt. It absorbs solar energy rather than reflecting it, so it runs hotter than clear glass and often needs heat strengthening to resist thermal stress.
Ceramic frit also fritted glass, printed glass: Ceramic ink screen-printed or digitally printed onto glass and then fired into the surface during toughening. Because the frit fuses into the glass it is as durable as the glass itself and will not fade, which is why it is the correct specification for anything exposed to weather.
Back painted glass also lacquered glass: Float glass with an opaque lacquer cured onto its rear face, so the colour is viewed through the thickness of the pane. It is an interior product: the coating is not intended for weather exposure.
Frosted glass also acid etched glass, obscure glass: Glass with one surface roughened by acid etching or sandblasting so it diffuses light and blocks line of sight while still transmitting daylight. It gives permanent privacy, unlike switchable glass.

Performance measures

U-value: The rate of heat flow through a glazing unit, measured in watts per square metre per kelvin. A lower figure means less heat transfer. It is improved most by a low-emissivity coating and an insulated cavity.
Solar factor also g-value, SHGC: The proportion of incident solar energy that ends up inside a building, counting both what passes straight through the glass and what the glass absorbs and re-radiates inward. In a cooling-dominated climate this is the figure to drive down.
Visible light transmission also VLT: The percentage of visible daylight that passes through a glazing unit. The design tension in solar control is that reducing solar gain usually reduces this too; a highly selective coating separates the two better than a tint does.
Selectivity: The ratio of visible light transmission to solar factor. A high selectivity means the glass delivers daylight without the accompanying heat, which is what distinguishes a purpose-made solar control coating from a simple body tint.
Sound reduction index also Rw: A single-number rating in decibels for how much airborne sound a glazing unit attenuates. Performance depends on mass, on the two panes having different thicknesses so they do not resonate together, and on whether an acoustic interlayer is present.

Processing and construction

Heat soak testing: A test in which toughened glass is held at elevated temperature so that panes containing nickel sulphide inclusions fail in the factory rather than spontaneously years later in the building. It is a test, not a treatment, and it should be specified explicitly for glass at height.
Nickel sulphide inclusion: A microscopic impurity that can occur in float glass and expand over time, causing a toughened pane to shatter without any external impact. The risk is small per pane but real across a large facade, which is what heat soak testing addresses.
PVB interlayer also polyvinyl butyral: The standard plastic interlayer used to bond laminated glass. It retains fragments on breakage, blocks most ultraviolet light, and in its acoustic formulation damps vibration to improve sound reduction.
Ionoplast interlayer also SentryGlas: A stiffer, stronger alternative to PVB used where the laminate has to carry structural load or survive severe exposure. It is the normal choice for structural balustrades, marine glazing and hurricane-rated assemblies.
Structural glazing: A facade system in which the glass is bonded to its supporting frame with structural silicone rather than clamped by a pressure plate, leaving an uninterrupted external glass face. Because the silicone bond carries wind load, the sealant type and bite dimension are engineered.
Spandrel glass: The opaque panel concealing the floor slab, ceiling void and services between the vision areas of a glazed elevation. Usually toughened glass with a ceramic frit or opacifying backing, colour-matched so the facade reads as continuous from outside.
Thermal stress: Stress created across a pane when one part is heated more than another, typically by partial shading from a reveal, a fin or an internal blind. It can crack annealed glass, which is why heat strengthened or toughened glass is used where shading patterns fall across vision areas.
Edge deletion: The removal of a coating from the perimeter of a coated pane before it is sealed into an insulated unit, so the edge seal bonds to bare glass. Without it the seal adheres to the coating rather than the glass and the unit can fail.