The Lime Principally Used for internal plastering is that calcined from carbonate of lime, in which the impurities do not exceed 6 per cent, and is known as fat lime, pure lime or rich lime. It is unfit for any purpose where strength is required, or in situations where it is exposed to the weather, as it has no setting power, and is easily dissolved by wet.
Hydraulic Limes are those which, in order to set, do not require any outside influences, their own chemical composition of lime and silica, when burnt, being sufficient for the purpose. The name is given for their capability of setting and hardening under water. Hydraulic limes are obtained mostly from the lias.
Good Hydraulic Limes are obtained from many places in the United States and Canada, the best known is "The Rosendale Hydraulic Cement."
Artificial Hydraulic Limes may be made by mixing a sufficient quantity of clay with pure lime to obtain a composition like that of a good natural hydraulic limestone. The lime, if soft, may be mixed with the clay and burnt raw, or, as is more usual, may be burnt, slaked, ground, and then mixed with the clay and reburnt.
The Purer the Lime the quicker will it slake. Great care should be taken that the lime is properly burnt otherwise it will not slake properly, and will probably "blow" in the work.
The Perfect Slaking of the burnt lime before being used is very important, as it will slake eventually, and cause blisters in the work. In order to effect thorough slaking, the lime should be "run" as soon as the building is commenced. It should not be used unless it has been slaked at least three weeks.
A Bushel of Lime requires in slaking about a gallon and a half of water.
Lime which Slakes Quickly and with great heat is generally considered to be the best for plasterers' work.
When Lime "Falls" in dry weather without any sufficient apparent moisture, it is considered to foretell rain.
The Lime Should Be Run in couch on the site, where it can be seen by the architect. Care should be taken that as much lime is run as is required for the whole of the building.
The Plasterer, partly, perhaps, to avoid the monetary outlay, and partly to avoid the necessity of having to cart away any lime, has a tendency to run an insufficient quantity of lime. The result of this is that he, commencing at the top, the usually less important part of the building, has used up his lime by the time he has reached the principal rooms on the ground floor, and has to have recourse to possibly insufficiently seasoned lime, with an unfortunate effect on the work, as stated above.