What's a stretcher bar?

A stretcher bar is used to construct a wooden stretcher frame used by artists and printers to mount their canvases. They are traditionally a wooden framework support on which a user 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.

The miter joint is the most popular method of adhering corners on a stretcher, although butt joints are also fine if used in conjunction with gussets. 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 frames, the corner joints in stretcher bars are not glued or fastened in any permanent way. This allows the canvas to be retensioned over time, as it has a natural tendency to stretch and sag over time. In contrast, strainer bars stretch canvas in a fixed (non-adjustable) way.

The use of stretcher bars in the print market has become increasingly prominent with inkjet-printed canvas prints becoming more popular in the home. Many print companies import pre-cut and hand-finished stretcher bars from areas of South America and China in timbers such as Rosewood Paulownia and Fir. The rot and warp resistant characteristics of these timbers, as well as their availability make them a widely-used choice in machined stretcher bars.