How to Stretch Canvas Without Slipping
The traditional method for stretching printed art on canvas has always been with a pair of canvas pliers. The stretching is done in quadrants and at opposite angles. This is a relatively slow process and though this can produce a tight stretch it can also be done incorrectly. A more expedient method is conducted with a canvas stretching machine. The following tutorial was performed on a Gallery Stretcher, manufactured by Canvas Stretching machine, LLC.
Problem: Canvas Slipping or Relaxing After Stretching
A common complaint is that the stretched artwork has a tendency to relax, usually with changes in humidity. Much of this is caused by the composition of the canvas because “cotton” fiber is susceptible to moisture. A lot of the modern canvas is made with a certain amount of cotton, vinyl or a combination of the two.
There are a number of factors that can contribute to canvas slipping but I will limit this tutorial to only the most common.
- Stretcher Bar With Little or No Radius
- Lack of Support Bar/ Gussets
- Stretching Too Quickly
Typical Solutions:
Stretcher Bar Selection:
In an attempt to save money some folks have constructed their own stretcher bars using MDF. Many people would argue that MDF is stable and resistant to warping. There are two problems with using MDF. The first is that MDF does not have the same lateral strength that a good wood stretcher bar has, even with gussets and support bars. The second problem, and perhaps the biggest, is that without some form of “radius” on the top of the frame the canvas cannot easily be pulled (stretched) over the edge. The sharper the edge the more resistant it is to being stretched. Pictured above is an ideal wood stretcher bar with a pronounced radius on the top and even a slight one on the bottom.
In most cases when I hear that someone’s canvas has become slack I immediately suspect their stretcher bar selection. There are a number of possible causes, but this is the first that comes to mind. Trying to force a canvas over a hard edge cause the canvas to gather on the sides which results in cupping. More on this in later steps.
Support Bars & Gussets:
An unwritten rule is to provide a support bar in the middle of the frame for frames greater than 30″ and more for larger frames. The use of Gusseted corners is also a advisable to fortify the frame. If you can flex the middle of the frame with your arms then it needs to be re-enforced.
Stretching Too Quickly:
Don’t be too eager to apply the staples. The fibers of the canvas need time to move. Some canvas will stretch very easily while others may not.
Tap, Tap, Tap
A technique I use to determine if the canvas has stretched sufficiently over the edge of the stretcher bar is with light “tapping” action on the side with the “pad” of my index finger. A low, dull sound indicates the the canvas is gathering and cupping. A high-pitched sound indicates that it is resting firmly against the side of the frame.
Do not staple until the canvas is firmly against the side.
Too Much Tension
Stretching with a pneumatic canvas stretching machine will certainly make your life easier but it is also possible to over-stretch a canvas. Some of the light canvas are very stretchable and you must be careful.
One indicator that you are using too much stretching force is that the image on the side walls starts to form “wrinkles”. This can be corrected if you quickly reduce the stretching force.
A common misconception when stretching canvas is that it must be as tight as a drum. If you are striving to make it sound like a drum then make it sound like a tom-tom and not a snare drum.
The Starter Course
The following steps were done with a machine with a good quality wooden stretcher bar with slow even pressure and are still taut today.
Lay the artwork “facedown” on a clean work surface behind the machine just of the deck. Fold the canvas along the image so that approximately 1/16” of the image is extending over the radius.
Turn the artwork 90 degrees and make a second fold along the image.
Alignment Is Critical
Place the assembled stretcher frame (flat side up) on the artwork and slide it against the first crease. Then slide the outside rail against the second crease. This is the fastest way to align your artwork.
Once the artwork is aligned create the “starter course” using a pneumatic staple gun. Be sure that the artwork stays parallel to your edge. Adjust air pressure of staple gun so that the staples due not penetrate the canvas.
If your artwork is not properly aligned then it will be obvious on the first stretch. If the artwork is slightly skewed do not continue with the stretch.
“Loaded Position”
Place the stretcher frame on top of the stretcher plates with the starter course away from the operator. This is called the “loaded position”.
Note the position of the stretcher bar against the “mechanical stops”.
From the front of the machine there should be enough of a gap between the angle bar and the clamping bar so that the canvas will “waterfall down” into the opening. The operator should not have to feed the canvas in by hand in most cases.
Be sure to position stretcher plates closer to the center of the artwork. Moving them toward the outside rails will result in an uneven stretch; a few inches left and right of center. Art larger than 30” may require moving the plates farther apart, but try to avoid moving the plates toward the outside rails. The outside of the stretcher frame already has strength from the (2) outside rails; fortify the center where it is needed.
Feed the loose end of the canvas into the gap and step on the “clamping pedal” once to engage the clamp. There should be at least 1.5” of excess canvas beyond the bottom of the stretcher bar to allow the clamp to get a good hold.
“Un-loaded Position”
Once the canvas is clamped and you are satisfied with the alignment move the entire assembly forward and off of the plates. This is referred to as the “un-loaded” position. The stretcher frame should be in front of the plates and the canvas should be held fast beneath the clamp. It is now ready to be stretched.
Depress the left foot pedal (stretching pedal). Be sure to maintain control of the stretcher frame with your free hand. It should rest on the outside rail applying only light pressure.
The Stretch
Initiate the stretch by flipping the toggle switch on the right side of the machine. The stretching pressure should be no more than 15-20 psi to start a stretch. Increase the pressure slowly using the regulator on the right side of the machine and let the frame rise no more than 3-4”. Do not push the frame flat against the deck. Pushing down on the frame during a stretch will promote slipping.
If you need more pressure you can always increase stretching pressure. Do not “overstretch”, a tight stretch can be obtained with only 20-30 psi provide you have a stretcher bar with a nice radius. Stretcher bars should always have a pronounced radius, the harder the edge the harder it is to get the canvas to stretch.
Stapling
When you are satisfied with the tension you can staple the canvas to the frame with a long-nosed pneumatic staple gun. The staple gun is operated upside down and the trigger is depressed with the thumb. Slide the nose of the staple gun along the angle bar beneath the stretcher frame. Place a staple at least 1 every inch.
Release the stretch by stepping on the stretching pedal. Depress the clamping foot pedal again to open the clamp and remove the artwork. Rotate the artwork 90 degrees; use the deck of your machine and the work surface to support the artwork. This is how you stretch each end.
You are now faced with the decision to fold the corners and continue stretching or cutout the excess canvas and then stretch. Removing the excess canvas is the recommended method and provides the best results.
Cutting Excess Canvas
After the first stretch, remove the artwork and lay it face down on your work surface as shown. Create a slight crease on the corner to define the edge.
Cut just inside the creased line and stop at the stretcher bar.
Make your second cut along the stretcher bar as shown in the photo below.
Rotate the artwork 180 degrees and repeat the process. Place the artwork back on the stretcher plates in the “loaded position” and continue with the remaining stretches.
In some cases you, or your customers, may not want to cut the excess canvas. The reason we cut the excess canvas is to reduce the build-up of canvas on the corners. Cutting the excess canvas will yield tremendous results.