The Destructive Power of HF

Hydrofluoric acid (HF) eats up most hard water spots based on a silicon chemistry with a voracious appetite. It is for this reason that commercial products based on HF have been created for clearing window glass damaged by ground water in sprinklers, and concrete efflorescence. Each can leave behind extreme mineral deposits based on a spiked silicon fingerprint. At times it might even appear that no damage has been done to the glass during the "restoration" process using HF. However looks can be decieving. The simple facts are that whenever an HF solution of as low as one percent makes contact with glass for as little time as thirty seconds, a chemical reaction takes place. From my own observations alone I would venture to say this is a stage two dynamic etch. That means that the silicate/silica matrix of the glass has been dissolved and dynamically removed, leaving behind a new surface. Which surface is much more rough on a microscopic level than that of brand new glass. But you won't need a microscope to prove this. Just lightly run your finger across this new dry surface. The surface will also cause pure water to sheet out very well without leaving hardly any drops.
Here are a couple of experiments you can do yourself in your own kitchen to prove to yourself what I have said is true. Take a small three by three inch slide of window glass, and dip it in a cup of HF at only one to two percent concentration. Only emmerse one half of it. Leave it there for a minute. Remove it, rinse it, and dry it completely. You will notice the "water line". This is a line showing where a stage two dynamic etch happened. Next take a six by six inch mirror square. Create a good four by four inch patch of scratches in the center. Now apply only a one to two percent solution of HF to one half of your scratch patch for thirty to sixty seconds. Rinse quickly, and dry. You will see that the scratches which made contact with the HF solution have been severely magnified. Another interesting test involves drawing a letter on a mirror plate using a paint marker from Faber Castell. Once drawn rub a dilute solution of HF over the entire area untill the paint has been removed. Now rinse and dry. You will be able to see where the paint was. There will be a clear distortion of the glass only where the paint was. So you will still be able to make out which letter you had drawn. I remember visiting a building once where some HF had been used to remove long drip lines of a concrete silicone sealant from some dark glass. The sealant was gone, but there were long band lines left as a clear distortion of the glass. Further there was a clear orange peal effect showing where the hard water spots were. If you drive down the highway and check out some of the older glass boxes that have had HF used on them for many years, you will be able to see this type of distortion. Any type of waviness on glass plates is much easier to see from a distance. Now in the case of scratches. How do you know that someone has not been there before you and removed the previous spots with a crude abrasive? You will remove the second batch of spots, but in so doing you will also severely magnify the scratches leftover from the first time. Not good. Since the owner (who never seen the scratches before) will now think that you caused them to begin with. I can go on. But hopefully this info should help anyone who reads this, to understand why I would never suggest that anyone use HF on windows, unless they are plexiglass. Then there are other dangers to contend with too. It is not a forgiving chemical. Further;...if you do any experiments with it always use adequate precautions. Gloves, eye protection, etc. Since there are some serious health risks involved too. My advice is always to stay on the side of caution. I hope no one misunderstands me. My intent has never been to do anyone harm. Also, ultimately it will be the Window Cleaner that has to make a decision as to what they will use to remove anything from the window. It is your company and you are the commanding chief.

Henry Grover Jr.
Glass Tech Consulting
gtcnews@hotmail.com