From the earliest single action harps made ca. 1720 until the last, made between 1830 and 1840, the number of strings steadily increased. Hochbrucker’s 1728 harp housed in the Musée de la Musique has only 34, while the latest known crochet harps, made in the early 19th century, typically had between 39 and 42 strings. Indeed, in span of only a few years method books tell us that the number of strings increased from only 32, according to Philippe-Jacques Meyer’s 1772 Éssai la vraie manière de jouer de la harpe7 to 42 according to Guillaume-Pierre-Antoine Gatayes’ method, ca. 1790.8
The question of stringing on single action harps with regard to gauges is a hotly debated one among scholars and historical performers alike. Indeed, little verifiable evidence remains of 18th century stringing practices. Few string gauges exist from the time and stringing is rarely discussed in method books. Fortunately, however, we are generally aware of the materials used. Unlike its sisters of the keyboard family, pedal harps were strung with gut strings in the higher range, while the lower few strings (the number of which is determined by the range of the individual instrument) were strung with silk-core strings overspun with a metal, often silver, much like those used on some modern acoustic guitars. The gut strings were not treated with lacquer in the manner of modern strings, and as such had to be regularly oiled.
One significant barrier to the scholar hoping to answer the question of 18th century stringing is the complete lack of standardization. Method books of the late 18th and early 19th centuries are arguably the most enlightening source of information on the subject. These methods paint a picture of a world of heated disagreement, both between harp makers and harpists themselves, on the proper stringing of the instrument.
In his Méthode, thought to have been published ca. 1810, harp builder Jacques-Georges Cousineau advocates for a strict adherence to the ton de l’Opéra de Paris, praising this diapason as the lowest of all orchestras and thus the most appropriate for the harp, and warns against tuning to a higher pitch.9 Mme. de Genlis, similarly advocates for tuning to the ton de l’opéra in her 1811 method, while noting that she felt the “standard” gauge of strings of the time to be too light, stating that she prefers that the strings be thicker, stating the following:
“Je voudrois que les cordes fussent un peu plus grosses au même ton et par consequent plus tendues. C’est ainsi que je fais jouer mes élèves. Il faut beaucoup plus de force dans les doigts, l’exécution est plus difficile, mais le jeus plus net, le son plus beau. Il en résulte que les cordes tirant advantage sur la Harpe il faut des tables d’une grande solidité, sans quoi ells ne résisteroient pas à cette forte tension.”10
It is telling that later in the same text, Mme. de Genlis also describes Cousineau’s harps thusly—
"Les Harpes de Mr. Cousineau ont en général un beau son, mais ses tables sont faibles et ses mécaniques d'une extrême fragilité."11 (Genlis 1811, p. 11)
Guillaume-Pierre-Antoine Gatayes echoes de Genlis’ sentiments in his method book, claiming that “all harps are strung with strings that are too thin; it is true that this fault is that of the instrument, as the soundboard is too weak to resist the strong tension of thicker string; one must therefore, when one can, choose only harps that are very solid…” (Gatayes 1790). Robert-Nicholas-Charles Bochsa similarly encourages the use of heavier-than-standard strings in his 1820 Petite Méthode pour la harpe—
“Il est indispensable pour obtenir un beau son de la Harpe de la monter généralement en cordes un peu fortes. Par là on évite le son criard et frisé qu’ont toutes les Harpes montées en cordes trop fines.” (Genlis 1811, p. 12)12
Given the clear lack of true standardization of stringing during this period, it is hardly surprising that modern performers using period instruments (or indeed reconstructions thereof) rely on a combination of information gained from period string gauges, advice from these method books, and personal experimentation, experience, and tailoring to the specific needs of each instrument. This being said, today's performers, harp restorers, and scholars have generally agreed on a standard of stringing for most 18th and early 19th century harps, with crochet harps usually strung about two full octaves lighter than modern standard strings, such that, for example, a modern C5 string would be used on a French harp as a C3.