Efflorescence

The most common forms of efflorescence are made up of whitish deposits with crystalline appearance, pulverulents, or sticky.
They can also appear in humid environments, such as cellars, caves and stables where they are generated by the action of nitrifying bacteria that convert the urea of organic compounds (e.g. manure) into nitrate salts. On the coastal facades you can also find the chlorides of the salsedine brought by the burrasche.

Efflorescence as intended in construction is a surface phenomenon caused by evaporation of water containing soluble salts outside a wall.
The amount of salt crystals depends on the two factors involved: the evaporation rate that depends on the climatic conditions (temperature, relative humidity current, exposure or not to solar radiation, wind, etc.) and the internal capillary structure of the surface material.
As these conditions may vary, the front of evaporation can move even within the surface, in which case will be there that will form the crystals. In this case there will be no more harmless efflorescences on the surface, but instead the harmful sub-efflorescences in the first 15 - 20 mm under the surface that, with the repeated accumulation of crystals, cause the detachment of the most superficial parts of the masonry, similar to the phenomena due to the action of frost, and break the material.
The more deep the salts are still in solution and therefore cannot do harm.

The water can have internal origin (building moisture which is disposed of in a period between 6 and 24 months) or external (water, rain, soil humidity, etc.).

In ancient buildings the water with dissolved salts is normally penetrated into the walls by a lift from the ground in the base of the wall. Or there may have been sewerage losses etc. Another source of salts is the excrement of birds (the "guano" which is very rich in nitrates) that rain dissolved from roofs and cornices and transported down on the facade. Or they can derive from the soluble salts that are contained in the materials with which the artifact is constituted, such as:

• the ligaments (cement^[2], hydraulic lime, etc.)
• the inerts
• the bricks
• the dough water of malts

In this case the efflorescences are aesthetically annoying, but harmless, as - once completely dried the masonry - will end the contribution of new salts from the same.
When salts come from external sources such as soil (soluble salts absorbed by the capillary ski lift) or marine aerosol or smog in areas of air pollution, efflorescences and especially sub-efflorescences that form always have harmful effects following the aforementioned wetting and drying cycles.

The only real solution for the restoration of wet walls from damages from salts from sub-efflorescence is to extract them and remove them with a bio-extractor feeder pack. The compresses used for the reduction of salts are dense masses made up of inert materials (cellulose plate, clay materials, etc.) soaked with water and/or solvents.^[3] So far it has been used, but only by the restorers, mixed silk clay with pulp of cellulose. Today they employ another bio-axles, also aqueous, based on cellulose and diatomite fibers which is much more effective than sepiolite and whose use has also entered the normal architectural building renovation on a large scale. The latter is specified by the Superintendencies in many chapters for the most important monumental restorations with the commercial name of Westox Cocoon.

The practice of normal washing with a 5-10% solution of hydrochloric acid is absolutely useless to remove the efflorescences of the most common salts, chlorides, sulfates, nitrates as the acid is ineffective on these salts: this acid only serves to dissolve carbonates or malt residues. The use of acids is absolutely to avoid, however, in the case of artefacts of historical-artistic interest, since it causes a serious deterioration of the rocks, both natural and artificial. The extraction of soluble salts must therefore be carried out with distilled water. Removal of non-soluble saline efflorescences in water can be carried out by mechanical means (drinks, microscals, etc.)