How to protect stained glass windows in religious buildings


Breaking glass with a crowbar

You are worried about vandals damaging valuable stained glass windows in your place of worship. What is the best way to protect stained glass windows in religious buildings? Should you use acrylic, plexiglass, Lexan, tempered, or laminated glass? Explore the complex details in this post.

Etched glass at Govinda's Restauran...
Etched glass at Govinda's Restaurant, Tucson

Extremely valuable & vulnerable stained glass should be protected with Lexan or acrylic, but glass (laminated or tempered) can be used for panels at less risk. Always consult experts for the best advice.


Table of contents

Beautiful stained glass windows have inspired congregations in churches and synagogues since the Middle ages – communicating the parables of the bible to illiterate peasants in glorious technicolor – transcendental symphonies of light worthy of heaven itself.

Unfortunately, the properties that give glass its translucency also mean that it is brittle and very easily broken. This has made glass a prime target for vandals – with stained glass windows being especially vulnerable because places of worship are generally not occupied at night and, regrettably may fall victim to ideological prejudice, racial hatred, or antisemitism.  
(As an aside, students of criminology and history may wish to ponder that broken windows are metaphors in criminology for urban decay and, in a more sinister context, symbols of unspeakable events like Krystallnacht in 1938).

Cathédrale de Chartres - Chevet - Vitrail de Charlemagne
Early 13th Century stained glass from Chartres Cathedral

Why protect stained glass?

Unfortunately, even a simple stained glass panel is expensive to repair partly because the construction method, with colored glass fitted into H-shaped came, means that most windows cannot be restored on-site – they have to be removed to be fixed.

"H"-shaped lead or zinc came.
Stained glass construction method using H-shaped metal channel

Typical restoration costs are in the $100 – $400 per square foot range + the costs to board up the opening and re-install – and a further problem is that it may not be possible to match antique glass exactly.

For these reasons, it is very important to protect valuable stained glass against damage by weather and vandalism.

Note that even Dalle de Verre – made from 1″ thick slabs of glass – can be damaged by a determined vandal.

St Francis de Sales windows
Even Dalle de Verre can be vandalized (before – left, after – right)

In addition to protection, it’s also important to catalog and appraise the value of your panels for insurance purposes. These estimates should be updated periodically (15-20 year intervals) because of the effects of inflation on stained glass replacement prices.

Example of stained glass condition report
Make sure to have your stained glass appraised by an expert

Study this video to learn about the conservation of stained glass in religious buildings.

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Summary of advice on stained glass protection

The choice of a protection system for stained glass windows is not a simple matter. The best solution for a humble south-side chapel will not be the same as that appropriate for a historic metropolitan cathedral.

This post explains that the most prudent choice can only be made by carefully analyzing the risks presented by each group of windows to potential damage. The proposed protection (expensive and unbreakable, or inexpensive and strong) should be chosen with these risks, together with the replacement value of the stained glass, in mind.

In some cases, different protective materials may be appropriate for different windows in the same building. For example, panels situated at a great height and out of reach of projectiles can be protected by tempered glass, whereas windows at ground level are best protected by unbreakable Lexan.

Study this post carefully for detailed advice.

Types of vandalism

It is vital for church custodians to understand that vandalism can take place inside as well as outside of your place of worship. There has been an epidemic of church vandalism over the past 5 years with the Roman Catholic Church being especially victimized.

More than 130 occurrences of vandalism have occurred in the US since May 2020.  Incidents include arson, statues beheaded, limbs cut, smashed, and painted, gravestones defaced with swastikas and anti-Catholic language, and other destruction and vandalism.

“Stop the press” – in breaking news today, a $2 Million gold tabernacle has been stolen from a Brooklyn Catholic Church.

No amount of exterior protective glazing will serve its purpose if criminal vandals are allowed access to the interior of your church, synagogue, or mosque.

Exterior damage

The most common exterior damage to church windows takes the form of breaking stained glass and other windows, destroying gravestones & statuary, and painted graffiti. This post concentrates on how to protect stained glass but custodians should be aware of the wider problem when devising measures to combat vandalism.

The causes of vandalism

In some cases, vandalism represents protests against perceived injustices. Sometimes there might be general anger against the Church, and in other cases, there is no reason at all, In many cases, the damage appears to be the result of opportunistic action by bored or alienated teenagers. Perpetrators may work alone or as part of a group.

Damage to stained glass

The most common damage to church windows is broken glass caused by impacts with missiles – most often rocks and similar hard objects. A second category of damage is graffiti generally caused by spray paint.

A well-aimed rock thrown at high speed will cause serious damage to unprotected stained glass. To understand the nature of the risk – consider Newton’s 2nd law of motion:

Isaac Newton's first law of motion
Force equals mass times acceleration

The implications of this famous law are that large massive rocks, accelerated by strong individuals at a close distance are much more damaging them small pebbles tossed at long range.

Large rock breaks laminated glass window
Don’t leave ammo handy for the opportunistic vandal!
Gravel
Small pebbles are less dangerous than large rocks

A lesson for church landscapers: many acts of vandalism are opportunistic – do not surround your church windows with ready-made ammunition in the form of large rocks or boulders!

Premeditated vandalism or attempted robbery

Equally damaging as rock-throwers are more determined and premeditated acts by criminals wielding baseball bats or crowbars. The motives may be robbery, revenge, or ideology.

Breaking glass with a crowbar

In the Sonoran Desert of Southern Arizona, Tucson landscapers minimize the risk of access vandalism by planting thorny cacti and succulents adjacent to exterior windows.

Window damage due to extreme weather

With the threat of extreme weather growing due to the effects of climate change, large parts of the USA are threatened by storm and hurricane-force winds. Coastal areas along the Gulf of Mexico, Florida, and the Atlantic Seaboard are vulnerable to extremely damaging hurricanes. In addition, a large section of the Southern States and the Mid-West is now subjected to violent tornadoes with 250 mph winds capable of leveling entire communities.

Hailstones can cause window damage
Golf-ball sized hailstones can break stained glass windows

Large hailstones and tornado debris traveling in excess of 200 mph can make short work of stained glass windows.

Framing for protective glazing

It is vitally important to make sure that your efforts to protect stained glass do not have the undesired side-effect of causing damage by generating excess heat or condensation.

Protective glazing must be installed with a substantial air gap and with ventilation holes at the top and bottom of the panels to ensure the circulation of air.

Ventilation for protective glazing
Simplified drawing showing ventilation system to prevent heat build-up

For major projects involving large windows with unusual shapes (like Medieval lancet or Quatro-foils), we recommend the patented Klear-Flo® protective glazing system created by Associated Crafts® & Willet Hauser®. These custom aluminum extrusions come with specially screened ventilation holes for efficient circulation of air.

New framing must match the old

The exterior appearance of many old church windows has been ruined by superimposing discordant new frames over the original Gothic designs – as in this egregious example:

Bad choice of frame
Exterior view of Neo-Gothic windows is seriously compromised by discordant protective glazing

The construction of custom framing is the most crucial first step in protecting historic stained glass. Make sure you choose a qualified and experienced contractor.

Having designed the correctly ventilated and coordinated framing, it’s now time to choose the glazing material. The pros and cons of the most popular choices are summarized below:

Pros and cons of protective materials

MaterialProsCons
1/4″ clear annealed glassLooks better than plastic – less deflection, easily cleaned, scratch-resistantHeavy, breaks easily into dangerous shards. All glass types are hard to drill
1/4″ clear tempered glassSame benefits as above but 4 X stronger than annealed. Optional low-E coating Heavy, shatters into numerous popcorn fragments
1/4″ Laminated glassSame benefits as above but hurricane-resistant. Vinyl interlayer holds panel together when glass is brokenHeavy, more expensive than annealed & tempered glass
1/4″ Acrylic “Optix-L” formerly LuciteImpact-resistant acrylic is 7 X stronger than tempered glass, Clear, and non-yellowing. Can be drilled for vent holesMore expensive than glass, Somewhat easily scratched & hard to clean
1/4″ Lexan XL10 or equivalentVirtually unbreakable – 200 X stronger than tempered glass. With UV surface protection guaranteed for 10 years. Can be drilled for vent holesMost expensive option. Yellows with exposure to sunlight. Easily scratched & hard to clean

Each of these materials has its particular benefits and there is no correct option for all situations.

The choice depends on the nature of the threat, the value of the stained glass, and the budget available. We distinguish these 3 main categories of risk.

  • High risk & high-value windows
  • Moderate risk, moderate budget
  • Low risk & low budget

High risk – high-value windows

The top-level of protection is justified if your stained glass is historic, precious, and very valuable. The risk is increased if such windows are located in a dangerous, high crime area, if you have a history of vandalism, or evidence of specific threats (from unhappy or vengeful congregants, for example). Further danger indications include local episodes of racial or religious hatred or antisemitism. Finally, top-level protection is essential if you live in an area prone to hurricane-force winds or tornadoes.

For this category of windows, we recommend polycarbonate with anti UV coating – known by the trade name Lexan XL10. This material is virtually unbreakable and is guaranteed against yellowing for 10 years.

The degree of yellowing will vary, being most marked in South-facing windows in sunny regions of the USA for example. Even under these extreme circumstances, the milky yellow tint, although visible from the outside, will be inconspicuous from the inside of the building for many years.

While we strongly recommend Lexan for the most vulnerable and most valuable windows, customers concerned about yellowing and wishing to economize slightly can feel reasonably certain to safeguard against breakage by using a strong type of acrylic called by the brand name Optix L (formerly Lucite L).

One strategy would be to use Optix L on lower risk panels and unbreakable Lexan XL 10 on heirloom and irreplaceable works of art. However, be aware that the exterior appearance will begin to vary as the different materials weather at different rates.

Moderate risk

Less expensive protection is justified if your windows are of less intrinsic value and are judged to be of less risk of damage. This important risk assessment must be carried out in the light of local knowledge but less vulnerable windows may be those in more affluent neighborhoods and those protected from intruders by fencing or security systems.

Options for protective material include Optix L acrylic, laminated or tempered glass. Note that glass has the benefit that it scratches less easily than acrylic and can also be cleaned more easily if deface by graffiti.

Low risk or low value windows

Low priority stained glass windows include simple, inexpensive panels that are protected from casual vandalism by locked courtyards or those that are high up and out of reach of large projectiles.

Windows that are not subjected to extremes of weather like hurricane-force winds or damaging hail are also under reduced risk of damage.

Here, the benefit of protective glass may be mainly to insulate the stained glass panels from extremes of weather and to protect them against gravel thrown up by careless lawnmowing or from water spray from a garden hose. Options for relatively inexpensive protective material include annealed or tempered glass.

In conclusion

There is no simple solution for all stained glass protection projects. Take local risk assessments into account and seek the advice of experts. Tucson or Southern Arizona customers can get free advice from Artistry in Glass.


Description of protective materials

Annealed glass

Annealed glass – the most affordable type of glass was formerly known as “plate” but is now made by the Pilkington float glass process. Annealed – glass that has not been tempered or heat-strengthened – is used for shelving and protective tabletops but is easily broken and shatters into dangerous shards.

broken annealed glass
Annealed glass breaks into dangerous shards

Tempered glass

Float glass starts out annealed and is then “tempered” or heat-strengthened in large ovens in the glass factory. Quenching the outer surface and then slowly cooling the interior creates a toughened compressed surface that is 4 -5 times more impact-resistant than annealed glass.

tempering plant
Conveyor carries sheets of glass through the tempering oven
Stresses inside a pane of tempered glass

Once tempered, glass cannot be drilled or cut – if you try to do so, it explodes into numerous harmless “popcorn” pieces:

Tempered glass breaks into popcorn fragments
Tempered glass breaks into “popcorn” fragments

Learn more about tempered glass in this post.


Laminated glass

Laminated glass – sometimes called safety glass is made by sandwiching a layer of vinyl (Polyvinyl butyral (pvb)) between two sheets of annealed glass. The annealed glass has the same low impact resistance as regular annealed glass but the PVB layer holds the panel together when the glass breaks.

Laminated glass
The commonest type of laminated glass is made from two sheets of 1/8″ thick annealed glass with a flexible vinyl interlayer
Large rock breaks laminated glass window
Broken laminated glass held together by the vinyl interlayer

The ability to hold the glass together after large impacts has made varieties of laminated glass the recommended glazing for hurricane-proof windows. (Keeping the house envelope intact is crucial in high winds because, if windows break during a hurricane the resulting funnels of air will blow away the roof and destroy the house.)

Laminated glass is a good choice for protective glazing if you have a local glass shop with reasonable pricing. Laminated glass stays clear with no yellowing and the vinyl interlay stops 97 % of UV light. Even though it will break with heavy impacts. the window will not give way and your stained glass will normally be safe.


Acrylic Optic L

Optix L is a type of acrylic related to plexiglass. The positives are that it is stronger than regular plexiglass, and stays clear even when exposed to sunlight. It is 20 times stronger than tempered glass so is really hard to break – although it is not as strong as Lexan. Unlike glass, it can be drilled to provide ventilation holes. 

For details about the pros & cons of plexiglass vs tempered glass read this informative post


Lexan XL 10 polycarbonate

Polycarbonate (PC) is an amorphous thermoplastic material that is virtually unbreakable. In addition to its toughness, the clarity (transmits over 90% of light) and the light weight make it very useful for protective glazing.

Lexan is the brand name of the type of polycarbonate developed by General Electric (GE) in 1960. The Bayer company in Germany had already patented their version of polycarbonate under the brand name “Makrolon” in 1955.

Lexan XL 10 is a special Lexan upgrade with a UV-blocking surface layer applied to one side or both sides. Manufacturers guarantee it against yellowing for 10 years with this UV coating.

To avoid high shipping charges, we recommend you buy large sheets of Lexan locally. But if you just need a small, 24″ * 36″ sheet the best deals are like these from this vendor from Amazon.


Insider tips from a glass professional

Money-saving tip! Glass and plexiglass are essentially the same quality from whichever shop you purchase them. Save money by using these glass-buying tips from the experts at Artistry in Glass.


Where to purchase plexiglass – top tips from the experts.

Numerous online vendors advertise “best prices” and “free shipping” but the bottom line is that shipping a full (48″ * 96″) sheet is prohibitively expensive.

For example, the price of a full (48″ * 96″) sheet of 1/4″ thick plexiglass from a top vendor like Fab Glass and Mirror comes to a whopping $736.25 with”free shipping”. This is a company that advertises “we will beat any price by 10%”. So, the #1 tip for purchasing large sheets of acrylic is to always buy locally.

In summary:

  • always buy large sheets locally to avoid shipping charges
  • for best pricing – purchase full sheets
  • learn how to cut acrylic – or get to know someone who can

The exception to buying locally is with sheets 24″ * 36″ or less. This size is affordable to ship because it falls under certain “oversize” UPS and FEDEX limits. So – explore your options on Amazon for these smaller sheets:


Who will cut custom sizes of acrylic?

Our latest inquiries reveal that neither Home Depot nor Lowes will cut acrylic but they do have competitive pricing on full sheets if you can figure out how to trim them to size.

Many local Ace Hardware shops will cut thin (1/8″) plexiglass and often include the cutting in the retail acrylic price. But check ahead because many stores are independently owned and services vary.

Specialist plexiglass wholesalers

Specialist acrylic outlets will cut sheets to size and we like the national chain “Calsak Plastics” for its great service and good pricing. They have a complete range of acrylic and polycarbonate. Follow this link to find branches in Atlanta, Charlotte, Los Angeles, Minneapolis, Portland, Seattle, Tucson, and Irving, TX

Look for specialist wholesale outlets on the web for a dealer near you.


Conclusion

There is no simple rule for protecting stained glass in religious buildings. Each project must be assessed individually to judge the risk of damage in the light of local knowledge.

Extremely valuable & vulnerable stained glass should be protected with Lexan or acrylic – panels at less risk can be protected with laminated or tempered glass.


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John

I was an exploration geologist and University Professor working in Botswana, Zambia, Brazil, Saudi Arabia, the Netherlands, Portugal, and other countries before opening Artistry in Glass in 1986. In my more than 35 years of experience, I have brought my technical abilities as a scientist to the trade of glasswork. During this time I have become an industry expert in glass and glass-related skills. Watch out for special insider tips developed from my detailed knowledge of the glass business.

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