What if the canvas gets loose?

When you stretch the canvas out in a humid environment, you run the risk of it slacking when placed in dry environment. Therefore, always stretch your canvas in an area with as little humidity as possible. Temporarily stretching the canvas and letting it dry thoroughly before affixing it permanently is another solution. The natural fiber is at the longest when it is dry and it shrinks when the humidity rises. Given the natural properties of its fibers, the canvas reacts to both climatic conditions and the paint which is applied.