Using Canvas Tightening Keys
Timber stretcher bars which are used in the construction of a canvas can sometimes contract in length due to the drying of the timber over time.
Tightening keys come supplied with most brands of stretched canvas and they can be used to increase the tension of the canvas material over the stretcher frame. Canvas keys - which are usually made of wood, but sometimes of plastic - are the small wedges which are either pre-fixed in the corners of the canvas or stapled to the inside of one of the stretcher bars in a small plastic bag. If they are not pre-fixed in the corners gently slot them in; 2 in each corner.
After you have gently inserted all eight tightening keys, apply a moderate amount of equal pressure to each of the pairs of keys. Now check the canvas to see weather it is tight enough. If the canvas requires further tightening a light weight hammer can be used to gently tap in the keys using a small piece of wood as a buffer. If this approach is used it is important not to use too much force as this will result in the stretcher bar splitting.
It is important at all stages to ensure the same amount of pressure is used on all eight of the keys as the frame will buckle is an uneven amount of force is used.
Using this method to tighten a canvas usually provides good results, although there is always a risk of damaging the frame if it is done incorrectly. If the painting which needs to be tightened is of some value (monetary or sentimental) and if you are not experienced in tightening canvases it is recommended to take the painting to a professional framer for it to be tightened.