This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Hand Mixing. The great bulk of plastering mortar, outside of a few large cities, is mixed by hand, and, for the best results, requires very close regulation of materials and careful manipulation. The mixing should be performed as much as possible outside the building to be plastered, so as not to make the interior damp from the steam of slaking lime. When mortar is mixed in cold weather, it should be done under shelter, and in no case should frozen mortar be used.
A good quality of lime having been selected, it is slaked with clean water in a tight box, and allowed to stand for at least 24 hours - a week is better - to allow all the particles to be acted on. If there is any residue after slaking, the lime paste should be run through a fine wire screen into another box. This paste forms the base from which the various coats are made by addition of the other materials; the mixtures are generally classified as coarse stuff, fine stuff, plasterer's putty, gauged stuff, and stucco.
226. Coarse stuff is used for the first, or scratch, coat, and is made by mixing with the slaked lime, reduced to a thin, cream-like paste, the proper quantities of sand and well beaten-up hair. (The sand is usually added last, but some authorities claim that better and tougher mortar is made by adding the sand soon after slaking.) On government work, the specifications require that the hair be not mixed with the lime paste until the plastering is to be applied. The materials are thoroughly incorporated by a hoe, and piled in a heap for a week. The mortar is then mixed with sufficient water to give it the proper consistency. Only a small batch should be wet at a time, and should be laid at once on the lathing.
The common method is to mix the hair and sand with the lime as soon as it is slaked, and then throw the mortar into a pile, the entire operation not taking more than a few hours. This is a very poor way, as the lime will not slake thoroughly in such a short period, while the hot lime and steam will char the hair, so as to materially affect the strength of the plaster.
227. Fine stuff is the pure lime which has been slaked to a paste in a trough, by the addition of a small quantity of water, after which it is further diluted by water until it is as thin as cream. The substance is then allowed to settle; when the excess of water appears clean, the lime held in suspension having subsided, it is drained off, and the moisture in the mass is allowed to evaporate until the stuff has become sufficiently stiff for use. When desired, a small quantity of white hair is added to it.
Plasterer's putty is practically fine stuff, but the creamy fluid having been strained through a fine sieve, the paste has been rendered much more velvety. It is always used without hair.
Gauged stuff consists of about \ of the foregoing putty and about 1/4 of plaster of Paris. The latter article causes the mixture to set quickly, and the composition must be immediately used, not more than can be applied in 20 or 30 minutes being prepared. An excess of plaster in the mixture will cause the coat to crack. Gauged stuff is used as a finishing coat for walls and ceilings, and also for running cornices; for the latter work, equal portions of putty and plaster are used. Stucco for interior work consists of 2/3 fine stuff and 1/3 sand; it is used as a finishing coat, the mixture being "whipped," and reduced, by the addition of water, to a thin paste.
Machine Mixing. The advantages of machine-mixed mortar are that sufficient time is given for the lime to thoroughly slake, the hair and sand are added just before delivery, and the mixing performed much more uniformly than when done by hand. In brief, the process is as follows: The fresh lime is placed in revolving pans, with enough water added to slake the lime slowly. When this process is completed, the lime and water are run through screens and pumped into tanks, and allowed to remain about three weeks to finish the slaking. When mortar is to be made, the paste is run into the mixing pans, where the hair and sand are added and the mass is thoroughly stirred until it is entirely homogeneous.