Continuous Bias Strips: Trick from the Quilters’ Bag
Pictures will be forthcoming.....
Quilters often use a bias strip to bind the edges of their quilt. Usually, they don’t want to use the commercially available ones: either they’re the wrong color, or the wrong size, or they want something with a pattern, or they want 100% cotton, etc., so they often have to make their own. I don’t know who invented this technique, but it was a huge boon when I was making a Victorian gown with applied decorative bias strips of velvet 5” wide. I needed a whole lot of this stuff, and had to go back twice to get more velvet, but it worked like a charm, and I now use it quite often to bind the tops and bottoms of corsets when I have trouble matching the color in a purchased bias tape.
The basic technique is this: a) take a true square of fabric and cut it on the true bias (corner to corner), giving you two right triangles. b) Slide one triangle past the other so that instead of the bias edges being next to one another, the straight edges abut. c) Now, flip one triangle over, laying it on top of the other, with the straight edges still aligned and right sides together (the other edges won’t line up; they’re not supposed to). Sew the straight edges together and press open or serge and press. d) Now, open your fabric out; you now have a parallelogram, with two edges on the bias, and two edges on the straight of grain. e) Determine how wide you want your bias strip to be (including seam allowances) and call it w; draw chalk lines parallel to the bias edges and w apart. (IMPORTANT: when you measure to put on these lines, make sure you measure perpendicular to the bias edge, NOT parallel to the straight-of-grain edge. Otherwise, your bias tape will be narrower than you intended. ) It may not work out evenly and the last strip may be too narrow. If this happens, simply trim off the last partial strip.
Now comes the tricky bit. f) & g) You need to fold the fabric so the remaining straight-grain sides are together (right side to right side) BUT offset by one width of your tape. This seam will have one width of tape hanging off each end of the seam. (Make sure you line them up so that the marked lines cross on the seam line, not the edge of the seam allowance.) Sew and press (or sew, serge, and press) this seam. h) The marked lines should now describe a spiral around your tube, roughly meeting up across the seam you just sewed. Cut along the marked lines and voila! You now have a continuous piece of bias tape. (Note: you have just repeatedly cut across two seams; those short seams will separate, if treated roughly. Also, you can easily stretch the strip out of shape (it IS a bias strip, after all), so treat it gently. Wind it onto a card or roll until used and try not to manhandle it when applying.)
The remaining question is, of course, if I need L inches of w-inch-wide bias tape, how big a square do I need to start? Turns out that the answer is pretty simple. Multiply the total length you need in inches (L) by the width (including seam allowances) in inches (w), then take the square root of that product. The resulting number (a) is the length of one side of the square you should start with (also in inches). (I’ll spare you the mathematical proof I did to convince myself that this is true.)
The trouble comes when a is larger than the width of your fabric. You then have to either break it down into several squares, (the most you can get out of one square of fabric of width W is (WxW)/w; you can then figure out how many of these squares you’ll need) or sew several widths together to make the required width. I think the second approach could rapidly get unwieldy and out of hand, so I would recommend the first approach, even if you don’t like the math. (I suspect there are spreadsheet programs out there that can do this calculation fairly easily, and make a chart of width versus total length. I know that there are such charts in various places on the web (don’t know where, off-hand, though quilting sites would be a good place to start looking).) But this method will give you the answer. (Remember, if you use the first method and you need a square of side a, and your fabric is only half that wide, you’ll need 4 squares of side (a/2) to make the same amount of bias, not 2 squares.)
I would also recommend that you assume you will need a little more than you think, and cut accordingly. While mathematics is exact, this method loses a little in the seam allowances, and in those instances where you end up with a partial strip in figure e). It would be a shame to come up 5” short, when you could have made your square just a little bit bigger to start with, with very little effort.