Canvas is a historic material for paint, and provides a pleasant surface to look on, but it is not completely stable. While modern materials allow for a fully synthetic weave, the traditional surfaces are still linen and the more modern cotton. Both of these react to the environment.

The strands of material are twisted to make thread from which they are woven. So, when the atmosphere is damp, the organic material takes in water and swells up. This makes the strands thicker, and so they distort each other more, tightening the canvas. In dry and hot weather, the canvas relaxes. In extremes of weather, it can sag.

It is possible to tighten or loosen your painting, if it is not jammed solidly into the frame. (For the love of heaven, if your painting is valuable or delicate, take it to a professional to do this.) Artists who paint on canvas do it before painting, to give them a good surface to work on. I like to paint on a canvas that is nearly drum tight, and gives out a low thud when tapped.

The way this is done lies in the construction of the prepared canvas. It is tacked to stretcher bars that fit into each other at the corners but are not fastened, so they can be pushed outwards. Sticking in wedges and tapping them will push the bars apart and tighten the canvas that keeps them all together. You’ll need a light hammer, the wedges, and a piece of cardboard.

Turn the painting over and lay it down flat on a clean surface. I use plastic bags, laid out on the floor. There are eight holes, because each corner can be pushed out two ways. Each corner will have one hole that’s lower than the other. The wedges go into those first. Choose one and put the cardboard under that, to protect the canvas, and then tap the wedge in until you feel it firm up and the corner starts to separate. Then do the same with the opposite wedge. Don’t stretch it out fully yet. Do all eight holes, always working in opposite pairs. Then work corner to corner, stretching it evenly. You don’t want to end up with a diamond-shaped canvas. Stop when you can lift the canvas and tap it to get a drum effect. You’re done. Tidy up.