Glass Types: What's on Your Windows, and Why It Matters

Annealed, tempered, low-E, tinted, laminated, frosted, and the half-dozen other glass formats that show up in residential and commercial buildings. What each one actually is, how to identify it without removing it, and the cleaning chemistry and tools that will damage it. The substrate reference.

I get a particular kind of email three or four times a year. It is always from a homeowner who has just discovered, painfully, that the cleaning method they used on one window — a method that worked perfectly on every other window in the house — has produced a permanent visible defect on this specific pane. They want to know whether the damage is reversible. The answer is almost always no. They want to know what they did wrong. The answer is almost always that they treated the window as if it were the same glass as every other window in their house, and it was not.

The single most consequential piece of knowledge a homeowner or working cleaner can have, before applying any cleaning chemistry or any tool with mechanical effect, is what kind of glass is in front of them. Glass formats differ in chemistry, in coating, in mechanical properties, and in the cleaning protocols that are safe to use on each. The same razor blade that is safe and effective on annealed glass will produce permanent fine scratches on tempered glass.¹ The same vinegar that is fine on uncoated glass will, with prolonged contact, damage some forms of low-E coating. The same glass-restoration polish that works on a window pane will destroy a window film.

This piece is the substrate reference — the encyclopedia entry on glass types. I am going to walk through the eight glass formats that account for the overwhelming majority of windows you will encounter, with the identification techniques I use in the field, the cleaning protocols that are appropriate for each, and the failure modes that result from treating one as if it were another.

A short note on what glass is

All architectural window glass is, at its core, the same material: soda-lime-silica glass. The bulk composition is approximately 73% silicon dioxide (SiO₂), 14% sodium oxide (Na₂O), 9% calcium oxide (CaO), and the remainder a mix of magnesium oxide, alumina, and trace iron compounds.² This is the chemistry that has been used for flat glass since the late nineteenth century. The primary mechanical and optical differences between glass formats come not from changes in this core composition but from what is done to the glass after it is made — the heat treatment, the coatings applied to its surface, the way it is laminated to other materials, and the films and tints that are added.

This is important, because it tells you that the cleaning chemistry that interacts with the bulk glass is largely the same across formats. The differences in how to clean different glass formats come from the surface treatments and the mechanical state of the glass, not from the underlying chemistry of soda-lime-silica.

With that established, here are the formats.

1. Annealed (float) glass

Annealed glass is the default. It is the float glass that has been cooled slowly from its forming temperature, leaving it in a relaxed mechanical state with no significant residual stresses. It is the glass that sits in the windows of most American homes built before 1990, in most older commercial buildings, and in the majority of residential and commercial windows that are not subject to specific building-code requirements for safety glazing.

Identification: Annealed glass has no manufacturer's etch mark in the corner. It produces a clean, flat reflection of point light sources without distortion. When broken, it produces long, sharp shards. Most importantly: annealed glass can be safely scraped with a sharp razor blade, and the cleanliness of the cut is the simplest in-the-field identification.

Cleaning protocol: Annealed glass is the most forgiving substrate. It tolerates the full range of cleaning chemistry — mild detergents, acids (vinegar, sulfamic, dilute hydrochloric in CLR), alkaline cleaners, solvents — with predictable results. It accepts mechanical action including razor scraping, abrasive pads (with care), and polishing compounds. The mistakes that ruin other glass formats are mostly forgiven on annealed glass.

Failure modes: Annealed glass is the most prone to etching under prolonged exposure to alkaline minerals (concrete leachate, hard-water deposits left for years), because the calcium-silicate hydrolytic alteration described in the hard-water articles proceeds more readily on the relaxed silica matrix of annealed glass than it does on tempered. The other failure mode is mechanical: annealed glass is the easiest format to break and produces the most dangerous shards when it does.

2. Tempered glass

Tempered glass is annealed glass that has been heated to roughly 620°C and then rapidly cooled with cold-air jets. The rapid cooling locks the surface of the glass into compression while leaving the interior in tension. The resulting mechanical state makes the glass roughly four times stronger than annealed and forces it, when it does break, to fail into many small relatively-blunt cubes rather than long sharp shards.³

Building codes in most U.S. jurisdictions require tempered glass in specific locations: any glazing within 18 inches of a floor, any glazing in or adjacent to doors, any glazing larger than nine square feet whose bottom edge is less than 18 inches from a walking surface, and most bathroom glass. If a window is in any of these locations, it is almost certainly tempered.

Identification: Tempered glass almost always carries a manufacturer's etched marking in the corner of the pane. The mark is small — typically about a half-inch wide — and includes the manufacturer's name or logo plus a tempering certification reference (often "ANSI Z97.1" or "16 CFR 1201"). Look for it. The mark is visible to the naked eye and is the definitive identification.

If there is no visible mark, a polarized-lens test can identify tempered glass through its characteristic stress pattern. Wear a pair of polarized sunglasses and tilt your head while looking through the glass at a bright surface; tempered glass shows a faint cross-hatched pattern called the quench mark or stress pattern that is not present on annealed glass.

Cleaning protocol: Tempered glass tolerates the same chemistry as annealed glass. The critical difference is mechanical: tempered glass should not be razor-scraped, except with great care and only after surface inspection.

The reason is fabricating debris. During the tempering process, small fragments of glass dust from cutting and edge-polishing operations sometimes get fused to the surface of the glass. These fragments are essentially small chips of glass embedded in the surface. They are invisible to the naked eye on an unlit pane but become visible at certain angles to a strong light source. When you drag a razor blade across tempered glass containing fabricating debris, the blade catches the embedded fragments and drags them across the glass. The result is a fine scratch with a small chip at one end — what the trade calls a comet — and the scratch is permanent.⁴

Failure modes: The dominant failure mode on tempered glass is comet scratching from razor work. The secondary failure mode is spontaneous breakage from nickel sulfide inclusions in the glass — a manufacturing defect that has nothing to do with cleaning, but that occasionally produces a tempered pane that fails into a thousand pieces with no apparent provocation. Cleaners are rarely blamed for this, but it is a useful thing to know about.

3. Heat-strengthened glass

Heat-strengthened glass is the middle ground between annealed and tempered. It is heated and cooled in a process similar to tempering, but at a slower cooling rate, producing a glass that is roughly twice as strong as annealed but does not fail into the small cubes of fully tempered glass. It is often used in commercial curtain walls and in cases where building code does not require tempered safety glazing but where the glass is exposed to thermal stress (large panes, west-facing exposures, glass adjacent to heat sources).

Identification: Heat-strengthened glass usually does not carry a tempering certification mark, because it is not certified safety glazing. It can be identified, like tempered glass, by the stress pattern visible through polarized lenses, though the pattern is less pronounced.

Cleaning protocol: Same as tempered. Treat as if it might contain fabricating debris; avoid razor work where possible.

Failure modes: Heat-strengthened glass shares the comet-scratch failure mode of tempered. It does not have the spontaneous-breakage problem to the same degree, because the residual stresses are lower.

4. Low-emissivity (low-E) coated glass

Low-E coated glass has a microscopically thin metallic coating applied to one surface of the glass. The coating is typically a stack of layers including silver, with antireflective and protective oxide layers above and below the silver. The coating's purpose is to reflect long-wavelength infrared radiation while transmitting visible light, which dramatically improves the thermal performance of the window without darkening it.

In modern double-pane windows manufactured after roughly 2010, the low-E coating is almost universally on the interior face of one of the two panes — that is, on the surface facing the sealed cavity, not on the surfaces that you clean from the inside or the outside of the building. This is by design; the coating is mechanically delicate and would not survive direct cleaning contact.

Identification: A handheld low-E detector — a small device with an LED that flashes a different color when held against coated versus uncoated glass — is the definitive identification tool, and they cost about thirty dollars. In the absence of one, the standard field test is to hold a flame (a lighter or match) close to one face of the glass and observe the reflections; a low-E coating will produce a noticeably different-colored reflection from the coated side compared to the uncoated side.

Cleaning protocol: For low-E coatings on the interior face of an IGU (i.e. inside the sealed cavity), no special protocol is required for cleaning the visible surfaces of the window. The coating is protected by the cavity. Standard cleaning chemistry is fine.

For the much rarer case of low-E coatings on the exposed face of a single-pane window, or on storm windows where the coating is on the outside surface: these coatings can be damaged by abrasive cleaning, by aggressive acids (the silver layer is chemically vulnerable to halides and to some acids), and by ammonia at high concentrations. Use mild detergent, deionized water, and soft cloths only. Do not razor-scrape exposed low-E coatings under any circumstances.

Failure modes: Low-E coating failure manifests as cloudy, milky, or iridescent residue on the coated surface that does not move and is not removable. Once damaged, the coating cannot be repaired. The pane has to be replaced.

5. Tinted glass

Tinted glass refers to glass that has been colored to reduce visible light transmission and/or solar heat gain. Tinted glass comes in two fundamentally different forms, and they require different cleaning protocols.

Mass-tinted (body-tinted) glass has the tint dissolved into the glass during manufacture. The color is uniform through the thickness of the glass. Common mass tints include bronze, gray, green, and blue. Mass-tinted glass is mechanically the same as annealed or tempered glass (depending on heat treatment) and is cleaned with the same protocols.

Surface-applied tint film is a polyester film with a tinted layer that is adhered to the interior face of the glass. The film is typically applied after the glass is installed. It is thin (1-7 mil), flexible, and mechanically delicate.

Identification: Look at the edge of the pane. Mass-tinted glass shows the same color through the full thickness of the edge. Tinted film shows a clear glass edge with a thin colored layer at the interior surface. Run a fingernail across the interior surface; tinted film has a slight texture and may have visible edges where it was cut to fit the pane. Mass-tinted glass is smooth and continuous.

Cleaning protocol: Surface-applied tint film is easy to ruin. The film cannot tolerate ammonia (the standard ingredient in many glass cleaners), abrasive pads, razor blades, or aggressive solvents. The protocol for tinted film is mild dish-soap solution applied with a soft cloth or non-abrasive sleeve, no scraping, no acid. The squeegee is fine if the rubber is soft and the technique is correct. The same is true for any window with a surface-applied film — security film, decorative film, UV-blocking film. Treat the film as if it were paint.

Mass-tinted glass requires no special protocol.

Failure modes: Tinted film failures include scratching, edge lifting, bubbling, color shift, and the "purpling" that older bronze films undergo as the dye degrades. Most of these are not cleaning problems and not the cleaner's fault. The ones that are: scratching from razor blades or abrasive pads, and the chemical haze that ammonia-based cleaners can produce on certain dye-based films.

6. Laminated glass

Laminated glass is two or more sheets of glass sandwiched around a polymer interlayer (typically polyvinyl butyral, PVB, or sometimes ethylene-vinyl acetate, EVA). The interlayer holds the glass together when it breaks and prevents the panel from coming apart. Laminated glass is used for safety glazing in some applications, for security glazing, for hurricane impact resistance, for skylights, and for sound attenuation.

Identification: Look at the edge of the pane. Laminated glass shows two distinct layers of glass with a thin interlayer between them. The interlayer is usually visible as a slightly different color or a fine line at the edge.

Cleaning protocol: The exposed faces of laminated glass are typically annealed or tempered glass; clean them according to whichever protocol applies to that face. The interlayer itself is not exposed and does not require attention.

The exception is at the edges. Some older laminated glass installations have exposed PVB at the edge of the pane, and the PVB can be damaged by aggressive solvents (acetone, MEK) or by water that gets behind the seal. In modern installations, the edges are sealed and the cleaner does not contact them. But if you are working on heritage commercial or older skylights, treat the edges carefully.

Failure modes: PVB delamination — visible as cloudy or yellowed patches at the edges of the pane — is the dominant failure mode and is rarely cleaning-related.

7. Frosted, etched, and acid-treated glass

Frosted glass has had its surface mechanically or chemically roughened to produce a translucent rather than transparent surface. Mechanical frosting is done with sandblasting; chemical frosting is done with hydrofluoric acid or fluoride salt etchants.

Identification: Frosted glass is visually obvious — it transmits light but does not produce a clear image through the pane. The frosting may be uniform across the pane or applied as a pattern.

Cleaning protocol: Frosted glass is the trickiest cleaning surface in residential work. The roughened surface holds onto soap residue, fingerprints, and water spots much more aggressively than smooth glass. Standard glass-cleaner spray-and-wipe will leave streaks every time. The protocol that works: dilute dish soap in warm water, applied with a soft cloth, scrubbed gently into the texture, then rinsed with clean water and dried with a microfiber. Do not use abrasive cleaners or pads — they will polish the frost out of the glass at high spots and produce visible bright spots.

Failure modes: Polish damage from abrasive cleaning is the main one. Also, water spotting is more visible on frosted glass than on smooth glass, because the spotted areas reflect differently from the unspotted areas, and the difference shows.

8. Wired glass

Wired glass is annealed glass with a wire mesh embedded in it during manufacture. It was historically used for fire resistance in commercial and institutional buildings; modern fire codes have largely moved on from wired glass to ceramic glass or other fire-rated assemblies, but plenty of wired glass is still in service in older buildings.

Identification: Visible wire mesh embedded in the glass. There is no other glass that looks like this.

Cleaning protocol: Standard annealed-glass protocol with one caveat: the wire mesh can rust if the glass is broken or chipped at the edge and water reaches the wire. The rust will bleed into surrounding glass and produce permanent staining that no cleaning will remove. Inspect the edges of any wired glass before applying water; if there is exposed wire or active rust, dry-clean only.

Failure modes: Rust bleeding is the dominant failure mode. The glass itself is mechanically weaker than ordinary annealed glass and is also more prone to failing dangerously (the wire holds the broken glass together, but the wire itself is sharp).

A diagnostic protocol for the first cleaning

When I am called to a property I have not worked before — usually as a consulting visit rather than a route stop — I run a five-minute glass identification pass before any cleaning chemistry comes out of the truck. The pass is:

Inspect the corner of every accessible pane for a tempering mark. Note which panes are tempered.
Look at the edge of each pane to identify laminated and mass-tinted glass.
Run a fingernail across the interior face to identify surface films.
Pass a low-E detector across the panes if any have visible reflective character.
Note any frosted, etched, or wired glass.

The result is a mental map of the property's glazing — this window is tempered with low-E inside, this one is annealed with surface film, this one is laminated, these three are mass-tinted bronze — that determines which chemistry, which tools, and which mechanical actions are appropriate where.

I have spent eight years on this map, refined it, taught it to working cleaners, and watched it prevent tens of thousands of dollars in glass replacement. It is the protocol I would teach first if I were starting a new cleaner today, before any chemistry, before any technique. Identify the substrate. Then clean.

Glass is not a single material. The pane in your bathroom is not the same pane as the one in your living room is not the same pane as the one in your front door. The cleaner who treats them as if they were is the cleaner who eventually causes the failure that ends up in my inbox.

Glossary terms in this piece

Annealed glass — float glass cooled slowly into a relaxed mechanical state; the default residential substrate
Comet scratch — the long fine scratch with a small chip at one end produced by razor work on tempered glass
Fabricating debris — small glass fragments fused to tempered-glass surfaces during manufacture; the mechanism behind comet scratching
Heat-strengthened glass — partially heat-treated glass; the middle ground between annealed and tempered
IGU (insulated glass unit) — the assembled double- or triple-pane sealed glazing unit
Laminated glass — two or more glass sheets bonded around a polymer interlayer
Low-E coating — microscopically thin metallic coating that reflects infrared while transmitting visible light
Mass-tinted glass — glass colored throughout its thickness during manufacture
Quench mark — the cross-hatched stress pattern visible through polarized lenses on heat-treated glass
Tempered glass — glass mechanically strengthened by rapid cooling; safety-glazing standard
Window film — surface-applied polyester film for tinting, security, or other surface treatment
Wired glass — annealed glass with embedded wire mesh; older fire-resistance assembly

Footnotes

Sources

International Window Cleaning Association and Glass Association of North America, Joint Position Statement on Fabricating Debris on Tempered Glass, 2022 update.
ASTM C1036, Standard Specification for Flat Glass.
ASTM C1048, Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass.
ASTM C1172, Standard Specification for Laminated Architectural Flat Glass.
Cardinal Glass Industries, Care and Cleaning Recommendations for LoĒ Coated Glass, 2024.
3M Window Films, Care and Cleaning of Tinted Window Films, technical bulletin, 2024.
16 CFR 1201, Safety Standard for Architectural Glazing Materials.

About the author

Easton Giordano is the contributing science editor at Window Washing Guide. He holds a PhD in materials chemistry from the University of Washington and spent eight years as a senior research chemist at a major architectural glass coating manufacturer in the Pacific Northwest before going independent in 2021. He now consults on glazing failure analysis and writes about the chemistry of glass for trade and consumer publications. He is, by his own admission, insufferable about vinegar.

All articles by Easton → · Editorial standards →

The mechanism by which fabricating debris produces comet scratches on tempered glass has been the subject of trade-association investigation since the early 1990s. The IWCA's joint position statement with the Glass Association of North America (GANA), updated most recently in 2022, lays out the mechanism in detail and recommends a specific inspection-and-disclosure protocol before razor work on any tempered substrate. The position statement is required reading for any cleaner considering razor work on potentially tempered glass. ↩
The exact composition of soda-lime-silica float glass varies by manufacturer and within tolerance ranges; the figures I cite are typical for North American architectural float glass. The composition has remained remarkably stable since the introduction of the Pilkington float process in 1959. Where compositions do vary meaningfully is in trace iron content, which determines the slight green tint of standard float glass. "Low-iron" or "ultra-clear" glass formulations reduce the iron content and produce a more visually neutral pane, often used in storefronts and high-end residential where color accuracy in the glass is desired. ↩
The strength factor of tempered glass relative to annealed is conventionally cited as 4x in trade literature, though the actual figure depends on the loading mode (tension, compression, impact, thermal shock). For thermal shock specifically, tempered glass tolerates roughly 10x the temperature differential of annealed before failure. For impact resistance, the factor is closer to 4-5x. For prolonged tensile loading, the factor is intermediate. The "4x stronger" shorthand is approximate; the technically correct framing is that tempered glass tolerates greater mechanical and thermal stress than annealed across all common loading modes. ↩
The fabricating-debris-scratch question is one of the more legally fraught issues in commercial window cleaning. A scratch on a tempered pane will be visible for the life of the pane, and the cost of replacement on a large commercial window is non-trivial. Most working cleaners and most commercial contracts now include a fabricating-debris waiver or disclosure protocol that allocates responsibility for the scratch risk before the work begins. The cleaner who skips this step is exposed to liability for damage that is, mechanically, not their fault. ↩

Glass types: what's actually on your windows and why it changes how you clean them

A short note on what glass is

1. Annealed (float) glass

2. Tempered glass

3. Heat-strengthened glass

4. Low-emissivity (low-E) coated glass

5. Tinted glass

6. Laminated glass

7. Frosted, etched, and acid-treated glass

8. Wired glass

A diagnostic protocol for the first cleaning

Glossary terms in this piece

Footnotes

Sources

About the author

Footnotes

Easton Giordano