If you were to look at a woollen fibre under a microscope you would see that the fibre is not smooth but made up of interconnecting scales or links.
If during handling, washing, or drying these fibres are rubbed together too much, the fibres interlock and ratchet up into tight links of fibre which produces the shortening of the length of the woven yarn. This produces the visible felting, or shrinkage of the garment, which is permanent and not reversible.
So, we choose a wool wash program, which is low temperature, high water level, so that as the drum turns the fabrics are given gentle mechanical action to remove soiling followed by a rest of nearly a minute.
The rest periods between wash drum action is very important as this allows any slight tightening of the fibres to relax back to their original length and shape.
Use a proprietary brand of woollen detergent which normally will contain silicone to aid the slipping of the fibres against each other without the felting. The biggest cause of damage to Wool and Lambswool garments when washing comes from using Biological detergents or detergents containing bleach or bleach activators. This can be avoided by only using washing powders intended for use with natural fibres (Lambswool, wool cashmere, etc).
Never when washing woollens use a cottons or coloureds program, these have a low water level and high mechanical action program. Permanent damage would almost certainly be done to the garments very quickly.
Most Wool wash programs will have an over-ride on the temperature so that even if you forget to reduce the temperature it will wash at a maximum of 40 degrees Centigrade.
When woollen garments are wet they can easily lose their shape, to avoid this always carry them and remove them with care from your washing machine to the drying area.
If possible dry in the open air lying them flat or supported on several rungs of an airer or wash line. Woolen and Lambswool garments must not be dried mechanically and should be air dried only. All Wool and Lambswool garments should be dried flat to retain their shape.
With some fashion items you may have to remove items before washing such as buttons which may cause damage to the garment during the wash. It is worth taking this precaution if you value your garment and wish it to last a long time.