It was recently discovered by Joachim Pissarro, great-grandson of Camille, who is himself a Pissarro scholar and co-curator of the RA exhibition. But "discovered", perhaps, is not quite the right word. Like the Americas before Colombus, the Pissarro - Seated Peasant Woman with Goats - wasn't quite lost; it just wasn't known about, a rather remarkable state of affairs considering the importance of the artist.
Painted by Pissarro in 1885 while he was living at Eragny-sur-Epte, Normandy, Seated Peasant Woman with Goats has been hanging in a private Swiss collection for most of the century, and Swiss collections are very private affairs indeed. They seldom lend to exhibitions and, in contrast to the ostentation of many British and American collectors, anonymity and secrecy are the Swiss by-words in art, just as they are in banking. Somehow, this picture managed completely to evade the scholarly net and has never been recorded in a catalogue of Pissarro's work.
As MaryAnne Stevens, another of the co-curators of the show explains: "In this country and America, the knowledge of what is in private collections comes from the fact that - with certain exceptions - collectors are terribly proud of what they have and wish it to be known. In Switzerland, with a few notable exceptions, it is the reverse.''
It came as a revelation to Joachim Pissarro, who was visiting Switzerland to select pictures for the RA show. "I happened to go and visit this very nice lady collector who I was introduced to by one of the auction houses. She just opened her door, and in her bedroom was this painting which was absolutely staggering. It was a marvellous discovery to make, not only a newly found Pissarro, which is rare enough in itself, but also a stupendously beautiful object."
Undocumented Pissarros are particularly unusual because the standard catalogue in 1937 was co-authored by one of the painter's sons, Ludovic Rodolphe Pissarro, who was naturally close to the source and missed little. This picture is out of the ordinary in technical terms too. It is painted on paper in gouache and tempera, and although Pissarro frequently worked on paper, it is extremely uncommon to find a tempera as big as this (80.5 x 81cm).
Joachim Pissarro says: "The sheer composition is so well thought out, so evidently the result of intense effort. It's very subtle in terms of gradation of hues and colours. It is beautifully painted and in impeccable condition. It is outstanding for its scale, its rarity, its condition and its simple beauty."
When Joachim Pissarro recovered from his excitement, and broached the subject of a loan for this RA exhibition, he met first with "an absolute, blunt no". But eventually, permission was granted to MaryAnne Stevens. But the loan was made on condition of strict anonymity for the owner; the RA has not even been allowed to tell Japanese museums, where the show goes next, who owns the Pissarro or any of the other little-seen paintings from the other Swiss collections.
From what little information that has been released, it seems the picture was bought by the present owner's father before the First World War, in the days when buying an Impressionist picture was still a bold, and affordable, thing to do. In this respect it is typical of many paintings in this exhibition. Swiss inheritance law places no death duties on works of art that stay in a family. Therefore collections tend to remain intact, and in quite ordinary, un-super-rich hands. The home where this Pissarro hangs, according to MaryAnne Stevens, is "modest, but not exceptionally so'' for a Swiss collection housing world-class pictures.
Elsewhere, particularly since the dizzy price rises of the Eighties, the collection of Impressionists has become the pastime of billionaires. But in Switzerland, Gauguins and Cezannes still hang on the walls of ordinary apartments, and their owners often regard them not as investments, nor statements of social status, but as objects for private enjoyment.
Hence, in part, the resistance to lending favourite pictures to foreign exhibitions. The owner of this Pissarro, for instance, says Joachim, "like many, many people I have seen in Switzerland, is absolutely not interested in being seen as a great collector, or in the money aspect of it.'' Seated Peasant Woman with Goats normally hangs opposite her bed. Presumably, while the rest of us are enjoying it at Burlington House, she will miss seeing it as she wakes up. It is hard not to sympathise.
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