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Like most of you, when I start a project one of the first things I consider
(after pulling threads and picking out the canvas color, of course) is whether to mount the canvas on scroll bars or stretcher bars. There are quite a few things to consider.
How big is the project? For small to medium projects, I like to use scroll bars. Although, small and medium projects can be easier to stitch on stretcher bars than a larger piece... For a really big canvas - painted OR counted - I waffle between scrolls and stretchers. Mostly I prefer scroll bars, because I can get closer to the work, which I REALLY like. Other times, I prefer to tack the big canvas to stretcher bars so I can see the whole design while I'm working on it...which becomes crucial if you're balancing colors and threads thruout the design.
What type of ground is it - mono canvas, Congress Cloth, or linen? I find that mono canvas is pretty darn tough and holds up well on scroll bars or stretcher bars - much better than rolling it up in your hand, that's for sure! Congress Cloth, while a bit stiff, can be easily creased if folded, so putting it on bars keeps the canvas flat and crisp. (I usually iron my piece of Congress Cloth before attaching to bars and stitching, so it starts out flat and crisp). And with linen, which I think is a pretty tough fabric as well, you may be an "in-hand" stitcher (which links you with a long tradition of stitchers stretching back thru time) or you may be a scroll bar stitcher. I have enjoyed cross-stitching in-hand, but I prefer putting my linens and other fabrics on scroll bars before I start stitching.
Will there be only flat stitches, or lots of raised, complex stitches? If you're going to be stitching lots of fancy stitches, or ones with long loose stitches, stretcher bars will be much easier on your canvas/work. That being said, I must admit that most all of my quilt designs (that use longish diagonal satin stitches) have been worked on scroll bars, and I haven't had any problems with the long threads being "rolled" on scroll bars. (On the other hand, I don't keep my quilt designs on the scroll bars very long; I usually complete the project in a few weeks or so.)
Other things to consider are:
What types of threads are you using - cottons or silks, rayons, metallic or wool?
Is it a travel project (one you'll carry around to stitch-ins or guild meetings, packing it in and out of your car)?
How quickly will you be working on it - finishing it in a week or a year (or two)?
All of the things mentioned above have to be considered. But, I have to confess: I usually go with the scroll bars. I know myself well enough to know that if I put a large piece on stretcher bars, chances are it will be more awkward to work on - and then I won't stitch on it as much. If I mount that same large piece on scroll bars, it won't seem quite so intimidating; I'm much more likely to stitch on it; and it will get finished faster (and I'm much happier as a result).
So I have a few techniques I use when I do mount a large piece of canvas (or fabric) on scroll bars. 1.) I'll cut long pieces of cotton batting and roll the hidden ends of the canvas against the batting, to protect the unstitched canvas, as well as soften and support the stitched portion of the canvas. 2.) I'll cover both rolled edges of the scrolled bars with fabric covers. Below is an illustration of how I quickly rigged up some muslin scraps (with safety pins, no less) to cover the top of a long project on white Congress Cloth:
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I confess I was so eager to get started stitching, that I couldn't be bothered to stop and baste the muslin (or heaven forbid, pull out the sewing machine and sew up the edges). The lower edge has a fabric cover I sewed from scrap fabric, with a plain ribbon closure that can be pulled and tied to any width of scroll bars....You can get as fancy as you want with these scroll covers, but hey, whatever works - that's my stitching motto!