Historical Restorations Foundation - Stained Glass and Leaded Glass Restoration

The European glaziers have 1,000 years of history in designing, painting, glazing and weatherproofing leaded glass windows. The learning process of these early glaziers 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, weatherproof and strong, thereby completing a structural web of GLASS and LEAD CAME.

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 providing little structural support.

By the 14th Century glaziers were using a mixture of Whiting (calcium carbonate), or Limestone Powder, Plaster Paris, Boiled Linseed Oil, and Turpentine. They found that this mixture formed a solid, lasting barrier against weather, bonded well to lead came and glass with good rates of expansion & contraction, and was renewable for future maintenance. This mixture is still in use today to re-cement the windows in the Great Cathedrals of Europe as each country in Europe has its own Bureau of Historic Monuments to oversee all work done on their historic structures. Most have licensing requirements for glaziers and exacting specifications on weatherproofing cement contents. Their precious stained glass is MAINTAINED and RE-CEMENTED regularly.

Only 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’s heat and contracting of Winters’ cold degrade this bond, new weatherproofing cement can be added (re-cementing) and the new weatherproofing cement renews the degraded mixture already present under the leaf (flange) of the came. The Linseed Oil bonds with the old aggregates bringing them into the new forming a uniform bond once again with glass and lead came. This completes the structural web and adds another 25 to 40 years of life to the overall window.

In the US during the 1950’s there was a big wave of covering stained glass with acrylic or plate glass coverings. 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 Sun’s heat to create temperatures inside this environment that easily exceed 185 degrees Fahrenheit on a sunny day. This environment also traps the ambient moisture / humidity inside this closed environment. With heat and moisture, the lead came will oxidize at a very accelerated rate, making the leaded glass panels of your window structurally weak !!

Any stained glass / opalescent glass / leaded glass window under these circumstances will show signs of structural failure in buckling, bowing, and settling of the glass pieces within the lead came matrix, and visually 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, giving protection from rock-throwing children. This also allows for easy removal of this wire-frame for repainting the wooden exterior surround and periodic re-cementing of the window. A stained /leaded glass window needs basic maintenance every few years (exterior repainting of the wood / steel sash, renew any failing glazing compound) and re-cementing every 25 to 30 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 pieces of glass - - - Weatherproofing Cement !!