Weezy Rider - You bring up a good point. The muslin I bought was never intended (by me) to be washed, or at least not that I would care. I was going to use it to practice then bind it and use it for dog blankets. If it scrunched up on subsequent washings, it really didn't matter. It was certainly not the highest quality muslin to begin with.
Lady Diana - The placemat was muslin, the quilt was a 108" wide backing from a quilt shop, so definitely 2 different backs.
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The batting in the placemat might not have been the Arctic Cotton. I don't really know what it was, it was just some left over something or other from a batch of charity quilts or something.
It's possible it was AC, but I suspect it wasn't. I think that's why I assumed that I had done something wrong. If the backing, batting and only some of the top fabrics were in common, and even the quilting was two different styles - I was the main constant in the equation.
I don't know that I have any AC left. I will definitely try what you mentioned if I do, but I don't recall now if it's technically prewashed either. LOL!