Heat is usually the culprit behind the shrinkage of clothes. Very hot water in the washing machine or high-heat drying, either from a laundry dryer or even hot heat from the sun will reduce the size of clothes, especially cotton fabrics. For natural animal fabrics, like wool or mohair, the simple agitation of the wet fabric inside a washer or dryer will create a reaction that can significantly shrink the material.
Cotton shrinks because of what is called relaxation shrinkage. During clothes construction, tension is applied to cotton yarns and fabrics. Tension is released under heat, whether it's steamed, washed or dried, causing the fabric to shrink to its natural size. Because of this process, most cotton fabrics will shrink during the first washing.
Animal (Wool) Fabrics
Wool, mohair or cashmere are essentially animal hair and shrink through a process called progressive shrinkage, meaning it will shrink a little each time it is washed. This is mainly why these fabrics are dry cleaned. The shrinkage occurs mostly due to the agitation process that happens in the washer or dryer, raising the scales of the cubicle layer of the wool fibres, causing them to catch on each other. The water and agitation cause the fibres to bind together, altering the size of the fibres.
Nearly all man-made fabrics, like polyester and acetate, do not shrink. Although, some blends with cotton or wool can shrink minimally. Usually, this does not occur unless the clothes are washed in extremely hot water, dried on the highest heat setting, and sometimes ironed wet on the hottest iron setting. The only reason a synthetic blend will shrink is if there are also natural fibres in the fabric. The more natural fibres, the more likely the fabric will shrink.
How to Avoid Shrinkage
To keep clothes made of natural fibres from shrinking, it is best to wash them by hand or on the delicate washing machine cycle in cold water. Avoid putting any clothes that are made of any natural fibre in the dryer. If you must wash clothes made of natural fabric, wash them in cold water and, when drying, It is best to lay out the items on a flat surface. If you are drying cotton material, hang it up. To avoid any possibility of shrinkage, have the clothes dry cleaned.
How to Control Shrinkage
First, wash the garment in the hottest but shortest time setting on the washing machine. The heat and agitation will help shrink the fabric, but the short time span will allow you to pay close attention to the quickness of the shrinkage. Then put the clothes in the dryer on the hottest setting. Alternate this until the desired size is accomplished. The amount of natural fibres in the clothes will determine how much attention must be paid to the shrinking process. If the fabric is 100-percent cotton or animal fibres, such as wool, less washing and time will be needed than if it is a polyester/cotton blend.