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See AllHow to Use Stretcher Keys
Ask the Expert: “I recently placed an order of standard wooden stretchers and they arrived with two little packages of wedge looking wooden things. What are these for, and how do I use them?”
The wedges are called “keys”, and they are inserted in the corners of the stretcher frame to re-tension a canvas that has gone slack.
The mitered mortise-and-tenon joints at the corners and the ability to “key out” or expand slightly is what makes the frame a “stretcher” rather than a “strainer” (a frame with fixed joints). It's a good practice to include keys when you transport or sell a painting for the convenience of collectors or gallery owners.
To properly insert the keys, place 2 at each corner in the small spaces that are found inside the frame. Most standard stretcher keys are inserted with the bevel (diagonal edge) against the inside of the frame.
Some light duty pre-stretched canvases use keys that are inserted with the bevel toward the joint. Using a small hammer, carefully tap the keys in until the canvas has reached desired tension.
Insert a small piece of cardboard between the back of the canvas and the frame to protect it while you are inserting them. Once the canvas has been restored to full tension, keys can be held in place with staples to prevent them from falling out in shipment.
Some stretchers are designed with keys that screw into place and expand with the use of a threaded rod. This design can exert a great deal of force, making it possible to adjust very large, heavy canvases.
It’s important not to over-tighten canvas or exceed the point of maximum sustained tension or damaged stretchers can result. Use caution when keying out older, more brittle paintings to avoid cracking.
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