As part of the conservation project for the Great East Window of York Minster, sometimes pieces which are not original are removed from the panels if they no longer fit or if they are very obtrusive. Gaps may also open up when pieces are returned to their original positions, and in both of these cases, new glass must be inserted to fill the spaces. So that the new glass does not draw the eye or detract from the original John Thornton design, it is painted to 'blend in' with the surrounding pieces. Newly painted pieces may continue original design features, and be copied from original pieces in the panel.
Conservators use traditional glass painting techniques, and mix their paints with vinegar or oil. The glass paint contains lead, so a mask must be worn and extractor fans used.