Salt Wines

Why add salt to wine? I am not quite sure why it was done in period, but I did find that a little pinch of salt in young red wines tends to smooth out the taste. So maybe it's as simple as that. Or does it have something to do with longevity, as salt has a preserving effect, aptly observed by Columella...

XI [...] Another wine of the sweet class is called honey-wine; it differs from mead because it is made from must, in the proportion of thirty pints of must of a dry quality to six pints of honey and a cup of salt, this mixture being brought just to the boil; this produces a dry-flavoured liquor. [...]

Pliny’s Natural History. Rackham H., Jones W.H.S., Eichholtz D.E. (trans.), Massachusetts: Harvard University Press, London: William Heinemann, 1949-54 (public domain)
https://ryanfb.github.io/loebolus-data/L370.pdf (Latin & English, the book)
http://www.masseiana.org/pliny.htm (English only, as website)

XXIII.—PREPARATION OF THASIAN WINE.
We insolate the grapes when ripe, laying the bunches in pairs during five days, and on the sixth day at noon we take them up warm, and immerse them in must and sea-water boiled to half its quantity; and we take them up and lay them in the press ; then having trodden them the following night and day, we pour the liquor into vessels ; and when it has fermented and is fined, we pour a twenty-fifth part of sapa into it ; and after the vernal equinox we rack it into proportionate vessels.

XXIV. PREPARATION OF COAN WINE.
Some indeed boil three parts of must and one of sea-water into a third of the quantity; but others mix with two measures of white wine one cotyla of salt, three cotyke of sapa, one cotyla of must, one cotyla of flour of orobus, one hundred drams of melilot, sixteen drams of Celtic nard.

Geoponika, translated from Ancient texts, by various authors, 10th CE
Compiled during the 10th CE in Constantinople for the Byzantine emperor Constantine VII Porphyrogenitus. Agricultural Pursuits, by Reverend T. Owen (trans.), of the Queen’s College at the University of Oxford, 1805.
https://books.google.com/books?id=0KZbAAAAMAAJ&source=gbs_similarbooks


CHAP. XXV. Of salt Water andJirong Brine for preserving Wines

FOrasmuch as some people, yea, even almost all the Greeks also, preserve their must with salt-water or strong wine, I thought that that part also of our care was not to be omitted. In the inland country, whither it is not easy to carry salt-water, strong brine must be made for preserves after this manner.
Rain-water is by much the fittest for this purpose ; if not this, then that which flows from the clearest spring. Therefore you must take care to place in the Sun, five years before, a great quantity of either the one or the other of these, and put it up in the very best vessels you have ; then, when it is  putrefied, you must let it stand so long, till it shall return to its former condition ; when this is done, provide other vessels, and strain the water by little and little into them, till you come to the dregs ; for there is always found some thick settlement in the bottom of water, which you let stand without stirring it.
When the water has been thus managed, it must be boiled into one third of the first quantity, after the manner of rob of grapes ; and, into fifty sextarii of sweet water they put one sextarius of salt, and
a sextarius of the best honey : these must be boiled all together, and all filthiness purged out of them ; and, when the water is cooled, a certain quantity of it must be put into an amphora of must.
But, if your land lie near the sea, the water must be taken out of the deep when the winds are silent, and the sea exceeding calm ; and it must be boiled into a third part, after you have put, if you think
proper, some of those spices into it, which I mentioned above, that so the wine may have a better flavour after it is cured. But, before you take up the must out of the vat which is under the wine-press,
suffumigate the vessels with rosemary, or laurel, or myrtle, and fill them up to the brim, that, when the wine ferments, it may purge itself well ; afterwards rub the vessels with pine-apples. The wine which you have a mind mould be sweeter than ordinary, you must preserve it the day after you have taken it out of the vat ; and that which you would have rougher, you must preserve it the fifth day, and so fill up and daub the vessels. Some also, having suffumigated the hogs heads, put the preserve in first, and so pour in the must. (p.533)


De re rustica, L. Iunius Moderatus Columella, about 60 CE.
L. Junius Moderatus Columella of Husbandry in Twelve books and his Book concerning Trees. London, UK: A. Millar (transl.), Book XII, p 517, 1745.
https://books.google.com/books/about/L_Junius_Moderatus_Columella_Of_Husbandr.html?id=qcNbAAAAMAAJ


Salt found to improve red wine flavour by Lucy Shaw.
https://www.thedrinksbusiness.com/2014/03/salt-found-to-improve-red-wine-flavour/
Wine Education Topic: Saltiness in Wine.
http://www.aromadictionary.com/articles/salt_article.html