A rehearsal pictured

In some ways it is strange that we feel the drawings of great painters bring us into more intimate contact with their individuality than the paintings themselves: strange because a drawing is partial where a painting is complete, and is often no more than a necessary stage in the process of preparing the final work. It can represent merely a fragmentary aspect of the artist's vision, a half-conceived notion, almost in private shorthand, or signify a purely practical need to firm up a detail, to scrutinize a structure more precisely, to consolidate the composition of a picture which had hitherto been set out loosely. To the artist himself the use of drawings may not be much less prosaic, in the orderly procedures of his studio, than the use of a recipe in a kitchen. Drawing is the same kind of groundwork for a painter that rehearsal is for an actor: the painting is the performance.

This certainly seems to have been the case with Andrea del Sarto (1486 to 1531), the most inventive painter of the High Renaissance working in Florence while Michelangelo and Raphael were in Rome. He appears to have run not only a very productive workshop, but to have been exceptionally efficient and thorough. John Shearman has calculated that the number of preliminary studies for one of Sarto's large altar pieces ''must have been in the order of a hundred and fifty'' and that most of these ''would involve drawing from the live model.'' There is evidence that unusually, for the sake of economy of action, he would sometimes turn to one of his drawings a second time, to provide a figure, for example, in a new painting, many years after he had first made and used it. In other words he kept his drawings in stock as a resource and for reference. Such a practise does not seem to have in any way diminished the care and quality of his work.

The fact that few, if any, of the Sarto drawings still existing are of a high degree of finish also indicates their mainly functional role. But in a different way, he valued them as art historians and connoisseurs have done, for their potential rather than their completeness.

It is Sarto's drawings rather than his paintings which have been most consistently studied and admired over the four hundred fifty years since his death. The taste for incompleteness in a work of art has been partly responsible for this, the love of the half-realized, and of the unhidden evidence of an artist's hand at work, rather than the smooth disguises involved - though often for the most expressive reasons - in the finished painting. In the case of the accomplished ''Study for the Head of the Virgin in the Pitti Pieta,'' there is also the exciting feeling that here, at first hand, we can see an artist studying the features of a living model, of a 16th-century girl, well before he has arrived at an idealized and perhaps frozen image suitable for a commissioned religious painting. There is an illogicality in this, perhaps, since the very purpose, in the Renaissance, of drawing from the live model was to invest the painted image with a natural vitality and warm identity. But this drawing, in the freedom and firmness of its red chalk lines, its rounded form, and its sensitive changes of light and shadow, has an intriguingly ambiguous expression, which the final picture does not.

The profound maternal concern, the restrained grief, of the Virgin as she cares for Jesus' body in the actual altarpiece, is hinted in this drawing, but far from entirely realized.

More than one writer has pointed out how Sarto drew with red chalk in a way similar to a sculptor chiselling marble. It is an apt parallel, because his drawings are indeed like tools. And they combine, stylistically, in an example such as this study, a high degree of sureness with careful investigation. The marks on the paper are both large and small: swift and sweeping in the larger aspects, scrupulously felt and intricate in the small. Bernard Berenson marvellously described Sarto's use of red (and not so frequently, black) chalk as ''an almost living instrument, a kind of prolongation of his fingers.''

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