This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
If in a position to coat the glass before putting in frame, excellent effects may be got by using ordinary shellac varnish (made with bleached shellac) tinted with aniline dye. The glass must be slightly warmed before applying the varnish. The strongest spirit of wine should be used for dissolving the shellac and the powdered (not liquid) aniline colours. Sufficient of the colour must be added to the varnish to give the required tint: 1 part of shellac to 8 of spirit is a good proportion. Methylated spirit will do. The varnish should be poured on and placed evenly over the glass (not painted on), and the superfluous quantity returned to the bottle.