Iron Piano Frames

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One piece iron piano frames were first patented in square pianos about 1825 in Boston. They were used in Steinway & Sons’ pianos from 1855. As the main structural element in a piano, these frames prevented the piano from going out of tune as a result of warping and shifting of wooden parts and allowed for an expanded range required by composers and performers. An important element of the frames used by Steinway & Sons was the quality of the metal.

Full iron frames were used in increasing numbers of pianos starting in the early 1820s. Full iron frames were first used in pianos in about 1825 by Alpheus Babcock, then working in Boston, and were subsequently improved by the Chickerings in the 1840s, also working in Boston. (3, pp. 79-80) They have been used in Steinway pianos since 1855.

Images of one of Babcok's pianos with the patentented iron frame can be accessed by cliking on  This site includes an image of Babcock's drawing of the iron frame.

When used, iron frames are the main structural element in pianos that resist the large loads applied by the tension forces of the strings. The incentives for use of iron frames, rather than using only wood for the piano’s structure, was to make pianos less susceptible to going out of tune due to the changes in dimensions that commonly occur with wooden frames and to allow for the expanded compass required by composers and performers. The dimensional changes of wooden frames were mainly associated with changes in humidity and temperature. Changes in dimensions of the wooden parts were also caused by creep of the wood due to the large loads applied by the tensions in the strings. Despite the advantages of dimensional stability provided by iron frames, there was considerable resistance to use of full iron frames until about 1855 because they tended to result in thin nasal tones. (4, p. 164)

In 1855 the new firm of Steinway & Sons won a gold medal at the American Institute Exhibition with a square piano with a full cast iron frame. (4, p. 217) This piano used overstringing, in addition to using a full iron frame. The use of a full iron frame allowed the tensile force of the strings in the piano to be increased, thereby allowing an increase in the volume of sound.

An important improvement to the iron frame was introduced by Steinway & Sons in 1872 via U.S. patents 127,383 and 127,384. These patents covered the use of “cupola” frames, especially for grand and upright pianos. The “cupola” frame had the same general layout as the previous flat frames but included curved or arched edges that greatly increased the frame’s resistance to bending, in the same manner that an angle iron is more resistant to bending than a flat strip. This allowed large tensile loads to be applied by the strings, up to about 30 tons (60,000 pounds).(3, p.122) In addition to the improved shape of the frame, Theodore Steinway spent several years working on optimizing the composition and casting methods of the cast iron used for the frames. (3, p.120) The composition of the cast iron was described as follows: “The metal employed contains 3.34 per cent of carbon as graphite and 1.20 per cent of silicon, besides manganese and sulphur. This in strength and lightness resembles cast steel, and is found to be especially well suited for its purpose.”(1, p.504) William made frequent mention in the diary of the frames of pianos with which he was involved.(Diary, 1872-3-30; 1875-6-6; 1876-9-24; 1884-5-9)

A Steinway brochure in 1877-1878 has the following interesting description of the iron frames used in their pianos: “All other Piano-Forte manufacturers—without exception—are compelled to have their Iron Frames cast at ordinary foundries, where they are often subject to the use of inferior, brittle, and second-hand metal, and insufficient care in casting, the chief aim, on account of close competition among iron foundries, being cheapness; hence, an imperfection in such Iron Frames is not an uncommon occurrence. The Iron Foundry of Steinway & Sons is specially built for the casting of full Metal Frames for their Piano-Fortes. Only the choicest brands of metal and coal are used, and after lengthy and costly experiments, Steinway & Sons have succeeded in producing Composite Metal, closely resembling cast steel, of almost double the strength of ordinary cast-iron, which is worked up and castings made by the most skillful artisans obtainable, thus insuring not only the greatest degree of strength, but the greatest uniformity and lasting qualities, in withstanding the crushing strain of 60,000 pounds of the strings under all circumstances, in any climate, and freedom from any imperfection in that most important portion of a Piano—the Iron Frame—and rendering crack, break, or any accident of that kind, as well as any injury to the vibration and tone by too great weight of Iron, an impossibility.”(2, p.1179)

In conclusion, the creative use by Steinway & Sons of full iron frames, and the design improvements that they developed and applied to the frames, were important factors in the commercial and artistic success of their pianos during the second half of the 19th century. The many changes to the pianos associated with full iron frames used by the Steinways involved improvements to the frames that had been developed earlier in the industry but had not been optimized. In this regard, the many detailed changes made by the Steinways eliminated the objectionable tinny tone of earlier pianos with full iron frames, and also allowed the adoption of overstringing, with resulting increased volume and purity of tone.

Additional material on this topic can be found by clicking on:

 a. Steinway Patents that Deal with Frames

b. Literature Related to Frames in Steinway Pianos


1. Appleton, D., Appletons’ Cyclopaedia of Applied Mechanics, Volume II, Park Benjamin, Editor: New York, D. Appleton and Company, New York, 1895.
2. Hutchinson, Thomas, compiler, “The Lakeside Annual Directory of the City of Chicago Embracing a Complete and General Directory,” Chicago: Donnelly, Lloyd and Company, 1877-1878. Available at:
3. Smith, Fanny Morris, “A Noble Art, Three Lectures on the Evolution and Construction of the Piano,” New York: The De Vinne Press, 1892.
4. Spillane, D., History of the American Pianoforte; Its Technical Development and the Trade, New York: D. Spillane Publisher, 1890.