We are facing an unusual problem in that mortar has been allowed to splash down over a plate glass window and to remain there, exposed to rain, for several weeks. We now find on removing the mortar that is has caused a staining of the glass, more visible from some angles than from others. Is there any way of removing these stains? We have tried washing, with and without the use of commercial window-polishing agents, and also a sulfuric acid solution, but without success.
This problem of laitance or mortar staining is not unknown, particularly with plant-precast units in which the glass is fitted before shipment. The staining is due to silicic acid attack; the longer the cement remains on the glass, and the more rain to which it is subject, the more severe will be the staining. The only way in which the stains can be completely removed is by getting a professional glazier to repolish the surface with a proper polishing agent dissolved in spirit or gasoline. An acceptable result, visible only very faintly at certain angles, is also possible by thorough rubbing with a commercial metal polish; a little experiment will soon establish the most suitable brand. Prevention is, or course, the best remedy. Splashes should be removed as they occur. If this is impractical and the chances of splashing are great, the glass can be smeared with a thin protective layer of vaseline.