Slavica Taseva, Vane P. Sekulov

The hypocaust in the sudatorium from the late Roman thermal spa in the village of Bansko, Strumica

The hypocaust in the sudatorium from the late Roman thermal spa in the village of Bansko, Strumica

With the excavations in 1965 in the thermal spa from the Roman period, in the vicinity of Stara Zagora, Bulgaria, among other artifacts, an inscription on a marble plaque hung at the entrance was discovered. It is an inscription dating back from the middle of the II century AD, unique of its kind, which contains a list of the premises and their arrangement in the building. Namely, it is stated in it that the spa had two apoditeriums, one frigidarium, two pools and a nimpheum (Д. Николов 1968, 43-48).

Although a hipocaust was not mentioned in the list, it was a construction element essential to Roman baths and therefore it must have been a component of the heating system in the spa in Stara Zagora.

The reason why the hipocaust was the most widespread heating system used in Roman times was its comprehensiveness - it possessed both hygienic and economical features. It was used in Italy and in all Roman provinces across Europe, North Africa and Asia Minor, in geographic regions with different climate in different seasons (Т. Иванов 1971, кн. 1, 23).

The origin and the use of the hipocaust have not been resolved yet. On one hand, the term "hipocaust" corresponds to Greek origin; while on the other hand, written sources by Roman authors refer to the Romans as the inventors of the hipocaust.

According to the writings by Cicero, Pliny, Valery, Maximus and others, it was invented by a person named Sergeus Orata (c. 80 AD), a rich citizen of Rome, who first constructed a hipocaust under a fish pool in order to heat the cold water (R. J. Forbes 1958, vol. VI, 40). This system of heating is referred to by authors as balneola suspendere or pensiles balineae.

The Roman architect and engineer Vitruvius, who lived and worked during the reign of Caesar (I century BC) in his "Ten Books on Architecture" (book V, chapter 10), gives a detailed description of the hypocaust construction in contemporary spas.

The basic function of the hipocaust was to heat the hanging floor and the walls from beneath. This is what the word hipocausis means in Greek, "heating from beneath". The Latin version of the term hipocaustum means heating installation.

Some Roman buildings used the hipocaust as a system of ventilation and dampness isolation, but most often, as we mentioned before, it was used as a heating system.

The rooms with a hipocaust had a favourable orientation in regard to the rest of the building, so that they could use the sun to a maximum, which was very often the case in ancient times, as suggested by Vitruvius (book V, chapter 10). Namely, he advised south-west orientation of the heated rooms (ad occidentem hibernum), with light coming from the north-west or from the south, as the baths were most often used in the afternoon until the setting of sun.

Such an orientation of the hipocaust was usual, which can be confirmed by the research into baths in Bulgaria made by T. Ivanov . It was noticed that the south and the south-west orientation of the rooms with a hipocaust depended on the continental climate characterized by moderately cold winters and hot summers.

The basic components of the rooms with a hipocaust system are: the furnace (prefurnium), the canals which supply hot air or vapour, the canals which let the hot air or the vapour pass, vertical canals to heat the walls, relief holes and canals to drain the hot air and the vapour.

The rooms with a hipocaust were heated with hot (dry) air or with vapour. When the rooms were used for perspiring by means of hot air they were referred to as lakonium, and when the perspiring was achieved by means of vapour, they were called sudatorium (A. Bace, 55).

The rooms were heated with dry air by means of furnaces which used wood and charcoal.  Three caldrons were placed above the furnaces, one for hot water, the other for warm water and the third contained cold water. The hot air from the furnace circled through the hipocaust heating the floor and the walls.

The thermal spas were most often built next to a spring of thermal mineral water and were accompanied by special reservoirs for hot water which was used for heating. The water entered the hipocaust to a certain height, and the vapour heated the suspensure and the canals in the walls of the rooms. Such were the Roman spas in Hisar (spa of Havuza), Bulgaria, the spas in Stara Zagora and some other spas (T. Ivanov 1971, 26).

Late Roman thermal spa

The village of Bansko is located 12 km south-east of Strumica, in south-east Macedonia (fig.1). The eastern part of the village, where the spring of thermal-mineral water is situated, is known among the local population under the name of Spa. Orographically it belongs to the north slopes of the Belasica mountain which has a general east-west orientation. In its geological structure the land has layers of the old Paleozoic, Neogen, and the Quarter. Tectonically, the site belongs to the Macedonian- Serbian tectonic massif, that is, The Belasica range which is 45 km long in the direction east-west with inclination to the north under an angle of 65°, along the length of which split zones and cataclizates-milonites were formed. All of this caused the appearance of significant springs of thermal water.

The archaeological excavations at the site Late Roman thermal spa in the village of Bansko, Stumica, were begun in 1978 and with short interruptions have continued until today. The existing campaigns include a space of about 1500m2 upon which a well-preserved Late Roman thermal spa has been found (fig. 2). The building was using the thermal-mineral water of the spring "Parilo", which is situated about fifty meters southwest from the bath. The capacity of the spring is 42 l/sec with temperature of water of 72˚C, enough to supply and qualitatively maintain the bath.

So far eleven premises are discovered with an area of 623m2 (fig. 3). Nine of them are excellently preserved. Premise 1 is especially well-preserved, where one cross and one semi-circular vault are preserved; Premise 5 is also well-preserved, where above the bathtub the semi-dome is preserved.  

Premises 1 and 4

Analyzing the plan of the Late Roman thermal spa in the village of Bansko, we noticed­ grouping of premises in three parallel rows following the configuration of the land (Ј. Ананиев 1989, 333). This ordering of premises in rows came as a consequence of the spa expansion, so several building phases can be clearly defined. In the first (oldest) phase of the building there are the three premises that form the first row, comprising the two premises with a hypocaust (С. Тасева, В. П. Секулов 2003, 265).

The two premises with a hypocaust are situated nearest to the spring of hot thermal water, so that the heat of the water would not be lost while using it as a means of warming.

Premise 1 is set in the direction NE-SW with length of 6.27m (fig. 4). The north-east half is with dimensions 3.90x3.10 m where the system of floor and wall heating , while the south-west half of the premise is a bathtub with dimensions 2.60x2.30 m and depth of 1.00m (fig. 5). These two halves are separated with a barrier wall which is 3.10m long, 1m high and 0.50m wide. At the east part of the pool under the wall and under the two steps passes the drainage canal which carries the used water to the main canal. The entrance to this premise is on the north side through a door which is 0.82x0.90 m wide and 1.85m high. On the north-east wall there used to be a door for entering this premise, but it was walled up in the phases of further spa expansion.

The completely preserved vault construction above the bathtub represents a semi-circled vault, while the space with the hypocaust is vaulted with a cross vault made of brick, which is 3.80m high. Vitruvius (book V, chapter 10) recommends vaulting of the laconium and the premises for perspiring, because the condensed vapor continuously flows down the vault. Communication with Premise 4 is via an arch door 2.35m wide and 3.00m high.

Premise 4 (fig.6) is with orientation NW-SE. It is 6.40m long and 3.15m wide. The east corner of this premise is 0.25m higher than the rest of it. This space with dimensions of 2.30x1.80 m is a marble plateau, and right next to it there is a walled-up bathtub with dimensions of 1.96x0.92 m and depth of 0.50m. Beside the north-east wall there is a bench 3.35m long, 0.35m wide and 0.54m high (fig. 7). Between the bench and the wall there is a row of 13 tubules. A bench of this type, only with smaller dimensions 1.70x0.30x0.30 can be seen in the west corner as well. In it there are preserved five tubules which, as opposed to the ones of the north-east bench, are walled-up and did not protrude above the bench. This bench served at the same time for easier entering the bathtub in the south-west part, which was in some later phase added  (Premise 10) with dimensions 3.20x 3.05m.

The height of the preserved walls in Premises 4 and 10 are 3.40m at the most. The roof construction of Premise 4 was a brick vault oriented along the shorter stretch of the premise according to the remnants of vaulting the walls, which offer to determine the height of the vault construction as well. On the south and the south-west side of both bathtubs in Premises 1 and 4 there are arch windows.

From Premise 1 the heating system continues along the side of Premise 4 (fig. 8), so that both premises are connected in a functional entity with a mutual purpose.

Elements of the heating system

Through a canal set along the side of the north-west wall of Premise 1 came the thermal water and through three openings entered the heating system, so that a great quantity of the vapor heated the suspensure, the walls and the benches in Premises 1 and 4. One of the openings for entering the water into the hypocaust is set beneath the door in Premise 1 (fig. 9), central to the main canal, while the other two are between the three rows of arches in the north corner of the premise. The heating system is located in Premise 1 in its north-east half and continues in Premise 4 up to the marble plateau and the walled-up bathtub, comprising 7.10m in length and 3.00m in width, or total area of 21.30m2, all of which has been completely preserved.

The carrier floor of the system is compactly built of regularly arranged bricks with dimensions of 51x33-35x4 cm. The floor is set on a flat surface because it should carry the big burden of the hypocaust construction (colons, hanging floor and floor finish) with a gentle fall towards the water canals with difference of 13 cm from the endmost points. Thus, the rule of Vitruvius was observed (book V, chapter 10) according to which the carrier floors in the hypocaust premises should be  so inclined towards the furnace floors that if a ball is thrown from the furnace upwards, it will slowly come back to the furnace. He assumes that with such a fall the warm air circulates and heats the best in the hypocaust system.

The substructure of the carrier floor is multilayered and is composed of several construction materials (fig. 10). By sounding the floor it was found that the bricks from the floor are fixed into a layer of white lime mortar with thickness of 4 cm. Beneath it pieces of broken tubules are arranged, with thickness of 2 cm, fixed into a layer of waterproof mortar with clay additions with thickness of 2 cm. Beneath this layer of mortar two layers of stone are arranged with thickness of about 20 cm and beneath them there is a layer of waterproof mortar with clay additions with thickness of about 20 cm. Because the area is not accessible for deeper sounding, it cannot be said whether the substructure ends by that point or continues deeper with its layers. Especially for damp buildings Vitruvius (book VII, chapter 4) as a first measure for dampness elimination recommends that there should be made a thick layer (three Roman feet, about 90 cm) of waterproof mortar with addition of pieces of baked bricks  to coat the whole of the floor surface, which as an example was used in this building.

Construction and construction material of the hypocaust

In the spa of Nikopolis ad Istrum in Bulgaria, on the east and on the west wall of premise A, in the still fresh waterproof mortar the ancient masters-masons drew out the sketch of the hypocaust construction. Two horizontal lines sketched at a certain distance from the floor, crossed by two perpendicular lines that form rectangles with width equal to the width of the distance between the colons and the half-cylindrical arches outlined between them, represents an original sketch and a proof of the plan for construction of the hypocaust and organization of the work of the ancient masters of masonry (T.Ivanov. 221).

In the Late Roman thermal spa in the village of Bansko in the premises with hypocaust a type of arch construction was made. The whole of the construction is made with brick arches and consists of an open central canal that stretches in the direction NW-SE along the whole length in the middle part of Premises 1 and 4 and goes out through the wall of Premise 4 through an opening that functioned as a regulator of the heat in the hypocaust. The canal has a width of 45-47 cm and depth of 70cm. On the left side of the canal there are three rows with three arches each, while on the right side there are two rows with three arches each (fig. 11). The last rows of arches next to the wall in both premises are 10 cm from the wall because of arranging the vertical bricks (tubules) along the height of the wall . The arches start from the carrier floor and are next to each other. The dimensions of the bricks they are made of are 25x13-14x4 cm. The thickness of the joints between the bricks is from 1 to 3 cm at the upper part of the arch. The space between the arches is walled by bricks with width of 4 cm and joints between them made of white lime mortar with width of 2 cm. The arches begin from the floor, so that the height of the light opening is 38 cm and its width is 68 cm.

Vitruvius (book V, chapter 10) mentions the construction colons of bricks. According to him the height of the colons should be two Roman feet, that is 60 cm. In our case the construction of colons is not made, probably because of the shortening of the height of the hypocaust space which has a small area of 21.30m2, so that a greater efficiency is received. As a finishing layer above the arches there is a row of bricks with thickness of 2cm. Above the smoothed area above the arches there is the hanging floor, which is designated with the term suspenzura or suspensio. It consists of an area of bricks with dimensions of 60x68cm and thickness of 7cm. These bricks are called bipedal bricks (bipedalis tegula) and are set from one row of arches to another occupying one half of the arch width. They are set above a layer of white lime mortar with thickness of 2.5cm. Above them there is a thick layer of 8cm of waterproof mortar with small stones and large pieces of red brick. The upper area of this mortar is with additions of small stones and brick dust, it is finely glazed, whereas at the ends by the wall it finishes with bended edges with width and height of 10cm. The same example of floor finish can be found in the spas in Pautalia in Bulgaria (T.Ivanov 1971. 38) 

The area above the central canal is open at present, and as there was found not an element that would be a proper evidence of its covering, we suppose that it was probably covered with bipedal bricks which could be lifted if there was a need to clean and mend the canals.

Heating of the walls

At the Late Roman thermal spa in the village of Bansko the system of heating the walls is composed of special bricks - tubules which are hollow, with rectangular form and thickness of walls of 1.5 cm. Their dimensions are 25x10 cm, with height of 42 cm in Premise 4, and 24x10 cm and height of 36 cm in Premise 1. In Premise 1 the tubules are set on the north-west wall on the left and right of the entrance door, in five perpendicular rows, with six bricks each, as high as 2.10m from the floor (fig. 12). On the north-east wall the tubules were set along the whole length of the wall as high as 2.10m, but only the first row is preserved. In Premise 4 the tubules are arranged behind the bench on the north-east wall in 13 rows, and according to the remnants of mortar on the wall they were very probably as high as those in Premise 1. On the opposite wall the tubules are arranged beneath the bench in front of the bathtub and end in the bathtub with a purpose of heating it.

The tubules with one side lie upon the bipedal bricks, whereas to the walls they are attached by mortar, in such a way that first on the wall was put waterproof mortar with thickness of 1 cm and above it was put a white lime mortar with the same thickness. To the wall they are attached with the help of iron "T" nails which are hit between them and probably ended in a horny form to hold both bricks at the same time. Such examples of attaching the tubules can be found in the spa in Bargala  (Z. Beldedovski 2003,57-69), in the ancient spa in Varazdinske Toplice (Belančiċ-Gorenc 1961,203-206), in the late-ancient balneum (the church of St Georgi in Sofia) (R. Ivanov  2002), in the spa in Nikopolis ad Istrum (Т. Иванов, 222) and other spas. On the north-west wall of Premise 1 there are more remnants of highly corroded nails set perpendicularly between the tubules.

In the middle of the side parts of the tubules, holes with dimensions 10x2.5 cm are made so that the warm air could move horizontally between all rows. Above the tubules two layers of mortar are laid, first white lime mortar with thickness of 1.5 cm and waterproof mortar with the same thickness which is finely smoothed along the whole surface.

Way of functioning of the heating system

The high temperature of 72°C of the thermal water from the spring Parilo in the village of Bansko in the proximity of which this thermal spa was built was high enough to be used for heating the floors and walls in the hypocaust premises without additional heating. Because of that, with the archaeological excavations done so far there have not been discovered furnaces (fornax) for floor heating and heating of the water. The hot thermal water which was directed through a canal into Premise 1 brought the vapor that expanded between the rows of arches and continued through the canals into the walls (tubules) heating them as well. Via the central canal the vapor continued into Premise 4, where circling through the canals it heated the floor and the walls of this premise. The length of the central canal from the hole for entering to the hole for eliminating the excessive vapor through the wall in Premise 4 amounts 9.90m. Its fall follows the fall of the carrier floor which, starting from the entering canal moves up towards the hole for eliminating the excessive vapor with difference in height of 13 cm.

The thermal water that was brought into Premise 1 via three openings with a constant gentle flow, ran out through the drainage pipe in the wall of the east corner of the same premise. This drainage ceramic pipe also served for the water to flow out from the bathtub in Premise 1, which has a drainage canal under the bench on the floor.

The excess of the great quantity of vapor that was running through the rows of arches in Premises 1 and 4, went out through a canal - relief which is set at the end of the central canal. It is a well walled opening with height of 55 cm and width of 42 cm (fig. 13).

A part of the vapor went out through the two ceramic pipes on the north-east wall of Premise 1 which are next to the drainage pipe at a height of 40 cm from the carrier floor and go out through the wall in the part of the bench in Premise 2.

The circling of the vapor through the hypocaust part warmed both the hanging floor and the walls and benches in the premises via the tubules through which it moved up and through which it came out heating the space inside.

The excess of heated air full of vapor came out through ceramic pipes - komini, (fig.14)which are set diagonally in the north-east wall of Premise 1 and in the south-west wall in Premise 4, at a height of 2.40m from the floor and come out through the roof of the premises.

The used water from the little walled bathtub in Premise 4, through a drainage canal ran into the main canal of the system and flew out into a drainage ceramic pipe which is situated in the wall at the north corner of the premise at a height of the carrier floor.

The warm floor, the warm walls and benches, as well as the warm and humid air created an ideal place for perspiring of the spa goers - sudatorium. Moreover, the two bathtubs with warm water where 2 to 3 people could stay, the smaller bathtub for one person and the plateau with marble tables where relaxation might have taken place with massages and smearing with perfumed oils was a place not only for bathing and curing with thermal water, but also a place for enjoying - a characteristic of the lifestyle in the Roman period.


With the excavations done so far in the whole area of the spa and the still not analyzed archaeological artifacts, we are not able to give the exact date of the premises with the hypocaust and the spa in general. However, using comparative methods and following the analogies of similar buildings across the Balkans and wider, still we can date them in the III century AD. It does not mean that this is a final date and that we cannot move it backward or forward. The research to follow will surely offer the answer to this very significant question.

The hypocaust system of heating was very much used in our country. It was used in public and private spas, for example in the thermal spas in Heraklea, the small thermal spa and the large one in Stobi, the spa in Bargala, the spa in the village of Banica in the Strumica region and other spas as well, but this type of arch system with a main central canal in the middle along the whole length of the system in the ancient thermal spa in the village of Bansko is the only one discovered so far and as an example is very rare in other countries. Its complete preservation makes possible its present functioning. The difference from other types of heating is, as we mentioned before, that there is no prefurnium, but the heating comes from the warmness of the thermal water.

The use of the system of hypocaust heating continues in the Middle Ages, for example, the small and big spa in the monastery complex of Vodocha near Strumica (Ј. Ананиев, 1991 57-64) , where the basic principles and the construction of the ancient hypocaust were used.


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