A stretcher bar is used to construct a wooden stretcher frame 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.
The mitre 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 home 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 in China make them a widely-used choice in machined stretcher bars.
Although artists use blank canvases and pre-stretched canvases in the art business, many photographers use stretcher bars for framing wedding photography and reproduction of photographic prints. Stretcher bars are also used in picture framing when framers are framing things like sport shirts etc. Stretcher bars are used extensively in theatrical productions for framing material backdrops.
When a photographer takes a picture then digitally transfers this onto a canvas (inkjet printing), he then stretches this over a stretcher frame. By wrapping the canvas all the way around the frame, known as gallery wrap, the photographer can then hang his picture on the wall - already framed.
The profiles on the stretcher bar should be slightly rounded. This has two advantages: 1) It allows the framer to see and obtain clear edges on images that have precise borders, and 2) It also allows the canvas weave to "roll over" the profile rather than snap over a sharp edge which is a major cause of canvas cracking.
Most imported Economy bars are available in three profile sizes:- 1) STF or SSB are a Standard Stretcher Bar profile which are 40mm wide and 18mm deep. The unique profile design and 40mm width gives extra strength over longer lengths. This helps prevent "bowing" when stretching the canvas. SSB's are packed 50 units per box. 2) HZF or GSB are a Gallery Stretcher Bar which are also a chunky 35mm wide and 38mm deep. Its sheer strength is obtained from a unique profile design. GSB's are packed 30 units per box. 3) Economy 2 way profile, which is a 42*20 profile and can be used as a standard or a gallery bar. These are available in lengths ready to be mitred.
Professional quality bars are mostly available in four profile sizes. Bars do not twist and will sit perfectly on a flat surface. 1) Pro-Bar Pro (H:18*W:42) - this bar is 18mm deep and 42mm wide 2) Pro-Bar Exhibition (H:38*W:33) - this bar is 38mm deep and 33mm wide. 3) Pro-Bar Museum (H:50*W:38) - this bar is 50mm deep and 38mm wide. 4) Pro-Bar 2way (45*32) - this bar can be used in either profile to create a 32mm or a 45mm deep frame.
These profiles have normally been imported in fir wood but they are now also available in better qualities.
Another way in which stretcher bars can be strengthened is by having a cross brace inserted. It is advised that lengths over 40" or 1m be fitted with a cross brace. By doing this it ensures the wood will not warp and will hang flat
To stop corners from breaking or warping Stretcher Bars should also be fitted with solid wooden inserts, preferably removeable ones