Singeing and mercerizing are in many cases related and done at the same time. Singeing is passing the fabric through a flame (fire) so that the hair and nubs of the fabric are burnt off to give it a clean surface. This is commonly done on most cotton fabrics including denim. It consists of the burning of fuzz on the fabric surface. Before singing the cloth is brushed to remove the loose fabric and also to remove the dust. The fabric is kept flat under tension and passed rapidly over an open gas flame. Later it is passed into a water to cool down.
Mercerizing means a treatment by soaking the fabric into caustic soda to give a shine to it. This process is not done on denim because it will hurt the color of indigo or sulphur. However it is mostly done on grey goods or dyeing or dyed goods which has a colorfast quality.