How was lead used in medieval stained-glass windows?

In medieval stained-glass windows, lead was primarily used to assemble individual pieces of colored glass into a cohesive panel.1

The process involved the following steps:

• Structural Assembly: Glaziers used grooved strips of lead, which featured an H-shaped cross section, to hold the glass pieces together.1
• Construction: The glazier would place pieces of glass between these lead strips on a workbench, cutting the lead to fit the contours of each piece. Once the panel was fully assembled, the joints between the lead strips were soldered to secure the structure.1
• Weatherproofing: To ensure the window was durable and protected from the elements, a putty compound was rubbed under the lead strips to waterproof the joints.1
• Support: Because lead is a soft and flexible metal, large windows required additional support. Glaziers divided windows into smaller panels and incorporated iron framing members, known as armatures, to prevent the weight of the glass and lead from causing the window to collapse.21

This technique of using lead strips, often referred to as leading, allowed artists to compose complex, monumental images from many separate pieces of glass.31