Stretcher bar

A stretcher bar is used to construct a wooden stretcher used by artists to mount their canvases.

They are traditionally a wooden framework support on which an artist fastens a piece of canvas. They are also used for small-scale embroidery to provide steady tension, affixing the edges of the fabric with push-pins or a staple gun before beginning to sew, and then removing it from the stretcher when the work is complete. Stretchers are usually in the shape of a rectangle, although shaped canvases are also possible.

Since a stretcher is simply a frame, it can be constructed in a variety of ways. The differences in construction have to do with how the corners are built. Commercially available pre-fabricated stretchers come in segments with interlocking corners, that can be fit together like puzzle pieces. Corner supports can also be made using simple woodworking techniques, however.

In the case of the French stretcher, a mitre joint is used to adhere the corners. "Keys" or small triangle wedges are inserted in the joint after stretching the canvas to give the canvas its final tension. When fastening the canvas, pressure should be distributed evenly around the stretcher to minimize warping due to unequal distribution of pull. Unlike other types of stretchers, the corner joints in French stretchers are not glued or fastened in any permanent way. This allows the canvas to be re-tensioned later, as it has a natural tendency to stretch and sag over time. In contrast, strainer bars stretch canvas in a fixed (non-adjustable) way.

A simpler form of stretcher employs butt joints to adhere the corners. The joint is pinned and glued into place and can not be expanded after assembly.

The profiles on the stretcher bar should be slightly rounded. This has two advantages:

There are many different stretcher bar profiles, and many different styles of cutting of the wood. So it is impossible to say anything is "standard." There are also many big regional difference in the style and cutting of the wood, due to the historical reasons. For the same reasons, the wood used for making stretcher bars differs a lot from country to country depending on the forest that is present. But most stretchers, to avoid warping is made in well dried Nordic pinewood sourced from Scandinavia, Russia, and Canada.

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Wikipedia