Late Iron Age iron smelting works found near Stockbury

By Tim Allen

During archaeological monitoring of a now completed new water main installation by South East Water near Stockbury, northeast of Maidstone, archaeologists from Kent Archaeological Projects discovered the remains of two ancient iron smelting works on the Clay-with-Flint surface of the chalk uplands. One of the works consisted of two separate pits, each containing two clay-built, funnel-like iron smelting furnaces. The other, much larger, better preserved and multiphase, consisted of five intercutting pits that together created a substantial sunken-floored structure containing at least ten in-situ and intact furnaces of a previously unknown type and two iron-ore roasting pits. Potsherds found within the remains dated the works to the Mid-to-Late Iron Age (c. 150 to c. 50 BC).

Following discussions with South East Water, the County Heritage Conservation Group and English Heritage, it was decided that the remains were of national importance and that sufficient excavation and sampling had taken place to characterise them in detail. It was therefore decided to cease excavation and preserve the unexcavated remains in situ.

Along with flint- and grog-tempered Late Iron Age pottery, the site produced nearly half a tonne of iron slag and smaller amounts of 'bloom' (the spongy conglomeration of iron particles produced from the ore at about 1200°C within the furnace during firing). The bloom would later be re-heated until red hot and hammered into the required shape by a blacksmith. The relative inefficiency of the method meant that the large amount of iron still present within the slag went to waste.

The furnaces were clay-walled structures of approximately 0.32m in height and 0.2m diameter at the top, narrowing down to half or less of that at the base, and looking like upside-down chimney pots. The walls were up to 40mm thick and were scorched deep red on their inside surface. Each furnace had a hole knocked out at the base, this being where the bloom was removed after the firing (until their final abandonment the access holes were re-sealed with clay prior to the next firing). The shallow roasting pits were used to heat the ore before it was packed into the furnace, along with much charcoal, which was then ignited. It is thought that air had to be blown with great force into the furnace to reach the required temperature, but no air holes were observed in the Stockbury examples, probably because they had been situated at the front base of each furnace, which had been broken out to remove the bloom. In fact, removal from anywhere else would have been impossible because the top-heavy furnaces were not free standing, as are other known examples, but were set into and supported by densely packed clay, with only the front part accessible from within the pit.

Two kinds of furnace were identified in the works, the 'slag-pit' type and the 'tapping' type, the former being the earliest known British type, the latter being a Late Iron Age innovation that remained in use into the post-medieval period. In the 'slag-pit' type the molten slag accumulated in the furnace base; in the 'tapping' type the molten slag was conducted into a basin-like hollow cut adjacent to the furnace base. In one example on the site the solidified slag, still with a liquid appearance, lay within the hollow as if frozen in time for over two thousand years.

The question of what kind of ore was used was quickly identified as of critical interpretive importance. Were these works part of a satellite industry associated with the well-documented Wealden Industry or were they part of a separate industry using a different source of ore? In the former case, a supply system and route serving the Stockbury area with Wealden clay ironstone could be postulated; in the latter case another source for iron-rich ore had to be identified. The answer was found when Sarah Paynter, the English Heritage expert on this subject, discovered partly fired pieces of ore amongst the piles of waste slag iron. The ore consisted of flint nodules surrounded by metallic iron-rich accretions. Such nodules occur commonly within the Clay-with-Flint surface geology of the area and enough evidence was therefore available to identify the Stockbury Industry as a separate and independent industry, and to provide the reason for its location (smaller amounts of iron slag and Iron Age pottery previously exposed nearby in a 'bloom pit' at South Street Road could now be understood in this context).

As an environmentally responsible organisation South East Water was quick to recognise the archaeological importance of this site and, following consultation with English Heritage and Kent County Council, agreed to fund technical analysis of the recovered materials prior to the production of a report for wider publication.

Key:

Highly scorched clay

Scorched clay

Clay support for furnace