Hair - particularly horse's hair - has long been used in plastering for two main reasons: it's easy to find and it works in much the same was as the strands you find in fibreglass resin. Remember, plaster has been around for quite some time now, so our ancestors needed to use whatever they could get their hands on in order to do the job right. Long ago, before everyone owned a car, everyone had a horse. Therefore, horse's hair was quite easy to obtain.
Choosing Your Hair
Today, ox hair is actually used more often than horsehair. It is possible to purchase 3 different qualities of hair. Whatever type of hair used, the goal is to help control small cracks that might develop when the plaster is drying or when bending is possible. Therefore, the hair needs to be strong and clean, which means it should be free from dirt and grease. It is also best for the hair to be long, which is one advantage that ox hair has over horsehair. Some people prefer to mix both horse and ox hair in their plaster.
Traditional alternatives to hair include chopped straw, reed, manilla hemp, jute, sisal, and even sawdust. Modern synthetic fibres such as glass and polypropylene which have been designed for use with Portland cement mortars have also been used successfully in pure lime mortars, despite their smooth and almost shiny appearance when viewed under a microscope. Natural animal hair, by comparison, have a rough texture, and are generally more suitable for old buildings. While woven hazel or willow spars work well and are often found in surviving wattle and daub, the practice of splitting oak and chestnut lath to produce riven laths became popular early in the 15th century. Oak and chestnut make particularly good riven lath as they both contain natural oils, thus ensuring long life. By the 19th century sawn lath started to be used, although there is no doubt that riven lath is stronger, and its textured surface and exposed grain affords a far better key
When taking plastering courses, you can learn more about hair and other materials that can be used as a binding agent for plaster.
So, the next time you clean your walls or hang a picture, think about what is inside the plaster and remember just how hairy your walls really are!
