Hair and other fibers have been added to lime and gypsum plasters to give greater strength to these nibs and stop them breaking off. For the reinforcement of lime plasters and renders, it is recommended that hair should be strong, long and free from grease or other impurities. Ox hair is the preferred choice but horse, goat, donkey and a variety of other hair including reindeer are suitable. Human hair should not be used as it is relatively fine and has poor strength.
Modern synthetic fibers 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 whereas natural animal hairs by comparison have a much rougher texture and are generally more appropriate for historic buildings.
The importance of ensuring that sufficient hair is evenly distributed throughout the mix cannot be over-emphasised. If lumps of hair are allowed to form in a plaster mix, this will cause the lumps to have no binding power and create weak spots in the plaster, causing it to fail.
Where haired lime plaster is to be applied to a masonry background, the plaster relies largely upon suction for its bond (as the masonry has a smooth surface), so it does not need as much hair compared to a plaster which is to be applied to lath, which relies almost entirely for its key on the nibs that protrude between the laths. Insufficient hair reinforcement in a plaster mix on lath will result in weak nibs and the risk of early failure.
Hair should be added to plaster just before spreading as the alkalinity of the lime attacks the protein in the hair. Millar, in his 19th century masterpiece Plastering Plain and Decorative (see Recommended Reading), writes of an experiment where haired lime plaster was stored in wet conditions for nine months: at the end of this period he found that the hair had ‘been consumed by the action of the lime’. Recent analysis of a failed ceiling plaster from an important civic building showed that, while the mortar mix proportions were adequate and the appropriate quantity of hair was almost certainly used, it had been introduced into the mix and ‘wet stored’ for several weeks prior to application. The weakened fibers were unable to maintain the integrity of the nibs which could then not support the weight of plaster, resulting in a failure that could have been catastrophic had the hall been occupied at the time.