Stretched canvas, mixed fabric, white, primed
Fluctuations in the humidity can occasionally cause a stretcher bar to warp. In order to prevent that, you should keep in mind that any change of your picture´s environment can cause damage. Most natural fabrics contract when wet. For that reason, a stretch canvas should not be wedged out at the corners before you do your painting. This contracting property of the fabric can be put to good use in instances when dents or light creases have shown up on your canvas: you can wet it yourself from the back to remove them.