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Why were iron armatures needed in large stained-glass windows?

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The north rose window at Chartres Cathedral in France, featuring intricate stained glass with vibrant geometric and biblical designs in a large circular and five tall vertical panels. — stained glass The stained-glass rose window in the north transept of Chartres Cathedral, France.

In the art of stained glass, iron armatures were necessary because of the inherent flexibility of the lead strips used to join individual pieces of glass. While small windows could be managed easily, larger windows required these iron framing members to provide essential structural support and stability for the leaded panels.1

As windows grew in size—such as the expansive examples found in Poitiers Cathedral and Chartres—glaziers developed elaborate systems of vertical and horizontal armatures. Beyond their functional purpose, artists ingeniously incorporated these supports into the overall design, often using them to create geometric patterns that delineated ornamental subdivisions. This transformation of a structural necessity into a positive design element was a pivotal development in the creation of Gothic medallion windows, allowing for a harmonious fusion of complex imagery and architecture.1