I have long been obsessed with sound. And as both performer and maker, this only become more important over the years.
Musicians have been muting their violins since at least the 17th Century. They used a range of different materials, the most written about being wood, metal, bone, and ivory.
I have seen two original leather mutes from the 18th century. The most beautiful (in a private collection) was found in the original leather cello case, with the 1770s cello for which it was probably made. The other was clearly ancient, but less clear how old. Leather is tricky to date.
In the 17th and 18th centuries it seems most instruments were sold with cases, usually leather, often with hammered rivets decorating and protecting it. They were often designed and made in the luthier's workshop, some of Stradivari's cases still survive. They were often ornately decorated, and if one extrapolates the curiosity of most makers I have ever met in this century, I would find it only common sense that if they were trying all materials to change the sound of a violin, viola or cello with a mute, they would have also tried leather. It was in the workshop, and they had the expertise of working with it. The only thing we don't know is when they started.
The lack of written evidence, whilst important, does not infer that it didn't happen, especially as there are extant examples, just that we know little about it. It may not have been common, but as leather has a far more limited life span than wood, metal or bone, it is not surprising more mutes didn't survive.
The first scores that specify muted violins in the parts appear in the 1680s: 1681 in Paris: Lully's ballet “Le Triomphe de l'Amour” in the 'Prelude for the Night', and 1682 in London: Purcell's Fairy Queen. Schmelzer in Germany, also uses “con sordino” in Le Memorie Dolorosa, slightly earlier in 1678. At this time they are used to emote Sleep, Night, Mystery, and Sorrow (amongst other affects); by the 1740s they are regularly donned for slow movements of symphonies.
The idea of changing the sound of a violin with something extraneous is already used by Biber in his “Battalia” of the late 1600s ,when he requests for paper to be put under the strings to create a drum like affect, as an early use of col legno. Composers were experimenting with different timbres then as now.
Richelet writes in 1680 in his Dictionary of Words and Things of a Mute, that its a violin thing, and made of a small plate of metal, curved in an arc to stop the instrument resonating loudly.
Rousseau a century later, writes of them changing the tonal quality, and its in this realm, that I make mine. In researching and testing many kinds of materials, and leathers, I was looking for a substance that produced a different sound world, not only making the instrument quieter. And leather does that- it creates a silky soft, dark timbre, which is unique.
I sell them worldwide, to orchestras as well as individual musicians, (even the virtuoso Alina Ibragimova has one!); and am proud that I have in a small way had effect on the sound of string players world wide.