Allgemeine Musikalische Zeitung Wien (1818)
The performer played on the flute a concerto by Schneider, an Adagio and variations of his own composition, and, together with piano and violin (J. P. Pixis, Böhm), took part in a set of variations for piano, violin and flute on the theme Nel cor più, etc., composed by Mr J. P. Pixis. Keller’s merits are well known: a gentle, mellow tone, a graceful and carefully studied delivery in the Adagio, a remarkable pianissimo, and an ease in executing the brilliant passages, arpeggios and trills that generally suit the instrument particularly well (although we must except the intervals B–C and C–D). These qualities make him a highly accomplished and particularly interesting flautist. Indeed, if Venus were to celebrate her festivals, Keller would surely have to play an Adagio there! As a chamber virtuoso, he deserves an outstanding rank; yet in a concert performer we also appreciate fullness of tone. His constant softness remains too uniform: it may well transport the listener into a momentary state of bliss, but it gives the art itself too little life to make a powerful impression on a wider audience. It is very much to be hoped that Mr Keller will take this into consideration, for we would gladly see him receive the distinction he deserves — all the more so given his modesty — a distinction which several of Vienna’s excellent flautists know how to secure for themselves.

Obituary, contributed by a student of the master (1855) (translation by Tom Moore)
The composer of songs, Carl Keller, died on July 19 at 8 o’clock after a long illness in Schaffhausen, Switzerland. He was born in Dessau in 1784, and thus reached the age of 71! He was also one of the few who had still received a certificate of instruction as a skilled musician. Such letters were formerly provided to tower musicians by the Kapellmeistern of the good old days. As a young man he entered the Chapel in Berlin, daring to take his first step into the wide world of music, full of intrigue. He often told his circle of friends of the terror he felt when he watched the crowds in the city and thought about his future. “Ah, a city which receives so many, will also receive you, and where so many eat, you will not go hungry!” That was his cheering thought as he made his first attempts to find a position. His courage did not disappoint him. Heaven’s blessing was with his brave heart to the end, but alas, the poet’s words came true: “the human heart must little by little be broken”. In the days of King Jerome the young artist came to Kassel, where he experienced the joys of a victorious artist, but suffered by the thought that he did not serve a German prince. Upon Jerome’s expulsion he certainly lost his daily bread, but rejoiced at the thought that his heart depended on Blücher [Gebhard Leberecht von Blücher, contemporary Prussian general] and Germany’s glory. Keller was in Kassel for seven years, then in Stuttgart for two years, until he found a carefree and comfortable position with the princes of Fürstenberg as a chamber musician, which was sweetened by his marriage to a lovely daughter of the Court Actor of the Duke of Baden, Mayerhofer. At Donaueschingen his Lieder began to follow, to pour forth “all of Germany”, fresh and inward. Here he also composed much for the flute, and songs that were adopted by the people. Who does not know the polonaise: „Kennst du der Liebe Sehnen? » and the song of the canteen: „Helft Leutchen mir vom Wagen! » or “Land meiner seligsten Gefühle! » He loved to worked and worked until the end of his life; for we have a guitar method and a flute method by him in the press. In 1848 there was a notable impact on the princely Chapel; most of the musicians were pensioned off, and only a chamber music ensemble remained. Keller, a devoted servant of his Prince Carl Egon, whom he would always refer to as “my dear Prince”, had painfully to leave Donaueschingen, because he wanted to supplement his pension through income from teaching lessons. He moved to the nearby Schaffhausen, where the leading families had welcomed him in happier summer months. Keller was noble in nature, through and through. His songs were sung gladly everywhere, and made his reputation. His true heart beat its last on foreign soil; thus, grant his name a place in your esteemed newspaper, a place for the amiable singer, who passed away on July 19^th, without the German press yet reporting it! A grateful student.