Stringing:
Neapolitan Mandolin (18th century) 

18th century Neapolitan mandolin

Left: Illustration of a 4-course mandolin from A Complete Introduction to the Art of Playing the Mandoline, (London: c.1785), which is a translation of Gabriele Leone's Méthode Raisonnée pour passer du violon à la mandoline et de l’archet à la plume (1768). 

Right: Illustration of a 4-course mandolin from Michel Corrette's Nouvelle méthode pour apprendre à jouer la Mandoline (1772).

String material and tuning

All source material from the time call for the 4-course mandolin to be tuned like the violin [from low to high]: g-d'-a'-e". Strings were tuned in unison, though the 4th (lowest) course bourdon could be paired with an octave string.

One of the earliest sources we have that discusses the stringing of the mandolin in detials is Giovanni Fouchetti's Methode Pour apprendre facilement á jouer de la Mandoline á 4 et á 6 Cordes, (Paris, c. 1770). [See note 1.] According to Fouchetti's instructions (p.5):

"A l'egard des cordes elles doivent être de laiton. L’on prend pour les La, des cordes de Clavecin du número 5. Les Re sont du numero 6, mais l’on en met deux tordues ensemble pour chaque Ré. A l’egard des Mi, l’on se sert de cordes de boyeau, l’on prend des chanterelles de Pardessus de Viole. Les bourdons, ou Sol, sont aus si des cordes de Boyeau, mas fileés; l’on prend des bourdons de Violon, mais plus-petits. On fait quelque fois filler des cordes de soyes, pour servir de bourdons; elles sonnent très bien."

["Regarding the strings, they must be made of brass. We take for the La [A strings], harpsichord strings of number 5. The Re [D strings] are from number 6, but we put two twisted together for each D. With regard to the Mi [E strings], we use gut strings taken from the chanterelles from the Pardessus de Viole. The bourdons, or Sol [G strings], are also gut strings, but spun [i.e. wound or overspun]; we take violin bourdons, but smaller. Strings of silk are sometimes made to serve as bourdons; they sound very good."]

In other words, the 1st (top) course was strung in gut, the 2nd course was strung in plain yellow brass, the 3rd course was strung in twined yellow brass, while the 4th course bourdons were gut overwound with wire -- though silk was another possibility for the bourdons. (Whether the silk was by itself or wound with wire is unclear. For information on strings made completely of silk, see note 2, below.)

Fouchetti's directions demonstrate not only that the twined string was known and used into the second half of the 18th century, but that it could be used alongside wound strings on the same instrument.  

Tuning diagram for 4-course and 6-course mandolins from Michel Corrette's Nouvelle méthode pour apprendre à jouer la Mandoline (1772).

Other instructions by Michel Corrette (1772) reiterate Fouchetti's stringing of gut for the 1st (highest) course, brass wire for the 2nd course, but suggest a combination of open- and close-wound strings on an unspecified core for the 3rd and 4th courses, respectively:

"À legard de la qualité des cordes, on se sert de chanterelles de Guitare pour les cordes F. L. M. Les autres se montent en cordes de Clavecin du No. 5 jaune G, N, les cordes R, S demie fillés et celles I, P, Q filées en entire."

["With regard to the quality of the strings, we use guitar chanterelles for the strings [labelled as] F, L, M, [in the diagram]. The others are assembled with harpsichord strings of yellow [brass] No. 5 [for strings labelled] G, N; the R, S [labelled] strings are half-wound; and those [labelled] I, P, Q are fully wound."]

Notes: