The archaeology department got two for the price of one last Friday with talks from Jeremy Hodgkinson and Jonathan Prus, both from the Wealden Iron Research Group (sponsors of one of the department’s PhD studentships).
View of furnace showing sandstone revetment and forging hearth (credit: WIRG)
Jeremy Hodgkinson’s talk was a background into the current understanding of the landscape of the bloomery iron industry of the Weald. To date there are 686 bloomery iron sites in the Weald ranging in period from the Iron Age through to the post medieval. Of these 686 sites only 183 have been dated of which 62% were Roman, showing when the industry was at its peak.
The Weald has been seen as one of the most important iron production sites in Roman Britain and even the empire with the waste heaps of some sites measuring over 10,000m3. Jeremy Hodgkinson created a classification of waste heaps based on their volume (Hodgkinson 1999) with the smallest site, Grade 1, being less than 100m3, Grade 2 sites being between 100-1,000 m3, Grade 3 sites being between 1,000-10,000 m3 and Grade 4 sites being larger than 10,000 m3.
Some issues that have arisen from this Grading system is the fact that it is extremely difficult to measure the volume of the waste heap with many measurements often unknown and in some cases slag has been removed so the volume is not correct.
Jonathan Prus gave a talk on his current ongoing research project into the ratio of Ore:Slag:Bloom and their relationship in the Weald.
In this project PXRF was used to measure the bulk chemical composition of samples and was crosschecked against SEM-EDS. The results showed that ore being used was all very similar in composition with an iron content of around 50-55 %. This showed that there was some way of measuring the ore to find the correct composition as there was ore available with a higher iron content.
This tests also showed that some ore that were low in calcium produced slag which was high in calcium. This shows that at some point in the smelting process calcium is being added. One way it is believed to be added is via the furnace lining breaking down and mixing into the process. Due to the geology of the Weald the furnace lining would have a large content of white sand which would be high in calcium.
Overall it showed that the industry had an efficiency of around 44% meaning just under half the iron in the ore is extracted with the other half being sacrificed to make the slag.
Written by Ethan Greenwood