Factitious: Not genuine, intrinsic, natural, or spontaneous; inauthentic. (Oxford English Dictionary)
The melancholic wisps of Brahms’s Intermezzo in B-flat minor, Opus 117, No. 2 are pianistic in a way that Brahms is often not. It feels like harpsichord music, which certainly fits when you consider that Brahms was a Couperin fan who edited at least one volume of the Pièces de Clavecin. But look to the coda of the intermezzo, and you imagine neither harpsichord nor piano:
You hear a horn.
Brahms was not a flashy orchestrator. Compared to the Russians and his younger contemporaries such as Richard Strauss, he was a stick-in-the-mud. In his 1925 History of Orchestration, Adam Carse quotes a nameless conductor describing Brahms’s orchestra: “The sun never shines in it.” Yes, Brahms was conservative. He never quite accepted the valve horn. All his horn parts––think of the alphorn call in the finale of Symphony No. 1, or the soulful solo in the Poco allegretto of Symphony No. 3––can be played on natural horn.
Consider the limitations of that instrument. A horn in C alto sounds a harmonic series that begins like this: 
The notes in between are not available as natural notes. Scales only become possible in the upper register. But in the days before valves, players devised ways to fill in the gaps, through hand-stopping and lip tension.
One vivid example of “lipping” occurs in the Scherzo of Beethoven’s 7th Symphony. The theme of the Trio swings between do and ti, a great contrast to the skipping main theme. (Note the fragrant tempo-typo from a French edition.):
Eventually that do-ti-do settles on the dominant, and Beethoven gives it to the 2. horn, which sits on the 3rd harmonic like a cow ready for milking. To sound the F-sharp on a natural horn (really a G-sharp, since the horn part is in D) the player must relax the embouchure and lip the tone down a half step, then back up. In case we don’t get the joke––this is a scherzo, after all––Beethoven repeats the effect 19 times, lapsing into hemiola:
The result is a “factitious tone.” Not fictitious; factitious. The OED defines factitious as “not genuine or natural…artificially created or developed; made up for a particular occasion or purpose.” The excerpt suits the definition precisely, for the player is sounding a tone that is not natural, is artificially created, and is indeed created for this purpose. You’ll find this detail in every orchestration manual.
But back to Brahms.
In the closing measures of the Intermezzo, Brahms––like Beethoven––settles on the dominant in the bass. That dominant F is decorated by a lower neighbor, a working out of the E-F neighbor motion in the left hand in the first full bar. (Which is a reflection of the opening half-step!)
Is it fanciful to hear a horn in B-flat basso droning away here, sounding the 3rd harmonic, just as in Beethoven’s scherzo? The effect is pastoral in quite a different way, a diminishing of light as the theme winds its way down to the tonic.
Often we think of orchestral music as orchestrated piano music. The Brahms Intermezzo in B-flat minor is an example of orchestral sonority influencing piano writing.
Listen to Paul Lewis play the Intermezzo on YouTube.