Box-Flue Tiles in Hypocaust Heating Systems

The Hypocaust Heating System was used to heat rooms.  In essence it consisted of a suspended floor (suspensura), built on piles of tiles (pilae), under which a fire was lit.  The hot gases from the fire circulated under the floor, thus heating the room above.  In order to allow the fire to draw, these hot gases had to be vented.  This was achieved by lining the walls with flues which conducted the hot gases and smoke upwards to a chimney.  As a result, the walls also became hot which served to improve the overall efficiency of the system.  Flues were very often made of stacks of rectangular section tubes known as box-flue tiles (tubuli).  In order to allow for the plastering of the walls, it was necessary to provide a keying surface that plaster could adhere to.  This was achieved by the use of combs and rollers when the tiles were made..  In the case of combing, the wet clay had a pattern impressed into it by a wooden or metal comb.  Rollers were also used and the distinctive patterns they produced enable archaeologists to relate tiles from many sites to a common source, or at least, to a common roller.

The tile floor was usually finished with a covering of mosaic, composed of small pieces (tesserae) of stone and other materials, laid in a bed of cement to form a decorative pattern.

 

The walls of tiles were finished with layers of plaster which was often painted with polychromatic panels and frescoes.  Fragments of painted plaster from the Easton Maudit villa reveal that there were at least 27 different colours of paint employed in the decoration of its walls.  Indeed, the excavation team found pots that still contained paint, the colours of which matched those on the painted plaster fragments. 
Examples of flue-tiles found at Easton Maudit