We would like to perform excerpts from Monteverdi's Orfeo on period instruments. According to recent research, what is the most authentic temperament that could have been used for the keyboard instruments in the Italian early baroque?
For a number of years I was publicity, marketing and business manager for a Baroque chamber orchestra in Atlanta, Georgia, United States of America. I have put the question of temperament and tuning for Monteverdi's Orfeo to several friends who perform with that group. If I hear back from them, I will post their answers here.
I fear that I may have confused things with several of my comments here, so I need to use an answer to explain myself. Temperament is one issue (the most important one to the person asking this question), and base tuning (in cycles per second, or Hz) is another. Throughout the Baroque period, different cities, nations and locations in Europe and elsewhere used different base tunings (referred to as kammerton in German and diapason in other languages), and sometimes within the same location an entirely different base tuning would be used depending on whether the music was for choir, for orchestra, or for pipe organ. There are even examples of pieces of music for combinations of choir, orchestra and organ where the composer wrote out the sheet music in different key transpositions for the different sections because of different base tunings employed between those sections.
While A=392 was the most common "early French Baroque" tuning, and A=415 was the most common "High Baroque" tuning, and A=466 was used for some pipe organs and instruments such as recorder, there was wide variation. Just as every town had their own official time and clocks, it was not possible for all the different nations of the world to standardize on a certain base tuning at that time in history.
In the modern era, the most common base tuning is A=440Hz. Early music groups today usually choose between A=392 (one whole-step below A=440), A=415 (one half-step below A=440), and A=466 (one half-step above A=440). Also, for Classical and early Romantic-period music, such as that of Mozart and Schubert, an agreed-upon base tuning is A=430.
Base tuning is a very important issue for musical instruments and for singers. Asking a modern singer or choir to "re-tune" the performance of a piece as much as a whole-step down or a half-step up can put a strain on the tone quality the singers can produce. Furthermore, brass and woodwind instruments are constructed in a way that severly limits their ability to tune up or tune down. There have been times and places in history when professional brass and woodwind players were expected to own two or even three sets of instruments, built for different base tunings, to accommodate whatever base tuning was required for different repertoire.
With regard to the tuning used by a particular composer some three hundred years ago or more, musicologists only know for sure if they can find actual tuning forks used in those places and times, or can take measurements from actual playable historical instruments whose tuning did not vary by much, such as recorders or organ pipes.