15 Dec HOW TO MAKE A FRESCO (SECOND PART)
The mortars are made by a binder and an agglomerate mix with water.
In the case of fresco aerial mortars are used, that set in contact with the air, whereas the mortars typically more hydraulic, that reinforce themselves even under water, are less right or totally unsuited with this pictorial technique, like, after all, all the mortars.
The agglomerates in the mortar have the function of support matrix; they can be made by sand, marble powder, cocciopesto, pozzolana, ecc..
The sand for the grooming of the mortars can be thin or coarse due to the cases:
- coarse: granules from 2 to 5 mm for the mortar layer
- medium: granules from 0,5 to 2 mm for the arriccio
- thin: granules till 0,5 mm for the intonachino
The sand has to come from the river because it needs to be without sodium chloride and potassium, which are present in marine sands.
The layers of the wall for the fresco are usually three:
- rinzaffo o rabboccatura
- arriccio o arricciato
- intonachino, tonachino, velo o intonaco di rifinitura
The rinzaffo (a mortar layer) is layed on the coarse wall carefully dampen, is made by coarse sand and grassello (a thick mixture) in proportions of 2 parts and 1 respectively, otherwise of three parts of sand and one of grassello when the latter is good quality made.
The application happens throwing the mortar hard against the wall by the trowel.
The obtained surface has to be undifferentiated but rough, so it can provide a sturdy hold to the arriccio.
The optimal thickness is included between 1 and 2 cm.
The drying has to be slow and homogeneous, to avoid cracks and uplifting.
When the surface is ready, you lay on the arriccio or the arricciato, made by medium sand and chalky grassello in proportions 3:1 or 2:1 if the sand is more coarse or the lime is more thin, the thickness is around 1 cm.
Subsequently it beats the spolvero and it draws the sinopia that will be useful for the realization of the days (giornate).
When the arriccio is strengthened enough, it goes to the execution of the days (giornate), it goes on from the top to the bottom, avoiding to dirty the days before.
The choice of the surface to treat day by day is done earlier based on the collocation, the complexity and the speed of execution.
When it applies the intonachino it is good to lay it on more copious as regards the border of the day selected.
The intonachino is made by thin sand and grassello in proportions 2:1 or 3:1 or even 1:1 due to the measure used for the arriccio, in this case, however, the surface has to be smoothed thinly and its thickness has to be appreciably thinner as regards to the arricciato (for example 4/5 mm in the case of the arriccio of 1 cm), as is easier to be cracked.