Are the outer panes right for our stained glass windows?

OF THE TWELVE OPENABLE PANELS in our south-facing stained-glass windows, 6 of them have protective glazing (an outer pane of clear or wired glass). Notably, the 6 that don’t are all bowed at the bottom. 

It may appear that the outside layer of glass is helping to preserve the stained-glass windows. More likely, the bowing on those panels occurred first, causing the outer glass to fall out. After all, our consultant 20 years ago warned us that the protective glazing was improperly installed—and was therefore shortening the life of our windows.

Below the jump, I excerpt (without comment) from a longer article on the topic, written by an expert.…

The following is excerpted from
“Is stained glass protective glazing necessary?”
by Julie L. Sloan
New York Landmarks Conservancy
Common Bond (Spring 1991) vol. 7, no. 2, pp. 4–7
Published by Partners for Sacred Places

Protective glazing is not an alternative to good maintenance and restoration. It will not correct existing deterioration and will not stop the deterioration process. Properly installed protective glazing may lengthen the life of a stained glass window, although this will not be true for all windows. Improperly installed protective glazing, however, will significantly shorten the life of a window and perhaps lead to irreparable damage. 

The putty with which a stained glass window is sealed is usually made with an organic oil medium such as linseed or soya oil. It has a life span of 25 to 75 years, depending on the installation. As the oil dries and shrinks, the putty hardens and cracks, losing its adhesion to the glass and the lead. It may eventually crumble and fall out.… Deteriorated putty also contributes to the sagging that is commonly seen in stained glass windows. Protective glazing is often installed to correct this problem. However, it is far more advisable to reputty and relead the windows

as needed.

Water is the worst enemy of stained glass windows.… A dangerously wet situation is rarely found in an unprotected window, but is frequently found in windows

with inadequately vented protective glazing. If the protective glazing is not adequately vented, it will trap moisture against the window [which leads to corrosion.]

Protective glazing will neither improve nor prevent [natural] deterioration. It may, in fact, accelerate it by allowing a build-up of heat between the stained glass and the protective glazing.

DESIGN OF PROTECTIVE GLAZING

If protective glazing is determined to be necessary [such as to protect from vandalism], its design and installation must be carefully considered and executed. The system must:

1. Serve its function of protection without causing harm to the stained glass.

2. Must be easily removable for future cleaning or restoration of the stained glass or frames.

3. Must be aesthetically sensitive to the building.

4. Must be installed with proper ventilation.

5. Meet all pertinent building codes and other regulations.

6. And be relatively maintenance-free.

The protective glazing must be installed at least one inch away from the stained glass, in most cases, and the cavity must be ventilated. All other compromises in the design and installation of protective glazing are acceptable as long as the protective glazing is properly vented.… Consequently, a consultant should oversee the design of the protective glazing.