I know it goes without saying;...but hard water spots come from hard water drops. Hard water drops are hard because they are loaded with silicate based minerals. Silicates are microscopic "rocks". Hence the word hard. These silicates usually include the elements silicon, oxygen, aluminum, iron, sulfur, phosphorus, potassium, calcium, and sodium. They are very similar to the chemistry of window glass, and so have the tendency to "lock on" to the microscopic surface. Once there they can initiate an exchange reaction with the atoms in the glass effecting a stage two static etch over time. So that when they are removed via a superabrasive based polish what remains is a visible distortion of the surface or a "fingerprint" of the spot itself. I can make this statement with absolute confidence since I have performed the experiment myself on a piece of dark glass cut from an existing window damaged by hard water drops. Dark tinted glass by the way is much more likely to become damaged by hard water, and is much more difficult to restore. Many of the videos on the internet demonstrating stain removal systems are performed on first surface reflective glass. This is the easiest surface to clear. I would like to see one done using dark glass!
So many people think that hard water spots are just a deposit. Just a simple stain. While this is usually true at the beginning, it is not always true. Over time serious damage can be done. The reason lies in the anatomy of the spot. The spot forms as has been mentioned from a hard water drop. As that drop evaporates in the hot sun the concentration of silicates increases, and so does the pH of the water drop. When the pH reaches 9 and above a chemical reaction begins leaching the hydrogen atoms from the glass surface. As time wears on other atoms/elements such as sodium, calcium, and oxygen are leached away. At this point complete dissolution takes place or what is called a stage two static etch. Both the reaction byproducts and the various silicates in the original drop are left behind as the water molecules evaporate. Hence it is considered static. It is also true that not all of the water molecules have left. You would have to heat the glass to seven hundred degrees to accomplish this. But from what we can see with the unaided eye;... the glass is dry.
The greatest damage or dissolution takes place around the outer edge of the spot. This is because as the drop evaporates it does so from the middle out. I call the middle area the "basin", and the outer edge the "ring". The drop ring rather than the basin carries most of the silicates and so reaches the highest pH first. So logically this is where most of the damage or dissolution takes place. The spot basin is virtually untouched. If you look close at the pictures shown on the side bar here you will see what I mean.
What is interesting about the potential of hard water drops to etch a glass surface is the fact that this action is increased by a few different factors. One is simple repetition. Every time a drop runs down the window, it is caught by a previously existing spot. The drop stops right over the spot, and evaporates right there when the sun comes out the next day. So spots will become more and more intense over time by this process. As each drop forms on top of each spot and evaporates, etching becomes more severe. Another factor to consider is how hot the glass is because this determines how quickly each drop evaporates. Which allows for many more drops to form and evaporate. It also means that the chemical reaction or dissolution of the glass will be much more pronounced. Heat always speeds up a chemical reaction. Another factor to consider, which has already been mentioned briefly, is the chemistry of the glass. Dark glass is much more likely to react with highly alkaline water drops. This is why water spots are much more difficult to remove from this type of glass surface. Dark glass also heats up more quickly in the sun. Ordinary clear glass is the next down. And the easiest to clear are reflective pyrolytic first surface windows.
I am quite sure that a deeper look into the "structural biology" of hard water spots would reveal some rather fascinating facts about the destructive nature of these "one celled" animals. Their "cellular walls" are in fact alive as each spot evolves through its life cycle. One tactic is to prevent the walls from forming in the first place by creating a hydrophylic surface by the use of a titanium dioxide coating. Of course hydrophylicity can be created in other ways too. There are various silanes that can be functionalized to form covalent bonds with the "free" oxygen atoms on the surface of glass, and at the same time hold onto water molecules strongly enough to form a film. In this way water drops are not so easily formed. I would like to point out that the ability of such designer coatings to resist the alkali attack of high pH water drops is not dependent on their hydrophobic/hydrophylic properties;...but rather their chemistry. It is also true that when spot rings have been eliminated by hydrophylic coatings, it is far less likely that hard water from either a sprinkler system or concrete efflorescence will do damage to such coatings. And so the holy grail of stain resistant coatings might not be hydrophobics at all. But rather just the opposite. You judge!
Written by Henry Grover Jr.
henrygroverjr@gmail.com
To receive these posts in your inbox just type your address in the box at the top right, "Follow by Email".
Also if you have any interest in any of the products mentioned or alluded to in any of these posts please just send me an email.
No comments:
Post a Comment