The remains of four furnaces were uncovered during the excavations. Three of the furnaces are similar to each other while furnace #3 has a different design and is interpreted as a smithing hearth.  

Furnaces 1 and 2.

                      Cross-section of Furnace 2 

              The length of the scale is one meter.

The average diameter of the three furnaces at the exterior walls was between 70 and 95 cm across, while the interior ranged from c. 30 to 50 cm producing an initial wall thickness of about 10 cm which was subsequently thickened by relining, thus reducing the interior diameter.  The smaller furnace could hold a maximum of 7 crucibles while the larger could have held 20 maximum. The number was probably one or two lower when one considers that the crucibles were not touching each other and probably had charcoal in between them, or at least ash, indicated by the thick glaze.

The walls were vertical and their height was at least 40 cm, as this amount is preserved in one of the furnaces. A complete furnace wall would have needed to be high enough to contain the necessary amount of (presumably) charcoal, unless more was added during the process but that would have been inefficient and the temperature would have decreased while it was being added, resulting in the use of even more fuel.  Excess in height would have also been inefficient due to a higher surface area and more air to heat to a sufficient temperature. Therefore, it is estimated that the height was probably not far above 40 cm. The height of a crucible is around 20 cm, half the height of the furnace, thus leaving the areas between the crucibles and around 20 cm above them for charcoal.

The design and operation of the furnace can be suggested by comparing the visual archaeological evidence to known furnace parameters. Although no archaeological evidence remains of the top part of the furnace, it is reconstructed as a closed top domed furnace with a central tuyére rising up from the floor of the furnace, and with a side exit flue, based on the following evidence.

Three of the furnaces had a central air pipe rising out of the centre of the floor of the furnace.  This is interpreted as the tuyére and presumably this was attached to bellows needed to blow air into the furnace to raise the temperature high enough for the crucible steel process to work.  The air would have come through the tuyére at the bottom of the furnace and traveled up through the charcoal.  It is unlikely that the necessary temperatures, at least over 1050^OC suggested by the presence of mullite and probably over 1450^OC would have been reached and maintained, presumably for hours, using an open top furnace. Furnace 2 had a section of the furnace wall that splayed out from the body of the furnace. The internal walls of the splayed area exhibited a high degree of vitrification, equal to the thickest glaze at the bottom part of the furnace near the floor. This indicates that hot air and ash from the fire was somehow directed into this area.  If the exit flue was directly above the tuyére and there was no roof, the air would have blown out of the furnace top and would not have vitrified the splayed area nor heated the crucibles sufficiently.  By having the flue on the side, the air would come up out of the tuyére, circulate around the crucibles and fuel before leaving through the flue, thus more evenly distributing the heat around the crucibles. The furnace works both as an updraft and a down draft furnace. Therefore, to satisfy the archaeological observations, and for the furnaces to reach and maintain the necessary temperatures, the top of the furnace had to be closed. A domed roof is suggested rather than a flat one because it is the most efficient shape for heat distribution and is mechanically more stable. In addition, there is archaeological and ethnographic evidence of pottery kilns that support the suggested furnace reconstruction.

After the firing and the cooling of the furnace, the furnace was broken into on one side and the crucibles were removed. The sharp breaks in the black glaze around the crucible pad imply that the glaze was brittle when separated from the rest of the furnace floor.  In addition, the evidence of slow cooled steel prills in the crucible slag indicate that the crucible contents, and therefore by association, the crucibles themselves were slowly cooled.  The furnaces were then relined and used again.  

After building the furnace, the crucibles were placed inside. Next, broken crucible pieces would have been placed in between the crucibles and charcoal would have been put around the crucibles. The furnace roof would have been sealed before the fire was started. Bellows must have been used to introduce air into the fuel bed via the tuyére, as no natural draft would develop. The most effective placement for the tuyére would have been to introduce the air a few centimetres in to the fuel, and this is where it is located in the Merv furnaces.  In this way more air is distributed in the fuel bed and less air escapes up the furnace wall (Rehder, 1987, 53). 

The design of the furnace would have been very efficient, utilising many of the same principles as deep fuel bed furnaces and modern gas producer furnaces. It is the oxidation and reduction zones of the burning deep fuel bed of the gas producer furnaces, which can be compared to the mechanism that would have occurred in the Merv furnaces. Evidence provided by the crucibles and the reconstructed furnace design can suggest the furnaces’ mode of operation and efficiency. After the furnace was fired for a period of time, an ash bed would build up at the bottom. This is evident from the black ash glaze at the base of the furnace and over the furnace floor. The introduction of air into the incandescent ash at the bottom of the fuel bed would produce an oxidation zone. According to the Ministry of Power (1958, 427) the depth of the oxidation zone is 3-5 times the average particle diameter of the fuel and the temperature reached by the fuel in this zone is the highest in the fuel bed, which may reach 1600^O C.

The evidence found in the crucibles supports this. The crucible bases have a white matrix, indicating that they were fired under oxidising conditions and the degree of vitrification and ratio of glass to ceramic in the matrix indicated that it was also the highest fired part of the crucible.  In addition, by measuring the average height of the white crucible matrix, the oxidation layer in the furnace is approximated at about 8 cm from the floor of the furnace.  Based on the Ministry of Powers claim the average size of the charcoal would have been between 1.6 -2.6 cm in diameter. 

All the oxygen would quickly be used up as the air travels up through the fuel bed and would enter a reduction zone where the temperature would drop from around 1,100^OC at the bottom of the reduction zone to around 800^OC at the top of the reduction zone (Ministry of Power, 1958, 430). This steep drop in temperature and the presence of a reduction zone can be observed as the dark grey, lower fired upper areas of the crucible wall and lid.

Although direct evidence cannot be observed from the archaeological remains, evidence that the furnace produced and burned its own gas can be deduced from the above evidence of a deep fuel bed. Modern gas producer furnaces usually used coal or coke but charcoal could also be used (Ministry of Power, 1958, 426). A gas producer furnace uses a deep fuel bed with added water vapour to assist in the production of hydrocarbons.

The Merv furnaces also utilised a deep fuel bed, indicated by the evidence provided by the crucible, and water vapour would have been present from the crucibles losing water when they were being fired, in addition, some water vapour would have come from the charcoal. Charcoal readily absorbs water vapour from the atmosphere. The amount it will absorb is dependent upon the size of the charcoal, the atmospheric moisture, and the temperature at which the charcoal was produced.  The temperature at which it was produced will also effect how easily the charcoal ignites.  Provided the charcoal was dry enough to fire, the amount of water vapour and steam present in the furnace would not have significantly affected the furnace operation however under certain conditions it would have contributed to the production of hydrocarbons as one of the gaseous products. When the blast of air reached the reduction zone, the water vapour and unreacted carbon would form hydrocarbons and carbon monoxide (Ministry of Power, 1958, 430). 

These hydrocarbons would be forced upwards by the new incoming blast of air, then, if the furnace had a closed top as suggested, the gas would flow downwards again before going out of the flue.  At least some of this hot gas would be forced back into the hot charcoal and would combust as a gaseous fuel providing additional heat to the furnace. These hydrocarbons would also transport carbon into the fabric of the top part of the crucible, giving rise to the black lid and upper parts of the wall.