I have carpet in my sewing area, and starch like crazy... it dries to a powder and vacuums up with no trace. Just to prove it to yourself spray a piece of paper .. let it dry and see how it reacts. Starch never truly goes into solution or dissolves, it is mostly suspended. That's why it will "fall" out when the fabric is handled or manipulated. Think about a shirt that has been starched ... at the elbow or any area that is bent or folded repeatedly will no longer be stiff.