Traditional raw materials for plasters in a workmanlike

Plaster as a sacrifice layer

The plaster is one of the surface finishes of buildings more widespread since ancient times, both for its low cost (much lower than that for example of a facade of stone or brick, or a coating of stone slabs cutting), and especially because it is an excellent system of protection of the walls, easily replaced when no longer able to fulfill its function because deteriorated.
From Vitruvius, the treatise writers of all ages have therefore paid particular attention to the drafting of recommendations for making plasters.

Types of lime

The most widely used binder from Roman times for plaster especially for outdoor, is lime, derived from carbonate rocks (ie rich in calcite) and of which there are two different types: aerial, able to hold only in the presence of anhydride carbon (and air) and natural hydraulic, capable of carbonate even in very humid environments or even underwater, and therefore particularly suitable for the construction of piers, dams, aqueducts, cisterns and fountains.

Very suitable for the preparation of lime are the pure limestone, ie with a high content of calcium carbonate, although sometimes marbles were also used, which, although they are practically only consist of calcite, are scarcely suitable for their microcrystalline structure. It is also possible to derive also from the bottom of reasonable quality travertine or limestone with mineral impurities, such as the red ammonite (the so-called marble of Verona). Other rock types exploitable are finally the dolomite or dolomitic limestone, with a lower magnesium content. From these rocks is obtained, however, a type of lime usually poorer: the lean lime or magnesium; limes fat, considered the best, are obtained from pure limestone. Natural hydraulic limes derive ultimately from marly limestone, ie with a content of clay minerals up to 25%.

Calcination of rocks

Once you have identified and collected the suitable rocks, it is necessary to proceed to their calcination, consisting of a long cooking time to about 900°, turning once in small ovens loaded with charcoal and supervised by skilled workers, and currently in industrial furnaces studied especially.

In addition, in order not to jeopardize the operation, the oven must be loaded with a lot of care and the combustion temperature to be adjusted continuously and kept constant.

Turning off the quicklime

From calcination, through the following chemical reactions:
- CaCO3 + 900° = CaO + CO ₂ (kicks fat);
- CaMg (CO _{3) 2} + 900° = CaO + MgO + CO ₂ (kicks lean)
we get the quicklime, which occurs in crumbly clumps, oily, and generally whitish. It is a compound very caustic, which to be used effectively must be converted into hydrated lime or calcium hydroxide (Ca (OH) ₂₎ through the process of putting out.

This operation, which is fundamental to have a binder of good quality, allows to obtain the pit lime, a creamy mixture based on slaked lime and water essential for the preparation of mortars and plasters. The putting out can happen in two ways: by immersion in basins or by sprinkling, that is spraying the plates of quick lime with water jets.

The reaction is rather violent and produce a lot of heat, while the pit lime freshly prepared in ancient times was left to mature for several months in special containers or in containers very close to the working site (the putty was in fact difficult to transport and therefore was prepared on the area), taking care to protect it from contact with air or water through layers of leathers: a not well seasoned pit lime can in fact contain lime crumbs, ie lumps of lime not put out that reduce the mechanical strength of the mortar (and thus lower the quality and the duration of a plaster). Nowadays, the now widespread use of pre-mixed plasters and sale of ready pit lime hermetically sealed in waterproof bags have now made theseattentions unnecessary.

Inerts for plaster

At this point, the binder to be mixed with inerts to prevent the phenomenon of shrinking, namely the reduction in volume that occurs after hardening, and likely to cause the typical webs of small cracks visible on the plaster not well done.

The most common is the inert sand, possibly from river or lake: sea sand is in fact unsuitable, and if it is essential to use it you have to wash it thoroughly to remove all traces of soluble salts, which can compromise the strength of the mortar or promote degradation.

Another inert pretty common in the past was the crushed rock, used when it was not possible to obtain a suitable sand or to obtain specific results, such as a colored plaster: this practice is still widespread, especially in the restoration sites when necessary identical to the original mortars for grouting and make good any gaps or have colored mortars for grouting injuries and defects in masonry or stone elements.

Very important are also its particle size, which as far as possible to be uneven (ie with clasts of different sizes) and the mechanical strength of litotypes departure: the sand is in fact better than silica.

Between the binder and the inert are possible both physical and chemical bonds: the first consist of the possible penetration of the binder into the pores of the inert, while the latter are established in the area between the calcite crystals and mineral inerts.