Ha ha, no, not something from Harry Potter, but one method I use to mark quilting patterns.
I use two different vanishing pens. The one that lasts me about an hour or so is a 'Collins vanishing fabric marker'. These seem to be available online all over the place for about £3 each.The other vanishing pen (the marks last a few hours) is a double ended, 'Chaco Ace disappearing marker'. I bought this at one of the quilt shows a year or two ago, and naturally I have no idea which stall it came from! I cannot seem to find anywhere in the UK, online that sells it either. Gah! Fortunately mine is still OK (a tip I learned is: always store pens horizontally) but I shall try and remember to keep my eyes open for another one at both Malvern/FOQ next year.
It seems bright light/sunshine makes the pen ink disappear much quicker, so try and keep the cloth covered.
I do also use 'wash away' pens: the 'Berol Handwriting' (red barrel, black or blue ink) which is designed to be washed out of clothes. I've never had a problem washing the ink out of any of my quilts, but I have heard of others that do (but then, I do tend to put my quilts on a 60 degree hot white wash) These pens are fairly cheap and available in most supermarkets.The other two pens I have are: the Chaco wash away pen and Prym wash away pen, though I tend to save these last two for trapunto and tracing out embroidery designs.
I have now marked the spines of the feathers I intend to quilt on the final border. I used the Berol handwriting pen for two corners and the wash away pens on the other two. The Berol is much easier to see!These are to be very informal types of feather, from the Fun with Feathers DVD, by Patsy Thompson. I use a bendy ruler to get the basic spine shape and draw in the curve. I can then quilt in the plumes free hand.
1 hour ago