.
Explore the Guide:
Canvas keys, also called stretcher keys, are thin, triangular wood wedges used to tighten loose canvas on a stretcher frame. Over time canvases can loosen their tension due to humidity or temperature changes. Canvas stretcher bar keys allow you to adjust the tension of the canvas to prevent sagging and prolong the life of your painting.
While extremely useful, they’re often not well understood. In fact, many artists neglect to use stretcher keys entirely, not realizing that they could potentially solve serious problems if a painting becomes loose on its stretchers.
Stretched canvases often include a small bag of canvas keys, usually stapled to the packaging. Sometimes, the keys are already inserted in the stretcher frame. If you buy stretcher bars to build your own canvas, you'll typically receive eight keys for every four bars. The size and shape of canvas keys can vary slightly, as manufacturers design them to fit their specific stretcher bars. Larger stretcher bars require larger canvas keys.
Strainer Frames: For many years, canvases were attached to strainers—frames with fixed corners that did not move or expand. If a painting needed to be tightened, the only options were to remove and reinsert the tacks that hold the fabric in place or remove and restretch the canvas entirely.
Early Stretcher Frames: By the mid-18th century, artists were using what could finally be called stretchers. Early versions used a variety of corner styles and had spaces for keys—small wood wedges that could be driven in to expand the chassis.
Modern Stretchers: In 1886, a design patented by Frank Pfleger of Joliet, Illinois looked much like the stretchers in use today. Pfleger’s design featured spaces in each corner for stretcher bar keys to be inserted, but his key was U-shaped, so only one per joint was needed. Companies including Friedrichs (later Fredrix) further improved and refined canvas stretchers, adopting the now-familiar, wedge-shaped keys.
Tension Adjustment: Over time, canvases can sag due to the weight of the paint, changes in humidity, or other environmental factors. Stretcher keys allow you to adjust the tension of the canvas, preventing sagging.
Preservation: Keys help preserve the structural integrity of the canvas and the paint layers. Maintaining proper tension reduces the risk of cracks or flaking in the paint over time.
Re-Stretching: Stretcher keys make it easier to re-stretch a canvas if it needs to be removed from the frame for any reason, such as cleaning or restoration.
Conservation: Proper tension, facilitated by stretcher keys, can extend the life of a canvas, making it a valuable tool for artists who want their work to endure.
If your stretched canvas starts to sag, don’t toss it—tighten it. Canvas keys are small but essential tools that help maintain the tension of your artwork over time. This video explains what canvas keys are, how they work, and why they’re crucial for keeping your canvas in top shape. Whether you’re a painter or a framer, understanding how to adjust a stretched canvas properly can prevent long-term damage and keep your artwork looking its best.
To insert the keys, place two keys 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.
Using a small hammer, carefully tap in each pair of keys. As the canvas tightens, check to make sure each corner is square. Don’t overdo it, or you could damage the artwork or overexpand your canvas.
Once the canvas is taut, you're done. Leave the keys in the frame, taping them in place so they don’t come loose. Store any extra keys you have in a safe place in case you need them for future adjustments.
Even if you've taken care to stretch your canvas properly, it can still become loose over time. Common causes for this include:
Humidity Changes: Linen canvas, especially when stretched in a humid environment, can loosen as the fibers contract and expand. If possible, stretch linen canvas on a dry day.
Over-Stretching: Avoid over-stretching the canvas initially, as this can weaken the fibers and lead to sagging.
Heavy Paint Application: The weight of thick paint layers can cause the canvas to sag. Use a heavier-weight canvas for techniques like impasto.
Canvas Keys: If your stretcher bars have keys, gently tap them in further to tighten the canvas.
Re-Stretching: If keys don't solve the problem, you may need to remove the canvas and re-stretch it. Use a staple remover and needle-nose pliers to carefully remove the staples, then re-stretch the canvas and secure it with new staples.
Avoid Water or Tightening Spray: For oil-primed canvases, avoid using water or tightening spray, as these can damage the priming layer. Tightening spray is best suited for minor dimples in acrylic-primed canvases.