Heritage Glass: Matching Original Cylinder and Drawn Glass

Before you order a replacement pane, go outside and look at the reflections in the rest of the window. It takes half a minute, it costs nothing, and it decides whether the new pane disappears into the elevation or announces itself from the pavement for the next fifty years. A cracked pane in a sash is rarely urgent, so there is time to do this in the right order.

Start with the reflections

Stand three or four metres back and find something with a long straight edge reflected in the glass: a lamp post, a downpipe, the ridge of the roof opposite. Move your head slowly from side to side.

In float glass the reflected line stays straight. It is a mirror. In hand-made glass the line bows, pinches and swims as it crosses the pane, and it does it differently in every pane, because every pane came off a different part of a different sheet. That movement is why a Georgian terrace glitters at dusk and a re-glazed one does not.

After dark you get the same information from inside, with a torch or a candle held a foot from the glass. Two reflections appear, one off each surface. In old glass they wobble independently of one another.

What the surface tells you close up

Once you know it is not float, the detail says which process made it.

• Concentric, curving ripples following the arc of a large circle, often stronger towards one edge of the pane: crown glass. Neighbouring panes cut from the same disc have arcs that run in sympathy.
• Ream running one way only, like very shallow corduroy, with a faint orange-peel feel to the surface and round or teardrop bubbles scattered at random: cylinder glass.
• Fine, regular parallel striations, much flatter overall, with small bubbles stretched out along the same direction as the striations: drawn sheet.
• Nothing at all, plus a slight green cast when you look at a cut edge: float.

Thickness is the other tell. Original glass in a Georgian or early Victorian sash usually measures around 2mm, occasionally under, and is not the same thickness across the pane. Late Victorian and Edwardian glass tends towards 3mm. A pane that measures a dead 4mm on a calliper all the way round is a replacement. Glass merchants settle the argument with a shortwave ultraviolet lamp: the face of float glass that sat on the molten tin bath fluoresces a milky white under it, and no hand-made glass does that.

Crown, cylinder, drawn: three processes, three periods

Crown glass was spun. A blown globe was transferred to a punty rod and reheated, then spun out by centrifugal force into a flat disc as much as five or six feet across. The outer part of the disc gave thin, brilliant, fire-polished glass. The thick lump at the centre where the rod had been, the bullion or bullseye, was the waste, sold cheap and glazed into back stairs and outbuildings. That is exactly the opposite of what the reproduction trade now sells as an "olde worlde" front door.

Cylinder glass was blown as a long bottle, the ends cut off, the tube slit down one side and then reheated in a flattening kiln until it unrolled into a sheet. Chance Brothers of Smethwick made the first British cylinder blown sheet in 1832, using imported French and Belgian glassblowers, and it was Chance who glazed the Crystal Palace in 1851. Cylinder gave much larger sheets than crown, which is why sash windows change shape in the middle of the nineteenth century.

Drawn sheet was the first machine process to matter domestically. A continuous ribbon was drawn vertically out of the molten glass and held at the edges by rollers, which is why the striations all run one way. Float came next: Alastair Pilkington began experimenting at St Helens in December 1952, and with his colleague Kenneth Bickerstaff spent the rest of the decade making it work. The first sheet produced entirely by the float process came off in July 1958, and the process was announced to the glass-making world in January 1959. Ordinary window glass changed over during the following decade, and every pane made since has been flat.

Why the old glass is so thin

Two taxes shaped what you are looking at.

Glass carried an excise duty from 1746, and after 1811 the duty was assessed on finished glass. It was charged by weight. A tax on weight is a tax on thickness, so British window glass was made as thin as the process allowed, and blown thinner still by makers being taxed on every ounce. The duty ran at rates that at times exceeded the value of the glass itself. It was repealed in 1845. Window tax, a separate thing entirely, was assessed on the number of windows and ran from 1696 until July 1851.

Within a decade of those repeals glass got cheaper, sheets got bigger, and the sash window changed with them. The six-over-six with slender glazing bars gives way to the two-over-two, and then to the single large pane with heavy meeting rails and horns to carry the load. The longer version of that story is in our history of the sash window, and the way pane divisions date a house is covered in glazing bar patterns.

What is still made today

Nobody has spun crown glass commercially for a very long time. What is available is mouth-blown cylinder glass and machine-drawn Fourcault sheet, both still made on the Continent, plus salvage.

Mouth-blown cylinder. Made exactly as it was in the 1830s: blown, slit, flattened. Glashütte Lamberts in Bavaria produce it under the Restauro name, and Tatraglass import a mouth-blown cylinder sheet averaging around 3mm into the UK market. It is the only thing that genuinely matches a pre-1850 sash, and it is expensive, irregular in thickness and full of genuine seeds and stones, because that is what it is.

Machine-drawn Fourcault sheet. Drawn by the same process used from the early twentieth century, and sold at average thicknesses of roughly 3mm and 4.5mm. It has real directional ream and elongated seeds, it is far more consistent to cut and glaze than mouth-blown, and it costs a fraction as much. On a late Victorian or Edwardian house it is not a compromise at all. It is the correct glass.

Histoglass is the usual UK route when the period glass has to be built into something else. They supply cylinder, hand-drawn, machine-drawn and float, along with a range of thin double glazed units, the HD10 to HD13 series, which matters if you are trying to get heritage glass and a thermal upgrade into the same sash.

Salvage. Reclaimed old glass exists, mostly cylinder and drawn, and it is the cheapest way to match if you only need a pane or two. Expect variable quality, and expect to buy more area than you need.

What it costs, and where the money goes

Those are orders of magnitude, not quotes. Delivered prices swing hard on sheet size, batch availability, how much you buy and the access on the day. The table is for the shape of the decision, not a number to hold anyone to.

Cutting waste is a real cost on hand-made glass, and the part homeowners never budget for. Thickness varies across the sheet, the score wanders, and a cutter used to float will break more than one pane learning the feel of it. That is why a heritage reglaze is priced on area bought rather than area fitted.

Where it gets difficult on site

Thickness and the rebate. Old sash rebates are shallow and were cut for thin glass. Going from 2mm original to 4.5mm drawn sheet eats 2.5mm of rebate depth, which sounds trivial until you are trying to run a putty fillet that covers the sprigs and still sits below the sight line of the moulding. On fine Georgian bars this single constraint decides everything else.

Cutting. Hand-made glass wants to be scored on its flattest face, in one confident pass, with a fresh wheel. A second pass over a wandering score is how you get a run-out.

Safety glazing. Requirement K4 of the Building Regulations deals with impact with glazing in what the guidance calls critical locations: broadly, glazing below 800mm from floor level, glazing within a door, and glazing in the side panel alongside one. A ground-floor sash with a low cill can fall inside that. Hand-made glass cannot sensibly be toughened, because toughening an uneven sheet distorts it further, so the answer in a critical location is a laminate, the heritage sheet bonded to a clear backing pane. It reads correctly from outside and satisfies the requirement. Raise it early, because it changes the thickness again.

Size. Mouth-blown sheets are small. A large two-over-two Victorian pane may exceed what a single blown cylinder can give you, and at that point you are into drawn sheet whether you wanted it or not.

Putting hand-made glass into a sealed unit

This is the question we get asked most often, and the honest answer is mixed.

It can be done. Tatraglass and Histoglass both build heritage sheets into double glazed units, and the outer face is what the street sees. Two things are worth weighing before you order.

The first is that a slim sealed unit with a heritage outer leaf still shows two reflections offset by the cavity, and that offset is the visual tell that gives modern glazing away on a period elevation. A narrow cavity reduces it. It does not remove it.

The second is that the moment you put a hand-made outer pane into a unit, you are asking a sheet of uneven thickness to sit flat against a spacer and hold an edge seal. It can be done properly. It is not the easiest thing to do properly. The wider trade-offs in fitting sealed units to original sashes, including rebate depths, sight lines and the extra weight, are set out in our piece on slimline double glazing for sash windows, and our double glazing work on period sashes treats the glass specification and the rebalancing as one job.

If the house is listed, the existing glass is protected fabric in its own right. Where crown or cylinder glass survives, expect a conservation officer to require you to keep it, and expect them to be right. Our guide to listed building consent for windows covers what does and does not need an application.

If the pane is already broken

1. Look at the crack before you touch it. A clean radial crack in an otherwise sound pane, with no piece missing, can often be left alone for years. Old glass that is still weathertight is doing its job.
2. Check whether the pane is original. Run the reflection test on the panes either side. If half the sash is already float, matching one pane to nothing is a waste of money.
3. Count how many you would need to match. One odd pane among eleven old ones looks wrong. Twelve new drawn-sheet panes in a rebuilt sash look right. There is a threshold, and it is usually cheaper to cross it than to sit just below it.
4. Save the broken glass. Sound offcuts from a cracked pane can be re-cut for the small panes higher up a Georgian sash. Stack it vertically, on edge, interleaved with card, somewhere it will not get knocked.
5. Settle the putty before the glass arrives. New glass in tired putty fails at the putty, not the glass. Linseed oil putty needs a primed rebate and a proper skin before painting, and the reasons are in our article on glazing putty for sash windows; the sequence itself is in how to re-putty a sash window.

Where to start on your own elevation

Wait for a low sun, or go out after dark with a torch, and survey the front of the house pane by pane. Sketch the windows and mark which panes still move the light and which sit flat. Most London houses come out mixed, and the pattern usually tells a story: the front bedroom reglazed after a burst pipe, the ground floor done wholesale in the 1980s.

That sketch is your brief. Where original glass survives, the right answer is almost always to keep it and repair around it, which means sound putty, sound timber and a sash that runs properly rather than new glass. Where it has already gone, specifying machine-drawn heritage sheet at each reglaze will quietly put the elevation back together over a decade at very little extra cost.

If you would like someone to look at it with you, our sash window repair service covers every London borough, and we will tell you plainly when the glass in front of us is worth matching and when it is not.