Stretching canvases is a skill that most art students in the past were very good at. As the cost of ready made stretched canvases has fallen, so has the general appreciation of the skill involved in stretching a canvas. In my day we made all our canvases and prepared the surface with Rabbit skin glue. Today of course there are Gesso primers that do the same job – its really not necessary to put Rabbits through such an ordeal any more!
There is no point in stretching canvas onto an unstable support. The first thing one has to do is to create a sturdy frame support. There are several good wood moldings used by professional framers as support frames. They are strong enough to support the weight and tensions when stretching even the largest of canvases. Large stretcher frames may need two cross bars to make them rigid enough. A stretched canvas when finished needs to be able to sit against the wall without any noticeable bending or distortion away from the wall. This is when the skill of the picture framer will shine through.
Stretching the canvas and measuring
The first thing one has to do is test the flexibility of the canvas. It may be the case that there is a ton of thick impasto paint and the canvas has no “give” in any direction. In this case a consideration to the brittleness of the paint is important. Oil paint takes a very long time to dry completely. Old oil paintings may crack if pulled in all directions when stretching. Fresh paint that is only touch dry will be less prone to cracking.
If the painting is long and thin in dimension, there may be more “give” on the long side depending on the canvas. When measuring the image size, this is important to take into account so that the painting does not wrap around the end edges. Usually an understanding is come to with the client as to exactly what they want. They may want the image to wrap around the edge anyway!
What I do then is lay the canvas flat on a work surface. I mark the size of the canvas, both its width and length. Holding one end of the canvas down against the work surface I pull the other end firmly and measure the distance it has moved from my mark. This distance is the “give” of the canvas. This is not taken as an absolute as canvas will stretch more in one direction than in two directions at the same time, but it does give one an idea of the general flex in the canvas.
Perhaps the most important things to do is check if the image on the canvas has parallel edges and will actually fit onto a oblong or square shape. This is often the case, so its always good practice to measure the image size along all sides looking for the shortest point and take that as a starting measurement.