11 Sep Like mystery lime creates the fresco
The lime was made by cooking at 900° C, in special funnel-ovens, some stones (generally river shingles) essentially formed by calcium carbonate (CaCO3).
At this temperature the shingles pulverize themselves turning into calcium oxide (CaO) with the elimination of the carbon dioxide (CO2).
The follow reaction happened: CaCO3 a 900° = CaO + CO2↑
The calcium oxide called “calce viva” must be “turn off” diving it into water for almost six months, before using it for the fresco, otherwise it could burn the colours. By diving in water, the calcium oxide became calcium hydroxide, Ca(OH)2, that is like a greasy mixture called “calce spenta”.
When the calcium oxide (that mix with the lime form the plaster) gets wet, lost by evaporation a lot of water (H2O) and it combines with the carbon dioxide of the air (CO2) coming back to form calcium carbonate (CaCO3) that is a compound tough and insoluble like the original shingles.
The follow reaction happened: Ca(OH)2 + CO2 = CaCO3 + H2O.
This process, called lime carbonatation, requests some months because it is not complete until most of calcium hydroxide turn in calcium carbonate.
Obviously this transformation happens quickly in the higher layers of the plaster, so the ancient artists mixed the sand of lower layers of “pozzolana” minced (a type of volcanic rock) that has the property to get hard even when there is water.
Now we see what happen when over the plaster there is the pictorial film.
In the technique of “buon fresco” the colours are spread over the damp plaster without binder, that is simply dissolved in water.
When the plaster gets dry, the calcium hydroxide contained in it, rises in the surface where, after the evaporation of the water and the contact with the carbon dioxide of the air, happens the reaction already described that carry to the transformation of calcium carbonate in the higher layers of the plaster.
The pictorial film is included in the carbonate layer and that gives to the colours the exceptional resistance that is the feature of the “buon fresco” and allows to this kind of paintings to be preserved even outside.
The colours are not absorbed by the plaster, like we thought in the past, but the calcium hydroxide rises in the surface and includes the pictorial film.
From what we said we can assume that is very important to paint with damp plaster: on the contrary, the particles of colours are not included by the calcium carbonate, and, when they are dry, “spolverano” (that is, passing the hand over the fresco, the traces of colours remain on fingers) compromising irreparably the pictorial result.