A stretcher frame is a mostly wooden frame on which a picture carrier (canvas, nettle, jute, etc.) is attached with tear pins or staples. In contrast to firmly glued or screwed wooden frames, the corners are pushed into each other with tongue and groove. With the help of wooden wedges, which are inserted into the slots at the corners (hence the name "wedge frame"), you can retighten the frame later if the attached fabric loses tension. Wedge frames are suitable as picture carriers for oil painting, gouache, tempera, acrylic, but also for watercolor painting. Nowadays also photos or printed canvases are mounted on wedge frames.
You can buy stretched and primed stretcher frames in specialist art supplies stores - or assemble them yourself. Both have advantages and disadvantages. As a rule of thumb, the larger the frame, the cheaper it is to build it yourself.
Profile of the wedge frame strips
The profile of a stretcher frame has a rounded elevation at one edge, which protrudes by approx. 2-4 millimetres. This round ridge forms the outer edge of the frame over which the screen is stretched. Thanks to the raised edge, the tightly stretched picture support does not rest on the wooden strip - otherwise the pressure of the brush would create a disturbing edge on the picture surface when painting.
Wedge frame corners
The corners of stretcher bars are machined in such a way that they can be pushed tightly into each other.
The stretcher frame then gains stability through the covering. The frame can be retensioned with the help of hardwood wedges. The wedges driven in with a hammer push the strips apart.
Different ledge types
There are a number of different profiles for wedge frames that are not compatible with each other. Depending on the manufacturer and region, different types of wood are used. But above all, the profiles of the slats differ both in terms of their dimensions and the height of the tongue and groove at the corners.
The quality of a wedge frame ultimately depends on the quality of the wood. Wood works (reacts to humidity) and can warp over the years. The longer the wood is stored, the greater the probability that straight strips will remain straight.
Often two wooden strips with opposite grain are glued together. This neutralizes the natural tension of the wood. In 90% of the cases this also works quite well, but if you buy or order strips for stretcher frames, you will notice that sometimes one or the other is not really straight. The larger the format, the stronger the effect. A warped canvas simply stands at a corner from the wall.
It is recommended to have your own stock of stretcher bars and to store them dry for at least one year. If you then select suitable strips for a new picture, you can separate the wheat from the chaff and only use the really good strips.
Assembling the stretcher frame yourself
With small image formats - approximately up to 50 x 50 cm - it is hardly worth building your own. Because you can get ready-covered and primed stretcher frames for just a few euros in the art supplies trade. This changes with larger formats: the costs for borders, canvas or cotton and the primer are clearly lower than with comparable prefabricated picture carriers. The work involved in assembling, covering and priming is about 30 minutes per canvas. In addition, there is greater control over the quality: especially with priming, you have more influence when building the canvas yourself.
To build a wedge frame you only need a hammer and a tacker (approx. 30 USD). A piece of wood is recommended in order not to damage the strips when hammering them together.
How it works exactly is described in the manual "Building the stretcher frame yourself step by step" (assembly takes about 3-5 minutes). Here is a video that illustrates the process (german, but you might understand it):
After assembly, the canvas, or in this case a cotton fabric (which is just as suitable), is stretched:
Then the canvas is primed. A 1 : 1 mixture of primer and acrylic binder is well suited for this purpose.