The photo (above, right) shows a pattern that has formed between the two panes of glass of a double-glazed sliding patio door. The pattern is on the inside of the inner pane, but the inside of the outer pane of the door carries a faint copy. It extends from the top of the glass to two-thirds of the way down and two-thirds of the way across from the right. The lock is on the left. The door faces north-west and gets intermittent sun. What caused it?
Colum Clarke, Bray, Wicklow, Ireland