A canvas goes slack because the fabric and the wooden bars underneath it both absorb and release moisture, and they do it at different rates. VanArtHub sees this constantly at its Davie Street workshop in Vancouver, where the swing between a wet winter and dry indoor heating drives the cycle hard. Most cases are corrected by tapping the keys. A minority need a full re-stretch, and telling the two apart is the first thing to do.
How to tell how bad it is
Take the painting off the wall and look across the surface from the side, with a light source low and to one side. This is called raking light and it makes any deviation from flat obvious.
-
A soft dip in the middle of one side, springs back under a fingertip. Normal relaxation. Keys will fix it.
-
Ripples running diagonally from the corners. Usually uneven original tension rather than age. Keying helps but may not fully clear it.
-
The fabric visibly hangs away from the bars, or you can push it and it stays. Beyond keying. This needs re-stretching.
-
The frame itself is not flat: one corner lifts when the piece sits on a table. The problem is the timber, not the fabric. New bars.
-
Fine cracking along the line of the slack. Stop. Do not key it. Tightening a canvas that has already cracked drives the cracks further.
Tapping keys, properly
Lay the painting face down on a clean, soft, flat surface. There are eight keys, two in each corner. Tap each one a small amount, in rotation around the frame, and keep going round rather than driving any single one home. A light hammer, small taps, checking the front after each full circuit.
Stop as soon as the surface reads flat. The instinct is to keep going until it is drum-tight, and that is how paintings get damaged. You are aiming to remove the slack, not to add tension the piece never had.
If the keys are already fully in, there is nothing left to take up and the answer is re-stretching.
When it is the humidity and not the canvas
A canvas that is slack in February and taut in August has not deteriorated. It is responding to the air. Before doing anything irreversible, note the season. Keying a canvas at its slackest point of the year means that when the humidity comes back the piece is now over-tensioned. If a painting swings noticeably, the better fix is to change where it hangs or to seal the back, not to key it twice a year.
What we do differently on an old painting
Re-stretching a painting that has been on the same bars for decades is not the same job as stretching a new one. The paint layer has dried in one position, the fabric has taken a set, and the tacking margins are often brittle and sometimes already torn. The aim is to return the piece to the tension it had, on bars that will hold, without introducing any new stress. Where the margins are too degraded to take staples again there are other approaches, and all of them start with looking at the actual piece.
Related
See canvas, damp and humidity for the cause, stretcher bars explained for keys and bracing, and canvas stretching in Vancouver to have it looked at.
Common questions
Why has my canvas gone slack?
Almost always humidity. The fabric and the wooden bars both take up moisture from the air and release it again at different rates, so the whole assembly moves with the seasons. Age contributes, since canvas relaxes gradually over decades, and under-tensioning at the start contributes too.
How do I tell if it needs keys or a full re-stretch?
Look across the surface in raking light. A soft dip in the middle of a side that springs back under a fingertip is normal relaxation and keying will fix it. If the fabric visibly hangs away from the bars, or you can push it and it stays, keying will not be enough.
How do I tap the keys correctly?
Face down on a clean soft flat surface. Tap each of the eight keys a small amount in rotation around the frame, checking the front after each full circuit, and stop as soon as the surface reads flat. Driving one key fully home before moving to the next pulls the frame out of square.
Can I over-tighten a canvas?
Yes, and it is the most common mistake. The instinct is to keep going until it is drum-tight. You are trying to remove slack, not to add tension the painting never had. Over-tensioning distorts the weave and, on an older piece, can crack the paint.
My canvas is slack in winter and taut in summer. Is that a problem?
That is the painting responding to the air rather than deteriorating. The mistake would be keying it at its slackest point, because when the humidity returns the piece is then over-tensioned. If it swings noticeably, change where it hangs or seal the back instead.
There is fine cracking along the slack area. What now?
Stop, and do not tighten it. Tightening a canvas that has already cracked drives the cracks further and the damage is not reversible. That piece needs looking at before anything is done to it.
The keys are already fully in and it is still loose. What then?
There is nothing left to take up, so the answer is re-stretching: lifting the canvas off, and mounting it again at the right tension, usually on new bars. It is the more delicate of the two jobs because the paint has already dried in one position.
One corner of the frame lifts off the table. Is that the fabric?
No, that is the timber. The bars have twisted, and no amount of tension work will correct it because the problem is the frame rather than the canvas. That piece needs new bars.
