Restoration of vandalised windows often requires replacing and re-painting glass. Above is glass from a window by Daniel Cottier in the Church of the Holy Rude, Stirling. The original head was smashed with some parts missing (left). This meant a new piece of glass of matching colour had to be cut. Following this, mutilple layers of paint are applied and kiln-fired to replicate the original (right).
Sometimes re-leading is all that's required. In a case like above, the panel needs to be dismantled with the old lead being scrapped, and any broken glass replaced with a good match. The panel is then reconstructed with new lead before re-installation.