Stretcher vs. Strainer
Today we want to talk about an integral part of most canvas paintings: stretchers and strainers. All paintings have a primary support, the layer upon which the paint is placed. The primary support can be made from a range of materials, such as wood, metal, or plastic. However, when most of us think of a painting we think of a fabric support, commonly known as canvas. Unlike wood or metal, canvas is not a stiff, hard support. In most cases, a canvas needs an auxiliary support to help make it a more paintable surface. The auxiliary support is usually a lightweight, wooden frame that the canvas is tightly stretched around and then secured to the edges or back using tacks or staples. The auxiliary support provides the necessary tension to the canvas, allowing artists to paint on a tight, flat surface.
Figure 1. Verso of a painting on a strainer
Strainers were the first type of auxiliary support used to support stretched canvases. A strainer is a rigid frame, meaning it is not expandable and its joints are fixed. Over time, the canvas may become slack and lose its planarity due improper environmental conditions and the degradation of the canvas fibers. This causes ripples in the canvas and can cause cracking in the paint layers. A slack canvas may also lead to the canvas touching the strainer bars, causing visible bar marks and cracks on the canvas. Obviously, not ideal. The only way to restore tension and planarity to the canvas is to remove it from the strainer and re-stretch the canvas onto a new auxiliary support.
People saw the disadvantages of strainers fairly early on and began looking for another option. The new option was what we now know as stretchers today, which are auxiliary frames with expandable joints. Stretchers were introduced around 1750 and had gained popularity by the late 1700s. By expanding a stretcher at its joints and increasing the overall size of the auxiliary support, you can return the tension and planarity back to a canvas, meaning the painting does not need to be re-stretched. Obviously, this is advantageous over a strainer. With the industrial revolution in the 19th century and widely available commercial art supplies came an explosion of new patents and designs for expandable canvas painting supports. The simplest design that artists continue to choose today are stretchers expanded with keys: small triangular pieces of wood slotted into the mortise-and-tenon joints to gently open the corners and hold the expansion in place. At first, only four keys were used, one for each corner of a rectangular stretcher. Eventually, people realized that the tension was more even with two keys in each corner. Today, most stretchers come with at least eight keys. The horizontal and/or vertical cross bar on larger paintings added for additional support can also be made keyable.
Figure 2a. Example of a stretcher with 8 keys
Figure 2b. Detail of the corner of a stretcher with keys
A keyable stretcher does have its limits. You can only expand the stretcher’s joints so far before it begins to lose strength and stability. You have to be careful when placing the keys, as they usually need to be tapped into place with a hammer. You don’t want to hit them too hard, as it might push them into the joints too far and over tension the canvas. Over tensioning leads to a distinct cracking pattern in the corners. You also want to make sure that you don’t hit the back of the canvas while installing the keys. If a key breaks, it can be difficult, or even impossible, to remove. Lastly, if a key falls out of its joint, it might get caught between the canvas and the stretcher, causing a bulge in the canvas.
Figure 3. Examples of old and new keys in various shapes and sizes.
For a more detailed discussion of stretchers and strainers, visit the Stretchers and Strainers section on the American Institute for Conservation’s Wiki. If you are feeling ambitious, check out Golden Artist Colors, Inc.’s article “Stretchers and Strainers: Secrets of the Trade”, which provides readers with a brief description on how to make their own stretchers and strainers. A third resource used for this post is “Stretcher or strainer?” by Bettina Ebert, written for Asiarta Foundation.
