Discovering Roman Hot Water Heaters
Government archaeologists in France discovered a
a Roman-era hot water heater, in a rural Roman farmstead near the
village of Saint-Pathus.
Today that village is 50 kilometers northeast of Paris
in the Île-de-France region.
They described their find in the article
miliarium from a private bath house in northern Gaul:
from water technology to ritual offering". 
1: Séverine Hurard,
Luc Leconte, Aurélien Lefeuvre, and Pascal Raymond,
Antiquity, 90 345 (2016): 1562-1575,
I described the Roman miliarium
As the authors put it, a water heater tank was
"an object at once ubiquitous and nearly invisible."
So why was this find such a big deal?
Water heaters were very common items, given the Roman fixation on bathing.
But the heater itself was a very valuable object,
incorporating a large amount of metal.
Very few water heaters have been found because they were
almost always recycled in order to reuse their metal.
Exceptions include those suddenly buried by volcanic eruption at
Pompeii and Herculaneum.
The authors of the paper report that "only a dozen or so" have been recovered
from all across the Roman Empire.
The bronze base of one, from a villa at Tourville,
near today's Apt in the Vaucluse,
Fragments from a bath complex in Longeas, and Charente,
Fragments from Toulouse,
A lead tank discovered when dredging the Rhône river,
A fairly complete tank in ancient Ambrussum, today's Villetelle,
Two fairly complete tanks elsewhere in Italy,
at the Villa della Pisanella at Boscoreale
and at Ostia, the ancient port of Rome.
A complete tank at the bath complex of Iuliomagus,
at today's Scheitheim in Switzerland.
A hypocaustum and miliarium in a Roman bath.
The hot-water heating tank discovered at Saint-Pathus is cylindrical.
It is 1.11 meters high, about 0.65 meters in diameter
and it weighs 250 kg.
It held about 350 liters.
The largest examples found at Pompeii and Herculaneum held
from 200 to 450 liters.
The cylindrical body is formed from two lead sheets,
each 1.2 meters high, 0.9 meters wide, and 55 to 77 mm thick.
These sheets were formed by open wet sand casting,
which produced a rough exterior surface texture.
One of the two sheets bears bas-relief decoration on its upper half.
The decoration is a grid of sticks or bars with 19 imprints of a
medallion of the head of Okeanos, or
Ώκεανός in the
original Greek, the divine personification of the world-encircling sea.
Okeanos is typically depicted with disheveled hair, beard, and
moustache, with each curl of hair representing a river.
Crab claws and crustacean legs protrude from the head.
The decoration was made by pressing bars and a form of the medallion
into the wet sand before casting the sheets.
The circular, concave, shield-shaped base or bottom cap is bronze.
That's a copper alloy with about 12% or so tin.
The base is 0.65 meters in diameter and 3 mm thick.
It was probably formed by casting.
The lead sheets were formed into a cylinder, joined with clamps,
and then welded together with a flattened lead weld bead.
The lead cylinder and bronze base were then joined by soldering.
The tank was place on metal bars directly over the hypocaustum
Toilets and Aqueducts in Roman Gaul
The technical details imply the existence of metal smiths in the area.
While no metal shops have yet been discovered in the area,
the widespread use of water heaters in Roman Gaul
mean that this was probably manufactured in the area,
likely in Meaux, 18 kilometers to the south-east and the nearest
sizable town at the time.
The water heater was decorated, but in a fairly crude and haphazard way.
Yes, a water heater was a vital component of a bath,
but it wasn't unusual to have one and you wouldn't care about having
special decorations as you wouldn't be showing it off to your friends.
The decoration was probably intended to help sell what would have been
a rather expensive piece of equipment.
The Bath House
The bath house at this site was a building 10.3 by 4.3 meters.
They describe it as a adaptation of the standard design to
local conditions and limited resources.
However, bath houses like this one are an exception in the rural settlements
excavated so far in the Île-de-France region,
meaning that the residents were wealthier than the average in the area.
The Ritual Decommissioning
The site was occupied from the second half of the third century until the
fifth century, with a short abandonment of perhaps just a decade in the
second half of the fourth century.
The bath house was disassembled during that decade or so.
The furnace area was excavated to form a pit, and the water heater tank
was buried in that pit.
The initial assumption was that this was done in order to hide the
valuable metal object from scavengers.
However, that hypothesis came into question once a large collection of
buried animals was discovered on the site.
Cattle were buried around the edges of residential buildings,
and juvenile pigs were buried in cellars.
They were buried entire or in parts, but none were partially eaten.
Half of the buried cattle and horses were in pits within 10 to 30 meters
of the bath house.
A complete cow was buried in a pit next to the north wall of the bath house,
and a complete juvenile pig and small owl were found in the remains of
the calidarium and tepidarium, respectively.
What Might It Mean?
The diversity and combination of species, the repetition of the burials,
and the completeness and frequent juvenile nature of the carcasses
suggest that this was part of some ritual.
Similar foundation and abandonment rituals were common in
the Rhine valley region during that time.
Juvenile pigs were often associated with cults of
Tellus Mater or Mother Earth, and Ceres, the goddess of agriculture.
Owls with associated with the cult of Athena or
known to the Romans as Minerva.
The combined burial of the sacrificial animals
along with a valuable object bearing the image of Okeanos
may have made this a symbolic offering made
in hopes of protection and prosperity.
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