I come from a recreational aquatics background (I am a Certified Pool and Spa Operator). Swimming pool water has to contain a certain amount of calcium (hardness) in order for the gunite or plaster to maintain its structural integrity. This is between 300-400ppm. This is higher than most tap water. Many pool owners have automatic pool sweepers whose sweeping tails often, if not always, shoot water on windows when they come close to the surface. In the heat on summer, water evaporation happens quickly enough to leave calcium deposits on windows. I have found here, during the infancy of my WC business, that these are sometimes difficult to remove with standard scraping. So now when I am getting pre-estimate info from a customer I ask if they have a pool. It helps to prepare me as to how I will price the job. And educating the customer about how those spots got there and why they should have it removed regularly (longevity of the glass) helps to get a commitment on scheduling future window cleanings.
Finally…I get to post about something I know alot about!