TAFA: The Textile and Fiber Art List

Wednesday, April 15, 2009

How to Make Clamshell Quilt Templates and Sew Curved Seams by Donna Hussain

"Lamplight" by Donna Hussain
Quilt completed after completing a clamshell class by Monica Calvert.

When learning to quilt I easily mastered sewing quilt blocks with straight line seams on my sewing machine. All I had to do was place the fabric pieces right sides together, then line up my ¼” sewing machine foot with the edge of the fabric for stitching. There are hundreds of quilt blocks that require such minimal sewing skill.

Straight-line stitching, the basic skill required to make any quilt.

To machine-piece a quilt block with curved seams, like Drunkard's Path, requires more skill. As a novice quilter, I joined a clamshell class given by Monica Calvert at a quilt shop near my home to learn her curved seam techniques. I had no intention of actually making a clamshell quilt until Monica demonstrated the amazing versatility and design potential of the block during class sessions.

Clamshell Row

A template is used to cut clamshell shapes. Clamshell templates can be made at home using graph paper and a compass. Draw three circles with the same diameter as illustrated below. Circles with a 9” diameter are recommended for starters. As you gain skill in sewing curves you can work with smaller clamshell shapes.

Drawing a Clamshell Template

Cut out the clamshell shape that you have drawn on the graph paper. This is your template but the graph paper is too flimsy for practical use. Use the paper cutout as a pattern to draw the clamshell shape on light cardboard, a manila folder, or plastic sheet. A clamshell template cut from these materials will be stiff enough to last for repeated use in drawing clamshells on fabric.

To make fabric clamshells place your template on the wrong side of fabric and draw the template’s outline with pencil or pen. This line will be your stitch line so make sure that the line is dark and easy to see.

In cutting out the fabric clamshell you must allow for a seam allowance, so cut the fabric ¼” away from the stitch line. For ease in later sewing, it is helpful to widen the seam allowance to 3/8” along the pointed tip.

Drawing and Cutting Lines

To practice sewing clamshell blocks together:
  • Select two fabrics. Cut out two clamshells with nine inch diameters from each of the two fabrics.
  • Sew the pointed ends together of one color set as illustrated below. (The red clamshells.)
Preparing to Sew Clamshells Together

  • Mark the mid-point of the convex curve of the clamshells to be inset. (Blue clamshells). You can determine this mid-point by folding the clamshell in half.
  • The next step is to inset the blue clamshells into the concave curves of the red clamshells, as illustrated. To do so, place the sewn-together red clamshells face up. Turn a blue clamshell face down so that the stitch line on the back side is visible. Poke a pin through the mid-point of the blue stitch line through to the midpoint of the seam joining the red clamshells together.
  • Join the ends of the red and blue seam lines together with pins. Divide the space to be pinned together in half again and again, each time poking the pins through the blue stitch line to the red stitch line. Many pins will be necessary to flatten out all potential puckers.
  • Machine stitch along the seam lines removing pins as you go.

Pinning Clamshells Together on the Stitch Line

The design possibilities using clamshell blocks are endless. Keep adding clamshells by sewing convex curves to concave curves until you reach the size you want for your quiltop.

Possible placements of clamshells in a quilt design.

You can vary the fabric and color of the clamshells or cut clamshells from fabric pieced together. I suggest that you draw a clamshell grid like the one below and experiment with different coloring patterns using crayons or felt pens.

Clamshell Grid

When using clamshells you may mix the blocks with other patchwork blocks or appliqué sections.
The shape of four clamshells sewn together reminded me of Indian designs, of oil lamps hanging in a mosque. In "Lamplight", I used a black clamshell to represent the lamp base. I visualized perforations in the lamp sides to allow light to pass through. I quilted a candle flame in the top clamshell which is pieced. All of the blocks beneath the arch are nine-inch clamshells, though some along the sides were trimmed to make a straight edge. I consciously used darker fabrics for the area under the lamps, and lightened the background colors close to the flame. The purpose of the Islamic arch in the quilt is to place the lamps in context.

Give clamshells a try. They are easier to sew than you may think!

California quilter, Donna Hussain, has exhibited in major quilt shows around the country, authored books, and is a regular contributor to Fiber Focus. Click on her name to see all of her past articles.

The photo shows Donna with her husband, Pascha.



  1. I don't understand this tutorial. What happens after the seam is sewn? There are two pieces of cloth sewn together with a circular seam. How does this become a quilt and not a pouch?

  2. Eva, I guess I need to add another diagram to my article to show you how to keep adding clamshells together, concave curves to convex curves, until your quilt top is the size you want. I'll work on that diagram today. I appreciate your comment. Thank you. Donna

  3. I to need one more step. I can get the four together, but then don't really understand how to add more. HELP! I can not find a pattern anywhere. I have many shells cut out and would like to sew them together. Thanks.

  4. If you need help in sewing together clamshells I suggest the following. First, study the diagram in my clamshell article titled Possible Placements of Clamshells. Then pin your cutout clamshells onto a design wall, fitting the convex curve of one clamshell into a concave curve of another clamshell.
    Once your clamshells are aassembled on the design board like a puzzle, start to stitch pairs of clamshells together. Return each sewn pair to the design wall. Then sew the groupings together to make larger units until the entire quiltop is sewn together.
    Happy Quilting, Donna Hussain


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