What causes a stained glass window to fail?
Window failure is usually due to heat and oxidation. Heat causes the window elements to expand and contract. These elements: lead, glass, cement, and window frame, expand and contract, often divergently from one another. This provides an opportunity for moisture to penetrate the window elements and their respective interfaces. Moisture dissolves and leaches the lead from its matrix; moisture replaces the oils that make cement strong, yet flexible; and rots and rusts window frames. The one-time monolith begins to return to its individual components, the window looses it structural integrity, and eventually becomes incapable of supporting its own weight. Human intervention can also hasten the deterioration of a stained glass window assembly. Improperly installed storm windows or other protective glazing can trap moisture and increase the temperature of space between the stained glass and the storm window by as much as 800 degrees. Such humidity and temperature extremes act as a catalyst
