The little-known Johann Kessel (c. 1766-1823) worked as a cantor and school lecturer. He was born in Lengenfeld in Thuringia, studied theology in Leipzig, and spent most of his working career in Eisleben as Cantor of the Andreaskirche and teacher at its choir school (GerberNL, vol. 3, col. 39). A guide to Eisleben describes him as a “great musician and excellent organist” (Berger 1827, 217).
Kessel’s Unterricht im Generalbasse is a late example of a thoroughbass treatise, although like many such treatises it addresses more topics than the title would suggest. A product of the author’s time in Leipzig, the first edition of Kessel’s book appeared in 1790, and includes the first Mozart reference (in the “Vorrede”). The following year Kessel prepared an expanded edition that includes the other Mozart references. Among the extensive list of subscribers was the Thomaskantor Johann Friedrich Doles. The book was unfavourably reviewed in the Allgemeine deutsche Bibliothek by Johann Gottlieb Portmann (attribution from Schütz 2007, 323).
In the Preface, Kessel justifies the need for a new thoroughbass treatise by noting the great changes in musical style and taste that have occurred in recent times, citing the works of contemporary composers including Mozart (see Christensen 2006, 21). Kessler was evidently fond of Mozart’s keyboard variations, and discussed and extensively quoted three of them in the book. The first quotation occurs in a discussion of musical rhythm and the building of a movement from cadences, phrases, and larger sections. Kessler quotes the beginning of the Piano Sonata in A, K. 331 (not attributed to Mozart here), and rather laboriously labels various points in the passage with his four-level division of Absatz, Einschnitt, Periode or Abschnitt, and Haupttheil. This quotation shows a number of variants from the usual musical text, which perhaps suggests that Kessler was working from memory.
A discussion of contrapuntal imitation includes quotations from the second variation of the Twelve Variations on a Minuet by J. C. Fischer, K. 179, and the ninth variation of the third movement of the Piano Sonata in D, K. 284 (quoted in its entirety). Fischer praises Mozart’s ability to maintain a flowing and pleasant melody even as it’s combined with the most artful imitation. These are among the earliest quotations of Mozart’s music in a theoretical treatise; all three works had been in print for a number of years by 1791 (K. 179 was first published in 1778, and K. 284 and K. 331 in 1784), and K. 179 in particular also circulated in manuscript copies (selective list in von Fischer 1962, 33-35). Kessel also notes that Doppelsonaten (probably sonatas for piano four-hands) by Mozart, Clementi, Kozeluch, Pleyel and others contain a great many pleasing examples of imitation.
The final Mozart reference occurs in a passage on the gebundene Fantasie (“strict” fantasy, understood to be a fantasy with a regular beat). Kessler may have had in mind the Fantasia in C minor, K. 475, which was the only Mozart fantasy in print at that stage, and the Haydn Fantasia in C, Hob. XVII:4.