Why Are Church Windows Divided Up? Stone Holding Glass Together

Glass in a medieval window came in small pieces, since producing large flat panes was beyond the technology, and those pieces were joined with lead strips into panels. A lead panel is flexible and cannot span a large opening without deflecting under wind, so a large window needs to be divided into sections small enough for a panel to hold, with stone members carrying the load. Vertical shafts and horizontal bars divide the opening, and the arched head above them must also be filled, which is where the decorative possibilities open up. Tracery is therefore the stone framework performing that structural job, and its development is a history of makers finding what the material allowed.

The sequence in English building is consistent enough to date a window:

• •Plate tracery, where openings are cut through a solid slab of stone in the head
• •Bar tracery, where slender shaped stone members are assembled, arriving in the thirteenth century
• •Geometric, with circles and simple shapes filling the head
• •Curvilinear or flowing, with continuous S-curves producing flame-like shapes
• •Perpendicular, with strong vertical mullions carried straight through to the arch and horizontal transoms
• •The continental sequence differs in detail and follows a comparable logic

Each development followed from what masons learned they could do. Cutting openings through a slab is straightforward and limits the pattern to whatever can be pierced without weakening the stone. Assembling slender bars instead removes that limit and allows far more glass, which was the point, since the whole direction of the period was towards larger windows and less wall. Once bars could be shaped, curves became possible and the flowing style followed, which is technically demanding since every stone is different and must be cut to a template. The later vertical style is in one sense a retreat from that complexity and in another a response to a different requirement, namely spanning enormous openings, where continuous vertical members carry the load efficiently and repeat identical stones.

The stonework exists to hold glass, and the panels it holds have their own construction worth describing. Coloured glass was made by adding metal oxides to the melt, with cobalt for blue, copper and iron for greens, and gold for the deepest reds, which is why red was the most expensive colour. Sheets were produced by blowing a cylinder, cutting it lengthwise and flattening it, or by spinning a disc, both of which give panes with visible variation. Pieces were cut to a design, painted with detail in a dark pigment fired onto the surface, and joined with grooved lead strips soldered at the junctions. Iron bars across the opening carry the weight of the panels. The lead deteriorates over centuries, which is why medieval windows have been releaded repeatedly.

Dating a building from its windows is a practical skill with a few reliable indicators. The overall shape of the arch narrows the period, with a sharply pointed head being earlier than a broad flattened one in the English sequence. Whether the head is pierced through a slab or assembled from bars is the first major division. Within bar tracery, whether the pattern is built from circles or from continuous curves separates two periods. Whether the mullions run straight into the arch or branch is the clearest single indicator of the latest phase. Caution is needed, since windows were replaced, enlarged and restored constantly, and nineteenth century restoration frequently rebuilt tracery in a style the restorers thought appropriate rather than what was there.

Small pieces of glass joined with lead cannot span a large opening, so stone members divide the window and carry the load. Piercing a slab gives way to assembling slender bars, which allows far more glass and then curved shapes. Whether the vertical members run straight into the arch identifies the latest phase, and restoration frequently rebuilt tracery in a chosen style.