In modern and specialist literature this indicates the thin upper and final layer of preparation, which creates a smooth and uniform surface to facilitate the subsequent application of brushwork. Depending on its characteristics, it gives the painted layers a transparent or opaque base colour. Traditionally the term ammanitura or imprimitura was used in a more generic sense to distinguish the glue and chalk preparation used on surfaces to be painted from mestica, an oil-based primer.
This is the most challenging traditional and technique, used by great Italian and European artists since the thirteenth century. Gold decoration adds both economic and aesthetic value to paintings, giving luminosity to the work and enhancing the colours of painted parts. Not infrequently whole paintings were gilded, with the exception of the saint being represented. In water gilding first the gold is applied to the panel, then it is painted. The panel is prepared by carving the border of the area to be gilded, then applying a gluey paste and finally the gold leaf. The substrate employed in the past was made of water, stiffly-beaten egg white and bole, a very fine oily clay; nowadays the ingredients have changed and two layers are normally used. First a layer of primer is applied (ammanitura or imprimitura). This is made of hydrated calcium sulphate heated in a bain-marie, and is also known as gesso di Bologna, Meudon white or Spanish white. Between two and five coats of primer are usually required, and it needs to be sanded very carefully to leave the panel perfectly smooth. The next step is the clay bole, which is mixed with water and glue. The bole affects the final colour of the gilding and can be yellow (for bright gold), red (for dark gold), or black (for antique gold), depending on the end colour desired. Only when the panel is perfectly smooth can the actual gilding be performed, by applying superfine sheets of gold leaf, transferred with the greatest care from their paper base to the dampened panel using a fine knife (gilderâ€™s knife) or a soft brush. Immediately before this step it is essential to apply a very fine layer of guazzo, or gilderâ€™s liquor: a mixture of alcohol, water and glue. Once the leaf has been applied, it can be shaped using the same knife. The gold leaves are overlapped by a couple of millimetres, to avoid imperfections and gaps. This process can be performed with different types of metal foil, from gold to bronze.
This is performed once the glue has dried, and consists of rubbing the gold leaf with a tool known as a burnisher (a shaped agate head on a wooden handle). The rubbing serves to smooth the gold foil and make it shiny. Burnishing is omitted when special â€śantiqueâ€ť effects are called for. It should be performed in different directions, so as not to leave traces or streaks.
The overglaze serves to protect the gilding from ageing and to reduce the shine of burnished gold. It can be performed by applying a fine layer of wax, shellac, mecca gilt varnish or other substances. It is not strictly necessary, but helps maintain the surface sheen over time.
The gold foil can be decorated with techniques dating back to the fourteenth century, using punches and gravers.
During restoration new gold leaf needs to be blended into what remains of the original. To do this, it is brushed with a solution made of bitumen diluted in turpentine, or with a special varnish of the correct colour, taking care not to leave streaks. Once the first coat has applied with a brush, the application is then rendered uniform using cotton wool.