The Hourglass Block is a Quarter Square Triangle unit where one triangle is one quarter of the square. Two different fabrics are set in opposite positions to create the design. This method makes two hourglass blocks at a time.
For this tutorial, I'll be making an hourglass block to measure a finished size (the size a unit or block measures once it has been sewn into a quilt top) of four inches.
The assembled hourglass block needs to measure 4 1/2" x 4 1/2" from raw (unsewn) edge to raw edge before it is sewn into the quilt top.
Here is my formula for determining how much fabric is needed to make an hourglass block.
Start with the finished size of the block. 4inches
Add 1/2 inch for the side seam allowances. 4" + 1/2" = 4 1/2"
Add 3/8" for every diagonal seam in the block. Two diagonal seams means 3/8" + 3/8" = 3/4". So, 4 1/2" + 3/4" = 5 1/4".
This formula works, but, your seams must be EXACTLY 1/4" and COMPLETELY straight.
Try as I might, some slight variation of a 1/4" seam can, and does occur.
So, I add an extra 1/2" for some wiggle room, and then trim the unit to the size needed.
The same formula works no matter what size hourglass block you want to make. (Finished size plus 1 1/4" for exact, or finished size plus 1 3/4" for trimming to size.)
With that in mind, start with two 5 3/4" squares of fabric.
You will need to mark a diagonal line on the wrong side of the lighter fabric square. There are a couple of ways to do this.
Some people prefer to draw a diagonal line from corner to corner and then sew 1/4" away on both sides of that line.
Others, like me, prefer to draw the sewing lines. I use a tool called a Quick 1/4 ruler. There are variations of this tool, but the mechanics are the same. It is 1/2" wide and the center is lined up corner to corner.
Then, draw a line on both sides of the ruler to mark the sewing lines.
Place the squares right sides together.
I sew just to the inside of these lines, not exactly on them.
Press the seams to relax the stitching. Cut the squares in half, corner to corner.
Press the seams toward the darker fabric.
This makes two Half Square Triangle (HST) units.
Draw sewing lines, corner to corner, on the back of one of these HST units.
Place the HST units right sides together with opposite fabrics facing.
Sew just inside both lines as before. Pull back the corner to check that your center seams line up correctly. Press the seams to relax the stitches.
Cut the units apart corner to corner.
Pick out the stitches in the seam allowance on both sides of each pieced unit.
Lay the unit flat, right side down. Push each seam allowance in opposite directions to open up the center intersection.
With your finger, press the center flat.
With an iron press these seams in opposite directions. You will see a tiny four-patch in the center.
Turn the unit over and press it flat.
Line up a square ruler so that the 45 degree line runs along a diagonal seam and the 2 1/4" mark is at the center.
Look to make sure that the edges of your block are within the 4 1/2" mark on the ruler.
Trim the side and top of the block.
Turn the block around one half turn. Again align the 45 degree line on the ruler with the diagonal seam. Make sure that the lower and left edges are lined up with the 4 1/2" mark on the ruler and the 2 1/4" mark is at the center.
Trim the side and top.
Repeat the process with the other hourglass block.