Buyers Guide Part 4 - What use is a stretcher bar?

Poor quality stretcher bars can leave your canvas like a fish out of water...very floppy.

Naturally, this subject is very close to my heart. Well, it is what I make after all!

Before the renaissance period (1400-1700) most all paintings were done on solid mediums, such as wood. Did you know the Mona Lisa was on wood and not canvas? Nowadays almost all canvas's are stretched on stretcher bars of some description. However, do you know how vital the wood behind the canvas is to the overall perception and longevity of your canvas? A good retailer should be able at the very least to tell you the type of wood they are stretching their canvas over.

How do you join a stretcher bar?

There are a number of stretcher bar joins on the market today, some vastly better than others. Here's a list of the most common ones and what they mean for you. Take a look at a stretched canvas on your wall. Most likely one of the below will be used in the join.

The pros and cons of a traditional stretcher bar joint.

Mortise and tenon join

Mortise and Tenon Join - Traditional stretcher bars - By far the most common joint used in stretched canvas. Joins are interlocked together using a soft mallet and squared. Your canvas was most likely stretched over the frame with some Stretcher Pliers, by hand. They then would have inserted the wooden wedges/keys (they may be plastic in cheaper frames) in each corner and tapped them out with a hammer to expand the join and stretch your canvas. We make these too and you can buy them at stretcher bars online.

The pros to a traditional stretcher bar

The most common method for stretching canvas and the staple (pun intended) of a picture framers canvas stretching diet. Widely available in a variety of wood types (some better than others) and generally come in standard sizes as well as some bespoke/custom made bars from some suppliers
You can stretch the canvas again if it goes slack. On the rear of the canvas you'll notice that there are some wooden "wedges" in each corner (that's if they've been supplied). Using a hammer, you can tap these into the corner joint and expand your canvas again to stretch it again.
One of the cheapest ways to stretch canvas. Stretcher bars, especially from the Far East are purchased by suppliers in bulk and the cost can become very cheap for some of the more poorer wood quality types.
Reusable - In some (not all) cases you can take apart the canvas from the frame and use the bars again.

The cons of a traditional stretcher bar

Cheap can actually sometime mean cheap for these bars. In a sense there are good cheap and bad cheap bars out there. Around 10 years ago the market was flooded with some real poor quality stretcher bars from the far east. These bars are generally made from Fir or Paulownia woods, which though are not as widely used nowadays, they are so cheap that if the quality of them is so bad it just as cost effective for suppliers and users to discard any defective ones. Not surprisingly a lot of sub-standard ones made it to market. More about these woods later.
You have to re-stretch the canvas yourself when they go slack. This can happen often, depending on the environment that the picture is hung. Canvas, and wood, contracts and expands when they get hot or cold. The little wedges on the rear of the frame have no way of staying in the frame when this happens and often fall out or just slacken off, loosening the canvas. If the wedges are not supplied, you'll either have to return to the seller for re-stretching or get some wedges from them.

The pros and cons of a cut and pin strainer bar joint.

Cut and Pin Pre-assembled stretcher or strainer bars

Cut and pin - Pre-Assembled Strainer Bars - This method of joining frames is relatively common, but you'll most likely find these being done either from some Picture Framers or large stretcher bar suppliers. They will generally have what is called V-Nails holding them together, a metal V shaped staple the same as what is used for joining picture frames. However, some more shady suppliers will just use a normal staple gun to put them together. If they are stapled take it back. It’s the absolute worst thing you can do to a stretcher bar as it may not hold the frame together square and become loose over time as the wood contracts and expands.

Pros of a cut and pin pre-assembled strainer frames

The cheapest way to produce and assemble a frame for stretching canvas for the manufacturer. You should not pay very much for any bars that are cut and pinned.
Depending on where you buy a canvas on strainer bars, depends on the wood quality. Some of the quality of the wood on these bars can actually be very good as a picture framer, for example, would buy lengths of stretcher bar moulding to produce them. Generally, these would come in Pine (finger joined or not) or Obeche woods.

Cons of cut and pin pre-assembled strainer frames

There is no way to re-stretch the canvas once it goes slack. The fact that these bars are often glued and V-Nailed means that the only way you can get your canvas to stretch again would be to lightly spray water on the rear. When it dries it should tension the canvas up a little, but over time the canvas will still go slack.
To give a tight enough stretch the manufacturer will often use a machine or tool to stretch the canvas to get it extra tight. This has an effect on the inks of the canvas and some cracking may occur around the sides of your picture. Canvas's should be tight but not over tight so as to damage the material. You may also find that the canvas used is a lower quality Poly canvas as this has a higher initial flexibility to help the initial stretch and is less prone to cracking inks - See the buyers guide Part 2.
The larger you go with your size on a cut and pin the more you'll notice the canvas sagging in the future. More surface equals more stretch needed.....
Although you could, in essence, use the frame again, re-using a cut and pin stretcher frame is not overly recommended, your stuck with the frame size and you'll need specialist tools to re-stretch a new canvas over it.
Some of the wood is of poor quality. MDF is widely used by some of the larger manufacturers and although can seem OK on a smaller canvas the strength of the wood for a large canvas may not be appropriate.

The pros and cons of using a Wunderbars joint.

Wunderbars Constant Tension Stretcher Bars

Wunderbars - What else?

Wunderbars Stretcher Bars - Well, what did you expect - Genuinely we feel that Wunderbars are a great choice for a stretched canvas. We designed them to be used with digital images as well as art canvas's. They can be used by even the most novice of canvas stretcher as they simply require stapling and pulling of a pin..like a grenade! If you buy a stretched canvas in some of the most major retailers in the UK, most likely they will be stretched using a Wunderbar.

Pros of using Wunderbars Stretcher Bars

They use a spring loaded corner piece. This means that you don't ever have to worry about them going slack. The piston in the corner is constantly sprung, pushing against both sides of the bar. So as the canvas and the bars contract and expand with humidity, so do your bars
You can re-use the bars. If you decide that you don't like the canvas you have you can simply remove it by removing the staples and reload the pins (If you need spare pins for reloading let us know). Following the instructions on this site even a canvas stretching novice can stretch a canvas, with no specialised tools needed.
They are gentle. With traditional stretcher bars to stretch the canvas you have to hammer the wedges in, quite tightly. With Cut and Pin you have to stretch the canvas extremely tightly so it doesn't go slack. With Wunderbars, once you pull the pin your canvas starts stretching. Noticeably immediately, but it continues to stretch until it reaches its optimum tension. This method is so gentle it causes no noticable cracking on the ink of your canvas and does not over stretch it, extending the frames longevity.
Because they are so fast and easy to make, by anyone, you'll find that you will get a faster service and turnaround without compromising on quality. Some retailers will even do this as an hour service for you.
They are made in the UK - We make them, in Cradley Heath in the Black Country, West Midlands. Quality is a priority for us too. Further-more we also only use local suppliers. All of the parts of your Wunderbars Stretcher Bar are sourced from companies within the Midlands. The only imported part of a Wunderbar is the timber used, and that's machined to profile in Midlands too.

Cons of Wunderbars Stretcher Bars

Wunderbars are high quality. You may pay a little more for your stretched canvas as they are built to last.
If you're planning on making them yourself then, as with all types of stretcher bars, you'll need to learn how to fold canvas corners.

Types of Wood Used in Stretcher bars

Which part of the tree is my stretcher bar from

Some timber for stretcher bars is a cut above!

Stretcher bars come in various types of wood and there is not enough time to cover all of them in this post, but we'll go through the most common types and what you should be looking out for.

Pine - Whole and finger Joined - Most stretcher bars are made from pine. All of our traditional stretcher bars and two of the Wunderbars range are made from pine. We use Unsorted whole Scandinavian Pine, but finger joined is also common for standard stretcher bars. Bear in mind though that there are grades of wood..some with more knots and defects than others. The majority of pine used in stretcher bars will come from Scandinavia or one of the Eastern Bloc/Baltic countries such as Latvia or Estonia. The quality of a pine stretcher bar is good and is the most cost effective wood for producing a stretcher bar.

MDF - MDF is much more common amongst the larger stretched canvas providers, though it's brief popularity has faded lately. It's cheap and you wont find too many larger canvas's stretched on MDF. It's very much prone to bending. These will be amongst the cheapest stretched canvas's available on the market and will be cut and pinned as its almost impossible to make a traditional stretcher bar from them. Where possible ask what type of MDF is used, there's moisture resistant MDF which reacts and performs slightly better than standard MDF.

Fir and Paulowina Wood -Turn over your canvas. What does the wood look like? Cheaper stretcher bars will be thin (about 18mm in depth, though there are 38mm deep stretcher bars available in Fir) and probably look a little greenish/browny/pale in colour. This is most likely stretched on Fir or Paulownia wood. They are extremely fast growing trees and are in general imported from the Far East. They should have (if the retailer has bothered to include them) plastic corner keys in all four corners of the frame, sometimes one per corner, sometimes two. The wood should have a brittle appearance and be extremely light.

These stretcher bars are cheap. As cheap as they get and as you'd expect the quality of them can be very poor. Check to make sure the sides of the image on the canvas have no dips in them as these bars are prone to cracking and having large chunks missing. If it has it's likely this will show through in time on your image. Like with MDF this is OK for a run of the mill, throw it away item, but long term purchases, you should generally try to avoid them. Its most likely the seller is trying to save money buying cheap. There are far better options from the UK or Europe than these bars nowadays for a similar price.

Tulipwood or American Poplar - One of the the Rolls Royce's of stretcher bars. American poplar or Tulipwood as its more commonly known is a straight grained hardwood. It's used on a lot of higher quality stretched canvas. You know the Mona Lisa, that I was talking about before? Poplar wood…..Quality with these stretcher bars can last a lifetime. They come very smooth and green textured, but you may find some other colours like brown or even purple and black in the timber. Very, very, few knots and as such are ideal for larger canvas's where warping is more of an issue

Bendy, warpy, twisty! My stretcher bars on my canvas frame are warped!

Types of warp

Larger canvas's are prone to bowing or warping. You'll see a dip at the top of your canvas frame. This is due in some parts to the person who stretched the canvas (uneven tension) or the wood containing a lot of moisture then drying out, a massive knot in the wood or even the join of the stretcher bar. To alleviate this problem cross bars or braces should be used to stabilise the frame. Generally any frame over around 30" x 30" in size should have cross bars in them. If yours hasn't then you run the risk of it warping. Make sure you have cross bars where needed on your larger frames and that the supplier has used straight wood.

A good stretcher bar wood should be "kiln dried" which makes them sturdier and less likely to warp too. Wood is a porous material and as such it absorbs moisture. As the wood absorbs the moisture it expands and then shrinks when it starts to dry (like when you put the heating on). All this expanding and contracting, over time causes the wood to warp. Kiln dried wood reduces this problem somewhat.

Wood is known as a live product and in general all wood moves and warps constantly. This will happen to a greater or lesser degree depending on changes in humidity in the room you've hung it in and the species of wood used to stretch the canvas. Thinner wood also tends to warp more than thicker wood, so the bigger the frame the bigger the wood should be. Size matters again!

Is a knot a good knot or is it not?

Knots can cause your wood to warp. This warping will be more evident and pronounced in larger canvas's, so smaller canvas's shouldn't have too many issues, unless its a massive knot! Wood, as you know is a naturally occurring product. A Knot can be caused from either a part of a branch that's been enclosed by wood or a dead branch that's just dropped off the tree. Knots in stretcher bars can cause concern the bigger the stretcher bar you go. So the larger the frame the less knots you should have in them to keep the stability of the frame. Larger knots are the worse and if you have a good supplier then these should be cut out when manufacturing the stretcher bar. The manufacturer should also take time to assess how straight the bar is in their manufacturing process. Basically, when timber dries, knots don't dry at the same rate, so it can crack, fall out or cause the bars to twist. In general all softwoods are prone to the types of warp and twist, with knots, or without, even after the wood has been kiln dried. However, a good stretcher bar manufacturer should reduce the risk drastically in their quality control systems.

Keep in shape - Why a stretcher bar should have a slope or a lip

An example of a sloped stretcher bar profile

An example of a lipped stretcher bar profile

All, and yes, I mean ALL stretcher bars on canvas should have sloped or lipped edges. If the canvas is stretched over a flat/Square bit of wood then flat piece of wood is going to bleed or show on the image side of the canvas. If you have a precious canvas that's been stretched on square piece of wood, get it changed! Your canvas should only touch the beveled side and the back of the bar, not the image.

What to look for in a retailer in regard to stretcher bars?

The verdict

The higher the quality stretcher bars the less problems you are going to have. When buying your stretched canvas you will need to decide how long you want the wood to last. Be prepared to pay more the deeper (protrudes from the wall) a bar you go and the higher quality of wood or the method you seek. Ask the seller what kind of bars they will be stretched on and what bar depths they go up to. Ask to look at their stretcher bars to examine the quality before you buy. If the retailer has an example on display, take it off the wall, turn it around, check the bars and how they look.

Stretcher bars may be unseen on the canvas but they are the engine room of what makes your picture stand out. If you want your canvas to last you need a good quality stretcher bar.

This is the final part to our guide

This is the final part to our buyers guide to framed canvas prints. Please let us know in the comments below if there's anything about your stretched canvas you want to ask or if there's anything we've missed. Also, if you get the opportunity, please share this post or link to it from your website, that'd be great!