The earliest reinforcements for string instruments were wooden props, designed to counteract the increasing tension of strings as their number grew, as seen in harpsichords. As instruments evolved, with more strings and higher tensions, additional wooden reinforcements were introduced, particularly in the fortepiano. Later, metal props were implemented, followed by the innovation of metal frames equipped with pins for string attachment. However, the rising string tension eventually rendered solid wooden reinforcements inadequate for maintaining the pins securely in place.

The development of a single metal plate became the solution to this issue. The first metal frame appeared in 1825, created by Alpheus Babcock for a table piano. By 1843, Chickering grand pianos featured cast iron plates, and Steinway followed suit in 1859. In 1855, a frame designed for crossed strings was patented. These metal plates allowed pianos to sustain high string tension, while wooden bracing at the back provided support against the soundboard's tension. By the early 1930s, advancements in metal plate design made robust wooden bracing unnecessary. Slimmer bracing reduced piano weight and size, enhancing portability. Alternative materials to cast iron were also explored, such as lightweight metal frames in the 1960s, though some, like welded steel plates, proved too elastic, making it difficult to maintain tuning stability.

Cast iron remains the standard material for piano plates due to its desirable properties. It is acoustically inert, damping vibrations effectively, and has minimal elasticity, both essential for maintaining stable tuning. Cast iron is produced using raw iron, scrap iron, and carbon, with the process involving layers of carbon, raw iron, and limestone in a furnace. The limestone creates slag, improving the fluidity of the molten cast iron. Typical cast iron composition includes 2–3?rbon, 1–3% silicon, minimal manganese, and phosphorus, with a melting point of 1300°C.

The manufacturing process begins with precise design calculations, which are programmed into a CNC milling machine to create a model from alder wood or Delignit. This model is made 1% larger than the final plate to account for shrinkage during cooling. The plate is cast using a two-part mold filled with a mixture of sand, soot, clay, and water. After the mold sets, rods are removed to create channels for pouring molten iron. The two mold halves are joined, and molten iron is poured into the mold. Once cooled, the plate is freed, cleaned, and prepared for further processing.

The next steps involve drilling approximately 500 holes for pins, attachment points, compressors, and staples. Modern CNC machines accomplish this efficiently and with high precision, automatically switching drill bits as needed. After drilling, the plate undergoes sanding, puttying, and painting to complete its preparation for installation in the piano. Vacuum casting is sometimes used in modern foundries to enhance the casting process, ensuring even filling of the mold by extracting air as the molten iron is introduced.