Introduction to stretcher bars
If you’ve decided you want to stretch your own canvas, you’ll need to select your materials. The bars you stretch the canvas on are just as important as the canvas for determining the final product. You may wish to use narrow bars so that the painting is easier to frame or deep bars so that it looks good unframed. If you are planning a very large canvas then you will want to consider the tension that all that tight canvas will exert on the bars – your canvas could be pulled out of shape – so stronger bars (thicker or with aluminium) would be appropriate. Here are some things to consider when choosing your stretcher bars.
What are stretcher bars?
All stretcher bars differ from plain timber in that they are shaped to have a flat back and a slanted front that has a higher edge along the outside of each bar length. This may be accomplished with bars that slope downward toward the centre of the frame or by adding a raised lip around the edge. You staple on the flat, back side and the canvas is stretched so that it floats over the slanted front and only touches the outer rim. This is to prevent a ridge of paint forming when the canvas comes in contact with the bar when the brush pushes the canvas towards the bar as you paint.
Characteristics of each type
Most artists use the Professional and Museum ranges of stretcher bars. The Alu-Pro range of bars is for those artists who wish to invest in the highest quality bars.
For very large sizes you will need to choose thicker bars for strength, so the thinner bars aren’t available in the largest sizes. The ‘depth’ refers to the profile, the measurement of how far the canvas will protrude from the wall when hung as a painting. These bars can be purchased in pairs for stretching your own canvas and they are also used in Jackson’s ready-made canvases and bespoke canvases.
Jackson’s Professional Range
- Wooden in three depths: 18mm, 21mm, 43mm
- Strong – but for larger canvases where the canvas tension will be greater, the thicker 43mm bars will be stronger than the narrower.
- The 18mm depth is available in inch sizes, in 2-inch increments up to 40 inches. These 2-inch size bars (and the 21mm bars as well) are for artists who might need to match an inch-sized frame or an existing inch-sized painting where cm-sized canvases would be in partial fractions of inches, or for those who prefer Imperial measurements.
- The 21mm depth is available in inch sizes, in 2-inch increments up to 54 inches. These two narrow bars are not available in as long lengths as the deeper bars because they are not strong enough to be that long without warping.
- The 43mm depth is available in 10cm increments up to 200cm long.
- Strong interlocking corners and a space for a wooden corner wedge.
- Made of wood: PEFC Certified kiln-dried fir
Jackson’s Museum Range
- Wooden in 2 depths: 20mm, 35mm
- Stronger because of the layers of laminated wood.
- Both depths are available in 5cm increments up to 200cm long.
- Strong interlocking corners and a space for a wooden corner wedge.
- Made of layers of wood: PEFC Certified kiln-dried pine
Jackson’s Alu-Pro Range
- Wood and aluminum in 2 depths: 25mm, 45mm
- The strongest because of the aluminum.
- Both depths are available in 10cm increments up to 200cm long.
- Corners need connector pieces to join them, which can be tightened in the same way that wooden wedges are used.
- Made of strong, light aluminum with a strip of wood on for stapling.
- Wood responds to temperature and humidity changes, it swells when it gets warm or moist and shrinks when it is cold or dry. Aluminum does not change its dimensions. This stability means the canvas stretched across the bars is not subject to changes in tension and the paint layer should not be pushed and pulled so it will be less likely to crack over time. The patented design creates a perfect framework for canvas stretching, free from deformation, twist, and warp. These are so sturdy that cross bars are optional, at any size.
The centre bar and crossbar
The centre bar and crossbar add rigid strength to help prevent warping (and are useful as a carry handle).
If a stretcher bar frame is very large it is a good idea to give it additional support by adding a bar across the back (a centre bar) or two bars across the back that cross one another (a crossbar). You can add a centre bar for strength (and to have a handle on the back) at any size that there is a slot in the bar to accommodate it. You will need to use a centre bar that fits the stretcher bar – there is a different size for each size of bar – so there are 7 sizes/types of centre bars.
If the stretcher bar can only accommodate a centre bar and not a crossbar, or if you only need the added strength of a single centre bar, choose the bar to go across the width, not the length. So a 50 x 100cm canvas with a single centre bar would have a 50cm bar across the width. This provides more stability than if the centre bar was to go lengthwise.
For the 18mm and 21mm Professional stretcher bars the use of a centre bar is recommended when any side of the canvas exceeds 30 inches. The canvas can only accommodate a centre bar, they are not made to use a crossbar. Sizes starting at 28 inches have centre bar slots.
For the 43mm Professional stretcher bars the use of a centre bar is recommended when any side of the canvas exceeds 100cm. Sizes starting at 80cm have centre bar slots. The 43mm bars are able to accommodate two crossing bars that pass one over the other for added strength (on two planes).
The Museum 20mm bars will accommodate a centre bar or a crossbar pair. Sizes starting at 50cm have centre bar slots. Centre bars 100cm and larger will have notch cut in the centre that allows two to be used in one layer (on the same plane) as the notched fit together.
The Museum 35mm bars will accommodate a centre bar or a crossbar pair. Sizes starting at 60cm have centre bar slots. Centre bars 100cm and larger will have notch cut in the centre that allows two to be used in one layer (on the same plane) as the notches fit together.
Museo Alu-Pro bars will accommodate a centre bar or a crossbar pair. These are called primary crossbars (a single piece plus the necessary primary hardware connectors) and secondary crossbars (in two pieces plus the necessary secondary hardware connectors). They are only needed for very large sizes unless you wish to use them as handholds.
The Length of the Centre Bar
Centre bars are smaller than the stretcher bars because they fit inside the rectangle, whereas the stretcher bars are sized based on the outside of the rectangle they produce. But we have already figured out the sizes that fit and they are labelled as the size needed for the rectangle, rather than the actual length of the bar. So a 50cm centre bar is not actually 50cm long itself, rather it is made to fit inside a 50cm stretcher bar.
How do I use the canvas wedges that come with a canvas or stretcher bars?
When you purchase a ready-made canvas it will usually come with a little plastic bag of eight canvas wedges, often stapled inside the wrapping. Occasionally the wedges are already inserted in the canvas frame. When you purchase stretcher bars to make your own canvas, you will receive eight wedges per four bars (to make one canvas). The size of the wedges varies – bigger bars will have bigger wedges. The shape can vary a bit as well, each manufacturer makes the ones that best fit the slots that have been made in the corner of their stretcher bars.
Many artists throw these small bits of wood away or start filling a drawer with them. But painter’s corner keys or retainers (two other names for them) can be very useful – if you don’t already use them you might want to give them a try. New canvases often need to be tightened and old paintings on canvas can become loose over time, usually because of humidity changes and temperature fluctuations, and your canvas that was as tight as a drum can loosen on its stretcher bars. They can easily be made tauter with corner tightening wedges, which expand the mitred corner of the bars slightly, enlarging the frame by a millimeter or so.
Wedges can be inserted into the canvas bars before painting or after.
They can be used to:
- Finish the stretching of a hand-stretched canvas – to give it that last bit of tightening. If you are priming your canvas, wait until you have primed it as priming will cause the canvas to shrink.
- Tighten up a ready-made canvas that has loosened up in storage or shipping (the tightness of a canvas will relax naturally over time). If your canvas arrived and is just a bit looser than you’d like, inserting the wedges will usually solve the problem. Often a couple of millimeters is all it takes to make a loose canvas taut again, or pull a sag out of a corner. If it arrives with the wedges already inserted in the canvas frame they may need a little tap to fully set them in.
- Tighten a canvas that was fine when you started painting but the weight of paint and pressing on it has caused it to loosen a bit. So when you are finished painting and varnishing, assess if it needs a bit of tightening now. This is a reason to keep the wedges for future use, because the fabric tightness may change. Take care to do it gently as over-stretching suddenly can crack an oil painting.
- Re-stretch a finished painting if it sags after many years. When a painting goes slack over time because the temperature and humidity in a building changes throughout the day and year, inserting wedges should allow you to tighten the canvas back up without having to re-stretch it. Take care to do it gently as over-stretching suddenly can crack an oil painting.
Inserting the Canvas Wedges
In each inside corner of a ready-stretched canvas or one you’ve stretched yourself, there are 2 slots in which to fit the wedges. Each slot reduces in size the further back it goes so that as the wedges are tapped into the frame it pushes the bars apart at the corner thus stretching the canvas a few millimeters. If you spread it far enough you can see the gap in the mitred corner expand. By the way, this doesn’t make the corner any weaker, there is still a full dovetail join inside the corner.
From the back of the canvas slide a wedge, point first (top of the triangle), into each slot, one at a time. Give them a tap with a small tack or finishing hammer. Be gentle. Bashing it could split the wood of the wedge, or you could bounce off and hit the back of the canvas, or overstretching of an oil painting could crack the brittle painted surface. Some artists tap downward with the side of the canvas on the table or floor, some tap upwards. I find upwards to be more difficult to aim, but try both and see which works best for you. Do one corner at a time and try to tap with the same force for each corner so the tension is even. There will be two in each corner, eight per canvas. Check the front of the canvas and if you need to, go around and tap them all again. I find that sometimes one corner is sagging more than the others and it might help to give that one corner a few more taps.
The wedges are asymmetrical and they work well in either orientation – if you insert them with the long side or the short side against the side of the stretcher bar. Some stretcher bars seem to accept the wedges a bit better in one way than the other – it must be how the slots are angled inside – Winsor & Newton brand canvases for instance work best with the wedges parallel to the bars. The wedges can also be angled out toward the centre of the canvas, and may work a bit better. But the orientation is up to you, try both and see which works better with your canvas.
It is good practice to try to get your bars to spread apart a bit before you start tapping in your wedges, rather than just using the wedges themselves to push the bars apart. Best practice is that you should do all the movement of the bars by pulling and tapping on the bars to expand them and only inserting the wedges after you have expanded the frame. They are called keys or retainers because they fit in the space you have created to retain the expansion of the bars.
Claire, our former bespoke canvas maker, explains: “If you need to tighten a canvas during painting, take a slip or block of wood, hold to the inside of the stretcher bar and tap this out with a light hammer (the wood stops you getting hammer marks on the stretcher bars) do this on all four insides, then push the wedges home, they are there to act as keys to stop the stretcher from contracting they are not to be hit with a hammer. Do not bang the wedges themselves in.”
This is what is meant when someone says you need to ‘bang your stretchers apart’, they mean to tap the stretcher bars in their centres, one at a time, away from the middle of the canvas. After you have expanded the canvas a bit then push your wedges in to hold them apart, like a doorstop wedge holds open a door, or a keystone holds a masonry arch in place. I find this allows the wedges to be pushed in further before you start tapping on them and gives a tighter canvas. But I usually do need to do some more pushing with the wedge to finish the job.
In case you were wondering – the wedges are meant to be left in permanently. They keep the corner of the stretcher frame pushed apart slightly. If you remove them the canvas will sag again as the bars move together.
Centre Bar Wedges
Centre bars also have a little gap and when you purchase a centre bar to stretch your own canvas you should receive some slim wedges to tighten up the centre bar.
Glossary
Canvas
A woven material used for centuries for painting. Usually made of cotton or linen. Can be stretched over strong wooden stretcher bars, glued onto a board or panel or used unstretched. Although acrylic can be painted on raw canvas, most artists prime the cloth with a ground that allows control over the absorbency, texture, and color of the surface.
Canvas Board
Canvas glued on to a hard board (thin MDF or compressed board). A rigid surface for oil and acrylic painting. Canvas board usually has shear edges (i.e. the canvas does not wrap around to the back, unlike a canvas panel).
Canvas Pad
A pad of unstretched, primed canvas sheets glued at one side ready for oil painting. Also available in blocks glued on four sides.
Canvas Paper
Pads or sheets of paper that are textured and coated to have the appearance and feel similar to primed canvas. Used instead of canvas for economy and convenience.
Canvas Panel
A piece of board or wooden panel on to which a piece of primed canvas has been glued to the front and wrapped around to the back.
Canvas Pliers
A tool which helps to stretch canvas tightly around a frame in order to make a satisfactory surface on which to paint. Canvas is usually fixed to the frame using staples on the reverse of the frame, or tacks on the side of the frame.
Canvas Sheets
Sheets of rectangular or square pieces of primed canvas that can be glued to a board to make a panel, or painted on as they are.
Cotton Duck
A heavy plain woven fabric that is a popular material for artist canvas as it is relatively low cost in comparison to linen. Cotton duck is most commonly available in 10oz or 12oz weights.
Gesso
Pronounced with a soft ‘g’ like gypsy or George. From the Italian for gypsum, a major component. This thick white liquid is primarily used as a ground for painting but can also be used to build up areas for carving on frames and is used underneath gilding. It can be colored. Gesso for gilding is often colored red. You can buy ready-made black “acrylic gesso”.
Gesso is made with calcium carbonate (also called whiting, chalk and gypsum) in a binder. It is painted on the canvas, paper or wood panel surface to create a ground on which to paint. Sometimes white pigment (usually titanium, sometimes zinc) is added to make the gesso very white.
Genuine gesso (also called true gesso) uses animal skin glue (hide glue or rabbit skin glue also called “size”) as the binder and the artist often makes the gesso him/herself, using a double boiler to melt the glue powder and adding the whiting. Rabbit skin glue is now also available ready made and just needs to be warmed.
One recipe for traditional gesso: three parts size, one part chalk (whiting), one part pigment powder. It is a rather lengthy, messy, smelly process of soaking, heating in a double boiler and mixing.
“Acrylic gesso” is more correctly called “acrylic primer” and should not really be called gesso. It uses an acrylic polymer as the binder for the chalky powder. It is made up of upwards of 14 ingredients. You can also buy ready-made black acrylic primer.
Genuine gesso is less flexible than the “acrylic gesso” and is usually painted on a non-flexible surface such as a wood panel rather than on stretched canvas, so that it will not crack. For paints that need an especially porous surface, like egg tempera, genuine gesso is usually preferred to the acrylic gesso/primer.
The acrylic primer varies a lot in quality and poor quality products can provide a less absorbent ground than is often preferred. Good quality acrylic primer is a very good product for oil painting and acrylic painting. It does both steps of the surface preparation in one- it both sizes (seals) the surface and gives a ground for painting. It can also vary in absorbency, with some products called “acrylic gesso” rather than “acrylic primer” being more absorbent and chalky and particularly suited to applications which require an absorbent surface.
Acrylic primer differs in thickness, opacity and grittiness of surface texture, depending on the manufacturer. It is usually too thick to use straight out of the bucket and should be diluted with water until it is the consistency of heavy cream. Most primers have instructions that advise you apply three thin coats rather than one thick coat. A very thick coat may crack as it dries. The first coat is often scrubbed into the weave of the raw canvas in circular motions to be sure that it is well sealed. The first coat will soak into the canvas or panel and act as its own sizing (sealer). Then subsequent coats are applied in alternating directions across the canvas. To get a very smooth surface you may wish to sand with sandpaper between coats. Some acrylic gessos are designed to have a harder surface specifically so they may be sanded smooth, but as they are less flexible they may crack on a movable surface such as stretched canvas, so should only be used on rigid surfaces.
For oil painting it is especially important that the oil never reaches the substrate as it will rot the canvas, paper or wood. Traditionally oil painters seal the surface with rabbit skin glue and then prime the surface with gesso (glue with chalk). Using these two layers assures that none of the oil will seep through. Some artists who use ready-made stretched canvases will apply an additional layer of acrylic primer to the surface to ensure that it is well sealed.
For painting on paper you may wish to prime both sides of the paper (one after the other dries) as the paper will curl when it is wetted by the primer. Painting the other side then uncurls it. For oil paint on paper you may want at least three coats.
Priming your painting surface is part of properly creating a painting. The underlying structure is very important to the longevity of the painting as well as to the appearance. Primer creates a surface that is sealed just enough to prevent the paint seeping through to the substrate (canvas, paper, wood), but is absorbent enough to hold onto the paint. If you were to paint on an unusual surface like a rubber toy, the paint might not adhere properly. But if you prime the surface with acrylic gesso/primer first, then your paint will go on properly and stay on. The primer is stickier than paint and will glue the chalk to your substrate and create a better surface to paint on.
While the gesso/primer is wet it may leach color up from the substrate and cause discoloration to the whiteness of the gesso. The glues in plywood, the resins in wood panels and in stretcher bars may be water-extractable. Sealing the wood or canvas first with a sealant medium such as Golden Acrylics GAC 100 will prevent Support Induced Discoloration (SID). Sealing (sizing) with rabbit skin glue does the same thing if you are using genuine gesso. Then prime as normal.
Some artists prefer that the substrate shows through underneath the paint and so they use a clear primer. This is usually an acrylic matte medium. This is a thick white liquid that dries clear so you can see the canvas. The texture is very different to gesso since it does not have the chalk powder in it, the surface is smooth and not as absorbent.
Be warned that priming can be a messy business. Gesso/acrylic primer dries quickly on brushes and can stain clothes. Be sure to use drop cloths and wash everything as soon as possible.
Many artists use the word gesso as a verb meaning “to prime” as in “I will be spending the day gessoing canvases in the studio”.
Some artists mix gesso in with their paint as a painting material.
Linen
A natural fabric made from long threads woven together which is stronger and more elastic than cotton duck. It is usually darker than cotton duck and can be stretched on a frame, glued on to a board or panel or painted on unstretched. Linen needs to be sized with rabbit skin glue or an acrylic substitute prior to painting with oils. Linens are available in a range of weights (the heavier the weight the tougher the fabric will be) as well as a range of weaves, from fine to coarse. Which you choose will impact the overall look of your painting.
Panel
A rigid painting surface for acrylics, oils, encaustic, pastels or watercolours. Made from solid wood, plywood, mdf, compressed card or aluminium. They are often braced to prevent warping both during the priming and painting period and over time. May also be wood with canvas or paper adhered to the front. Especially useful for encaustic or oil painting where a rigid surface helps prevent cracking of the paint surface over time.
Primer
A surfacing material used to coat a support to get it ready for paint application. Acrylic primer is made from calcium carbonate suspended in an acrylic binder. It can be applied directly to a support without the need for a prior application of size. To create a very smooth surface apply 2- 3 coats and allow to dry fully and lightly sand between applications. Gesso is a more absorbent variety of primer. Multiple coats of acrylic gesso will increase the absorbency of the surface, and light sanding between layers will optimise the smoothness.
Acrylic primer varies a lot in quality and poor quality products can provide a less absorbent ground than is often preferred. Good quality acrylic primer (can contain upwards of 14 ingredients) is a very good product for both oil painting and acrylic painting. It does both steps of the surface preparation in one- it both sizes (seals) the surface and gives a ground for painting.
Priming Brush
Usually a flat wide brush, made with synthetic or hog hair. For an even application, load the brush and apply whilst holding it at around 45 degrees to the support. Brush the primer on in all directions to make the coverage even. Allow each layer to dry fully before applying the next layer.
Rabbit Skin Glue
A strong glue made from animal parts, that is an ingredient in genuine gesso, is used for sealing (sizing) panels and canvas before priming and is used as sizing for papers. It stiffens canvas in preparation for gesso primer in oil painting. Also called hide glue.
For preparing canvas and panels the usual method is to soak the pellets or powder overnight, the next day heat in a double boiler and brush onto the canvas while still warm (do not overheat as the glue will be weak). Two coats are preferred to seal the canvas well, the first being scrubbed into the canvas to get well into the weave. Discard any left over as it does not reheat well. Then prime the surface as normal.
Stretched/Unstretched Canvas
A piece of linen, hessian or cotton duck that has been tightly wrapped around a frame made of wood or aluminum and fixed at the back. Sizing and priming the stretched canvas will increase the tension in the stretch. This creates a vibrant, drum-like surface to paint on. Stretched linen and cotton duck canvases can be bought ready-made. They are available unprimed or primed with acrylic or oil primer. Unstretched canvas can be purchased from and by the roll, ready to be stretched on to a frame at home or worked on unstretched.
Stretcher Bars
Stretcher bars will assemble to make a frame onto which canvas can be stretched over. They are available in pairs and made of wood or aluminum.
Support
A general term for a surface ready to be painted on. A support can be anything from a canvas to a wooden panel.
Tacks
Tacks are a similar shape to drawing pins and are made of metal, and are used to fix canvas to the sides of a stretcher bar. Staples are generally considered to be more successful at keeping canvas fixed to the stretcher, but tacks are often still used to add a traditional aesthetic to the overall look of the support.
Weave
The weave of a canvas can be completely smooth or very prominent, depending on how it was made. It will have an effect on how your painting looks. Artists who like to explore textures in their work might prefer a coarser weave, whereas artists who paint very fine detail may prefer a finer grain. The set of threads that are aligned lengthways in fabric is known as the warp, and the weft is the set of threads that weave in and out of the warp. In painting it is important that the warp and weft are similar so that when the canvas is stretched it will do so uniformly, without inconsistencies such as wrinkling. This is particularly worth noting if you are working with a linen that was not purchased from an art supplier.
Wedges
Wedges are supplied with ready made canvases and stretcher bars and provide a way of making your canvas even more tightly stretched. Simply insert the wedges into the corners of the frame of your stretched canvas and gently tap in with a hammer. The wedges will push the stretcher bars outwards and keep them in place. If the same amount of pressure is put on each wedge then the canvas will remain square. By pushing the bars outwards you will be tightening the tension of the canvas stretched on the other side of the frame.
Whiting
Calcium carbonate or calcium sulfate – also called chalk, marble dust, calcite, and gypsum depending on its source, use particle size (which affects both the texture - gritty or silky smooth - and the absorbency) and the purity level. (Talc is not included in this group and is too soft for use as a painting ground unless it is used in a blend with one of the above minerals).
It is a white powder ground from fossilized shell deposits (limestone), marble or made synthetically. It is made synthetically by precipitating fine particles of calcium carbonate making the most uniform, smoothest variety that is used for the preparation of panels for egg tempera for example.
Its use for artists is primarily as an ingredient in genuine gesso and acrylic primer.
It is also the opacifier that makes gouache an opaque form of watercolor paint. It is used in some soft and hard pastels. It is used as a filler for some paints. It is used to enhance the brightness of some paint colors, Viridian for example. It is the inert base onto which lake pigments are precipitated. Some forms are used in modeling paste in acrylics and mixing with oil makes oil painting putty.
It is not strong enough to be used as a white pigment. It is added to paper pulp as a buffer, to counteract reactions with pollution that would cause acidity in paper. It is also used as a polishing powder and in ceramics as a flux.