After the cloth had been made it was usually bleached. Traditionally, this was achieved by soaking the cloth in sour milk (lactic acid) and spreading it out on tenterhooks in the open fields to allow the sun to complete the bleaching process. Using this method it could take up to eight months before the cloth was ready to be sold at the market.
In 1746 John Roebuck began using diluted acid instead of sour milk. This halved the time required for the bleaching process. At the end of the century, Charles Tennant developed a bleaching powder at his chemical works in Glasgow. This not only speeded up the bleaching process but produced a whiter cloth. The increasing use of dash wheel machines in the 19th century also reduced the costs of bleaching.