Experimental Iron Smelting at Rievaulx Abbey 

Further experiments on 20 & 21 July 2002 as part of the National Archaeology Days

Aims:  To build and operate a bloomery shaft furnace  To smelt ore to produce iron and other metals and the waste products (slag) To monitor the furnace as it decays over time to provide a model for furnaces that are excavated.

The Furnace The experimental furnace is a cylinder built of clay around a structure of willow withies. It has an internal diameter of about 40 cm and the walls are about 20 cm thick. This size is based on previously excavated furnace bases. Although the furnace has been built 1.4 meters high, the original height of furnaces is not known. At the base of the furnace is the ‘tapping arch' or a hole through the furnace wall about 20 cm square through which any slag formed is tapped out. The two smaller holes 40 cm up from the base are each about 1 cm in diameter, these holes pass through to the centre of the furnace and are used to direct air from the bellows into the furnace. The furnace was built 2 weeks before the smelting began, so that the clay could dry out slowly, reducing the cracking when the fire was lit in the furnace. The inner surface of the furnace was lined with a mixture of clay and charcoal dust ( 'lute' ), which reduces the cracking as the temperature increases.

Furnace Construction at Rievaulx Abbey 8 June 2002

English Heritage Archaeology Day 22 June 2002

The Fuel The temperatures required range from 600oC to 1500oC. The best fuel is charcoal, since it is clean, virtually pure, carbon containing no significant other elements, in particular sulphur, reaches a high temperature, and is strong enough to support the burden of ore and fuel. The fuel has been supplied by Yorkshire Charcoal, made from locally managed hardwoods such as oak and beech. The pieces of charcoal have been cut to about 2 cm long to facilitate the movement of the fuel down the furnace and circulation of hot gasses within the furnace.

		l: Chopping wood for kindling r: Breaking charcoal
		Charging charcoal into the furnace

 

Charcoal To fuel the furnace, 3 tonnes of wood were required to produce 300 kg of charcoal. Typically beech, oak and alder were used in the medieval period as they produced the hottest temperatures when burned.

On-site charcoal-making demonstration

 

Preparing the Iron Ore Iron ore is a rock that is very rich in iron, although it also contains some other metals or elements such as silicon and aluminium. The Jurassic strata of the North Yorkshire Moors contain six beds of ironstone, which have been worked along where they outcrop at the surface as well as being locally mined. Bog iron ore (re-deposited iron minerals) may also have been worked and smelted, although no positive evidence of this ore being exploited has been found in this area. The ore being used for the experiment is a low-phosphorus Australian bog ore (red Goethite). The ore has been broken down to small pieces. Due to the high purity of the ore and its natural porosity, which helps the smelting process, it was not necessary to roast this ore in advance. Ten kilograms of Australian ore was smelted, along with a further three kilograms of high-phosphorus Dutch bog ore, also porous Goethite and un-roasted. The smallest particles (ore fines) that remained after crushing the ore were reconstituted into round pellets, so that they could be added into the furnace without falling straight through.

		l: Sorting the ore r: Knocking ore down to size with  hammer on tree-trunk anvil
		l: Charging the ore into the furnace r: Pellets made from ore-fines

 

The Smelting Process There are two main aims in metal smelting: Reduction of the metal oxide (the ore) to metal, and Formation and separation of the gangue (non-metal) oxides, as slag. The principle of iron smelting appears straightforward. The aim is to create a reducing atmosphere of carbon monoxide in the furnace, by the reaction of oxygen in the air with the carbon in the fuel. The furnace is filled with lighted fuel; bellows blow air into the base, then more fuel and ore are put into the furnace, throughout the operation. The flame at the top of the furnace will burn blue, indicative of carbon monoxide. The carbon monoxide penetrates the ore particles and reacts with the iron oxide to form carbon dioxide, reducing the iron oxide sequentially to metal. In a bloomery furnace some of the iron oxide reacts with the other oxides present (e.g. silica and alumina) to form slag, the waste product of iron smelting. This slag may be run out of the furnace or left to cool in the base of the furnace. It is the most important indicator of smelting activity. For these chemical reactions to occur the temperature in the furnace needs to be higher than 800oC. The temperature is increased by blowing more air (oxygen) into the furnace, although this can stop the formation of the reducing gas carbon monoxide. The thickness of the walls of the furnace helps to stop heat loss, and using charcoal as the fuel helps to keep the temperature high. Another problem with iron smelting is that pure iron melts at 1500oC. This temperature cannot be reached with these early bloomery furnaces while maintaining the necessary reducing conditions. It is believed that in early bloomeries the metal particles coalesced to form a pasty lump called the ‘bloom', which included much trapped slag. The smelting process, from first lighting the furnace, to removing the bloom at the end, will take about 8 hours.

		l: The tapping arch sealed with turf   r: Wires up the front and at the back of the furnace are thermocouples to measure the temperature inside
		l: Moving red-hot charcoal from  bottom to top of the furnace, to speed up heating   r: Adding more charcoal
		l: Bellows-blowing. A thermocouple is suspended down the centre of the furnace, attached to stepladder behind r: Taking a temperature reading
		More bellowing
		View through the tapping arch: yellow-hot slag can be seen forming inside. Thin metal rod is a thermocouple.

 

The Products The Bloomery Process produces a malleable iron (often mistakenly described as Wrought Iron), which was used from the Iron Age until the medieval period. Blast furnace technology that produces cast iron was introduced into Britain probably earlier than the accepted date of 1496. Documents indicate a blast furnace was built at Rievaulx in about 1570. However the transition from bloomery to blast furnace is complex and not fully understood. Cast iron (carbon content about 4%) will melt at around 1200oC, but cast iron is brittle and cannot be used for tools such as nails or knives.

		Opening the tapping arch
		Removing the iron bloom
		l: The hot bloom   r: Cooling it in a bucket of water
		l: View from the top of the furnace as it cooled r:  Iron bloom

Iron Iron is an abundant element in the universe, it is found in many stars including the sun. It is the most abundant element in the earth's crust, although the majority of this is in the form of iron ores and minerals, such as Hematite and Magnetite. The smelting or extracting of iron from ore has a long history, and one of the earliest known iron objects is a dagger shaped by hammering from Egypt dated to 1350 BC. Iron is one of the most useful metals ever discovered and smelted. Iron ornaments and ceremonial weapons were being produced in the Middle East by about 1100 BC. Celtic migrations in the 5th century BC (or earlier) spread the use of iron across western Europe to the British Isles. Although iron has a long history of use it comes in several forms and the complicated methods used to produce each of these forms causes further confusion. Pure iron has less than 0.1% carbon in it and is only moderately hard. When it is heated to red hot at about 700oC it can be easily bent and formed into various shapes. This is wrought iron (wrought from weak, to bend or twist). Unfortunately it is only moderately tough; it can be easily bent when being used. It also loses any sharp edge very quickly under the pressure of work. The melting point of pure iron is around 1530oC.  Cast iron contains between 3-5% carbon, and is very hard, but brittle. It takes its name from the fact that when it emerges from the smelter in liquid form it can be cast into moulds. Unfortunately, it's brittleness means it cannot be forged, bent or shaped in any way once it has solidified. Steel is iron with a small amount of carbon dissolved inside its structure (c. 1%C). This combines the best properties of both pure iron and cast iron. It is shaped when red-hot and it holds an excellent edge when it has been sharpened. Steel has been in utilised since the Iron Age, and a number of different methods have been used to make the steel. The problem is that steel was not easy to make, and was therefore expensive.

 

Coming Soon ... Results of Iron and Slag analysis!

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Copyright © 25 June 2002 Evelyne Godfrey, Department of Archaeological Sciences, University of Bradford, ArchSci-WWW@Bradford.ac.uk