Canvas stretcher bars for oil painting; milling, joining techniques questions

[Sean Rainaldi]

Hi. My other hobby is doing oil paintings and I was thinking about trying my hand at making my own oil canvases. Anyone have any suggestions as to the kinds of joints to make? What type of corner bracing (if needed) and joint type for corner bracing? Cross bracing - at what canvas size is cross bracing needed - and joint type? Do you use your router table and specialized bits to make the joints, or is that overkill and does a table saw suffice? Is pocket hole joinery sufficient? Also what's the best kind of wood to use for stretcher bars, what sizes of board are recommended? How do you taper your stretcher bars for the canvas side of the bars - table saw (http://javascript%3Cb%3E%3C/b%3E:showCorrections%28%27table%20saw,%20table-saw,%20tables%20aw,%20tables-aw,%20tables,%20tablas,%20table%5C%27s,%20tableau, %20tablets,%20tubeless,%20tabled,%20dabbles,%20tab la,%20tabla%5C%27s,%20table,%20tales%27,%27w0%27%2 9) or router? What's the best kind of canvas stretching tool or pliers? Will any kind of home depot style stapler do for stapling canvass to the bars? Suggestions on stapling technique? Thanks for any suggestions, input.

[John Downey]

Wow Sean, that's a whole bunch of really specific stuff! :D

I'd start by buying or borrowing a stretcher and proceeding to copy it. I've not looked closely at them, but I do have some idea what you want to make.

Joints are dry right? I believe they're supposed to come apart for replacement, and I seem to recall that the joints are just a large tenon and an open or slot mortise. I don't know how its fastened though, perhaps just the canvas tension.

Keep the pieces squared up until the joints are cut, then I'd add the bevels using a table saw. Joints should be easy to cut using a table saw and tenon jig.

I don't think wood variety will matter much, though I can ask my sister - she's a conservator (paper rather than painting, but she'll probably know). Any of the pines would do fine I'd think, obviously you want straight grain and no knots or defects. Size? Well about 1x3 is what I remember seeing, though I think the commercial ones cheat down on the thickness. My mother has a painting that is on a 1x2 rough sawn stretcher, seems fine.

[Glen Butler]

Wow that is a lot of questions. I do oils also and I will try to hit all your questions.

1: The best joint I have used is pictured below. It is two overlapping saw kerfs using a full 1/8" blade. The canvas holds the frame square so no cross or corner bracing needed other than the way you cut the joint.

2: You will need a table saw to cut the joints and you will need a jig to hold the pieces, because there is a finger the full length of the bar and there is a 45 degree slot to accept a 1/8 ply "pegs" that will do your final stretching.

3: Pocket holes are unnecessary.

4: Basically 1x2 Cedar makes up all the parts. The span bracing is undersized and mortised in. Span bracing occurs when spans reach 2' thus reducing the span to 1'.

5: Taper using the table saw. Only 3 degrees is sufficient. Leave an 1/8" that is not tapered and make sure the outer edges are round.

6: Your fingers should do a sufficient job of stretching. This video does a good job answering most of your questions. http://www.youtube.com/watch?v=QkToBRa-O-U

[Barry Vabeach]

Sean, I made a ton of those for friends, I use a bridle joint, which is an open mortise tenon joint, if you have a table saw it is very easy to set up, and I can run through a bunch pretty quick with a tenon type jig. The best part about the joint is that when you put the canvas on, you are tightening the joint, so even if the glue fails, it will still stay together. Half laps would also work as well. I got away from tapering and instead used hide glue to glue very thin strips to the outer edges . The idea of the taper, or the strips, is just to keep the canvas off the stretcher. I use 1/2" ply for the stock, it is very light and stable. I usually go with stock about 2 inches wide and can run about 30 inches without intermediate bracing. If you go wider, you might go a little longer, but I often but in a middle brace at 36 or more. You don't need much in the way of a joint for the bracing, nails, or a shallow mortise or half lap will work since the tension of the canvas will keep it in place. I use the full size canvas stretcher ( I didn't get mine from here, but this is the style http://www.enasco.com/product/9700720) , and use a pnuematic stapler - the hard part is deciding how to deal with the corners.

[Sean Rainaldi]

Thanks so much you three for your kind suggestions! I guess my only question is, because of what John said above I never thought of this - all the joints in the construction of a canvas need to be glue-less - is this correct? For repair and or archival restorations?

[Stephen Tashiro]

I'm interested in the archival aspects of canvas stretchers too since I've made some. I have some additional questions and speculations.

1. Is it possible that the unfinished unglued stretcher is popular merely because it is inexpensive? I haven't taken ancient canvases off of stretchers, but I have taken recent canvases off of stretchers. The basic job is to remove the staples and tacks from the stretcher. To me, this is easier if the stretcher stays together. The only implication I see about glue is that it might emit chemicals that would harm exposed canvas. The back of a canvas is often not gessoed, so raw canvas is exposed. The canvas may be cotton or linen. Do people who make furniture worry about glue harming the upholstery? (I don't know.)

2. Why not use acrylic varnish to seal stretchers? You could paint layers of gesso (also acrylic based) on the stretchers over the varnish (and any glue). One worries that chemicals may damage the canvas, but the canvas already has gesso on it. If gesso is going to hurt then you are out of luck anyway. Won't a sealed stretcher expand and contract less than one of raw wood? Expansion and contraction can be one factor in cracking the surface of a painting.

3. You can find a great variety of stretcher making methods on the web. Some artists use door moulding mitered and stapled at the corners. Some make mitered corners with wood biscuits.

[John Downey]

Here's what sis says:

"Always best to ask a real specialist, and as you know I'm not that with paintings. I'll tell you what I do know, though, and if you want to pass along my email address (probably not for posting to the list though) I can try to link them up with a paintings person.

As far as I know there's no specific wood recommended, but pine is probably not the best choice (they often are pine, though). It should be as dimensionally stable as possible.

If the members are nailed or glued together, it is called a strainer rather than a stretcher. Stretchers do come apart and are generally designed so that they can expand and contract with changes in humidity. However, it is not the canvas tension that is holding them together; they should have joints constructed in such a way that they will hold together on their own and support the canvas as it is stretched. They generally have a method of expanding them a little bit once the canvas is stretched, too, so that small inconsistencies in the stretching can be evened out. Older styles have "keys" to do this, and some higher end new ones have a mechanism embedded in the corners. These can be useful for evening out the initial stretching, but my understanding is that conservators prefer to see a good initial stretch without using them, and if there is buckling in the corners of the canvas once it is painted (including old paintings), this is an indication of changing humidity and shouldn't be keyed out at that point because that would introduce new stresses when the humidity inevitably changes again. (Indeed, canvas paintings are so dimensionally unstable it's a wonder they survive as well as they do).

Once the canvas is stretched and painted, it is a good practice to attach a sheet of foam core, cardboard, coroplast or what have you over the entire back of the stretcher. This creates a "pillow" of air behind the canvas that helps keep it stable and prevents excessive flapping around."

[Dave Sepucha]

Interesting discussion.

My wife is an artist and often does larger pieces (up to 4'x8'), typically oil on canvas. Out of convenience, she orders a whole slew of stretcher bars from places like Dick Blick. This is a typical example... http://www.dickblick.com/products/blick-heavy-duty-stretcher-bars/

No glue or fasteners are used, but the joints are pretty tight, so the stretchers stay put pretty well. For larger canvases, she'll use a brace in the middle so that the sides of the painting don't sag or bow. Never any corner braces though. (Some stores sell pre-stretched canvases with corner braces.)

We've been discussing having me make stretcher bars for her because she'd like something much thicker than she typically finds for sale. So this is a long winded way of saying that I'd also love to know a relatively easy way to reproduce this joint.

[Dave Sepucha]

One more thing... I'd like to know an easy (read "safe") way to make this joint on the end of an 8 foot board.

[Sean Rainaldi]

Just curious about one more thing. Does anyone make router bits to cut either the stretcher bar joints and/or stretcher bar bead / taper?

[Stephen Tashiro]

Here is a link to some information on the history of stretchers and strainers. http://www.conservation-wiki.com/index.php?title=PSG_Stretchers_and_Strainers_-_I._Introduction

[Glen Butler]

One more thing... I'd like to know an easy (read "safe") way to make this joint on the end of an 8 foot board.


It is simply a matter of making a sled for the table saw. A 1x2 is not heavy enough to be unwieldy.

[Glen Butler]

Just curious about one more thing. Does anyone make router bits to cut either the stretcher bar joints and/or stretcher bar bead / taper?

I have never come across a stock router, shaper, or moulder knive with the profile like those on the dick blick style stretcher bar. There just wouldn't be the demand for companies to produce them. But this profile, though pretty, is not necessary. A simple chamfer starting 1/8" inside the outer edge will suffice, and it will save you having to buy an expensive bit. As long as the canvas is not touching the wood, the stretcher bar has done it's job.

[Don Jarvie]

A guy in my town made canvases for a few artists and here's what I remember, it was about 14 years ago.

He used 1 1/2 x 1 1/2 hardwood, maybe oak, definitaly wasn't pine or plywood. Used 45 miters and glued and screwed them. I don't remember any tenons.

There were no cross braces and these were usually 4 x 4 to 6 x 6. Stapled the canvas like you would upolster a chair. Start in the middle of each side and work around.

Finally he used rabbit skin glue to coat the canvas and he said when the glue dried it would tighten the canvas so it would be very stiff.

He moved to Chicago and his clients were mostly in Boston and he said he was just going to crate them up and ship them so they must have been very sturdy.

[Dan Forman]

It is simply a matter of making a sled for the table saw. A 1x2 is not heavy enough to be unwieldy.

For an eight foot painting, you would want more than a 1x2 though.

Dan

[Dan Forman]

As John pointed out, stretchers are made with non-glued joints so that the canvas may be tightened if necessary due to sagging of the canvas. There are little triangular wedges that will fit into the slots in the stretcher to enlarge it and pull the canvas tight. There is some controversy as to whether this is a good practice.

Dan

[Dave Sepucha]

It is simply a matter of making a sled for the table saw. A 1x2 is not heavy enough to be unwieldy.

I agree. However I cannot make the joint I posted using only a sled on the table saw. The difficult portion is the deep groove which is similar to cutting a cheek for a tenon. I don't think I'd want to attempt this on a table saw with a 8 foot board.

The solution I've come up with is to punt in a sense. Basically use a much simpler joint and fasten the joints... so I'm talking about building a "strainer" (new term to me) versus a stretcher. I'd also do a simpler profile as mentioned earlier so that the face of each piece isn't flush against the canvas.

[Glen Butler]

I understand not. The deepest part of the groove, is about 2.12 from the proudest part of the finger. A table saw can easily cut that depth. Is it that you dont understand how to make such a sled, so as to cut the joint on a table saw?

[Glen Butler]

True. Didn't think about that. I wonder what is typical for a painting that large? Span bracing holds the frame also. It seems that you would want spreaders not strainers, especially on something that large. Time to reinvent the wheel.

How about a deeper frame with box joint style joinery (basically a 2x4 on edge). The ends of each spreader would have somewhat of a sliding dovetail, with a corner brace to match. Then threaded rod would push outward on that corner brace.

Dave what happened to your post?

[Dave Sepucha]

I think all of my posts are still around.

I'm pretty sure about the sled usage but I'm not sure how it would help to make the deep grove that goes alway through the stretcher end. ( the part that mates with the square " tongue" that remains)

However I think we've come to the same conclusion, reinvent the wheel. I'd probably just come up with my own joint to match my skills, tools, and size of the painting.

[Sean Rainaldi]

WOW, I find a month later there is much discussion thanks so much for the advice guys!

Yes Glen I have not been able to come up with any stock bits for artist canvases either after searching on the net. Maybe the manufacturers have them custom made.

Revisiting this canvas build idea, I was thinking of using the TS to cut the 3 degree chamfer you recommend, and then just use a quarter round router bit, maybe 1/8", to round over the outside edge of the frame. Think that would be sufficient?

Also Glen, how bout non blued pocket holes for the 45 degree miter joints for the very large canvases? Because to cut those slots on a table saw for very long boards would not be possible for me, the boards would hit the ceiling in my garage...

[Stephen Tashiro]

I've asked various artists that I know if they've ever used the adjustment that is supposedly supplied by the non-glued corners of stretchers. None have. It's also not clear to me how you would use such an adjustment if the canvas is stapled or tacked all along the stretchers as it normally is. Would you remove the staples? Or is the corner adjustment done before any tacking begins?

[John Downey]

I've asked various artists that I know if they've ever used the adjustment that is supposedly supplied by the non-glued corners of stretchers. None have. It's also not clear to me how you would use such an adjustment if the canvas is stapled or tacked all along the stretchers as it normally is. Would you remove the staples? Or is the corner adjustment done before any tacking begins?

I believe tightening the canvas is needed on the multi-decade to 100+ year plan. I can ask about it, but my sister is really more of a photograph and paper specialist. I think they'd just un-tack the corners to get at the kerfs for adjustment.