Monday, October 10, 2011

Mind your seam allowance...

This past weekend I taught a class on scrap quilts. I had prepared a bit of history of the scrap quilt, a bit of color theory, and I had spent considerable time on an exact description of how to calculate seam allowances for certain patterns, especially the “Flying Geese”. The triangles used in Flying Geese need to be cut differently, depending on whether they are to be used as the ‘goose’ or as the background triangle. A goose must have the straight grain on the long side of the triangle, whereas the background parts must have the straight grain on the two shorter sides of the triangle. So while the goose triangle must be cut by cutting a square on the diagonal twice, the background triangles must be cut by cutting a square on the diagonal only once. Naturally, these would be different size squares for one and the same flying goose:

I have never been a wizard at math, but I did learn some important formulas in school, such as the rule of three, which has always helped me find the needed number of stitches in knitting, and I sort of know what to do with Pythagoras’ theorem. However, Pythagoras had not yet encountered sewing machines. And surprisingly many women in quilting have not yet encountered (or ‘only’ completely forgotten?) Pythagoras...

My motivation to start this calculation was the fact that I always sew patchwork items with a ¼ inch foot, whereas I know that most people here still use the standard foot that comes with the machines, which would be in the metric system. First of all, I wanted to be able to tell them the different amounts of seam allowance that had to be calculated into their cutting when you compare inch to metric. But I also wanted the participants to be able to calculate their individual sizes. Personally, I have never been one who would just copy an instruction as given in a book or magazine, but would want to decide on my own size, e.g. 3 7/8 inch as the base length for my flying goose, and not 4 inch...  And of course I have long since been very much aware of the importance of seam allowance, which you simply can’t ignore when you are doing free-hand cutting and parts tend to shrink on you in the finished part when you don’t allow enough fabric to go into the seams.
In any case, it was a first challenge to get the participants to understand the difference between deciding on the size of a completed block and calculating the size of the parts to be cut. This was learned when some of the participants chose a pattern where two equal sized rectangles sewn together on their long side form a square of the same size as the long side:

Here, of course, the cut parts do not exhibit that same characteristic that two of them put next to each other on the long side will form a square, due to the seam allowance.

And it was a second challenge for the participants to understand the differently calculated seam allowances for the goose squares and the background squares. The seam allowance for the long side of a right angle triangle is a different value than the seam allowance for the short sides in a right angle triangle. However, some participants successfully completed a few self-calculated "Flying Geese" blocks:

Two of the thirteen participants did choose a Flying Geese pattern for their project, though, the others stuck with rectangles and triangles cut from the diagonal in a square.

In addition, we found out that seam allowance is not equal to seam allowance. Most of the participants were indeed simply using the standard foot that came with their machine. This being Europe, they all assumed that their feet would have the standardly quoted seam allowance of 0.75cm. Big surprise when almost none of the sewn parts ended up having the same size!
So I made everybody take the ‘seam allowance test’: sew a straight line, and then sew a parallel line next to it as you would when sewing along the edge of your foot – and of the thirteen participants less than half came up with that standard width, the others had 0.8cm, or even 0.9cm. Interestingly enough, the 0.9cm was a model by a brand which had another model with an allowance of 0.8cm. Now wouldn’t you assume that standard sewing feet on the various machines of one brand would have the same seam allowance?
In the end all of the participants sort of understood what they were doing – I hope. But it was certainly a tough lesson. 
And it has taught me, too, that I will stress that factor much more in my teaching in the other classes as well.

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