Christopher Whall,
Stained Glass Work: A text-book for students and workers in glass
“ The painting would remain for years, probably for centuries yet, if untouched, just as dust, without any attachment at all, will hang on a vertical looking-glass. But if you scrape it, even only with the finger-nail, you will generally find that that is sufficient to bring much—perhaps most—of the painting off, while both sides of the glass are covered with a "patina" of age which is its chief glory in quality and colour, and which, or most of which, a wet handkerchief dipped in a little dust and rubbed smartly will remove. ”
