How to Use Stretcher Keys


What are Stretcher Keys?

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.

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 wafers of wood inlay that could be driven in to expand the chassis. 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 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.

Keys are sometimes included with pre-stretched canvases, but almost always included with stretcher bars. While extremely useful, they’re often not well understood. In fact, many artists neglect to use keys entirely, not realizing that they could potentially solve serious problems if a painting becomes loose on its stretchers.

Stretcher keys are useful for several reasons:

  • 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.

How to Use Stretcher Keys:

    • Step 1 - 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.

    • Step 2 - 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.

    • Step 3 - Finally, leave the keys in the frame, taping them in place so they don’t come loose.