We get this question a lot more these days for some reason. If you have ever wondered what makes a piano sound bright or warm, watch this video I made on plate manufacturing. This is a huge factor that defines a higher end piano from a lower end one.
Plate casting is one of the most important parts of piano manufacturing. The plate (or as some people call the "harp") is what gives the piano the strength to hold the pressure of all the strings (close to 2 tons when fully pulled to pitch).
For decades piano plates have always been made the "wet sand casting" way. This involves a sand cast form where the worker will pour the hot cast iron to form the shape of the plate. Once the iron is in place, they will cover it with wet sand to allow it to slowly cure- a process that can take up to 6 months. This is how all high end pianos are made (Steinway, Bosendorfer, Mason & Hamlin, Seiler, Bechstein, Bluthner, etc).
In the 1970s Yamaha came up with a faster way to make plates that they called "V-pro". This involves a "closed form" meaning they poor the iron into a small hole in the top of the form and allow it to fill the form. After the form is full, they vacuum suck the air out of it and dip the form in water. This causes the iron to cure very quickly (in about 4.5 minutes!) but requires putting more additives in the iron. Some people will argue that the additives (some manufactures have admitted to putting tin in the iron as a filler) in the iron makes the piano sound thin or too bright.
Now a days most Asian manufacturers are doing v-pro but many are switching back the wet sand casting as the end result is usually more pleasing.
As always, if you have any questions on an of this, please feel free to contact me at any time! Michael@StilwellPianos.com