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The anti-reflective coating expands and contracts differently with heat than the plastic lens underneath. A big temperature difference will cause it to crack. So don't use ice cold water, use water that's at room temperature, which is usually what comes out of your cold tap.

> water that's at room temperature, which is usually what comes out of your cold tap.

Is that really your experience where you live? In mine, the cold tap is cold - once allowed to run through whatever's in the pipes in the building, since it's come from underground.

It depends on where you live and the time of year.

I've lived places where in the winter, the tap water is just a couple of degrees above freezing. I've lived in places where in the summer I've measured the tap water at over 90 degrees.

He must live somewhere that doesn't get to any real temperature extremes.

Yeah, I used to wash my glasses in hot water. I didn't notice an instant change, but it noticably degraded their quality after a while. Water straight from your cold tap is the way to go.

I've ruined my coatings by wearing them in the sauna before. Sauna temps tend to be around 180.