i learned about scouring when i started dying indigo. in order to achieve a uniform color and great result - you must first scour the clothes. This is easily done by boiling them in washing soda (aka soda ash, sodium carbonate) to remove all the waxes and oils. It's actually really important to do this to even new cotton as it contains a lot of impurities and waxes etc... from the manufacturer. if you end up trying this method you will be blown away by all that is still in your "clean" clothes...i scoured a pillowcase and was shocked at how dark and dingy the water became.
*this method is for cotton only as wool requires a different treatment...
xo n