Conservation and restoration of stained glass
It involves any and all actions devoted to the prevention, mitigation, or reversal of the processes of deterioration that affect such glassworks and subsequently inhibit individuals' ability to access and appreciate them, as part of the world's collective cultural heritage.
The fabric of the glass itself, the paint or stain used to decorate it, and even the metal framework used to hold the design together are all at risk of deterioration, and will likely require conservation work to ensure their long-term survival.
Historic glazing is subject to damage caused by continued exposure to pollution and the elements, on top of that resulting from inherent problems, such as the innate fragility of glass and any potential chemical instability of the materials involved (Brown et al. 2002, xi; Rauch 2004).
Owing to the delicate nature of the materials, and the incalculable historic and aesthetic value of stained glass work, any and all treatments should be planned and performed by professional conservators and craftspeople, who have been specially trained in the peculiarities of the medium.
While preservation is the shared responsibility of all involved, including visitors, caretakers, and other stakeholders, it is imperative that professionals are consulted to ensure the continued integrity of the physical materials and their associated significance.
The primary aim of conservation is to slow the rate of deterioration, caused by various factors, to the point where the loss of significance, such as historic information and/or aesthetic value can be kept to a minimum (Pye 2001, Ch.
Over time, corrosion, marked by pitting or pocking of the glass, can deepen and spread until the surface layer itself breaks apart, uncovering the vulnerable, damaged core, until it becomes opaque (Rauch 2004, 3).
This opacification occurs as water seeps through the subsequent cracks, causing the formation of opaque syngenite and gypsum crystals that block light from filtering through the previously translucent glass (Rauch 2004, 3).
As these crystals continue to grow, they pose an even greater danger to the glass, eventually breaking through the surface and dislodging the delicate paint layer, or even causing cracks (Rauch 2004, 4).
For example, the cames that make up the matrix of a stained-glass window, for which lead and zinc were most commonly used, undergo quite a bit of thermal expansion and contraction, eventually resulting in metal fatigue, which in turn weakens the joints between the plates, causing whole panels to deform or simply fall apart (Vogel et al. 2007, 8).
Additionally, metal matrixes and frames may weaken with stress, such as that which has occurred at Holy Trinity Church in Stratford-upon-Avon, UK, where years of wind buffeting the leaded clerestory windows has endangered the structural integrity of not only the glazing but also, the upper portion of building itself (fig.
Of utmost importance in this list are the fundamental ideas of minimal intervention and reversibility of treatment, both of which are necessary to ensure that the overall integrity of the windows, as historic and artistically valuable objects, is not compromised.
Owing to the incredible amount of chemical and methodological variability to be found when dealing with stained glass, research into the general production habits of pertinent regions or eras may provide clues that can help conservators understand the overall significance of the piece, anticipate what to expect from the materials, and determine how to proceed (Sloan 1993, 28; Vogel 2007, 5).
All aspects of this process including preliminary research, condition surveys, conservation plans, and all the methods and materials used throughout treatment should be duly recorded, and the documents are preserved and made accessible in the long-term for future reference (CVMA 2004).
Thus, cleaning efforts should not necessarily be concerned with the complete removal of all encrustations, but rather the careful thinning of these layers to a point where light can be transmitted through the glass at an acceptable level (Rauch 2004, 5).
The simplest cleaning can be performed using carefully applied deionised water, although other mechanical or chemical means are often necessary, and must always be done slowly, in a controlled and focused manner (Rauch 2004, 5–6; Vogel et al. 2007, 9–10).
Conversely, poultices or gel pads steeped in a non-ionic detergent or EDTA can be applied to the surface of the glass for long periods of time for "deep, focused cleaning" (Rauch 2004, 6).
If these layers appear particularly friable, then it is necessary to clean the glass delicately with cotton swabs, and in more extreme cases, manually affix the original paint lines to the surface, under a microscope, by applying small tiny drops of resin at specific points (Rauch 2004, 6; Vogel et al. 2007, 10).
Unfortunately, in practice, this is not always the case, and it is well known that unsuitable, or improperly ventilated protective glazing can actually create an overly hot or humid microclimate around the historic glass that increases the rate of deterioration (Vogel et al. 2007, 13).
