Within a room there may be pockets of air with different humidity levels due to the presence of windows, heaters, air conditioners or light fixtures. For example, heat from sunlight lowers the humidity near windows. In cold weather, moisture may condense in the area around them. (Hot air can hold more water vapor than cold air. When the temperature drops suddenly water in the air changes from a gas to a liquid, forming condensation.) This can cause an unstable environment for any pictures hung nearby.
The movement of paper resulting from environmental changes
is a natural occurrence and should not be restricted in the mounting process.
Restricting a work on all four sides prevents this natural movement. If artwork,
so mounted, is subjected to extreme humidity changes over a period of time,
the paper may buckle and remain
permanently deformed. When it comes to elements effecting hygroscopic materials,
all facets of the environment work together (pollution, heat, humidity and light).
It is necessary to try and control these forces as much as possible
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