Canvas Keys: What They Are, What They Were For, and Why We Don't Use Them
By Graham Sanderson, founder of ACF Canvasses. I make canvases with no keys, so I have an interest in this subject. Every claim below comes with its source, linked wherever it is online, so you can check it.
The short answer
Canvas keys are small wooden wedges that push a stretcher's corners apart to tighten the canvas. They are appropriate on bare canvas, before it is sized or primed.
On a primed or painted canvas, the Canadian Conservation Institute says keying out is highly inadvisable, for oil and acrylic paintings alike. It is likely to cause cracking, and the pattern conservators recognise runs diagonally out from the corners.
If your canvas has gone slack, check the room's humidity first. Slackness usually comes from the environment or the fabric, and keys do not fix either.
What artists are usually told
Most stretched canvases arrive with a small bag of wooden triangles taped to the back. The message is simple: if the canvas goes slack, tap the keys in.
It is common practice. CCI notes that keying out is frequently done to fix slackness, and says that is the wrong reason to do it (CCI Notes 10/9). The rest of this post looks at why.
What are canvas keys?
Keys, also called wedges, are thin triangles of wood that sit in slots inside the corner joints of a wooden stretcher. The glossary of Art in Transit (National Gallery of Art, Washington, 1991) defines them this way: tapped into the corner, they force the stretcher members apart and tighten the canvas.
Most corners take two keys, so a four-bar stretcher usually has eight. Some older stretchers were built with one key per corner (CCI).
Keys only push outwards. CCI is clear that they cannot close a joint once it has opened.
What are canvas keys for?
They were designed for one early moment. When raw canvas is first stretched, the waviness in the threads (the crimp) slowly pulls flat and the canvas loosens. CCI encourages artists to correct the tension at that stage, before the glue size goes on, to allow for the slackness that is coming.
CCI's summary: keys are appropriate for the artist before the painting is finished. Used long after, they can cause more serious problems than they were meant to solve.
Should I use the keys that came with my canvas?
If it is a ready-primed canvas from a shop, the window CCI describes has already passed. The canvas was sized and primed before you bought it.
The one job the keys still have is keeping the corners stable. If a key is loose, CCI says to press it back with finger pressure only, then secure it so it cannot drop out.
How do you put keys in a canvas?
This is the method for the situation keys were designed for: bare canvas you have stretched yourself, before sizing. It follows the procedure in CCI Notes 10/9.
Fit. Keys go in the small gaps along the inner edges of each corner joint. They should fit tightly without forcing the joint open.
Small canvas. Stand it upright. Using a small tacking hammer, tap one key in each of the two bottom corners, keeping close to the surface it rests on. Rotate the stretcher a quarter turn and repeat, working all the way round.
Large canvas. Lay it flat. Tap both keys in one corner, then move to the next.
Go gently. A keyed corner opens one direction at a time, which is why CCI describes the bolt-type stretcher, which opens both directions together and holds 90 degrees, as less aggressive. Keys cannot pull a corner back once it has moved.
Secure them. Unsecured keys can work loose and fall behind the bottom bar, where CCI says they can cause bulging and cracking along the lower edge.
Can keying out damage a painting?
Yes. Here is the research behind that, including what it is and what it is not.
In 1991 Marion Mecklenburg and Charles Tumosa published a computer model of a typical oil painting on canvas, 76 x 102 cm (30 x 40 in) (Mecklenburg and Tumosa, 1991). They moved each stretcher bar outwards by 0.1 cm, which opens the gap at the key by about 1.4 mm. Then they dried the modelled painting from 70% to 10% relative humidity.
The peak stress in the paint reached 10.4 MPa. The paint in the model, a Naples yellow, breaks at 2.5 MPa. That is just over four times its breaking strength. The paper's figure caption rounds this to five times.
A plain way to picture it: when paint and ground are cold and dry they behave less like a rubber band and more like a biscuit. They cannot stretch much, so any sudden pull cracks them. CCI says the lower the humidity and temperature, the more brittle these layers become.
Mecklenburg and Tumosa make the general point in another chapter of the same book: any distortion of a painting, accidental or deliberate, and they give hammering out corner keys as their example, induces stress, and if it is high enough the paint cracks and flakes (Mecklenburg and Tumosa, 1991).
What do keying-out cracks look like?
The same model predicted the crack pattern: lines radiating diagonally from the corners, reaching about 20 cm (8 in) in on a painting that size. The caption in Art in Transit says this pattern is found on paintings throughout the world.
CCI describes it from the conservator's side: diagonal lines running from the corners towards the centre, quite distinct from other cracking caused by dryness. Once the paint and ground have cracked, it cannot be undone.
CCI also notes it can take only a single keying out, early in a painting's life, to cause it.
Why do the corners take it worst?
Try this with a tea towel. Pull it along the threads and it barely gives. Pull it corner to corner and it stretches noticeably. Woven fabric is strongest along its threads and weakest on the diagonal.
Christina Young makes the technical version of this point in The Conservation of Easel Paintings: on a stretcher, stress in the canvas is uneven, especially near the corners, because the pull there does not line up with the directions of the weave.
The lining chapter in the same book adds a long-term figure. Repeated keying out over many years can grow a canvas by about 1% overall, more at the corners, which is close to the breaking point of old, brittle paint and ground.
Can keys damage a painting even if you never touch them?
They can. Timothy Green of the Tate, writing in Art in Transit on vibration in transport, describes keys shaking free, dropping, and getting trapped behind the canvas. He calls it a familiar problem, and says the sharp distortion can cause cracking and even paint loss.
Barbara Buckley's chapter on stretchers in The Conservation of Easel Paintings records that keys are often lost or replaced, and may split or fall out unless secured with wire, tape or adhesive. She notes they are usually softwood, and that a hardwood such as beech or cherry would split less under the hammer.
When is keying out the right call?
The literature does not say never. Two cases stand out.
First, the one keys were designed for: bare canvas you have just stretched, before sizing.
Second, a case from Timothy Green. If a painting's canvas has gone completely slack and it is about to travel, he advises keying out just enough to give it some tension. A loose canvas flaps, and each flap ends in a sudden stop that is likely worse than the vibration. He adds that you should measure the humidity first, to make sure the slackness is not simply dampness.
That second case is a museum conservator weighing two risks before a painting goes on a truck. It is a different decision from tightening a canvas at home because it looks a little loose.
Why is my canvas sagging?
CCI lists the usual causes: poor or changing humidity, too few tacks or staples, stretcher bars that have shrunk, a painting squeezed too tightly into its frame, or brittle tacking edges that have broken. It states that keying out corrects none of them.
The fabric matters too. CCI says canvases made from natural fibres are very unlikely to hold an ideal tension, because of stress relaxation and their ongoing response to humidity. Christina Young records that both linen and cotton respond significantly to changes in humidity.
What to do instead
From CCI's guidance on finished paintings:
Check the room. Aim for 40 to 60% relative humidity and avoid sudden swings. A cheap hygrometer will tell you where you are.
Give it a season. Slackness after a dry spell often corrects itself as humidity rises again.
Check the basics. Loose or missing keys, gaps between staples, torn edges, a frame that is too tight.
If it stays slack in all conditions, or it is a painting you value, ask a paintings conservator.
What do conservators use instead of keys?
Buckley writes that all conservation-designed stretchers reject keys for applying tension. From the second half of the twentieth century, conservators including Richard Buck, Franco Rigamonti, Gustav Berger, Franco Del Zotto and Giorgio Staro designed stretchers that hold even tension without them. The Lebron expansion-bolt stretcher was developed in the late 1950s after conservator Caroline Keck complained about keys falling out during travel.
It is worth being precise here. Those designs dropped keys, but they did not drop adjustable tension. They use springs or bolts instead.
The key itself has a long history of fixes. Antoine-Joseph Pernety described keyed stretchers as a new invention in 1754. Aaron Draper Shattuck patented a cast iron key in 1883, partly to stop keys dropping out. Frank Pfleger's 1886 patent included a U-shaped key for the same reason; Buckley notes his stretcher bars sold well and his key did not.
Why don't ACF canvases have keys?
We took a different route from the conservation stretchers. Instead of a better way to add tension, we built a canvas that does not need it.
The fabric. Our canvas is 100% Polyester PET. The lining chapter of The Conservation of Easel Paintings describes polyesters as chemically stable materials that do not respond much to moisture changes. That is the main cause of seasonal slackness in linen and cotton.
The strainer. We design and build our own MDF strainer. The corners are fixed, so there is no joint to hammer open and no slot for a wedge to fall out of.
How it happened. We did not read the conservation research first and design to it. In 2013 we CAD-designed the strainer with the artists we were working with in South Africa, and it solved the sagging problem. We have not had a canvas sag since. Keys never came into it. The research came years later, when we expanded into the UK, and it has mostly confirmed a design we already had.
We have also supplied these MDF strainers to the photographic industry since 2013, and none has ever come back to us.
What artists tell us
We do not ask about tension in our review requests. It comes up anyway.
“Remains taut and square... I have not painted on better in 40 years of working.” Marc Yeats
“As a fluid artist I find it difficult to maintain the tension with traditional canvases.” Sarah Garrett
“The canvas never sags.” Marie Theron, Durban
“I really love the solid corner pieces on your frames and know that they are solid and that the canvas will not bend or buckle.” Jill
What this post does not show
The damage figures come from a computer model, not a real painting keyed out in a lab. It is finite element analysis built on measured material properties. CCI calls it an experimental study. We would rather you knew which it is. Stefan Michalski notes in Art in Transit that an earlier humidity model from the same research group predicted crack patterns seen on real paintings, including keying-out cracks, which is why conservators take this work seriously.
The model uses an extreme. Drying to 10% relative humidity is far below the 40 to 60% CCI recommends. The model also used one paint, a Naples yellow oil. Other paints will differ.
Polyester still relaxes a little. Christina Young records that every canvas material tested, polyester sailcloth included, shows some stress relaxation and creep. Polyester's advantage is that it responds far less to humidity, not that it never moves.
We have not run our own keying-out test. Our evidence is our design, twelve-plus years of production, and what customers tell us.
When ACF is not the right choice
Our strainer corners are fixed, so there is no way to add tension later. We have not needed to. But if adjustable tension matters to you, for example because you stretch and size your own raw linen, a keyed or bolt-tensioned stretcher is the right tool for that job.
FAQ
What are canvas keys?
Small wooden wedges in the corner joints of a stretcher. Tapping them in widens the frame, which tightens the canvas.
How many keys does a canvas need?
Normally eight, two for each corner. Some older stretchers take four.
Should I key out a primed or painted canvas?
No. The Canadian Conservation Institute advises against it for both oil and acrylic paintings.
What should I do with loose keys?
Press them back by hand, no hammer, and tape or wire them in place so they cannot fall behind the canvas.
My canvas is sagging. What is the first thing to check?
Humidity. Slack canvas is most often a response to the room, and keying out will not fix that.
How can I tell if a painting was keyed out?
Look for cracks running diagonally in from the corners. Conservators recognise the pattern as distinct from ordinary drying cracks.
Why doesn't ACF supply keys?
Our strainer has fixed corners and our polyester canvas responds far less to humidity than linen or cotton, so there is no joint to adjust.
If you have keyed out a canvas and have a view on any of this, or think we have something wrong, tell us in the comments. I read and reply to them.
Read next
- Polyester Canvas: Honest Answers to the Hard Questions Traditional Painters Ask
- Does Paint Stick to Polyester Canvas? The Honest Answer
- Art materials were reinvented by science. Canvas wasn't.
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Sources
- Canadian Conservation Institute. Keying Out of Paintings, CCI Notes 10/9. By Debra Daly Hartin; originally published 1993, revised by Debra Daly Hartin 2016 and Wendy Baker 2019. Read online
- Mecklenburg, Marion F. and Charles S. Tumosa. “Mechanical Behavior of Paintings Subjected to Changes in Temperature and Relative Humidity”, pp. 173–216, and “An Introduction into the Mechanical Behavior of Paintings under Rapid Loading Conditions”, pp. 137–171. Michalski, Stefan. “Paintings: Their Response to Temperature, Relative Humidity, Shock, and Vibration”. Green, Timothy. “Vibration Control: Paintings on Canvas Supports”. All in Art in Transit: Studies in the Transport of Paintings, ed. Marion F. Mecklenburg. National Gallery of Art, Washington DC, 1991. Free full text
- Buckley, Barbara A. “Stretchers, Tensioning, and Attachments”; Young, Christina. “History of Fabric Supports”; Hackney, Stephen, Joan Reifsnyder, Mireille te Marvelde and Mikkel Scharff. “Lining Easel Paintings”. All in The Conservation of Easel Paintings, ed. Joyce Hill Stoner and Rebecca Rushfield. Routledge, 2012.
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