Conservation and Restoration of Historically Significant Stained Glass and Leaded Glass Windows

By B. Gunar Gruenke, Vice President of Conrad Schmitt Studios
Published in "Stained Glass Quarterly" Magazine


Conservation and restoration are two distinct approaches.  Starting in 1983, my father Bernard E. Gruenke, Jr., president of Conrad Schmitt Studios, Inc., began a series of presentations about that very subject to not only clients, but also to state and local preservationist organizations across the nation.  Bernie, Jr., has been a conservator and member of the American Institute for the Conservation of Historic & Artistic Works (AIC) for decades.  Conservation projects have included thousands of murals, painted and stained glass panels, sixteenth-century paintings, religious relics, statuary, art installations and entire interiors. 

In the conservation of a mural, the goal is to stabilize and conserve everything that can possibly be preserved.  Every piece of the artist’s original intent is saved.  Heated spatulas, vacuums from the back side and consolidents are used.  Any work that is infill painted – where perhaps significant water damage caused the loss of a portion of the painting – is applied over a barrier varnish of Acrylic B72 or equivalent.  All work performed must be 100% reversible.  However, it is important to note that even in the conservation approach, yellowed varnish is removed, and warped stretchers on which murals are mounted may be replaced.  This helps protect the artwork, and the stretchers are considered support members rather than part of the artist’s original intent. 

The same philosophy applies to stained glass conservation.  Like our studio’s approach, the Stained Glass Association of America’s (SGAA’s) Standard and Guidelines for the Preservation of Historic Stained Glass, Revised 1998, provide that “Any treatment should be the minimum and least invasive necessary to satisfy the needs of the project.” 

Here I call attention to some of the points as brought forth by others in recent articles pertaining to the question of lead conservation versus releading.  One author mentions that the lead is a part of the window and should be preserved.  I would counter that the lead is comparable to the warped stretchers or the yellowed varnish and is not essential to the artist’s original intent.  Even Michelangelo’s Sistine Chapel ceiling, during conservation, had the varnishes, and in some cases, I believe, the colored shadow glazes removed due to aging. 

The fact is that the artist of a stained glass window is, more often than not, designing with glass and light.  There may be a rare occasion, outside of the norm, where the lead is the integral and most important part of the design.  But characteristically, for most, the light, colored by glass, is the “artwork.”  Think of the Judson Studios centennial history entitled Painting with Light.  Given the understanding that the glass rather than the lead is the artwork, one must agree that the lead was, typically, merely the means to hold the artwork together…the framework per se. 

Purists would agree that conservation and preservation go hand-in-hand and that restoration is a bit more aggressive approach to “restore’ the artwork to its “original artist’s intent.”  It is true that a purist of conservation in an archeological sense of the term would require every microscopic element of the original work to be kept.  In fact, a true purist would require absolutely no work at all except a cleaning and non-evasive means to stabilization.  They may even call for a bowing window to be removed, stored flat and placed in a UV-filtered, environmentally controlled enclosure. 

That approach truly does not provide for the original artist’s intent.  At the Conrad Schmitt Studios, we have seen projects in which the consultant elected not to back plate the original faces of the figures with new faces on painted slide glass, even though all facial features were missing.  Now the project is a faceless room of windows.  That does not offer our generation or the next the visual beauty of the original artist’s intent. 

Going back to the question of releading, the critical issue must be raised of damage to the glass since the glass itself is the real artistry of a stained glass window.  When a window is bowing, it is critical to replace the lead and not attempt to flatten the panel with weights or any other means.  When a window bows, the lead is stretched. 

It is a well-established fact that lead does not have a memory.  Lead is not elastic.  Once lead is stretched from a 24” length to a 26” length, it is absolutely impossible to push it back to 24”.  It is guaranteed, every time, that by attempting to push the 26” length back to 24”, the lead will buckle out to one side or the other.  The stained glass will be required to absorb the pressure, and it will eventually break.  The end result is damage to the intrinsic value and what the artist really cared about: his or her glass choices. 

I have written an article on the subject of damage caused by flattening stained glass windows, “Choose Wisely among Stained Glass Restoration Methods,” which was published in Church Executive magazine.  It makes mention of an incident in which we lost a full releading project to the lower-priced option of stabilization, flattening and in situ repair.  Needless to say, about 15 years later, we were called in to perform the full releading because the damage to and bowing of the glass had persisted.  Of course, the cost of the two projects together was more for the client than if they had chosen to do the correct method the first time.  If the client cannot afford the full project of releading, then the worst windows are to be done first, and the project is completed in phases at the client’s discretion.

Cement putty does break down.  In our studio’s testing, leading brands of cement putty were put through our own two-year “weather test” in which “12” sample strips of multiple types of lead were secured to a board and placed on the roof of our studio for two years, through hot sun, 100+ degree temperatures, ice, rain, wind, snow, and 30-below-zero temperatures.  None of the putties would need to be re-oiled on an annual basis.  However, this process has proven somewhat useless as well. 

In a related experiment to see if old putty can be rejuvenated by adding new putty to it, we took multiple parts of old, broken-down putty and saturated them in a slurry of new mix.  While the surface of the old putty has regained a “pretty” look, the condition of replenishment was only on the surface.  These putties did not contain the plaster of Paris, as was mentioned in a previous article; we would enjoy the opportunity to “test” this putty as well.  Of course, everyone is aware that there are thousands of different putty formulas used in historic stained glass.  We have even run across a pure cement mix – what a bear that was to delead. 

In short, there is no basis to say that recementing in place on a regular schedule will do any good to any window unless perhaps it is your own window constructed with your own special formula of “regenerating putty.”  And, even in that instance, additional testing must be performed to prove that the value is anything other than surface aesthetics. 

For any professional to allude that reputable studios are promoting full releading as a means to drive up bid-dollar contracts is ludicrous.  That would be saying that to promote full releading is unethical.  Perhaps one could also find it unethical to perform on site repairs and “detrimental” flattening as a means to get a quick contract.

Every stained glass window in every setting does require its own investigation and procedural recommendations.  Countless times, we have told clients that their windows are fine and can last another five, 10, 20 or more years before work is needed on them. 

The final and most important fact that must be understood and kept at the forefront of every stained glass restoration project is the client’s needs.  Sure, there may be a museum where the original lead should be maintained, but most often the stained glass industry is serving churches.  The images in stained glass were often referred to as “The Living Bible.”  Churches today are living churches.  The term relates more to the people than the building and implies that the buildings are not museums.  They are active, with participants celebrating beliefs and sacred services.  The windows were designed to teach the story of our Lord and installed to be beautiful transmitters of light that enhance the worship celebrations within.

Releading and conserving the painted elements of bowed and buckled windows is providing a service that must be performed to maintain these most important, and quite often, one-of-a-kind sacred spaces.

[Editor’s Note: Publication of this and other articles on the techniques of restoration should not be construed as official endorsement of the methods and methodology presented in these articles by the Stained Glass Association of America.  Articles on restoration methods and philosophy are being presented here to promote a dialogue on proper restoration techniques and an advancement of the understanding of proper stained glass restoration practices.  Comments are welcome in the form of letters to the editor (which are presented in “FYI: People”) or as articles on restoration.  The Stained Glass Association of America’s current publication that describes the restoration practices and methodology endorsed by the Association is The Standards and Guidelines for the Preservation of Historic Stained Glass, Revised November 1998, which is available from the SGAA Headquarters by calling 800.438.9581.]

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