Should I prewash dyed yarn cotton fabrics?
Maybe.
It isn't always necessary. It depends on the quality of the fabric and the purpose of your project. Wall hangings that will never be washed don't need pre-washing. Tote bags and quilts will be washed a lot, so pre-washing is good insurance that they stay beautiful :-)
The Japanese dyed yarn cotton fabrics I sell at A Threaded Needle have had the shrinkage done in the manufacturing, and the yarn (threads) they are woven from are dyed with colourfast dyes before the fabric is woven. This means they won't bleed or fade when you wash them. So you don't have too pre-wash these fabrics.
But I still think if you are going to be washing the project after you make it, then you should wash the fabrics pieces before you cut them, in the same you want to wash the project after it is finished and in use.
There is what I think is best...
and then there is what I do in real life...
This quilt, which lives on my couch and gets pulled around the floor by grandkids (and me 😂) is made from 4" blocks of Japanese dyed yarn fabrics which were not pre-washed. It was quilted by machine sewing over the seams only, and has a cotton batt filling. It goes in the washing machine and dryer (though I shake it out before it is totally dried, cotton hates to be hot dried totally - so don't do that to it!! :-)
BUT there are buts involved :-)
The fabrics were all the same weight and texture and good quality. If there were mixed fabrics - say a cotton/linen stitched to a cotton, or different textures fabrics - say some blocks were a bit thicker or softer than the others- then pre-washing would help ensure that any changes washing caused would already have been 'got out' of the fabrics.
How to pre-wash dyed yarn cotton fabrics:
I think some people soak them gently in a sink, air dry and press. Me? I throw them in the washing machine with my towels. The only concession I give them, is I dry on low heat and take them out before totally dry, then shake and hang. I press with a steam iron before I sew.
And I never pre-wash the really small pieces that I am using for applique or for tiny pieced things, but if for some reason I wanted to, I would soak those in a bowl, then iron them dry.
