Glass

Glass is made mainly of silica, which usually comes from sand (other raw materials, like flint, were also used).

Perfectly flat or transparent glass couldn’t be made using traditional manufacturing techniques.

Traditional glass:

• often contains air bubbles
• may be tinged with colour caused by impurities in the sand used to make it – e.g. metal oxides
• has variations in the surface

Sash and case windows were popular across Britain by the late 1600s, which led to greater demand for window glass. Since then, different types of plain glass have developed in line with the fashions and technology of the time.

Glass can be made – or altered after being made – to be decorative glass. It can be etched, coloured or rolled to create richly ornamental surfaces. ‘Stained glass’ is a term often used to describe all kinds of coloured glass. But only glass that has been stained or painted after its initial manufacture is true stained glass.

Glass was usually held in place in windows by either:

• strips of lead known as ‘cames’
• a series of small brass nails called ‘sprigs’ and linseed oil putty

Cylinder glass

Made by blowing a cylinder of molten glass, which was then cut at the ends and along its side and flattened out in a furnace. Also known as ‘broad, ‘sheet’ or ‘muff’ glass, cylinder glass was popular until the mid-1700s.

Cylinder glass has a:

• slightly rippled surface in generally the same direction
• pattern of long air bubbles that lie in straight, parallel lines in the glass

Crown glass

Made by blowing and then spinning molten glass into a large thin disc known as a ‘table’. This was then cut into smaller panes. Crown glass became more and more popular from the mid-1700s.

Crown glass:

• is slightly curved
• often has distinctive semi-circular lines (the ‘ream’)
• is thinner, brighter and shinier than cylinder glass
• has a pattern of concentric circles – often giving window panes a ‘bellied’ look
• may have small thick circle of glass or 'bullseye' from the centre of the 'table'

Plate glass

To make early plate glass, thick cylinder glass or cast glass was ground down to make it flat and then the surface polished until smooth. Plate glass was very expensive as a result of this labour-intensive process. So it was used, from the late 1600s onwards, mainly for mirrors and to glaze high status buildings.

Patent plate glass

James Chance invented the process for making ‘patent plate’ glass in 1839. This process made it possible to grind and polish thinner sheets of glass than the traditional plate glass technique allowed. Manufacturers could then make more finished glass from the same quantity of raw materials.

Drawn flat sheet

Glass production became more mechanised from the early 1900s. Various methods were invented to allow a continuous sheet of glass to be drawn out of a furnace of molten glass. Each sheet was passed through a series of rollers and cooled, and then mechanically ground and polished.

Modern float glass

First made in 1959, this is the standard type of glass used for glazing today. Molten glass is floated on a bed of molten tin to produce perfectly flat glass. Until quite recently, it was produced to different grades, and 'horticultural glass' had slight imperfections. It was used for greenhouses and industrial buildings.

Coloured glass

Glass can be coloured as part of the manufacturing process by adding chemicals to the raw materials. For example, adding cobalt produces blue glass, while adding manganese creates purple glass. Great skill is needed to ensure consistency and depth of colour.

Stained glass

Only glass that has been stained or painted after its initial manufacture is true stained glass. For a more robust finish, the glass was usually fired in a kiln after the coloured material had been applied to its surface.

Materials used to create stained glass include:

• enamels – used from the 6th century to colour clear glass
• silver stain (silver sulphides and nitrates) – used from the 1300s to give yellow and orange colours
• metal oxides – a range was used from the 1500s to offer a much broader colour palette

Etched glass

Etching techniques were developed in the later 1800s to give glass a highly detailed decorative finish. A stencil is used to protect those areas of the glass not to be etched, and the desired technique is then applied. The result is a frosted surface that diffuses natural light and gives a degree of privacy.

Etching is done by either:

• blasting an abrasive material against the glass
• using hydrofluoric acid on the glass

Both methods are still used today.

Engraved glass

Using rotary grinding wheels, engravers can create various profiles – square, round and mitred – to be cut into the glass. Engraved glass was particularly popular in the late 1800s, and was often used alongside etched glass.

Rolled glass

Running hot glass through shaped rollers enabled a variety of regular patterns to be formed on the surface. Glass featuring geometric forms or plant themes could be easily created and mass produced in this way.

Use of scaffolding

If scaffolding is to be used for general repairs, ensure that:

• no part of the scaffold is likely to damage the glazing
• protection is put in place to stop debris from falling and hitting the glass
• window panes aren't broken to let a scaffold tube enter the building interior – a sash should be instead slid open

Window repairs and replacements

Glass may be damaged during window repairs, particularly if the frame has to be taken apart. Where a window must be replaced, it’s usually best to retain and reuse the glass from the original if possible.

Inappropriate replacement glass

Using modern float glass to replace broken or missing panes of traditional glass can create a patchwork effect in windows. This disrupts the view out of the window and the look of the building as a whole. Replacement glass should match the original glass as closely as possible, which may call for the reuse of salvaged material, or modern versions of crown glass.

Window alterations

Owners may sometimes wish to reinstate an earlier glazing pattern – e.g. where sashes glazed with a single pane of glass were once divided into six panes by glazing bars. But it’s important to consider the historic value of any existing plate glass, as this is now irreplaceable.

Ultraviolet filters

Modern UV filters applied to glass may degrade over time and require removal, but the adhesive used may prove difficult to remove. It may be better to use a UV blind with historic glass.

Stone cleaning

As well as damaging the masonry itself, chemical cleaning products can etch glass.

Added risks for decorative glass

Decorative glass is most at risk from water – whether rain, condensation or relative humidity – which can speed up decay. Combined with deposits of dirt, it can also encourage biological growth, which further attacks the decorative surfaces.

Even dirt left on the surface will eventually dull the glass. Air pollution can cause deterioration by forming weak acids on the glass surface. Over a long time, sunlight can also cause changes in the glass chemistry and colour through direct exposure to UV light.

Keeping windows in good order usually only involves:

• cleaning the glass
• replacing missing or dried glazing putty
• regular painting of the windows and putty – the paint should overlap slightly from the putty onto the glass

Glass is best cleaned with water and a soft cloth. Avoid using abrasive cleaning agents as these may damage the surface of the glass. Traditionally damp newspaper and vinegar was used and is still effective.

Chemical and caustic paint stripping of window frames can cause irreversible damage to old glass. Be careful when removing paint as scraper and abrasives will damage and scratch glass.

Original glazing should be kept wherever possible, as most types of traditional plain glass are irreplaceable except from salvaged sources. Any repair work should guard against breakages or loss of glass.

Small cracks in the corners of panes can be left in situ unless they pose a danger by letting in air or water. A specialist can use epoxy techniques to repair larger cracks in very valuable glass.

When planning glass repairs, you must also consider the:

• timber frame
• brass ‘sprigs’ used to hold the glass in place
• stone walls around the window

The condition of these surrounding elements may add to, or directly cause, problems.

Dismantling windows

Many repairs can be carried out with the glass in place. However, sometimes the glass needs to be removed.

To ease the removal of glazing and reduce the risk of damage, putty should first be softened. You can use an alkali-based paint stripper or an infrared lamp designed for this purpose.

All sprigs and any other fixings should also be removed with care before taking the pane out of its frame.

It may be best not to remove glass panes where the bedding putty is difficult to extract.

Find out about the repair of sash and case windows.

Replacement of traditional glass

Every care should be taken to keep original panes of traditional glass. If damaged traditional panes must be replaced, you should source as close a match as possible.

Several specialist companies make replacement glass using traditional techniques. This is likely to be a better match than float glass.

Spotting issues early and having any remedial work done quickly can reduce damage to decorative glass and keep down maintenance costs.

Most coloured glass or stained glass windows were put together using a framework of lead ‘cames’, which were soldered together. The many joints mean that it’s important to check windows often for loose or disturbed sections.

Loose or rattling glass can be a sign of a more serious problem. You should have any such issues fixed as soon as possible – particularly where the glass is part of a door or an opening sash window. Avoid slamming doors shut.

Stained glass windows can also become bowed (bent) due to the distortion and fracture of the lead cames and iron bars that should hold the window flat.

A tight or swollen door or window should be fixed by a skilled joiner before damage to the glass occurs.

How to clean decorative glass

Extreme care should be taken when cleaning painted glass, and harsh detergents must not be used.

You can use water and a soft cotton cloth to clean:

• coloured glass
• rolled glass

Stained glass should only be cleaned by an experienced conservator. This is because the stability of the applied staining could be at risk. Generally only the external side should be washed.

Make a photographic record

It’s a good idea to take detailed photographs of decorative glass for reference in case serious damage occurs in future. This will allow an accurate reconstruction.

Where glass has been broken, retaining the original material is important, especially if it's decorated. A specialist can often repair broken glass using epoxy-based materials developed for this purpose.

Replacement glass is a last resort, not least because it’s unlikely that you’ll be able to match the original colours or patterns perfectly. Take care not to mistake original rolled glass for modern replacement glass. Where you need to replace a broken pane of rolled glass, choose the pattern closest to the original, even if the replacement pane is not historic glass.

The most common causes of breakage are:

• direct impact to the glass
• stress caused by structural movement or vibration

Decorative window issues

You should also check the window assembly regularly for signs of any problems. For example:

• thermal movement can cause a window assembly to fail over time
• lead ‘cames’ will corrode slowly – white powder on the surface is a sign that this process has begun
• soldered joints may break apart
• the surface sealant between the glass and cames can be worn away or washed out
• the weight of glass in the frame can put too much stress on its supports, causing the window to buckle

Seek advice immediately if you spot any such defects. Early attention may help to keep repair costs down and prevent more serious problems. In extreme cases, a specialist may need to remove the entire window, take it apart, fit new cames and put the window back together again.

Secondary glazing can be a good way to protect historic glass and improve energy efficiency. Care must be taken, however, to ensure that the system has appropriate ventilation.

Without this, a microclimate can be created between the secondary glazing and the historic glass. Moisture levels and temperature ranges in this environment can vary more widely than usual due to a greenhouse effect. This can speed up decay.

Specialist advice should be sought to help overcome such difficulties.

You should check whether you must first apply for planning permission or any other type of consent before making any changes to sash and case windows or decorative glass windows. Contact your planning authority to find out more.

Listed building consent

Plans to carry out any work that may change the appearance of windows in listed buildings will require listed building consent.

Works requiring listed building consent include plans to change:

• window frame material or operating method
• timber astragal (glazing bar) profile or the arrangement of panes
• glazing panes to double glazed units
• paint colour or finish

Plans to install secondary glazing or special ventilation arrangements are also likely to require listed building consent.

Other consents

Your home may be unlisted but within a conservation area where planning controls over window alterations are in effect. Your planning authority may also have planning policies controlling alterations to windows in other locations such as flatted properties or properties along important routes. These may include acceptable paint colours for external joinery.

Find out more about listed building consent and conservation area consent.

Advances in glass-making technology have had a profound effect on building design over the centuries.

It was only in the 1950s that modern float glass was first made. Before this time, traditional manufacturing techniques couldn’t produce sheets of glass that were perfectly flat, transparent or of any great size.

Glass made before the 1950s often had air bubbles and surface imperfections. Very early glass was also tinged with colour caused by impurities in the sand used to make it.

Sash and case windows became popular across Scotland from the end of the 17th century. Early versions had to be subdivided by astragals into as many as 16 sections since only small panes of glass were available at the time.

But from the later 1800s it was possible to produce far larger sheets of glass, known as plate glass, at an affordable price. Many Victorian windows only have two subdivisions as a result. Fashion conscious homeowners often revamped their outmoded Georgian windows. After the timber astragals had been cut off, the sashes would be re-glazed with new plate glass.

The availability of affordable plate glass also had a significant impact on shopfront design. Larger windows let more natural daylight into shops, which was a big change in the days before electric light. It also became fashionable to create far more elaborate window displays, as shoppers could now see these clearly.

Get in touch if you have any questions. We will be happy to help.

• Technical Research Team
• 0131 668 8951
• technicalresearch@hes.scot

Professional institutions

Heritage Windows Group
Glass and Glazing Federation

Publications

Inform guides

An introduction to conservation and maintenance of traditional buildings for complete beginners.

Maintaining Traditional Plain Glass and Glazing
Domestic Decorative Glass

Short guides

Information on building conservation topics for home owners, trades people and building professionals.

Sash and Case Windows

Other resources

Building Scotland – Celebrating Scotland’s Traditional Building Materials
The Building Conservation Directory