Recreating a medieval blast furnace

Furnace Cottage
Rob Chapman, of Furnace Cottage, Oakamoor, Staffordshire, took part in the Time Team Big Dig in the summer of 2003. The back garden of his cottage was one of the sites specially featured in the live Big Dig programmes because it seemed very likely that remains might be discovered of the Elizabethan blast furnace known to have stood here.

Though the test pit excavation for the Big Dig didn't reveal the location of the blast furnace, it did uncover an iron 'bloom'. This would have been produced by an even earlier method of iron smelting practised for centuries before the more sophisticated Elizabethan blast furnaces were developed. Time Team's excavation in Rob's garden revealed that, before the Elizabethan furnace was built, an earlier medieval furnace – or bloomery – stood on the site.

In an attempt to recreate medieval iron working Time Team called in iron smelting expert Tim Young and blacksmiths Aaron and Toby Peterson.

The blooming process
The blooming process begins with the construction of a furnace. 'We have limited time with Time Team to build an exact replica of a medieval bloomery and prepare it properly,' said Tim Young as he set to his task. 'What we've done is to construct the furnace with fire bricks and then line it with clay.' An original medieval furnace would have been constructed completely out of clay.

The three-day time limit for a Time Team programme also meant that there was not time for the clay to dry out naturally. So, once it had been built, the bloomery was fired to dry it more quickly. While this was being done, constant attention was required to rub wet clay into any cracks that appeared to ensure the furnace was totally airtight.

Bouncy-castle bellows
After firing with wood for the drying process, on day three of the dig charcoal was loaded into the top of the furnace to prepare for the smelting. After constant blowing with bellows (and a bouncy-castle air machine – a 21st-century 'cheat'), the temperature inside the furnace began to rise. 'We should be hitting about 1,400 degrees Celsius when this is running properly,' said Tim.

With the air blasts bringing oxygen to the heart of the furnace, the charcoal was soon glowing white hot. The furnace was then loaded to the top with the first charge of equal amounts of charcoal and iron ore. Additional charges were added as the level dropped. 'All the time the ore is running to the bottom and the iron bloom is settling with slag on the top,' said Tim.

Fingers crossed
Towards the end of the day, after around six hours of firing, the furnace was shut down. Fingers were crossed that it had worked and everybody waited to see if an iron bloom could be lifted from the base of the furnace.

'We're going to have to use an iron bar to break the surface slag to see if our firing has worked,' said Tim. 'We're going to try to lift it from the top using long tongs. If that doesn't work we'll go in from underneath.'

After some apprehension about whether the reconstruction had worked, the chunks of slag were steadily lifted away and a cheer went up as a crusty lump of iron bloom finally emerged from the heat-hazed furnace.

'Well, it worked,' said Tim. 'But I think we could have done a better job given more time. The brick lining of the kiln didn't allow for enough silica to be incorporated into the firing, which you would get with a furnace constructed completely out of clay. I think we've still got a good few kilos of iron though,' he said with a smile.

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