This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
For the best result the lime should be thoroughly slaked at least twenty-four hours before adding the hair, which must be thoroughly beaten up and mixed with the lime paste with a hoe, and the necessary amount of sand added. This mixture should then be stacked outside of the building as long as possible before being used, in our case at least ten days. When ready for applying small quantities of this mixture are wet up with water to the proper consistency, tempering, this is called. Unless particular care is taken the sand and hair will be added as soon as the lime is slaked as it is much more convenient to do; but this should not be allowed as the lime does not always get wholly slaked and the steam and heat of the slaking lime will burn the hair and destroy its strength.
Another practice which should be avoided, is that of mixing the mortar in the basement of the building as the steam and moisture will penetrate to all parts of the building at a time when the immediate application of the plastering gives no opportunity for drying off. In regard to the proportions, about one and one-half bushels of hair and three barrels of sand to a cask of lime is a good ratio, but the amount of sand will in ordinary cases be determined by the judgment of the mixer, who should be a competent and experienced man.