https://escholarship.mcgill.ca/downloads/c247dv781

Very good information! Thanks for putting this together. I read most of the article and it clears things up a lot for me. I like the info where they say that a tuning system has impact on what modulations Chopin would choose because of the different sound of the major 3rds in different keys.

I checked the first recording by Raoul Koczinsky and it seems to be tuned to 405 HZ. I assume that the instrument was tuned to perfection for this recording. In my ears though it sounds very out of tune, being used to the well Tempered tuning system. It comes close to the Werckmeister III patch.

Above is an interesting scholarly article trying to dissect the question you raise (that I've honestly only skimmed).  I assume you are referring to Pleyel No. 13819 (1848) which is currently in the Cobbe Collection of Composer Instruments at Hatchlands Park in East Clandon in England.  There is a second piano, which is Pleyel No. 13214 (1847), which I believe is the same 1847 Chopin Pleyel used in the 1948 Polish radio concerts by Raoul Koczalski (and is certainly the one used recently by Hubert Rutkowski for his Chopin recordings).  I have no idea off hand how much the Cobbe or Warsaw Pleyels are rebuilt versus restored or how much we can accurately anyone can say the instruments sounded like from the factory compared to how they've aged and been maintained (with any wood-built instruments--especially violins--changing tone drastically every few decades as wood hardens, varnish breaks down into different chemical constituents, parts are modified or replaced, etc.).  Pleyels had very different hammers (materials, tension, feel) compared to modern pianos, and some technicians will try to modernize when maintaining or rebuilding an instrument, and many such modifications are as hard to reverse as they are to do.  As to what temperament is used on any of the recordings of the 1848 or 1847 and what historical accuracy there is (or isn't) about their tuning choices, I'm not certain as I've only started into the question.  When I have more time, I'll trying to dig deeper into this, but my schedule is a bit full at the moment.

Once we've nailed down which temperament is most appropriate, there are plenty of forum users (myself included when I have time) who can put together a preset to match current sound of the Cobbe Pleyel or the 1847.

Discussion of Neidhardt versus Bach temperament: http://www-personal.umich.edu/~bpl/lari...rison.html
More on Neidhardt: https://polettipiano.com/wordpress/?page_id=659
Cobbe Piano: https://www.cobbecollection.co.uk/colle...and-piano/
Cobbe Piano recording: https://www.youtube.com/watch?v=3PoSlDhoTJ4
Another Cobbe recording (lower audio quality): https://www.youtube.com/watch?v=LyurmW7EaK0
1847 Pleyel recent recording: https://www.youtube.com/watch?v=EGwNj7fas5I
1847 Pleyel vintage recording: https://www.youtube.com/watch?v=fcV3P6zS30Q
More from the vintage concert: https://www.youtube.com/watch?v=0Mlgjg2CqY8
Hubert Rutkowski album recording on the 1847: https://www.piano-classics.com/articles...-on-pleyel

Interesting question! I don't know if the actual tuning system of Chopin's Pleyel has been documented, but I think it likely that he may have used some sort of near-equal system such as Jousse2 or one of the Hummel tunings (these can be found in the Scala archive, but you would need Pianoteq Standard or Pro to access them). If anybody knows any different, please enlighten us!

(edit) The preset Neidhardt III is not very far from equal. It's quite an old temperament (1724) and I have no idea if Chopin might have used it. However, there must be a good reason for it appearing in the preset list, along with the others. Maybe Chopin experimented with different tunings? I wouldn't be surprised if he did.

Thanks! I noticed that when choosing a Pleyel preset it also changes the tuning and temperament system, so maybe that is a good guide.

Above is an interesting scholarly article trying to dissect the question you raise (that I've honestly only skimmed).  I assume you are referring to Pleyel No. 13819 (1848) which is currently in the Cobbe Collection of Composer Instruments at Hatchlands Park in East Clandon in England.  There is a second piano, which is Pleyel No. 13214 (1847), which I believe is the same 1847 Chopin Pleyel used in the 1948 Polish radio concerts by Raoul Koczalski (and is certainly the one used recently by Hubert Rutkowski for his Chopin recordings).  I have no idea off hand how much the Cobbe or Warsaw Pleyels are rebuilt versus restored or how much we can accurately anyone can say the instruments sounded like from the factory compared to how they've aged and been maintained (with any wood-built instruments--especially violins--changing tone drastically every few decades as wood hardens, varnish breaks down into different chemical constituents, parts are modified or replaced, etc.).  Pleyels had very different hammers (materials, tension, feel) compared to modern pianos, and some technicians will try to modernize when maintaining or rebuilding an instrument, and many such modifications are as hard to reverse as they are to do.  As to what temperament is used on any of the recordings of the 1848 or 1847 and what historical accuracy there is (or isn't) about their tuning choices, I'm not certain as I've only started into the question.  When I have more time, I'll trying to dig deeper into this, but my schedule is a bit full at the moment.

Once we've nailed down which temperament is most appropriate, there are plenty of forum users (myself included when I have time) who can put together a preset to match current sound of the Cobbe Pleyel or the 1847.

Discussion of Neidhardt versus Bach temperament: http://www-personal.umich.edu/~bpl/lari...rison.html
More on Neidhardt: https://polettipiano.com/wordpress/?page_id=659
Cobbe Piano: https://www.cobbecollection.co.uk/colle...and-piano/
Cobbe Piano recording: https://www.youtube.com/watch?v=3PoSlDhoTJ4
Another Cobbe recording (lower audio quality): https://www.youtube.com/watch?v=LyurmW7EaK0
1847 Pleyel recent recording: https://www.youtube.com/watch?v=EGwNj7fas5I
1847 Pleyel vintage recording: https://www.youtube.com/watch?v=fcV3P6zS30Q
More from the vintage concert: https://www.youtube.com/watch?v=0Mlgjg2CqY8
Hubert Rutkowski album recording on the 1847: https://www.piano-classics.com/articles...-on-pleyel

(edit) The preset Neidhardt III is not very far from equal. It's quite an old temperament (1724) and I have no idea if Chopin might have used it. However, there must be a good reason for it appearing in the preset list, along with the others. Maybe Chopin experimented with different tunings? I wouldn't be surprised if he did.