Presidio Chapel Historic Structure Report - Presidio Trust

PRESIDIO CHAPEL (BUILDING 130)SAN FRANCISCO, CALIFORNIATREATMENT OF STAINED GLASS WINDOWSProtection, Maintenance and InvestigationThe amount of cleaning, repair, or restoration recommended for a stained glasswindow depends upon the condition, quality, and significance of the glass, asdetermined by a stained glass professional. Minor cracks, sagging, and oxidationare part of the character of historic leaded glass, and require no treatment. Moreextensive cracks, major bulges (generally, more than 1” [38mm]), and similarsigns of deterioration may require more significant intervention.CleaningDirt, soot, and dust can build up on both sides of the glass as a result of pollution,smoke, and oxidation. Gentle routine cleaning will remove harmful deposits, whilealso providing an opportunity to inspect the condition of each window. The typeof cleaner to use depends on the glass. Water alone should be tried first (soft wateris preferable). If water alone is insufficient, the next step is to use a non-ionicdetergent. Most unpainted art glass can be treated with acetone, ethanol, isopropylalcohol or mineral spirits to remove yellowed coatings or grime in cases wheregentler methods have failed. Acidic, caustic, or abrasive cleaners should never beused, nor should common household glass cleaners, which contain ammonia thatcan adversely react with the putty or metallic cames. All residue leftover from thecleaning solutions must then be removed with a non-ionic detergent, and the glassrinsed with water. Painted glass must never be cleaned before the stability of thepaint is confirmed, and only then with great caution.RepairIt is essential to properly maintain the window regardless of the type ofglazing. Routine painting and caulking, and periodic replacement of the glazingcompound, will go a long way towards preserving wood window frames, andmasonry frames should be kept well pointed and caulked to prevent moisturefrom corroding the steel armature and anchors within.A very common-but extremely harmful-practice in the American stained glassindustry is performing major window repairs in place. A window cannot beproperly repaired or restored in place if it is bulging or sagging far out of plane, ifover 5% to 10% of the glass is broken, or if solder joints are failing.Leaded windows will generally outlast several generations of waterproofing, butwhen the waterproofing has failed, the window should be removed from theopening and waterproofed on a bench. Sealants (e.g., putties, caulks, and silicones)are used to seal the leaded panel against the sash, and to seal any open jointsaround the window frame. Some sealants release acetic acid as they cure, whichcan harm lead, and thus should never be used. The appropriate type of sealantwill be determined by a professional, as it depends upon the types of materials tobe bonded and on the desired appearance and longevity of the window.Repairs/Replacement of glassGlass cracks will enlarge over time as the contacting edges grind against eachother whenever the window is subject to vibration, thermal expansion andcontraction, and other forces such as building movement. Therefore, it isimportant to repair cracks across important features as soon as they are detected,and while a clean break remains. There are several techniques used to repairbroken glass, each of which differ in terms of strength, reversibility, and visualeffect, and the appropriate repairs must be selected on a case-by-case basis by arestoration specialist.Minor repairs, such as replacing a few isolated pieces of broken glass, can beperformed in place as a reasonable stop-gap measure. This work, typically calleda “drop-in,” “stop-in,” or “open-lead” repair, entails cutting the came flangearound the broken piece of glass at the solder joints, folding it back to repair orreplace the old glass, and resoldering the joints. Repairing a zinc came window isnot as easy. Zinc cames are too stiff to open up easily, so they must be cut openwith a small hack saw and dismantled until the broken area is reached.ReferencesSee “Preservation Brief 33: The Preservation and Repair of Historic Stained andLeaded Glass.”Page & Turnbull89PART III: TREATMENTRECOMMENDATIONS