Felt is tangled fibres of wool, hooked together to a non-woven piece of fabric.
The surface of a fibre consist of scales. When the wool is wet, the scales rise and interweave with each other. The more the wool is being worked with the stronger and more durable it gets. Felting is the extreme shrinkage of the wool fibres, and when it has gone through the process, the fibres cannot return to its/their original position.
Not all kinds of wool are equally good for felting, and the qualities differ.
Felt has been used in many places around the world for centuries and is considered to be the oldest known textile.
In Mongolia, the nomads have lived in felted tents (Yurts) for thousands of years.
In Central Asia 2,500-year-old felt was found, and in China making and using felt dates back even further.
In the Nordic countries, Norway and Sweden, felted boots have been worn outdoors in winter, because they were warm, hard and dense. Felt was also used as lining in unlined leather boots.
The sails for the Viking ships were woven wool and then felted in order til become tight and durable.
In Denmark there has not been a tradition for felt making as in the other Nordic countries, but fulling of the knitted and woven textiles has been common. In Danish open air museums, you can see many boards for fulling, which tells us that felting and fulling have been used for clothing in Danish households.
The picture is from Gunilla Paetau Sjøberg's book: Filtning - gammal teknik på nytt sätt