Capo d' Astro

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The capo d’astro in a piano is a fixed metal bar used in place of agraffes in the treble region to hold the strings in place. They were introduced in Steinway pianos in the 1870s to simplify construction and, more importantly, to balance the upward and downward forces applied to the frame by the tension in the strings, thereby minimizing the tendency of the frame to distort and cause the piano to go out of tune.

There are three entries in William’s diary regarding capo d’astros. The last entry notes that a Style 1 Grand Piano with a capo d’astro bar was a great improvement.(Diary, 1878-04-09) The earlier two diary entries note that capo d’astro bars had started to be used in upright pianos in 1877 and 1878.(Diary, 1877-11-15; 1878-03-17)

A search of Steinway patents issued through April 4, 1885, when the last patent taken out by Theodore Steinway was issued, indicates that the following patents dealt with Capo d’astros:

* Patent 170, 646, C. F. T. Steinway, dated November 30, 1875, Improvement in Agraffes for Piano-Fortes. This patent describes the use of capo d’astros in conjunction with agraffes for positioning the strings. The capo d’astros are described as follows in the patent: “The capodastro consists of a screw-shank and a bead of brass with a groove to receive a steel face, and also to give a hold for a screw-driver, by means of which the capodastro can be conveniently screwed home, and after it is secured in position the steel face is inserted, and thereby the bearing-points for the strings can be made narrow without permitting said strings to wear in.”

* Patent 204,108, C. F. T. Steinway, dated May 21, 1878, Improvement in Piano-Fortes. This patent describes how a capo d’astro bar is used in upright pianos. The capo d’astro bar has cavities which engage with projections on the metallic bridge so that the correct position of the capo d’astro bar is determined. The patent does not discuss how the capo d’astros themselves function, i.e., how they position the strings.

* Patent 204,111, C. F. T. Steinway, dated May 21, 1878, Improvement in Capodastro-Frames for Piano-fortes. This patent describes a capo d’astro frame that includes capo d’astro bars that are designed in a manner such that the desired tension is achieved in the strings, and such that the distances between up bearing and down bearing contact points form aliquot parts (parts that divide evenly) of the main scales of the corresponding strings, so that these parts are in harmony with the main portions of the strings.

The changes to the pianos covered by the patents listed above can be summarized as follows: Initially, in 1875, the capo d’astros were individual screws with slotted heads through which strings passed. They served, together with agraffes, to locate the strings in the desired locations. Later, starting in about 1878, the individual capo d’astro screws were eliminated, and replaced with bearing strips or bars, generally two down bearing bars and one up bearing bar. The locations of the bearing bars are such that the lengths of the string between the down and up bearing strips form aliquot lengths of the main portion of the string, so that their tones harmonize.

Literature Related to Capo d’astros in Steinway Pianos

* A Steinway brochure in the 1877-1878 Lakeside Directory notes that “The space gained by the use of the Capo d‘Astro bar permits the use of more heavily felted hammers, whereby a pure, rich quality of tone is retained much longer than heretofore.”(2)

* An 1892 book by Smith contains an illustration showing the presence of the capo d’astro bar in the treble region of a Steinway piano(3).

* A book published in 1893 notes the following regarding use of the capo d’astro bar: “Capo D'Astro Bar, Patent No. 170,647, Nov. 30th, 1875, in Grands. By means of this invention, the suspended wrest-plank can neither be raised nor depressed by the great tension of the strings. Although this patent was granted for Grand pianos, it has been adopted for the construction of Upright pianos also.”(4).

* Appletons Cyclopaedia of 1895 notes the following regarding the duplex scale that makes use of the capo d’astro bar: “The most notable feature in the Steinway scale is ‘the duplex’ arrangement, which is described as “a second scale of exactly and mathematically proportioned and shortened length, added to the principal scale. This second or duplex scale is applied between the tuning-pins and the end points of the strings upon the wrest-plank.” . . .”The duplex scale of a Steinway grand piano is here between” . . . “the agraffe pin, and” . . . “the ‘capo d’astro’ bar. The advantages gained by this device are more full and harmonious tones, and the avoidance of dissonances in the partial overtone vibrations.”(1)
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Sources:

1. Benjamin, Park, ed. Appleton’s Cyclopaedia of Applied Mechanics, Revised and Improved Edition. Vol. II, New York: D. Appleton and Company, 1895, p. 505.
2. Hutchinson, Thomas, Compiler. The Lakeside Annual Directory of the City of Chicago Embracing a Complete and General Directory. Chicago: Donnelly, Loyd and Company, 1877-1878, p. 1145.
3. Smith, Fanny Morris. A Noble Art. New York: The De Vinne Presse, 1892, p. 149.
4. Steinert, Morris. The M. Steinert Collection of Keyed and Stringed Instruments. New York: Published by Charles F. Tretbar, Steinway Hall, 1893, p. 92.