Cellulosic browning occurs when a cellulosic fiber si socked with water and allowed to dry slowly, leaving a borwn discoloration where the evaporation of moisture occurs. Because cellulosic molecules break down as fibers age, older cellulosic fibers possess greater potential for browning. Even if a carpet facing contains no cellulosic fiber, browning could develop if cellulosic fiber are present in the backing. When cleaning cellulosic fibers, detergent solutions produce more browning on fiber tips, acids may be applied using a rotary brush machine. The most effective acids are 1 percent concentrations of acetic acid or a fluoride salt. The use of bleaches and strippers should be a last resort only, since they may damage both the fabrics and dyes.