Clavis Artis is the title of an alchemical manuscript published in Germany in three volumes in the late 17th or early 18th century, attributed to Zoroaster (Zarathustra). It features numerous watercolor illustrations depicting alchemical images, as well as pen drawings of laboratory instruments.
Three copies of the manuscript are known to exist, one at the Biblioteca dell’Accademia Nazionale dei Lincei in Rome, one at the Biblioteca Civica Attilio Hortis in Trieste, and one at the Bayerische Staatsbibliothek in Munich. There is no information about the author and the origin of the manuscript, but there are references to a Rosicrucian order (Orden der Gold- und Rosenkreutzer). If you speak Italian, there is more information available on the Italian Wikipedia.
There is even debate whether the author might be “Abraham Eleazar”, who wrote a well-known alchemical text, the Uraltes Chymisches Werk. A non-illustrated book under the same title is digitally available at SLUB Dresden, published in 1738 in Jena, Germany.
Source: The images were taken from the copy at the Biblioteca Civica Hortis in Trieste and made available on Wikimedia Commons, where they are considered as public domain.