What is Coated Glass and Why Do We Care?


Coated glass is very simply that...it is glass which has been literally coated with various metallic compounds that only permit specific portions of solar radiation to enter a building. This keeps the furniture and carpets from fading, helps with air conditioning and heating, and adds to the beauty of the entire building. You might say it is indeed quite critical to the overall performance of the building. Especially in certain parts of the world where very large structures are constructed in actual deserts where the temperature soars during the day.

If we look closer at coated glass we learn that it is a rather finely tuned masterpiece of metallic compounds. Which are sprayed on when the glass is moving down the bath of molten tin, or evaporated on in a vacuum. So the coatings are either pyrolytic (made in fire), or vacuum sputtered. Either way they are Low-E. The E stands for the word emmisivity. Which means that only certain parts of the suns light are emitted or we might say "permitted" to enter. It is also true that these coatings are extremely thin. Hundreds of times less than the thickness of a human hair. Which means it wouldn't take much to waste them. That is why we care. Can you imagine the loss, if a very large building couldn't keep itself cool anymore? Or just simply the beauty of the building were in some way seriously marred? Please check out the GANA Glass Information Bulletin "Proper Procedures for Cleaning Architectural Glass Products". GANA 01-0116 It plainly states that, "...glass products can be permanently damaged if infrequently or improperly cleaned,...". Did you notice the word 'permanently'? When restoring plain glass surfaces whether clear or tinted (all the way through), we have much more than 40 nanometers to work with. A typical procedure for repairing glass surfaces can involve grinding with a silicon carbide technique;... and then polishing with cerium oxide. You can't 'grind' coated glass. Further, you have to be extremely careful when polishing it or using any type of chemical. Repairing Low-E surfaces is very limited. This is why we and our customers should care about the frequency and quality of the routine cleaning. GANA does specify in the bulletin cited here that metal razors should not be used during the routine cleaning of coated surfaces. Just soap, gently agitate with a soft mop, and squeegee off. Therefore when coated surfaces develop construction debris, or even pollution, which does not come off with the procedure described;...it is no longer 'routine cleaning'. Which means there is a potential of doing damage that could be permanent. It is suggested that various nonpermanent protective films be used at construction sites to protect the window surfaces during storage and when they are installed on the building. It is also suggested that the frequency of the routine cleaning be increased if pollution is more of a problem. I personally think it important to look for ways to check the integrity of such surfaces when they are being cleaned. Also when using water fed poles we should be looking at the silica content of the rinse, not just the TDS! Silica deposits begin invisibly and become more intense with repeated cleanings.

One type of coating that came out over twenty years ago was a silver and bronze pyrolytic first surface reflective Low-E. When perfectly clean it is a beautiful coating. But when allowed to go uncleaned for months and even years it will turn brown. To 'clear' such windows of this pollution many have used hydrofluoric acid or other very strong acids. Which has many times actually stripped off the coating in patches. If this has not happened then it has degraded the integrity and strength of the coating. Which means that the next time the same acid, a different one, or even a light cerium polish is used it might strip off then. This creates a hidden land mine! For the next guy hired to restore a Low-E reflective surface. Remember he does not know who was there before him and what they used. Probably the new owner of the building or maintenance manager doesn't even know. Great care must be taken when working on these surfaces. Which again only emphasizes the necessity of frequently and correctly maintaining coated glass.

Written by Henry Grover Jr.
Member of the Glass Committee of the IWCA
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