There are a number of reasons you may want to remove the sizing or stiffening from fabric. This is a guide about removing sizing from fabric.
By fulgrove1 1
How do I remove sizing that makes my fabric very stiff. Laundering with detergent does not help. Any ideas?
February 24, 2014
Soak your fabric in a brine solution of salt and baking soda for 5 days or more. If you heat the brine with the fabric, it will work more quickly. However, hot water can also cause the fabric color to bleed or for the fabric to shrink. A generous amount of vinegar added to the wash has been suggested elsewhere. Vinegar is supposed to prevent color loss. Regardless of the method used, (whether brine or vinegar) shiny fabric is likely to lose it's shine-coating, as it's likely to dissolve in the brine or vinegar.