Artist canvases will inevitably become loose. The materials tend to sag and loosen over time, which can be upsetting for a collector's painting, an heirloom or a canvas that is still being painted on. Often, a freshly stretched canvas can become loose overnight and be found in the studio the next day sagging on the stretchers. Thankfully, there are things you can do to save the canvas while saving you money, or for valuable pieces, there are professionals who can correct this without damaging the piece.
Canvas tightening keys are wooden wedges, usually in a plastic bag that is attached to pre-stretched canvases, or they can be bought separately. These are placed in between where the stretcher bars meet. They work by pushing against the stretchers and adding length. By doing this, the stretchers are made to be slightly larger, making them fit the canvas better. Fit the two keys into all four corners and add slight pressure. Do this to all four corners evenly to avoid warping and cracking. This is the easiest way to fix a loose canvas. Tightening keys cannot be used on a canvas with crossbeam or corner supports.
Water can be applied to the canvas to tighten it if canvas key did not work well enough or cannot be used. If the canvas has not been primed yet, apply water to the front of the canvas with a brush. Apply water to the back of the canvas if it has already been primed and worked on, and make sure to avoid touching to wood to prevent any warping of the stretcher bar frame. The canvas should tighten up once it is dry. This step is not recommended if this is an old or valuable piece.
Commercial liquid canvas tightener can be bought at most art supplies stores in the canvas accessories section. It usually comes as a spray that can be applied to the back of the canvas, and it should remove any sagging that water did not remove. If the canvas is small, this is a fairly inexpensive way to tighten it; however, this method can get expensive for large canvases. Although most sprays claim to be 100 per cent archival, don't use them if the painting is valuable.
Re-stretching may be a last resort. This will be very easy if the canvas has not been primed. Just remove the staples from one side and restretch that side as tightly as possible. It will be a bit more difficult if the canvas has already been primed. Remove the staples from one side, then, using canvas pliers, pull the side as tight as possible when stapling. The paint is at risk of being cracked if the canvas has been worked on and the paint is dry. Do not pull too tight when restretching in this way; alternately, take the piece to a professional.
You should take a valuable or old piece to a professional for restretching. Many frame shops offer this service, but if the piece is worth too much to risk damaging, take it to a professional conservator or restoration shop. Try contacting the artist for help if you bought it from an artist who is still working.
Humidity can loosen a canvas. If the canvas becomes loose during the summertime, it may resolve itself in the winter. Stretcher bars may warp if a canvas is stretched too tight.