What is a canvas with "wedges" and how does it work?

WEDGES — also known as spacers, chocs, retainers, or ‘painter’s corner keys’ — are small adjustable pieces of wood fitted into the corners of traditional stretcher bars. Their purpose is to expand the stretcher slightly, allowing you to re‑tighten a canvas that has become loose or “bouncy” over time, during transit, through environmental changes, or simply from pressure applied during the painting process.

"Why would you choose a canvas with wedges? Surely if it is stretched properly, they're unnecessary?"

This is true, to a degree. Ultimately, whether to choose wedges or not depends on several factors, including

-          Profile / depth (deeper profiles are more stable but still benefit from wedges for long-term tension control

-          Dimensions (larger canvases loosen more easily and require adjustable tension)

-          Medium & painting style (heavy impasto, oils, gels, palette-knife work and vigorous brushwork may loosen canvas over time)

-          Transport (movement, vibration and handling can relax tension – wedges allow correction after arrival

-          Environmental Conditions (Sunlight, cold, heat, humidity and seasonal changes all affect canvas fibres and stretcher wood)

-          Weight of the painting as it develops (As paint layers build, the cloth can shift and relax even if it was perfectly stretched at the start)

But that’s not all they’re good for.

A properly made, installed, and functioning wedged stretcher can be disassembled and reassembled. This is one of the biggest advantages of a traditional stretcher system. It means the canvas can be removed from the frame, rolled, exported in a tube, and then re‑stretched at its destination with far less effort and far greater accuracy. This is especially valuable when the receiving country has limited stretching facilities, inconsistent quality, or extremely high framing costs. A wedged stretcher makes the entire process smoother, safer, and more professional.

Most, cheaper pre‑made canvases have rough wedge slots cut into the corners, with loose wooden or plastic chocs rattling around in the packaging. Artists are then left trying to force badly‑fitting wedges into those slots — often cracking the frame, splintering the corners, or ending up with uneven tension.

ETH Canvas does things differently.

We use a precision dovetail system, which allows all wedges to be pre‑fitted during manufacture:

• 8 wedges per corner

• 2 wedges per cross‑bar

• No glue

• One stabiliser staple that pops out cleanly when tapped

This means your canvas tightens evenly and safely, without fighting with mismatched chocs.

Our stretchers are made from kiln‑dried, knot‑free obeche, which resists splitting when wedges are tapped in — unlike cheaper pine frames that often crack at the exact moment you try to tighten them.

Each wedge is individually cut to match its stretcher, forming a true functioning pair. They slide and expand smoothly, without damaging your cloth. Mass‑produced canvases simply can’t offer this.

ETH Canvas is one of the only local suppliers able to produce deep‑profile box frames with a full wedge system, up to 50 mm.

A properly tightened canvas should vibrate like a drum membrane when flicked. If it doesn’t, it needs tightening — and our wedge system lets you do that safely and precisely, expanding the corners of the stretcher bars slightly and enlarging the frame by 1-2mm.     

When to insert my wedges?

Wedges can be used at several stages in the life of a canvas: 1. To complete the tensioning of a hand-stretched canvas

Once priming is finished (priming causes the cloth to contract and tighten naturally), wedges give the canvas its final, controlled bit of tightening.

2. To tighten ready-made canvas that has relaxed in storage or during shipping  

It’s normal for pre‑made canvases to lose a little tension over time. A millimetre or two of expansion in the corners is often all that’s needed to pull out a slight sag or firm up a soft corner. If wedges are already in place, they may simply need a gentle tap to activate them.

3. To correct tension that has dropped during the painting process

Heavy paint, pressure from brushes or knives, and the gradual weight of layers can cause the canvas to loosen. Once the painting is dry (and varnished, if applicable), you can assess whether it needs a small adjustment. Always tighten gently — sudden over‑stretching can crack dry oil paint.

4. To restore tension in older paintings

Over the years, changes in temperature and humidity can cause a canvas to slacken. If the sag is slight, carefully tapping the wedges will usually bring the canvas back to proper tension without needing to remove staples or re-stretch the work.You only need a tiny amount of controlled expansion in the corners to correct a light sag. If the canvas doesn’t respond to gentle tightening, it means the fibres have relaxed too much for wedges to fix, and the only solution is a full re-stretch

How to use a wedge-system to re-tighten a stretched canvas

If your canvas is unprimed, don’t touch the wedges yet. Priming pulls the cloth tighter on its own, so any adjustment should only happen once the primer is dry and the canvas has settled.

1. Seating the wedges

Every stretcher corner has two tapered slots designed for the wedges. From the back, slide each wedge in point‑first and give it a gentle tap. As they start to move, you’ll often see the mitred corner open slightly — that’s exactly what should happen. The internal join keeps the frame stable, so you’re not weakening anything.

Unlike most suppliers, ETH uses zero staples in the construction of our wedge frames — only a single stabiliser staple to keep the wedge in place during manufacture. This means the wedges seat cleanly and respond exactly as they’re meant to, without fighting against hidden metal fasteners inside the frame.

You can tap downward with the canvas resting on a table, or upward while holding it — whichever feels more controlled. The important part is consistency: work one corner at a time and use the same force so the tension stays even. If one corner is softer than the others, it may need an extra nudge.

2. Orientation

Since they aren’t symmetrical, wedges will seat differently depending on the stretcher. Some frames accept them better long‑side against the bar, others short‑side. There’s no “correct” orientation — use whichever slides in cleanly and feels right for that particular canvas.

3. Expanding the bars first

For the best result, expand the stretcher bars before you rely on the wedges. Use a piece of wood against the inside of the bar and tap lightly to move the bar outward without marking the timber. The wedges then act as retainers, locking that expansion in place. They’re not meant to be the main force — they hold the adjustment you’ve already created.

This gives a cleaner, more controlled tightening and avoids over‑correcting with the wedges alone.

4. Small vs large canvases

Use a rubber hammer. On smaller canvases, simply work around the frame, gently tapping both wedges in each corner and checking the tension as you go. Stop as soon as the canvas feels firm and responsive. As you tap the wedge, the space at the corner of the stretcher may increase. Don't be alarmed. This means it’s working.

On larger canvases, you need to expand the facing stretcher bars first, to keep the tension on the canvas even. So, tap the wedges that push one of the stretcher bars in one direction, then rotate the canvas and do the same for the wedges pushing the opposite bar, in the opposite direction. Then, repeat for the other sides. This prevents the canvas from pulling in one direction and keeps the tension uniform.

5. How hard to tap

A wedge system is designed for precision, not force. “Tap” means tap — not bludgeon. Too much impact can split the wood, distort the frame, or over‑tighten the cloth. It’s always safer to tighten gradually and go around a second time if needed.It is advisable to check the progress of your tightening after each wedge position and that you stop once the canvas is taut enough – once the wedge is embedded, it cannot be removed and if you "over-tighten", it can create too much pressure within the stretcher structure and end up warping your frame.

6. When wedges don't work

A tiny amount of controlled movement in the corners is usually enough to correct a light sag. If the canvas doesn’t respond to gentle tightening, the fibres have relaxed beyond what wedges can correct, and the only proper fix is a full re-stretch.

7. Leave them in

Wedges are part of the stretcher system. Once they’re in, they stay in. Removing them allows the bars to contract again, and the canvas will slacken.