The term fresco is used to indicate mural painting in general. In reality, that type of wall painting made with coloured pigments mixed or diluted with water, spread on a fresh plaster, using lime as white, is properly called fresco. The fresco technique is based on the property of lime which, together with the river sand, slowly dries to form a plaster with a golden and crystalline surface into which the colour penetrates and remains so fixed as to resist not only washing, but also, and for centuries, the weather. This plaster is composed of two layers: the curled and the plaster. The first is placed directly on the supporting wall, and is formed by a part of lime and two of sand. The second is made up of equal quantities of lime and sand, finer than the one used for the curling, combined in a mixture to which is sometimes added the powder of very fine marble. The drawing is usually done by brush. The painter can only paint on the damp plaster, so only the part that will be sure to finish will have to be taken up. Once the drawing has been drawn, the artist begins to spread the colours. The most suitable colouring materials for frescoes are iron oxides. Excellent are also the cobalt blue and green.
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