Introduction
You just stretched a canvas using a canvas stretcher and days later it begins to lose tension. Before you curse the canvas, the canvas stretcher machine, the stretcher bar or blame the weather consider the following first. The canvas may not have been fully stretched tight in the first place.
Problem
A canvas is dynamic, it is effected by moisture and temperature so it is certainly a prime suspect and should always be considered. The canvas, depending on the type you are using can range from very heavy to very light and this too will impact the stretch. It is also worth noting that the artwork is applied to the face of the canvas and then laminated, (if it isn’t it should be). The back of the canvas is untreated and is exposed to the elements, this in itself should give food for thought. Why would we partially encapsulate a material that repels moisture on one side and absorbs it on another?
Solution #1
My recommendation would be to direct your attention to the stretcher frame construction, the type of profile and the spacing of support bars. Naturally, the stretcher frame must be sturdy and capable of withstanding the stretching force but the profile is the single most critical component that must have a pronounced radius on the top (and even a little radius on the bottom) to allow the canvas to be pulled over two hard edges. During a stretch we are attempting to pull a canvas 180 degrees away from the applied force. The harder the edges the more difficult it is to pull.
During a stretch the canvas can actually gather on the sides resulting in “cupping”, this can be detected during a stretch by tapping on the sides of the art with the pad of your finger. If it is not drawn tight it will sound rather dull, a higher pitch sound would indicate a tight stretch. This is when you staple.
Solution #2
Another area to focus attention your attention to is the placement of cross-bracing whenever the span is greater than 30″. We strongly recommend cross-bracing to fortify the middle of the assembled frame because without it the middle will deflect inwardly creating an “hour glass” effect and the image will move. Gussets or corner bracing helps but they are not as effective as a cross brace.
Summary
Don’t be too hasty to staple the canvas next time you are conducting a stretch, give the artwork a chance to be pulled over the radius and verify that it is pulled tight against the side using the tapping method. I would argue that in most cases the canvas was never fully stretched to begin with and with all of the other factors to consider wouldn’t it be nice to eliminate just one.