EN
Two almost identical portraits of Jan Heweliusz, the celebrated astronomer who was born and died in Gdańsk ( l6 l 1-1687), have been subjected to technological investigations. One of the portraits is featured in the Library of the Polish Academy of Sciences in Gdańsk , and the other is displayed in the Bodleian Library in Oxford where it was sent by Heweliusz himself in 1679 as a gift for the Royal Society of London for the Promotion of Natural Knowledge, whose member he became in 1664. Upon this basis, the origin of the portraits is placed in 1678-1679.Some historians of art believe that the authors are two eminent artists working at that time in Gdańsk: Daniel Schultz (1615-1683) or Andrzej Stech ( 1635-1697). Other experts support the authorship of only the second of the two. Both painters modelled themselves on art from the Low Countries whose influence upon the Gdańsk artistic milieu was considerable. It is difficult to distinguish the works by Daniel Schultz and Andrzej Stech due to a certain thematic and plastic similarity, this task can be completed only upon the basis of historical investigations. Consequently, technological research was needed to explain at least some of the problems connected with the technique applied in case of both of the portraits. The preparation of a copy of the Gdańsk portrait for the Mikołaj Kopernik Museum in Toruń (1994) not only offered an excellent opportunity for writing this article but additionally provided extensive technological experience pertaining to the manner in which the portrait was painted. Research has disclosed that both depictions were executed according to the principles of seventeenth-century Dutch painting: on linen canvas with red ground and grey oil underpainting which determined the actual shape of the composition. The finishing layers were realised in oil with the „alla prima” method and glazing. The surface of the Oxford painting is smoother as a result of thinner canvas and better polished ground. The texture does not show distinct traces of brush strokes. On the other hand, the Gdańsk portrait reveals bolder and wider brush strokes with impasto effects. These features bring the Gdansk painting closer to the works of Daniel Schultz. The Oxford portrait is characterised by a subtle, smooth painting manner, more typical for the compositions of Andrzej Stech. X-ray tests show that the author sought an ultimate painting and drawing form in the course of the creative process. Visible changes testify to the fact that the portrait is neither a copy or a replica but an original portrait of the astronomer realised upon the basis of an authentic drawn sketch which also served as a model for the author of the Oxford portrait. Computer analyses indicate that only the faces in both depictions are relatively identical but the remaining fragments, despite an illusory similarity, differ as regards shape, size and arrangement. In conclusion, it must be said that the Oxford and Gdańsk portraits, albeit similar in colour and partially from the point of view of their draughtsmenship, present a basically different painting technique which demonstrates that they were executed by two artists: Daniel Schultz (the Gdańsk portrait) and Andrzej Stech (the Oxford painting), whose painting temperaments varied but who remained followers of identical plastic patterns.