rowuk

JD,
the mouthpiece is out of metal, nothing slow about that. The lips, if trained, speak when airflow of sufficient pressure is available. Many players do not take in enough air and play more "carefully" when slower or legato playing is required.
The basics of good tone production are identical in fast staccato and slow legato passages. You need to be as aggressive in filling up regardless of the playing style. A positive attack also helps when playing softly. Legato just means no space between the notes, that sounds most convincing when there is clean articulation. Slow is not castrated long. Here great breath control is required to maintain the energy of the note!
Perhaps best explained: if slow is just a weaker fast and legato is just a weaker form of tonguing, than the end result is just weaker........

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Whenever I feel blue, I start breathing again.