The Four Stages of Etched Glass

Glass is made up primarily of silicon, oxygen, calcium, and sodium. That is window glass. It is otherwise called soda lime glass. Glass can be etched with various chemicals. Three of which are sulfuric acid, hydrofluoric acid, and ammoniumbiflouride. There are three different effects I have noticed indicating an etch has occured. One is a clear distortion of the surface where reflections are changed. This can be in the firm of long lines. Where silicone sealant had run down prior to removal with a hydrofluoric acid based product. Or a type of orange peal effect showing where each hard water spot was prior to removal with a hydrofluoric acid based product. Another is a whitening effect that can also appear as a slight or very strong white haze. This can happen when the tin side of tempered plates makes contact with hydrofluoric acid. It can also happen when HF makes contact with a surface that has been severely physically abraded on a microscopic level. Then a third effect involves scratches. Which otherwise might never have been noticed. Scratches are usually severely enhanced (made much more visible). Scratches that were very slight but hidden by mineral deposits will always be magnified many times over when they are revealed after the HF takes off all of the mineral deposits. All of this is what I have observed in the field. These different types of etch fall into one of the four different stages of etched glass surfaces.
These are stage 1 static, stage 1 dynamic, stage 2 static, and stage 2 dynamic. Stage 1 whether static or dynamic, only involves the calcium or sodium atoms. These are usually replaced with the hydrogens taked from water. Stage 2 whether static or dynamic involves what is called total dissolution of both the moderator and builder matrix of glass. In other words, all of the atoms have been reacted with. If we are talking static the reaction byproducts have remained on the glass. In the case of a dynamic (in motion) etch, all of the atoms involved in the reaction have been removed from the surface. To apply. Most types of etch we observe in the field via the inadvertant use of glass etchants in restoration work are both static and dynamic. The whitening effect is static. But usually a dynamic etch has occurred as well. When scratches have been intensified, this is a static etch. When the clear distortions of mineral deposits have been left behind, this is definitely dynamic. If these distortions are severe enough, restoration of the surface might not be practical at all. But there are certain static etched conditions that are restorable. Especially if that static etch has resulted from vapors instead of direct contact with the acid. An interesting case in point involved a crystal glass shop. I was called in to identify and fix a white haze on the inside of every window in the shop. The man was moving out and feared his landlords anger. It turned out he was dealing with a vapor etch from the fumes of hydrofluoric and sulfuric acid. They use vats of both at extreme concentrations, and high temps. The acids release gases that travel through the air, and become acids again when they recombine with water vapor that condenses on the inside of window glass. This happens more in the winter because of the difference of the temperature between the outside and the inside. We were able to restore the surface without any signs of clear distortion (stage 2 dynamic). However, the amount of time necessary to accomplish the task was greater than this man wanted to give. So a quick job was done. One that was apparently acceptable to the landlord. I could go on forever with this topic. So I will sign off now, and come back to it later in another post.

Henry Grover Jr.
gtcnews@hotmail.com