I recently experienced one of our beautiful clients calling me to say there were weird marks over her photos. It was framed series of 9 images and the frame was one of our usual black box frame and the photos were set back (so no matt board).
It sounded very unusual and I asked her to bring them in so I could see.
We studied this together and then she thought she was going crazy because she could not see the same marks,
Finally we noticed them , they were not on the prints at all but they were on the inside of the glass.
To cut a long story short they were hung in area where the down light above was incredibly hot and way too close to her art and this caused the glass to sweat.
Our framer Pete from Framecraft suggest you stand beside your art and hold your hand up towards the light, if you feel any heat this is not going to be ideal for your artwork.
He recommends that if you have down lights in your house that they be 12 volt halogens or leds.
"Light Emitting Diodes (LEDs): LEDs are the next generation of lighting. They don’t generate heat, take no time to reach full brightness, contain no chemicals such as lead or mercury, and emit no UV rays or infrared radiation."as quoted on the Beacon Lighting Website