One of the members of my quilt group is very exacting. She does her own drafting, precise cutting, meticulous quilting. I am sure her blocks come out to the exact measurement she intended when she started out.
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Me, well... this is not always the case! Here are my patriotic heart blocks. The brown fabric I used (the words are the Pledge of Allegiance) is a bit stiff and I think that fabric was the problem because when I 1st pressed it - there was a little ridge. Which I didn't really pay much attention to at first. But... it threw the length of my block off by enough that the blocks needed to be "stretched" length-wise to equal 12.5". They weren't off a huge amount, but enough that X5 rows - well, it just wasn't going to work.
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Now, this particular member of my quilt group - she was taught to gently press with a DRY iron. Yup - I got the dry iron alright. Hopefully my quilt teacher won't see my water bottle, ha, ha. It dispenses a fine mist - much faster and more even coverage than that silly sprayer on the iron could ever deliver! Besides the fact that the directions for my iron instruct to use some sort of fancy water that you have to PAY for... I don't think so! Water bottle uses straight tap!
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Here is how I square up my un-even quilt blocks... 1st I cut a square of freezer paper the size I want the block to be. In this case - 12.5" - because these block are off just that smidge and will be easy enough to stretch to the intended 12.5" length - the width is already perfect, phew!. This square is ironed onto my ironing board so I can quickly just pop each block on that square to see if it is perfect... or not. If not - than a light mist with water and gently pulling while ironing... usually will bring the block up to the right size. If you have a fabric that tends to "shrink" back down when it drys - a little spritz of spray starch will fit that problem (shhhh, don't tell the quilt teacher!).
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Now my blocks are ready to be brought over to my cutting mat and using my 12.5" square ruler (I love this ruler), I can quickly trim anything that needs to be trimmed. If you used spray water or starch - make sure the blocks are completely dry first. I use the back side of my rotary cutting mat for this step. Flipping over the mat to trim and cut angles - it helps the front of the mat have a longer life.
Buy Your Rulers Here!
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The next set of blocks I have to size are my courthouse steps blocks (all done, yeah!). These blocks came out to some odd size that I can't remember off the top of my head, so I will cut a new piece of freezer paper to that size (saving my 12.5" square for later use) and repeat the process. I don't really care what the unfinished size of my blocks are - as long as they are all the same size so that they fit together!
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Heart block pattern is in J. Hopkins book - Once More Around The Block. I have used this particular block in several different quilts (various sizes - the book gives you directions for all different sizes!).