What are pianos made of?

How often do we think about all the materials that pianos are made of? Here is a quick reminder in this list of materials:

Wood

Spruce: 
Used for the soundboard, which is crucial for amplifying vibrations, and sometimes for key and action parts. 
Maple and Beech: 
Hardwoods that are strong and rigid, making them ideal for the rim, action components, and bridges. 
Mahogany, Walnut, and Birch: 
Often used for the outer case and rim, with veneers providing the visible finish. 

Metal

Cast Iron: Forms the heavy internal plate that supports the tremendous tension of the strings. 
High-Tensile Steel: Used to make the piano's strings, which vibrate to create sound. 

Other materials

Felt: 
Used to cover the hammers and other parts of the action mechanism to cushion and control the strike. 
Plastic: 
Many modern pianos use plastic for keytops (in place of ivory) and for some action components to ensure durability and consistency, especially in humid conditions. 
MDF: 
Medium-density fiberboard, an engineered wood product, is used in some modern pianos to make them more affordable and durable.