These bits of wood are canvas tightening keys or wedges, used if the canvas begins to sag a little on the stretcher. This can happen over time due to temperature changes, humidity, and aging. You put them into the slots made for them in the corners of the stretcher bars, then tap them in further to tighten or square up (align) the canvas. Most canvases that come with canvas keys come with eight of them, two for each corner. If you’ve bought an unprimed ready-made canvas, or are stretching your own, don’t use keys until after you’ve primed the canvas, as priming makes the canvas more taut. Some canvases are made with rigid corners, without the option of hammering in a wedge or key. If you’re using one, just check the canvas is very taut before you buy it; it should stay that way.
How to Use Canvas Keys
If you find that your canvas has become loose or sagging, slide the canvas keys by hand into the slots in the stretchers in the corners. The wedges are oddly shaped, pointed on the end, and longer on one side than the other. The pointed end goes into the slot, but some people put them in with the long side flush against the side of the stretcher bar, others prefer to place them with the short side against the side of the stretcher bar so that the wedges are angled out toward the center of the canvas. Either one works, so try both methods and decide which works best for you. Place a small piece of cardboard between the canvas keys and the canvas to protect the canvas when you tap the keys into place with a hammer. With a smaller canvas, you can gently tap the end of both wedges, or keys, into the corner with the hammer, working your way around to each corner. You may see the mitered space at the corner of the stretcher increase as you tap the wedge. With a larger canvas, in order to keep the tension on the canvas even, you will want to push out the opposing stretcher bars first. In order to do that you will tap the wedges that push a stretcher bar in one direction, then rotate your canvas so that you can tap the wedges that push the opposite stretcher bar in the opposite direction. Then do the same for the remaining sides. Tap the wedges gently a few times only. You may need to go around again, but you don’t want to knock them too hard at first. You run the risk of splitting the wood of the stretcher, overcorrecting, or tearing the canvas. By gently tapping the wedges you will eventually achieve the desired tautness of canvas.
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