Stretcher keys are an integral part of accurate canvas picture framing during the lifespan of the canvas material. They are commonly wedge-shaped and fit into the corners on the back of the frame to help adjust canvas tension. Canvas material slackens over time and in varying humidity, so it is important to know how to use stretcher keys to keep the picture in optimum tensile condition.
The wooden stretcher bars that hold the canvas in position are prone to expansion and contraction with changes in ambient temperature, so this additionally impacts on the canvas tension. By using stretcher keys, you can tighten the canvas.
Locate the bag of stretcher keys on the back of the frame and remove them. Generally canvas frames come with a small bag containing eight stretcher keys stapled to the back of the frame. Buy a set of eight stretcher keys from an art supply store if they are missing from the frame.
Place a plain white tablecloth on a smooth tabletop to protect the front face of the canvas. Position the canvas frame on the tablecloth facedown. Check that there are key slots in each of the corners of the stretcher bars that frame the back of the canvas. There are two in each corner, one to each side of the stretcher bars.
Using your fingers, push a stretcher key pointed end first into a corner slot. Push the second key into the slot on the adjacent stretcher bar that makes up the corner. Repeat the procedure for each of the corners by pushing in two stretcher keys per corner before moving on to the next.
Use a length of 1-foot by 1-foot wood and place it at the protruding end of the stretcher key to provide a hammering surface area. Using a small hammer, gently tap the wood against each stretcher key only once to begin to drive the key into the slot. Turn the canvas over and look at the tension of the material.
Turn the canvas facedown again on the tablecloth if the material is still slack. Tap once only with the hammer on the wood that you positioned against each stretcher key, and recheck the front of the canvas for tightness. Repeat the procedure until the canvas is tight.
Tap only once per key with the hammer to maintain even installation and prevent the frame from warping.