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The only other thing we have done is to mount a laminate finish on top. We have found that will stop sticking. Be aware that depending on how glossy the picture is you may dull the finish.
We only do this if the frame the customer wants is to small for a spacer and they don't want mats.
Condensed moisture is what makes the photo stick to the glass. Condensation happens whenever the temperature of the glass is warmer on the inside than it is on the outside. (Remember this by visualizing a glass of ice tea in summer. When the ice melts and the tea achieves air temperature, the condensation goes away.)
Anything that makes an air gap between the glass and whatever's in the frame will work -- mats or spacers.
The air gap insulates, just as a double-pane window works. It slows the rate of temperature and humidity change inside the frame -- a good thing -- and provides a way to evaporate the moisture when it condenses.