We know a great deal about the cuisine and agricultural methods of ancient Rome from the writings of Lucius Junius Moderatus Columella. Columella’s principle work De Re Rustica, “On Rural Matters”, includes a recipe for making mead.
The recipe, De aqua mulsa facienda, was translated into English in 1745.
Therefore having set apart this bees-wax-water, and destinated it for preserving of fruits, mead must be made by itself of the very best honey ; but it is not made after one manner : for some, many years before, put up rain-water in vessels, and set it in the Sun in the open air ; then, having emptied it from one vessel to another, and made it very clear, (for, as often as it is poured from one vessel to another, even for a long time, there is found, in the bottom of the vessel, some thick settling like dregs) they mix a sextarius of old water with a pound of honey.– Of the Way to Make Mead – from De Re Rustica translated 1745
Nevertheless some, when they have a mind to make the mead of a rougher taste, mingle a sextarius of water with three quarters of a pound of honey ; and after they have, according to this proportion, filled a stone bottle, and plaistered it, they suffer it to be forty days in the Sun, during the rising of the Dog-star ; then they put it up in a lost, which receives smoak. Some, who have not been at the pains to preserve rain-water till it becomes old, take that which is fresh, and boil it in to a fourth part : then, after it is grown cold, if either they have a mind to make mead sweeter than ordinary, they mix a sextarius of honey with two sextarii of water ; or, if they would have it rougher, they put three quarters of a pound of honey to a sextarius of water ; and, having made it according to these proportions, they pour it into a stone bottle ; and, after they have kept it forty days in the Sun, as I faid above, they put it up in a lost, which receives smoak from below.
Take a sextarius of aged rainwater and mix with a (roman) pound of the best honey. Place in a covered stone bottle and leave in the sun for 40 days. Then smoke for some unspecified period.
Boiled water may be used instead of aged rainwater.
A “sextarius” is thought to be equivalent to around half a litre. The roman pound is believed to be equivalent to around 300g.
Pliny the Elder, Gaius Plinius Secundus, also mentions mead making in his work Naturalis Historia. The relevant passage in translation:
There is a wine also made solely of honey and water. For this purpose it is recommended that rain-water should be kept for a period of five years. Those who shew greater skill, content themselves with taking the water just after it has fallen, and boiling it down to one third, to which they then add one third in quantity of old honey, and keep the mixture exposed to the rays of a hot sun for forty days after the rising of the Dog-star; others, however, rack it off in the course of ten days, and tightly cork the vessels in which it is kept. This beverage is known as “hydromeli,” and with age acquires the flavour of wine. It is nowhere more highly esteemed than in Phrygia.– Naturalis Historia by Pliny the Elder
Notably in neither passage is there is a mention of using yeast. The ancients were accidental users of yeast. By exposing sugar containing substrates to the air they could haphazardly contaminate it with a wild yeast organism. The contaminated substrate would naturally ferment under warm conditions.
It is not recommended to try this method of making mead. Modern methods are far more reliable and hygienic.