Preshrinking fabrics removes excess dyes and chemical finishes used in the manufacturing process.
Most cotton fabrics shrink 2-3% when washed and dried, so if they are not preshrunk, the fabric may pucker at the stitching lines the first time it is washed.
Some dark or vivid colours need to be checked for colourfastness to determine whether the dye bleeds to light coloured fabrics or colours the water.
How To Pre-wash Fabric
Use a mild soap to machine wash fabric before you cut it for sewing.
Do not use soap intended for fine woolens as it may yellow cotton fabrics.
If the colour bleeds use a colour catcher in the wash, some people also recommend using household vinegar to prevent colour bleeding.
How To Test for Colourfastness
Fill a glass with warm water and a few drops of mild soap, such as dishwashing liquid.
Cut a small square of fabric and place in the water. Allow to soak until the water has cooled slightly. Swish fabric.
Place wet fabric on a sample of light-coloured fabric; blot dry. If dye transfers to the light coloured fabric, repeat the test; if fabric still bleeds, it is probably not colourfast.
