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I just attended an afternoon concert given by Geoffrey Lancaster in Canberra, Australia, the first of a Complete Haydn Sonatas series, where he used two fortepianos "from Haydn's time." The brochure also billed the pianos as "18th century instruments." I supposed that meant rebuilt rather than original instruments, as they looked relatively new both inside and out.

An interesting observation I made was regarding the tuning stability of these instruments. The concert was an extended one, comprising two programs with an hour long break in between. Each program ran about 100 minutes with a 20-minute intermission. Hence there were four sessions in total, during which ten sonatas were played.

The technician came up to adjust the tuning in both intermissions (and presumably during the hour long break as well although I wasn't there to observe it), which I understand is normal even for concerts using modern pianos.
What I thought was interesting was that twice *during performance*, the pianist stopped, citing an out-of-tune note. He then picked up a tuning hammer and fixed the note before resuming the performance. The first time it was in the middle of a movement (hence he restarted the movement after the fix) and the second time between movements.

Observing this I then supposed that the reason for using two pianos, which sounded alike and between which he alternated throughout the concert, might be that it would allow the piano being played to be as in tune as possible. This would imply that moderate playing (as was the case with these sonatas), not just the environment, could also significantly affect the tuning on these instruments. We didn't have the opportunity to ask questions so I'm not sure if there were other, perhaps musical reasons for using two pianos.

Another interesting observation was that in both cases, the pianist re-tuned the note (fairly quickly) by sounding that note alone. I'm pretty sure at least one of those notes was a bi-chord, as it was in the middle of the keyboard and more than half of the notes on the piano were bi-chords (which I observed during intermission). I'm hence curious how he decided which string of the unison was out...

On a related note, neither pianos had pedals, but both appeared to have two knee-operated levers. The music did sound like a sustaining pedal was used at times, although I couldn't tell for sure whether he was operating the levers. There were also passages that sounded markedly mellower then usual, not much unlike with today's una corda pedal. Does someone know if one of the knee levers could have been the equivalent of a una corda pedal?

I understand fortepianos are indeed much less stable: wood frame vs. iron plate, less torque on tuning pins...

Perhaps the two instruments (replicas?) were made by diffferent makers? Have different characteristics? Have been made during different decades?

About which string to adjust: perhaps the pianist had already had problems with that instrument and knew which string to blame?

Knee levers were used for a while. Neither the una corda or sustaining mechanisms would be rare. A "moderator" could also explain the mellower sound. See this link: http://en.wikipedia.org/wiki/Piano_pedals

Here's a crash course on fortepianos: http://www.mattbengtson.com/fortepiano.html

You can tell which string is out of tune with an una corda pedal.

Fortepianos are not as stable partly because the hammers hitting the strings lift them above the nut.

Fortepianos are unusal instruments by our standard of modern pianos. These like harpsichords, were tuned a lot usually by the owner. Mozart could quick tune but not do repairs and Beethoven- well enough said.......

Simply expect these to re-act like a harpsichord. It helps some to gently take a felt wedge mute and seat the string on the nut if the side bearing is a little on the weak side.

I listened to a night time concert by Geoffrey Lancaster with the same program you have mentioned, there is a reason for having two pianos, and it isn't for tuning purposes. The two fortepianos used were a Stein copy and a Walter copy. Each of these pianos show different characteristics, requiring different touch to play. The Stein copy is used for more delicate, expressive sonatas. It requires a very delicate touch because it does not catch the hammer upon rebound from the string. This means that if you play the Stein too forcefully, the hammer will strike the string, bounce back and strike it again. Beethoven preferred Streicher pianos which were based on the Stein model. The Walter piano is more suited to fast scale runs and virtuosic passages, double octaves, trills etc because unlike the Stein, it DOES catch the hammer.