If you buy a canvas from a store you may notice that it comes with small wooden wedges. These wedges are called “canvas keys” and they’re for tightening the canvas. Canvas keys are optional and they don’t work if the corners of the stretcher bars are stapled or glued.
Art stores usually have a bucket of canvas keys next to the stretcher bars. If you decide to use them, you’ll need eight of them–two for each corner. Some store bought canvases have them stapled to the back of the canvas in a bag. I’ve even seen a few brands that use plastic canvas keys.
Use Caution When Tightening a Canvas
Disclaimer: Using canvas keys to tighten a painting is an extreme measure and can damage the painting if done improperly. It’s best to consult with an art conservator and have them fix it for you. Damaging a blank canvas is a small loss but ruining a valuable painting is a disaster. Proceed to tighten a canvas at your own risk!
A Slight Amount of Slack May Be Acceptable
The canvas and the wood stretcher bars are both affected by the weather. The changes to the temperature and humidity throughout the year make them expand and contract. A small amount of slack in the canvas is acceptable and it may even tighten as the weather changes. Canvases that have large bulges in them may need the attention of an art conservator.
I don’t recommend that you tighten a canvas with canvas keys. The brief overview provided below is just to demonstrate how they work, in case you’re curious.
How to Insert Canvas Keys into the Stretcher Bars
Insert the canvas keys into the slots on the inside corners of the stretcher bars. Position them so the edge with the angle is facing away from the corner as shown in the pictures below. I removed the canvas in the photos so that it’s easier to see how it works.
Some artists insert the keys into the corners after they gesso the canvas, in case it needs tightening in the future. I put mine in a drawer and forget about them. I don’t install them on my canvases because I don’t want to encourage anybody to to pound wedges into the corners of my paintings with a hammer.
How Do Canvas Keys Work?
The way that canvas keys work is the wedges force the stretcher bars apart slightly when you tap them lightly with a hammer. It’s best to slide a piece of cardboard between the canvas and the stretcher bars to protect the canvas. When you drive the key into the slot, the angled edge pushes the stretcher bars apart, which tightens the canvas. If I haven’t made this obvious by now, this is not a subtle action. Forcing the stretcher bars apart is severe. This is something you should avoid if possible.
Do I Have to Use Canvas Keys?
You don’t have to use canvas keys, in fact I recommend leaving that to art conservators who know what they’re doing. Many times the slight slack in a painting is acceptable and may change with the weather.
I should also note that canvas keys will not work if the corners of the stretcher bars are have staples in them. Sometimes artists will staple the corners of the stretcher bars so that they don’t come apart while they’re stretching the canvas. Stretched canvases from a store may have corners that are glued or stapled. In either case, pounding a canvas key into the corner will not force the stretcher bars apart.
I never use canvas keys. When I stretch a canvas, I try to get the proper amount of tension in the canvas right from the start. If I’m stretching it myself, I usually wet the canvas with a sponge and let it dry. If I need to adjust the tension I remove the staples and try again. When I think the tension is correct I then coat it with gesso. I have yet to have a canvas slacken to the point it was a problem when using this method.
Store bought canvases often have loose canvases but fortunately there’s an easier way to tighten them.
How I Tighten Store Bought Canvases
When I order canvases online, they occasionally arrive with a loose canvas. This is the result of poor packaging. They might stack other objects on top of the canvas in the same box and the weight can stretch it out during shipping.
Mist the Back of the Canvas with Water
Before you contact customer service and ask for a refund, try to tighten the canvas by spraying the back with water. Use a household spray bottle filled with plain water. The canvas in this example has a lot of slack in the corners and near the stretcher bars. Use a paint brush to wet those areas with water.
I let the canvas dry overnight by leaning it against a wall. It’s best to position the canvas so that air can flow around the back of it so it will dry faster. Allowing the canvas to remain wet for extended periods can cause mold growth. You can use a hairdryer if you’re in a rush.
When it dries it should be tight. Water causes the canvas to shrink and this should be enough to take the slack out of it. If the canvas is still loose, spray the back of the canvas again with water. Maybe try spraying a little more water this time. Another idea is to apply warm water to the back of the canvas with a sponge. The warm water might encourage the canvas to shrink more than usual.
Wet the Front of the Canvas or Apply a Coat of Gesso to Tighten It
It may help to spray the front of the canvas or wipe it down with a wet sponge, but only do this to a blank canvas. I’ve noticed that applying a fresh coat of gesso to the front of the canvas will tighten it somewhat.
Spraying the canvas with water to tighten the canvas has worked for me many times. It probably won’t work on dings that were created by some sort of impact or other damage to the canvas. If the canvas is still too loose to paint on, then at least there’s no harm to the appearance of the canvas and you should still be able to return it to the store.
I’ve only used the water spray method on blank canvases and I don’t recommend spraying finished paintings with water! The moisture may affect the adhesion of the paint, especially if it’s an oil painting.
Blank canvases are easy to tighten. If a canvas arrives in the mail with slack in it, the simplest solution is to spray the back with water.
Expensive paintings or paintings that hold a lot of sentimental value to you should be fixed by a professional.