Expert: Mike Fulcher - 2/13/2009

Question
QUESTION: Sprinkling a lawn with alkaline water forms hard spots on windows that seem to bond to the glass.  What is taking actually place?  What chemicals would be involved?  How would one reproduce this crystal bonding in a home workshop for experimenting?  Thank you

ANSWER: David, I apologize for the late reply.  The water spots are the result of the minerals in hard water being left behind after the water evaporates.  The minerals vary by location, but are typically calcium, iron, or even sulfur based salts.  They aren't chemically bonded to the glass, but instead are hard to remove due to their small crystal size fiting well into the microscopic surface defects in the glass.  The easiest way to remove them is with a dilute acid solution (sulphonic acid cleaners are what's usually available at the local store).  To try to replicate this, all you really need to do is allow water to evaporate on a glass pan or plate.  Once the water is gone, to increase the amount of material left behind just add more water and evapoarte it again.  Heating the water will speed this up, but try to avoid boiling it as this will affect the crystals you end up with.  I hope this helps.

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QUESTION: Can this be replicated with sodium carbonate or sodium bicarbonate?  I have experimented with bicarbonate and have got it to stay crystallized forever so long as it stays dry.  When soaked again, I guess it dissolves (or something) and doesn't stay permanent like the water spots do without the acid treatment you suggested.  I'm trying to get something that is impervious to water afterwards. What I know about chemistry you could put in your pocket and I was glad you explained the adhering process because that helps me narrow things down.  Can you suggest a way to get these crystals so water won't effect them after they are dry (like on a window)?  Could the infrared and ultra violet light of the sun on the window make any difference as opposed to just heat?  Thanks again.

Answer
I believe that the most common type of water spots are calcium based, in the form of calcium hydroxide, oxide or carbonate.  You will need chemicals to replicate this.  The easiest way would be to dissolve a small amount of calcium carbonate in water, then evaporate the water to form spots.  If you add a small amount of sodium hydroxide (lye) you can make more calcium hydroxide/oxide which is less water soluble.  If you make too much however, you will precipitate the hydroxide/oxide out and it won't form a spot on the glass.  By experimenting you should be able to find amounts that work for you.  Have fun.