Often associated with historic buildings, horse hair plaster is used as an intermediate layer of plaster when mortaring walls. Horse hair plaster is made my adding horse hair to lime putty and sand and the horse hair works as a binding and reinforcing agent. The horse hair and lime create a more flexible mortar that is not as likely to crack as cement mortar. Horse hair can be purchased in bulk from animal hair suppliers. Use horse hair plaster on masonry or lath to create walls that have a unique, rustic texture.
Combine lime putty and sand. Use one part lime to three parts sand. Mix together lime and sand using a cement mixer. Mix thoroughly, for at least 20 to 30 minutes.
Add water only if necessary and if the mixture is overly dry and crumbly. Add just enough water to create a thick, spreadable consistency.
Mix in the horse hair just before the end of the mixing process. Sprinkle in the hair gradually and break up any clumps with your hands. Add horse hair until you see the hair evenly distributed throughout the mixture. Use about three cups of hair per gallon of plaster. If you are not using the plaster right away, wait to add horsehair until just before using, as the alkaline properties of wet lime putty can deteriorate the hair.
Store plaster (before adding horse hair) in a plastic, sealable bucket. Lime-based plaster will keep for months if sealed in an airtight container and actually improves with age. Mix in just a bit of water after reopening to reconstitute the plaster, if necessary. Keep the plaster (with horse hair) covered during the mortar project.