European Stained Glass / Structural Theory and Maintenance
In the Great Cathedrals of Europe one will see huge, spectacular stained glass windows of every design, and most have been viewed for many centuries.
The stained glass windows in the Cathedrals throughout Europe vary in age, the oldest dates to the 11th Century (Germany and France), a few were installed during the 12th Century, and some date to the 13th Century. With the invention of Silver Stain the 14th Century saw a huge production of new stained glass windows, now boasting deep yellows and orange tones. The 15th and 16th Centuries gave way to a new Renaissance of stained glass with larger windows and the advent of fired-enamels, giving the designers a color palette like never before. The 17th Century through the 19th Century brought about new styling following architectural classic trends and stained glass ‘murals’ resembling an oil painting.
The Europeans have 1,000 years of history in designing, painting, and glazing leaded glass. This experience taught them the rule of ‘9 square feet’ per panel. They learned early on that to exceed this size led to structural failure in the form of buckling, sagging, and bowing. The learning process of these early glaziers also included the development of Weatherproofing Cement. This is the material applied to the completed leaded glass panel – pushed under the leaf of the lead came, on both sides, to make the panel waterproof and strong, thereby completing a ‘structural web’.
9 square feet structural ‘rule’
The magnificent windows we see are composed of many smaller leaded glass panels, none of which exceed 9 square feet in size, and all are separated by structurally anchored T-bars that transfer the weight of each individual panel to the stone wall or steel / wood sash. A leaded glass ‘panel’ is a structural ‘web’ where three structural ‘members’ work in unison. Lead came, glass pieces, and weatherproofing cement combine together to make a structurally sound leaded glass panel. The cement adheres to the glass and the lead came to form a strong weather-tight bond, with the correct rate of expansion & contraction to last for many years.
The weatherproofing cement used in early times was mostly potash, finely-ground limestone and suet. This mixture did not fare well under severe weather and didn’t bond well to either glass or lead came.
By the 14th Century glaziers were using a mixture of Whiting (calcium carbonate), or Limestone Powder, Plaster Paris, Linseed Oil, and Turpentine. They found that this mixture formed a solid barrier against weather, bonded well to lead came and glass with good expansion & contraction, and was renewable for future maintenance. This mixture is still in use today to re-cement all windows in these Great Cathedrals as each country in today’s European Union has its own Bureau of Historic Monuments to oversee all works done on these lovely structures. Most have licensing requirements for glaziers and exacting specifications on weatherproofing cement contents.
This formula of Weatherproofing Cement has a unique ability of ‘renewing’, in that the 2 aggregates (Plaster Paris & Whiting) are suspended in the linseed oil, and bonding with the oil. As years go by and the rain erosion and overall expansion from summer heat and contracting of winters’ cold degrade the bond, new cement can be added (re-cementing) and the new cement renews the degraded mixture already under the leaf of the came. The Linseed Oil bonds with the old aggregates bringing them into the ‘new’.
Throughout this 1,000 history of stained glass the windows needed to be maintained continually. Each generation of glaziers would teach the next generation their trade and the importance of maintaining the windows built by previous generations.
Here in the United States this glaziers trade has been degraded to a mere ‘hobby’, displayed at craft fairs, where stained glass craftsmen have gone by the wayside. The trade was never taught to a new generation as the European immigrant glaziers (1875-1910) that came to this New World and built our church windows passed away in the 1940’s. The big demand for church windows in America came from 1880 thru 1929 as most of our churches were built during this period. With little or no demand for their skills immigrated stained glass designers, painters, and glaziers simply got old and passed away without ever passing on their knowledge and skills to a younger generation.
Today in the US we have nearly 1,000,000 churches of all denominations that occupy historical buildings, constructed during the hey-day of stained glass, that have lovely stained glass / opalescent glass windows that desperately need maintenance and for many . . . . complete restoration!!
In the US during the 1950’s there was a big wave of covering stained glass with acrylic or plate glass coverings. This came about because of an ignorance of basic maintenance & re-cementing, as well as greed on the part of glass and acrylic vendors & installers.
With exterior glass or acrylic coverings the stained glass window is trapped in a nearly ‘sealed’ environment where the exterior covering acts like a car windshield and multiplies the Suns’ heat to create temperatures inside this environment that easily exceed 185 degrees Fahrenheit. This environment also traps any ambient moisture from humidity on the interior. With heat and moisture the wooden sash will surely rot away, and the lead came will oxidize at a very accelerated rate, making the leaded glass panels weak. Now we can multiply the moisture and humidity as the window needed a structurally supporting, thorough re-cementing when the coverings were originally installed!! A stained glass / opalescent glass / leaded window under these circumstances will show signs of structural failure in buckling, bowing, settling of the glass pieces within the lead came matrix, and evident settling of the leaded glass panel in its sash.
Many churches throughout Upstate New York (our home area) that we’re restoring windows for are opting to remove their exterior coverings as we restore or re-cement their windows. We fabricate a wooden frame to fit inside the sill of the window and fasten ‘galvanized hardware cloth’ to this frame. This allows the leaded glass window to ‘breathe’ and allows for easy removal of this wire-frame for repainting the wooden exterior surround and eventual re-cementing of the window. A stained /leaded glass window needs basic maintenance every few years (exterior repainting, renew any failing glazing compound) and re-cementing every 25 years on the exposed side (West facing) and every 40 years on a protected side.
For a simple test: Take your fingertips and tap firmly on your windows. You’ll hear rattling of the glass within the lead came matrix – -This means your window is missing the main structural ingredient needed to support the leaded glass window – – – Weatherproofing Cement!!