One of the mysterious green rocks – slag from an iron bloomery.
Isn’t it amazing what you can find just lying around? A friend told me about a field near my village where he had found strange looking green stones. I searched where he told me and found a rich scatter of them – some glassy and vitreous, some cinder-like or slag-like.The stones seemed to be the residue of some smelting process – but what metal was smelted, who smelted it – and where did they get metallic ore in the South Downs?The reason for my surprise is that the Downs consist of chalk, capped by clay-with-flints – not geological formations that I had ever associated with metallic ores. But here I was mistaken. There are numerous brick works along the line of the South Downs, built to take advantage of the outcropping clay of the Reading Beds.Among these brick clay beds, I now learned, are found nodules rich in iron – as much as 35% or 40%. The nodules supplied a whole iron-making industry in the Wealden area for centuries, from the beginning of the Iron Age, through the Roman occupation until late medieval times.
The manufacturing process was relatively simple. For two thousand years, people have been building a small kiln-like structure from clay (or digging a smallish hole in the ground and lining it with clay) to create a simple smelter known as a ‘bloomery’.
A ‘bloom’ of iron – note fragments of flint and slag embedded in its surface.
A fire is started in the bloomery and charcoal and crushed iron ore are added through the top – just like a modern blast furnace. The process is continued for six or seven hours, adding charcoal and iron ore, removing the slag from the bottom and topping up the furnace with it. The end result is a roughly circular lump of iron at the bottom of the furnace called a ‘bloom’, weighing several kilogrammes.
The bloom is later processed in one of a number of ways to purify it further, either by hammering or by heat treatment, and is ultimately made into tools, harness, weapons and a dozen other household items.
I went back to the site of my earlier finds and looked more closely. I found more green glassy rocks and more slag. But this time I found something else – a rusty lump weighing almost one kilogramme. It took me some time to be sure –checking weight, density and filing off a corner to expose bright metal – but my rusty and eroded lump was an iron ‘bloom’. Not only was it a solid mass of metal, some of the smelting impurities were visible in the form of small fragments of flint and slag buried in the surface.
Unfortunately there is no way of dating the iron other than its archaeological context and this is ambiguous. However, in my village there is evidence of at least five Roman buildings including one in the field where I found the bloom. So until I unearth further evidence, my money is on it being Roman.
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Fascinating! Thank you so much for sharing.
I think the books you need are Ernest Straker’s Wealden Iron and Cleere’s Inron Industry of the Weald for good studies of the Wealden iron industries – not just small bloomery furnaces, there were large blast furnaces during the medieval and early post medieval periods (making, amongst others, cast iron firebacks and cannon and cannon balls for the Civil Wars.). Most of the industry was water powered. Not sure if the local ore nodules from the chalk smelt terribly well though, they are an iron/sulphur mineral and sulphur is not a good thing to have in iron smelting. I’d need to check but its possible a lot of the ore was imported – took several tons of charcoal to smelt one ton of ore, so it was worth taking the ore to the charcoal (the oak woods of the weald) rather than the charcoal to the ore.
You say that the iron bloom can’t be dated: in fact it can, although dating the slag is easier. Even bloomery iron contains small amounts of charcoal and if you’re willing to sacrifice some of your sample you could get an AMS carbon-date for the smelt. However it is lkely there is more charcoal in the slag and for the ball-park date you’re looking for half a kilo or so of slag would be plenty to get a date. You should contact a radiocarbon lab and get more up to date info, as I’ve been out of this sort of archaeology for 20 years: Oxford University was once involved with work in this line when they first set up their accelerator back in 1981, and they still have an active research programme into C-14 work. Try them for starters.
I have found some like this in my garden in Crawley West Sussex (Green shiny stone and large and small pieces of slag. I am a keen gardener and have lived here for nearly 17 years. Its only a small patch, but flints, chalk, charcoal and Iron slag have all come out with some other oddities. Last November I decided to look up any information about local soils and rocks and I came across your site, which has been most helpful. I now have a rather large selection which require further investigation when I have time.