Most of the instruments that come to the shop for restorative work have been up to pitch for a long period of time. The entire piano framework has been under tremendous strain for many years; decades in fact.
When letting the tension off the plate, this must be done safely as the sudden release of pressure could cause an unbalance of the plate or other structural components of the instrument. Very much like unbolting a car tire the weight or pressure must come off evenly distributed. With almost 20 tons of pressure on the framework one cannot tell if major structural components have deteriorated to the point of collapse and are being held in place only by the tension applied.
Over the years I have used this procedure successfully time and time again. It is the safest way I have found to let down the tension from the piano. This task is best completed using a 4ths and 5ths temperament.
First take middle C and tune down so that middle C becomes the same as the B below. Then tuning in octaves tune all of the C’s to that pitch or tune all of the C’s to the B below. The result is that middle C is now a full tone lower than originally.
Next tune the F below Middle C to the E below F. Then follow the same procedure used for C. Next, tune all of the F’s by octaves or tune the F’s to the E below. After F, then the G below middle C down to beat less, then the D above middle C down to beat less, then A below and so on. Work your way around the temperament until you have completed the temperament. Each time you lower a key in pitch do all of the same keys of the keyboard. It is not important to fine tune as you go. Just rough in the keys and make them close to the pitch you are looking for.
The end result will be that the entire piano will now be a full tone lower that A440 which means that you have released approximately 4 tons of weight off the iron frame.
Now return to middle C and begin again by lowering the pitch to the B below. Work your way around the temperament using this new pitch. At completion of this second step the piano will now be two full tones lower than A440.
From here you can take a T-handle or a stringing hammer and turn the tuning pins one half turn so the remaining tension is released. This process once learned can be completed in about an hour.
Some photos of the process; please begin at photo # 47