STRETCHER KEY: WHAT ARE THE WOODEN THINGS ON THE BACK OF A CANVAS FOR?
I often observe mistakes related to the placement of stretcher keys. Some articles briefly touch on the subject without explaining in detail when and how to use them optimally. They are not just pieces of wood to fit into the designated slots and be done with it. It’s more subtle and technical! You shouldn’t do anything haphazardly; the quality and lifespan of your painting depend on it. To fully understand, here are detailed explanations on how to use them optimally, along with problems and solutions…
STRETCHER KEYS: WHAT ARE THEY USED FOR?
Canvas keys are small wooden triangles essential for stretching a canvas. Inserted into slots at the corners of the canvas, they allow for a taut, flat working surface.
Canvas keys are useful during the preparation of the canvas for painting, and also over the years. Over time, the canvas can loosen. Cotton canvases will stretch more compared to linen canvases. For paintings intended for sale, the latter are often preferred.
Typically, there are eight keys, with two keys per corner. Large canvases may have additional keys to be wedged at the base of the uprights or wooden support crosses.
WHEN TO INSTALL CANVAS KEYS?
Install the canvas keys before starting to paint, to work on a perfectly flat surface and to prevent some paints from cracking if the canvas is stretched too much afterwards.
HOW TO INSTALL CANVAS KEYS?
When the canvas keys are very sharp (which depends on the brand), it is essential to cut the tip off with cutting pliers before inserting them! In some cases, if the canvas key is driven in too deeply, it can puncture the canvas… Which would be quite unfortunate!
It is important to be cautious when inserting the keys. A key that is driven in too deeply cannot be removed!!! Therefore, it is necessary to proceed gradually!
It is a common mistake, but it is essential that the striking area of the keys forms a right angle with the frame’s upright! This way, the hammer strikes directly along the slot’s axis. When the keys are placed in the wrong direction, the part where the hammer hits is angled relative to the wooden upright and the direction of key insertion. This leads to an off-center movement that is much less effective, so one tends to hit harder, increasing the risk of breaking the keys.
Think of the example of a nail being hit at an angle: the same principle applies here.
An exception applies to very small frames, for example, 15×15 cm. There is not enough space between the keys to strike perpendicularly. In these specific cases, the keys will be reversed to be at an angle, but since the striking angle is not optimized, it must be done carefully.
Once the keys are in place: tap with your hammer to drive in the canvas key. Do not hit too hard at once, balance all the keys.
First test:
Once the canvas keys are driven in, to check that the frame is well balanced, the opening between the wooden uprights should be balanced at all four corners.
Second test :
Lay the canvas flat. All four corners should touch the table or the ground. If one of the corners does not touch, and you can rock the canvas on the other two corners, it indicates an imbalance in tension. (I will address this issue further below).
Then…
Once your painting is completed, or between layers, it is possible that the canvas has loosened a bit, so you can slightly re-tighten it!
Over the years, the canvas may loosen a bit more, so the same principle applies.
HOW TO KNOW IF THE CANVAS IS WELL STRETCHED?
When the painting is finished, and to know if your canvas is properly stretched, tap it and listen! The sound changes. If poorly stretched, the sound is flat. If well stretched, the sound is more percussive and resonates more. Do not excessively stretch your canvas to make it resonate, we’re making a painting, not a drum!! Excessive tension will put too much pressure on the material and wear it out prematurely! So, just a slight resonance…
LARGE CANVASES AND GESSO:
Be careful with particularly large formats and if you use Gesso (a white primer to protect the fibers and smooth the support): I recommend inserting the keys before applying the Gesso, but to stretch the canvas VERY SLIGHTLY at first: just enough to space the canvas from the uprights or wooden crosses and have a flat surface when you apply the Gesso (otherwise, the wooden upright behind will be felt through the canvas).
The moisture brought by the Gesso will temporarily stretch the fibers quite tightly. This phenomenon is amplified on large surfaces, potentially causing deformation of the frame.
I myself had a case with a frame where the wooden cross of my 100×100 cm canvas had bent, stretched like a bow… The center of the cross had moved about 10 cm away from the back of the canvas! Believe me, it’s scary!
Once the Gesso is completely dry, the canvas has relaxed, and before painting, you can then slightly re-stretch it to have a taut and flat surface.
LARGE PANORAMIC CANVASES:
When driving in your keys, ensure balance: Large panoramic rectangular canvases tend to stretch more along the longer sides than the shorter ones. Take this into account by balancing the keys on the widths and lengths to achieve a uniform tension of the canvas!
IN CONCLUSION
It is necessary to re-tension tactfully at each stage of the painting’s life and consider the canvas keys as a true technical tool, not just as a “stick it in the corners and go” 😉
HELP! WHAT TO DO IN CASE OF PROBLEMS WITH CANVAS KEYS?
No canvas key with my canvas:
It can happen that some frames are sold without the necessary keys for stretching the canvas, even though there are slots for them… (often very low-quality material). If you find yourself in this situation, and your new cotton canvas is already a bit loose, you have the option to make your own keys or buy them separately.
Tip: Really don’t buy too low-quality material! Choose your equipment wisely for a better artistic experience!!
Canvas keys don’t fit into the slots:
This problem is common, even with quality linen canvases. To solve the problem, use a cutter to reduce the thickness of the wood. Be careful not to remove too much. Then, finish the job with sandpaper. Be delicate in this operation to preserve the strength of the piece and that it is not too thin to come out on its own.
Key driven in too far, my frame is twisted:
If you drive a key in too far, it can cause the frame to deform.
If the deformation is not too severe: place the two corners that are in contact with the ground on supports about 1 cm high. Use heavy objects to press down the other two “air corners” to reverse the curvature of the canvas. Use a hairdryer to heat the entire canvas, especially the tense areas, to relax it.
If the key is really driven in too far, follow the same advice as for a broken key.
The canvas key broke while hitting it, I can’t drive it in or pull it out:
Tip number 1: Make sure this doesn’t happen by going gently when driving in the keys. 😉
Some keys are (often) made from low-grade wood that is quite fragile. If you end up with a key broken flush with the slot: the only solution is to detoil your frame by removing all the staples and/or seeds (nails). This way, you can separate the wooden uprights to retrieve the problematic piece.
However, be careful!! This operation is very technical and requires appropriate equipment! Especially for correctly and uniformly re-stretching the canvas, which is not at all obvious when you’ve never done it before…
My keys keep popping out, what can I do to prevent them from falling?
There are “key blockers” that are fixed with a nail to prevent the canvas key from coming out and falling.
AND YOU?
Was this article helpful to you? Have you ever had issues with your canvases? Do you have any other tips? Do you prefer working on cotton or linen canvases? Small or large formats? The comment section is for you…
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