Fenestration Knowledge Series · Article 02
Stand in front of an old window and move your head slowly from side to side. A roofline bends. A reflection shimmers. The view appears to move across the pane.
That waviness is one of the most recognisable features of historic window glass. It is also one of the most misunderstood.
Old glass usually looks wavy because of how it was made—not because it has flowed downwards over time.
Those ripples, bubbles and variations in thickness can be evidence of a hand-made or early industrial process. In the right building, they contribute to the character of the complete window just as much as a slender glazing bar, an original handle or a carefully formed steel profile.
This is Article 02 in the D&G Window Solutions Fenestration Knowledge Series. In Article 01, we traced the history of glass from ancient making to modern float glass. Now we look more closely at what old panes can tell us—and why that matters before historic glass is removed.
First, the myth: old glass is not slowly melting
A familiar story says that medieval and historic windows are thicker at the bottom because glass is a liquid that has flowed downwards over centuries.
It is a persuasive story, but it is not the reason old panes are uneven.
At normal building temperatures, glass does not continue to flow in any meaningful way. The Corning Museum of Glass notes that ancient glass objects far older than historic windows show no evidence of gravitational flow. The variations we see in old window glass were already present when the sheet was made.
Where a pane is thicker at one edge, a glazier may also have installed the heavier edge at the bottom for stability. That practical choice can make the myth appear convincing, but it does not prove that the glass moved after installation.
The real answer lies in manufacturing.
Crown glass: a window cut from a spinning disc
Crown glass was made by transferring hot glass to a pontil rod and spinning it rapidly. Centrifugal force opened the glass into a broad circular disc. After cooling, panes were cut from that disc.
The process left recognisable characteristics:
- curved or concentric ripples;
- small bubbles or “seeds”;
- variations in thickness;
- a thicker centre where the glass was attached to the pontil.
That thick centre produced the familiar bull’s-eye. Genuine bull’s-eye pieces were a consequence of the process rather than a decorative idea invented for later windows.
Historic England records that crown glass was widely used from the fourteenth to the nineteenth century. Original examples are now rare, and the distinctive movement of their light and reflections cannot be matched exactly by ordinary modern float glass.
Cylinder glass: flattening a blown tube
Cylinder glass was produced differently. A glassmaker blew and elongated a large hollow cylinder, removed its ends, cut it lengthways and reheated it until it opened into a sheet.
Flattening a curved, hand-blown cylinder was never perfectly uniform. The finished panes could show long, gentle waves, parallel lines, bubbles and slight changes in thickness.
These distortions often look different from the circular movement associated with crown glass. They can stretch in one direction, causing straight objects outside the window to appear to bend as the viewer moves.
Cylinder glass increasingly displaced crown glass during the nineteenth century. It allowed larger sheets, but it still carried visible evidence of the maker and the process.
Drawn sheet glass: industrial, but not perfectly flat
Mechanised drawn-sheet processes pulled a continuous ribbon of glass vertically from a furnace. They increased output and pane size, helping glazing become more accessible through the late nineteenth and early twentieth centuries.
The glass was more consistent than many hand-made sheets, but it was not optically perfect. The drawing process could create waves, draw lines and subtle distortion. Many older shopfronts, factories, houses and early steel windows contain glass from this industrial transition.
This is why “wavy glass” does not automatically mean medieval glass—or even hand-blown glass. Different processes produced different forms of distortion, and panes were frequently replaced, moved or reused.
Plate and float glass: the pursuit of a flat view
Plate glass was cast and then ground and polished to create flatter, clearer surfaces. It made large, high-quality panes possible, but the process required substantial labour and energy.
The decisive change came with float glass. Developed commercially by Pilkington in the 1950s, the process forms a continuous ribbon of glass over molten tin. The surfaces become remarkably flat and parallel without the extensive grinding previously required.
Modern float glass established the visual standard we now take for granted: clear, uniform and relatively free from optical distortion.
Place float glass beside an old hand-made pane and the difference can be immediate. One reflects a façade almost like a mirror; the other makes the reflection gently move.
What creates the waves, bubbles and changing reflections?
Historic glass can contain several clues to its manufacture:
- Surface ripples: created as hot glass was spun, stretched, drawn or flattened.
- Thickness variation: the sheet did not form with the tight tolerances of modern float production.
- Bubbles or seeds: small pockets of gas remained within the glass.
- Lines and striations: the movement of the glass during forming left visible directional patterns.
- Colour: iron and other ingredients could give a subtle green, blue-grey or amber cast.
- Changing reflections: uneven surfaces refract and reflect light at slightly different angles.
These are not all signs of damage. A manufacturing characteristic is different from a crack, an unstable pane, failed putty or a glazing detail that no longer keeps out water.
Can wavy glass date a window?
It can provide a clue, but it should never be treated as proof on its own.
A crown-like ripple, a cylinder line or drawn-sheet distortion may suggest a manufacturing family or period. However, buildings evolve. Glass breaks. Openings are altered. Old panes are salvaged and fitted elsewhere. A frame can be older or newer than the pane it holds.
A responsible assessment considers the evidence together:
- frame material and profile;
- glazing-bar proportions;
- putty and fixing details;
- hardware and opening method;
- glass thickness, colour and distortion;
- archival drawings, photographs or alteration records;
- signs of previous repair or reglazing.
The glass is part of the story, not the whole date stamp.
Why old glass matters in heritage steel windows
Historic steel windows are defined by proportion. Their slender frames and glazing bars divide a façade into a precise rhythm of light and shadow. The glass changes that rhythm again through reflection.
Replace one original wavy pane with perfectly uniform float glass and the difference may be noticeable. Replace every pane and the whole elevation can become flatter, brighter and more reflective—even if the steel frame is retained.
That does not make modern glass wrong. It means the visual consequence should be understood before a decision is made.
When D&G Window Solutions assesses heritage steel windows and other metal windows, we look at the complete opening: frame condition, corrosion, glazing bars, hardware, drainage, putty, glass, exposure and current use. The correct approach may include retention, careful removal and reinstatement, selective replacement or a more extensive reglazing strategy.
When should historic glass be retained?
Historic England advises that original crown and cylinder glass should be retained in place wherever practical and protected carefully during repair work. This matters because genuine early panes are finite: once broken or discarded, they cannot simply be ordered again.
Retention is particularly worth investigating when:
- the pane is sound and securely glazed;
- its waves, bubbles or colour contribute to the building’s character;
- it is part of a coherent group of historic panes;
- the window can be repaired without imposing unacceptable risk on the glass;
- safety and performance requirements can be satisfied through an appropriate design.
Removing old hardened putty can place thin historic glass at serious risk. Repair should therefore be planned by people who understand both the frame and the glazing—not treated as a routine strip-out.
When might replacement be necessary?
Retention is not an absolute rule. Replacement may be required where glass is cracked, loose, badly damaged or unsuitable for the risk and use of the opening. Doors, low-level glazing, areas exposed to human impact and other critical locations may require a modern safety-glazing solution.
The decision should account for:
- the significance and condition of the pane;
- the safety requirements of the current application;
- wind, weather and exposure;
- security, acoustic or thermal objectives;
- the strength and condition of the frame;
- whether a compatible restoration glass is appropriate;
- whether significant panes can be recorded, salvaged or reused safely.
For a new custom steel window or metal window, the specification may deliberately use modern high-performance glass. For a sensitive historic opening, the priorities may be different. The right answer comes from the building, not from a one-size-fits-all product list.
Repair the opening, not just the pane
A window works as a system. Glass cannot compensate for a corroded frame, failed perimeter seal, blocked drainage path, loose hinge or poorly formed putty line. In the same way, a beautifully restored steel frame can be visually compromised by an inappropriate glass choice.
Before work begins, ask four questions:
- What glass is present, and is it likely to be significant?
- Is the pane actually defective, or simply characteristic of its manufacture?
- What safety and performance duties must the opening meet now?
- Can the frame, glass and glazing method be repaired as one coordinated system?
That is the difference between replacing a component and making a sound fenestration decision.
The beauty is evidence
The view through old glass is imperfect because the process was imperfect. Yet those imperfections record heat, movement, gravity, breath, tools and skilled hands.
They soften reflections. They animate a façade. They connect a window to the technology and craft of its period.
Old glass does not look wavy because it is slowly melting. It looks wavy because its manufacture is still visible.
Before that evidence is removed from a heritage steel window, it deserves to be understood.
The D&G Fenestration Knowledge Series
This is Article 02 in our recurring series of practical, technically grounded guidance on glass, windows, doors, custom steel and aluminium systems, and heritage repair.
- Article 01: The History of Glass—How One Material Changed the Way We Build
- Article 02: Why Old Glass Looks Wavy—and Why It Matters
- Coming next: The History of Steel Windows
- Planned: Why Heritage Steel Windows Should Be Repaired Before They Are Replaced
- Planned: How Modern Safety Glass Actually Works
Working with original glazing, custom steel windows, modern aluminium systems or a heritage refurbishment project in Cape Town or the Western Cape?
Request a proper window and glazing assessment from D&G Window Solutions.