Expert: Robert Lamb - 6/28/2010

Question
We have a problem where some mysterious scratches have appeared on a pane of tempered glass.  Our window fitting sub-contractors fitted the glass and everything was fine, nobody noticed any scratches, however there are now two parallel scratches running horizontally along the glass (about 1m long). The fitters say it could have been due to window cleaning damage.  Do you have experience of such an occurrence or any advice you can give on this please?

Answer
Ana,

In my experience, very few, if any window cleaners world wide would cause, or even could cause such scratches. I am associated with national and international window cleaning groups and am aware that there are some instances where glass, especially tempered glass can be scratched.

The reason why tempered glass is scratched more often than plate or float glass is that during the tempering process, if the temperor does not keep his plant meticulously clean, glass fines (very minute glass particles usually) are sometimes baked into the surface of the glass. When some unsuspecting cleaner, or painter, home owner or building maintenance personnel comes along after the glass is installed and runs a razor blade, paint scraper or really most anything over the surface of this defective glass, these glass fines break off and scratch the glass.

To explain this further, on the Mohs scale, which defines a substance's (minerals usually) hardness. Stainless steel, and even non-stainless are not hard enough to scratch glass, for example razor blades and paint scrapers meet this criteria. Glass, on the other hand WILL scratch glass. (A diamond is also much harder than glass and will scratch glass.)

So back to the first explanation about tempered glass, if glass particles are baked into the surface of tempered glass (defective glass really), and someone breaks that almost invisible to the naked eye baked into the surface glass during a cleaning process, those microscopic chips WILL scratch the glass. Those scratches will though will be characteristically hair line scratches emanating from the point where the glass fines were baked into the glass. (The scratches look almost like a comet under a microscope.)

This website, with illustrations, will explain it further: http://stopscratchedglass.com/ see also http://scratched-glass.net/

We have seen very deep scratched caused by other trades. For example, if cement or mortar from a brick layer is splashed onto a window and left to dry, and then an unskilled laborer tries to remove that mortar, he may scratch the glass very easily and there are several reasons why that is so. Mainly improper removal methods and the fact that mortar has sand in it that will scratch the glass. Again, refer to the above website.

Using normal cleaning methods, a skilled window cleaner will not scratch glass.

If you have any other questions or concerns, please do not hesitate to contact us.

Sincerely,

Robert Lamb

Window Masters
www.window-masters.com

Rob's Window Cleaning
http://www.robswindowcleaning.com