The bloomery furnace at its most basic is a container and reaction vessel for both the fuel and metal oxide charge, which is used in the production of metallic iron from iron ore. It provides insulation to the fire within, retaining heat and allowing significantly higher temperatures to be achieved when compared to those achieved in an open fire or shallow pit furnace. The bloomery furnace more importantly precludes atmospheric oxygen from reaching the fuel and ore in an uncontrolled manner, thereby allowing the regulation of the internal redox conditions. This controlled
environment also reduces fuel consumption, increasing efficiency and economy.
Bloomery furnaces were typically formed from a sunken pit with a stone and/or clay superstructure, the base of the pit was often lined with clay. Bloomery furnace designs can vary significantly, however, they are generally divided into two morphological types; the bowl furnace, which exhibits a minimal superstructure and has a width greater than the height; and the shaft furnace which is characterised by having a hight greater than the width (Dungworth 2012). Very few bloomery furnaces survive intact in the archaeological record, and it is often only the furnace bottom that is preserved. As such it can be difficult or impossible to determine if a furnace bottom was associated with a bowl or a shaft type furnace. It is also possible that furnace variants with no substructure were used, these furnaces are completely superterranean, which means they will leave very little, or no significant evidence in the archaeological record.
Fig.37. Idealised cross-sections of a simple stone-lined pit or bowl furnace (left), as well as a simple superterranean shaft furnace (right). Both furnaces are blown from the left. (After Buchwald 2005).
Fig.38. Idealised cross-sections of both a bowl furnace (left) and shaft furnace (right). The diagram shows the clay dome or shaft superstructures, along with the clay lined furnace bottoms. Both furnaces are blown from the left. (After Dungworth 2012)
One common feature of all bloomery type furnaces is an opening or series of openings near to the base of the furnace through which a controlled flow of air is allowed to enter, this opening is known as the blowhole, tuyère opening, or less commonly just the tuyère (Tylecote 1980). The pipe or nozzle that feeds air through this opening is more generally termed the tuyère. Air could enter the furnace through the tuyère via natural draft, that is, the furnace was positioned to take advantage of the prevailing wind, or the air could be forced in to the furnace using bellows. In smelting furnaces the reducing atmosphere is the most important aspect for a successful smelt, rather than maximum temperatures (McDonnell 1986). The temperatures within the furnace only have to be sufficiently high to allow for a low viscosity free flowing slag to form and separate from the bloom. This means that forced air is not necessarily required for bloomery furnaces, and a natural draft can be exploited.
Another aspect of the bloomery furnace which can be applied to their classification is that of waste material separation and removal. As a metallic bloom forms within the furnace, the gangue minerals from the ore begin to form a liquid slag, which separates from the bloom and flows to the base of
the furnace under the influence of gravity, this type of slag is known as smelting slag or production slag. The liquid slag then coalesces and accumulates in the bottom of the furnace where it is either released or extracted through an opening, or is allowed to settle in a chamber in the furnace bottom.
If the slag is released from the furnace it is said to be a tapping furnace, if the slag is left to settle in the furnace bottom it is said to be a non-tapping furnace (Dungworth 2012). These two different methods of dealing with the slag waste will result in two distinct morphological types of solidified slag. Tapping furnaces produce characteristic tap slags, these display flow structures and resemble lava, whilst non-tapping furnaces produce furnace bottom slags which are often planoconvex, forming in the shape of the furnace bottom. Both of these slag types can experience intimate contact between the ash, fuel, and furnace lining during their formation and movement, and this can dissolve and incorporate additional elements and inclusions into the slags from these sources.
The bloomery process was started by preheating the furnace from within using a fire, with either dry wood or charcoal being used as the fuel. Once the furnace was sufficiently hot it was filled from the top with a mixture of crushed charcoal fuel and crushed roasted ore, this mixture was known as the furnace charge. The ratio of the charge mixture was generally one part fuel to one part ore by mass (Buchwald 2005). Once fully charged the furnace was left to react the mixture, or in some cases it could be periodically topped up with additional charge.
The limiting factor in the amount of charge reacted by the furnace was often determined by the quantity of slag produced by the process. When the slag accumulating in the bottom of the furnace reaches the level of the tuyère opening, either the process had to be stopped or the slag had to be tapped off. If the slag was tapped, then the process could be continued for an extended period, simply by adding more charge to the furnace and tapping the slag as required. If the slag could not be tapped and blocks the tuyère opening, no more oxygen can be blown in to the furnace and a gas equilibrium will form. This gas equilibrium will prevent any further heat production and reduction
reactions from occurring in the furnace, and the smelting process stops. The smelting process would normally take between five and seven hours to complete (Thiele 2010).
When the smelting process was complete, the charge inside the furnace would have burned down, resulting in the formation of a bloom, which consists of a consolidated mass of porous iron known as sponge iron, and a quantity of slag, which would have mostly separated from the bloom. At this point the bloom was ready to be removed from the furnace. The raw bloom was generally removed from the furnace whilst still red-hot, and it was worked immediately. To reheat such a large piece of sponge iron would require a lot of additional fuel and it is unlikely that a metalsmith would waste the heat that was already present.
When the bloom was removed from the furnace, any bloom slag adhering to its surface would be removed with a wooden mallet, then the bloom would be hot-worked to expel any remaining smelting slag from within its structure and to close and weld any pores in the metal, this process is termed primary smithing (McDonnell 1986, Blakelock et al 2009, Thiele 2010). This would consolidate the sponge iron bloom into a monolithic iron billet and the high temperature oxidising conditions would decarburise the iron, reducing the amount of dissolved carbon present making it softer, easier to work, and less prone to cracking. During this primary smithing process, the metalsmith would be able to judge the quality of the iron and determine its workability.
The bloom would rapidly cool once it was removed from the furnace and would require regular reheating to keep the metal soft enough to work and to melt slag inclusion aiding in their expulsion.
As such the bloom would have been maintained at yellow-heat during the primary smithing process, at around 1100˚C, probably using the remains of the smelting furnace itself to re-heat the bloom (McDonnell 1986, Buchwald 2005). Some larger scale production sites may have used a dedicated hearth for this bloom consolidation and slag expulsion process, known as a cleaning or purification
hearth (Buchwald 2005). This process of bloom preparation and consolidation can produce two types of slag; bloom slag and purification slag.
Bloom slag is generally low in wüstite, and high in fayalite and glasses of near fayalitic and/or anorthite composition. As bloom slag forms within the furnace during the smelting process it can be considered as a component of the smelting slag. Purification slag is made up of a consolidated mixture of residual and newly oxidised iron from the bloom's surface; flux, usually of silica or calcium oxide/silicate to prevent excessive oxidation and loss of the newly formed metallic iron;
and remnants of bloom slag and smelting slag, dislodged and extruded from the bloom during consolidation. Purification slag is considered to be primary smithing slag.
Iron ores are considered to be self fluxing, but in situations where the ore used was particularly high in gangue minerals or low in silica, additional smelting fluxes may have been added to the initial furnace charge to aid in the successful slagging of these additional undesirable minerals. Smelting fluxes could take the form of silica, calcium carbonates, or calcium silicates. Silicate fluxes would be used to form a more acidic slag, removing basic gangue minerals, whilst calcium fluxes would be used to form a more basic slag removing acidic gangue minerals. The addition of calcium minerals may have also been used to reduced the loss of iron to fayalite, forming calcium silicates rather than iron silicate.