Outside access to the Chapel of St. Nicholas windows is not easy, so I have never previously viewed them from that perspective. However, closer examination of one of the windows dew me to venturing outside. It appeared that the glass was layered. Certainly not immediately obvious, but there are sections that clearly have more than one layer of stained glass, producing a three dimensional feel that is most apparent on this tiled floor section from ‘The Lord is my refuge and fortress’. However, trying to photograph this effect as intended, is close to impossible. The best I could manage was to demonstrate that there is a lower layer of lead.
From the outside, such features are evident on only two of the four accessible windows, but are significantly more obvious (and more abundant those windows than is apparent from the inside). Some of the double layers are in vivid, dark coloured locations – perhaps two layers of glass are necessary to achieve the deep blues, for example (although it is worth noting that not all vivid blues areas are double-layered).
Seen in its entirety from the outside, with the benefit of some highlighting, the images below show the extent of the double-layering. When overlaid with a mirror image of the edited window, it is clear that this feature exists only in the darker areas or vivid colour.
UPDATE: November 25, 2018
Further research reveals this process to be ‘plating‘ – a stained glass technique perfected by L.C. Tiffany in the early 1900’s. Plating is the process of layering glass, one piece over another, to create shadows, contour and add depth to compositions.