Cotton yarns and fabrics can be given a lustrous sheen by altering the cross-sectional shape of the cotton fibres. This is done by soaking the yarns (or fabrics) in caustic soda and then neutralizing them in acid. The fibres become swollen making them much more cylindrical which gives them their characteristic lustre. Mercerised cotton as it is known is also stronger and has an increased affinity for dyes. Read more..