| STRETCHING CANVAS | Stretching your own canvas not only offers a saving in cost, but also means that you can prepare a canvas to your own specifications. |
Stretcher bars
Wooden stretcher bars are sold in most art-supply stores and come in different lengths. They have premitred corners with slot-and-tenon joints. The face side of each stretcher bar is bevelled to prevent the inner edge of the stretcher creating ‘ridge’ lines on the canvas. Stretcher bars come in varying widths and thicknesses, depending on the size of support you wish to make. For a work under 60 × 60cm (24 × 24in), use 45 × 16mm (1¾× 5/8in) stretcher bars. For larger works, use 57 × 18mm (2¼ × ¾in) bars.
Wedges
You will also need eight wedges or ‘keys’ for each stretcher. These fit into slots on the inside of each corner of the assembled stretcher; if the canvas sags at a later date, the wedges can be driven in further with a hammer to expand the corners and make the canvas taut again.
Canvas-straining pliers
Canvas-straining pliers are especially useful for stretching ready-primed canvases. They grip the fabric firmly without any risk of tearing, and the lower jaw is bevelled to give good leverage when pulling fabric over a stretcher bar; the correct tension is achieved by lowering the wrist as the canvas passes over the back of the frame.
Other equipment
Use a heavy-duty staple gun and non-rusting staples with a depth of at least 10mm (3/8in) to fix the canvas to the frame. You will also need a rule or tape, a pencil and a pair of scissors to measure and cut out the canvas; a wooden mallet to tap the stretcher bars together; and a T-square to check that the frame is square (or you can use a length of string to ensure that the diagonal measurements between the corners are the same).
Large canvases
A support that is larger than 80 × 100cm (32 × 40in) will require an extra crossbar between the two longest sides, to support them when the canvas contracts during preparation, exerting a great deal of force.
Tacks
Using a hammer and non-rusting tin tacks to fix the canvas to the frame is more economical than stapling, but means more work.
Pliers
Canvas-straining pliers stretch ready-primed canvases firmly and without tearing.
Cutting the canvas
Use pinking shears to cut canvas; they avoid the need to fold the edges over at the back of the frame to prevent the canvas fraying.
Assembling the stretcher frame
Slot the stretcher bars together, checking that all the bevelled edges are at the front. Tap the corners gently with a wooden mallet or a piece of wood for a close fit.
Checking for square
Use a T-square to check that all the corners of the assembled frame make right angles. Double-check by measuring the diagonals with an expanding tape measure or a length of string; they should be of equal length. If the frame is out of true, correct it by gently tapping the corners with the mallet.
Stretching the canvas
(1) Cutting out the canvas
Working on a large table or the floor, lay the frame bevel-side down on a piece of canvas. Cut the canvas to fit the frame, allowing a margin of about 5Omm (2in) all round for stapling (1).
(2) Securing with the first staple
Ensure that the warp and weft threads of the canvas run parallel with the sides of the frame. Fold the canvas round to the back, and secure with a staple at the centre of one long stretcher bar (2).
(3) Tensioning the canvas
(4) Continuing to staple
Reverse the frame, pull the canvas firmly and evenly, and secure a staple opposite the first one so that consistent straining is obtained. You can use canvas-straining pliers, if necessary, to grip the cloth and pull it taut over the frame (3). If glue size is to be applied, the canvas should be taut, but not as tight as a drum, to allow for possible shrinkage. Repeat the process on the two short sides, so that one staple holds the canvas to the centre of each stretcher bar (4). Check the parallel alignment of the canvas weave.
Securing the canvas
(5) Stretching and stapling
Now add two more staples to each of the four stretcher bars – one on either side of the centre staples – following the sequence shown in the diagram (5). The staples should be evenly spaced at 50mm (2in) intervals. Continue adding pairs of staples to each side, gradually working towards the corners. Insert the final staples about 50mm (2in) from each corner. Note that working systematically out to the corners keeps each side in step with the others. Fastening the canvas completely on one side before doing the next stretches the canvas unevenly.
Finishing off
(6) Fixing the first corner staple
(7) Folding the flaps
(8) The finished corner
(9) Inserting the wedges
The corners should always be finished off neatly; if they are too bulky you will have difficulty in framing the picture. Pull the canvas tightly across one corner of the stretcher, and fix with a staple (6). Then tuck in the flaps on either side smoothly and neatly (7) and fix with staples. Take care not to staple across the mitre join, as this will make it impossible to tighten the canvas later on. Then fix the diagonally opposite corner, followed by the remaining two. If necessary, hammer the folds flat to produce a neat corner (8). Finally, insert two wedges in the slots provided in each of the inner corners of the frame; for correct fit, the longest side of each wedge should lie alongside the frame (9). Tap the wedges home very lightly. The canvas is now ready for sizing and priming.
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