Margaret,
known enemies such graphs (as well as photos, oils, etc.):
1. too high or low humidity;
2. Variations in temperature (especially dangerous crossings at zero);
3. Ultraviolet, especially - direct radiation (eg, on the wall next to the window)
Particularly dangerous (compared with a system of canvas, oil) to store graphics, I did not see it as a standard graphic work is protected by glass (a glass and plastics with a high absorption in the ultraviolet range) and the frame with a backdrop. Including - from dust.
Chistotsellyuloznoy Watercolor on paper, I think, can be stored no less than the canvas, oil. And soon - longer.
Tempera - the same, plus - it does not darken.
Bed and sanguine can be fixed chemically and physically.
I think the danger of storing graphics, compared with oil paintings, exaggerated.
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