At this point, most high-end piano makers still use wooden action parts in their best pianos. Steinway & Sons takes the stance that you don't argue with success:
Steinway & Sons has a long and successful history with the use of hard rock maple in our action parts. For more than 150 years, artists around the world have chosen the Steinway tone and touch as the standard of excellence.
These artists expect a certain sound and touch each time they play a Steinway piano that is unique to our instruments. We have a strong commitment to the world of music to ensure that these performance expectations continue to be met. With respect to this commitment, we at Steinway feel that our choice of materials in action parts influences not only the touch of the instrument but also uniquely contributes to the tone. The use of all natural hard rock maple action parts is a time-tested ingredient of the Steinway performance experience and we remain very comfortable with maple as the material of choice in our Steinway pianos, as we continue to build our pianos to a standard, not to a price.
Similarly, Bösendorfer states that the company "has always used traditional components and methods in crafting its instruments. There has been no reason to change the 180-year-old tradition of using organic and natural componentry."
Even when piano manufacturers have experimented with using plastic or composite action parts, they have usually stayed with traditional action centers — the pivot points on which individual action parts swing or rotate. Traditionally consisting of a center pin held fast in the wood of the main part, and surrounded by a wool cloth bushing in the smaller wooden flange (hinge), the action center's proper operation critically depends on the flange being able to move freely around the pin in a controlled manner. But since felt is highly susceptible to changes in humidity — it swells up and tightens in higher humidity and loosens in lower humidity — the feel of a piano action can vary dramatically from season to season. In extreme cases, an action exposed to high humidity will become heavy and sluggish, and keys may actually stick, while in very low humidity the flanges may become so loose that action noises develop.
This tendency of piano actions to react poorly to extreme humidity or dryness led Steinway & Sons, whose pianos were shipped to destinations as hot and humid as the tropics and as cold and dry as Siberia, to look for ways to protect the action centers from changes in humidity. Their initial solution, dating back to the late 19th and early 20th centuries, was to treat the wooden action parts with liquid paraffin, thus sealing the parts and at least slowing the passage of moisture into and out of the flanges. The problem with this approach was that, over a long period of time — decades, in most cases — the paraffin migrated into the wool bushings of the action centers, where it reacted with the metal center pins to create verdigris, a type of corrosion. Eventually, the verdigris would render the action sluggish, even unplayable.