Glass Etchants

Glass etchants such as hydrofluoric acid, sulfuric acid, and ammoniumbifluoride;...should NEVER be used in glass restoration work! Many people will however use them because they are an exceptionally easy and inexpensive, but temporary "fix". Therefore several commercial products have been developed which are based on such chemicals. The reason why they work is the same reason why they should never be used. They go after silicon based molecules with an insatiable hunger. When water spots will not yeild to some of the other acids such as hydrochloric, sulfamic, citric, and acetic it is probably because they are based on silicate minerals. Spots that are based mostly on carbonates will yeild to mild acids like acetic which is in vinegar. Glass is based on a silica/silicate matrix. So acids such as hydrofluoric will attack hard water spots and the glass surface simultaneously. Every time. HF will actually break the SiO2 bond in glass.  There are four different stages of etched glass. Each one has its own appearance. These four are divided into two groups of two. Static and dynamic. Static etch stage one and two shows up as a white cloudiness. Stage one and especially two dynamic etch shows up as a clear distortion of the glass surface. I have seen many times what I call spot imprints. Having the same exact outline shape as the spots that were removed by the etchant. As it turns out the etchants used tend to degrade glass more where the largest concentrations of minerals are. Leaving an imprint. I have proven this in many different ways.

Here is an interesting experiment using hydrofluoric acid and a small light bulb.

Another interesting thing happens when glass etchants are used on solar hardcoats such as PPGs Solar Cool glass. HF in particular has the ability of degrading the physical integrity of the thin film. Sometimes without even showing it. Not until someone comes along with an optical grade polish such as a cerium to remove the newly formed spots, does this become known. It could be that the hard coat will strip right off in patches. One company I worked with had this problem. They had to turn to a very fast machine, a feather touch, and an optical grade submicron aluminum oxide powder.
Anyhow, my techniques are a lot more involved than doing a quick acid wash. The goal is to remove the mineral deposits without doing any damage to the glass surface. At times it is necessary to create a new "near surface". Yet this should be done in a controlled way. Such work requires a price of between ten to twenty dollars per square foot. Most people simply choose to look out dirty windows. It is also true that most times the cause of the stains cannot be completely eliminated. So instead of a one time fix forever, the building becomes what I call a maintenance restoration job. This involves correctly and safely removing the spots, sealing the glass to keep future spots from sticking as tenaciously to the windows, and removing all future spots on a periodic basis along with maintenance of the sealant.

Written by Henry Grover Jr

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