What are stretcher bar wedges?
Canvas stretcher bar wedges are the small triangular wooden (or plastic) triangles inserted into the four corners of a stretcher bar frame. By tapping these into the corners using a hammer they open up the join to stretch a canvas print. These may or may not be included by the supplier dependent on where you purchase them. Generally made from plywood or plastic there can be either one or two per corner.
Stretcher Bar wedges - material
Wedges/keys can take on a variety of forms dependent on the supplier. Most commonly the wedges will be in plastic or plywood, however beech, solid timber and oak are also used, amongst other woods. This generally depends on the image that's being stretched. You'll find that museums use high quality wooden wedges made from high quality timbers to complement the artwork. Wooden wedges of any sort tend to contract and expand with the stretcher frame depending on the rooms environment, reducing the likelihood of them falling out in the future. As plastic wedges do not have this trait you'll find small "lumps" in them to hold them in the corners.
Wedge shapes and quantity
Wedges will generally be triangular in shape. However, also common, especially in plastic wedges, is having one of the side flattened to slide across the frame when inserted. Flat sides are also usually found on "one wedge per corner" stretcher frames. There is no real difference in having one wedge in the corner, though there are some schools of thought that this may cause uneven tension across the frame. UK stretcher bar manufacturers largely use two triangular plywood wedges, whereas Baltic or Chinese purchased stretchers tend to opt for one. Here at stretcher bars online, being a UK manufacturer we use two, supplied free, plywood wedges per corner.
Wedge insertion and use
Common stretcher bars like the picture on the right have two gaps to fit the wedges. If you look inside the gap you'll notice that the hole reduces in size the further back it goes. As the wedges are tapped into the frame it pushes apart the corners thus stretching the canvas. Although most canvases will be stretched initially using stretcher pliers, wedges can be used to also stretch the canvas. Typically this will happen when the canvas becomes slacker over time. By tapping in the wedges once more re-stretches a canvas