Vitrified forts — How and why?

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jimit J
jimit May 13, 2002 18:47:25
Replying to FourWinds

Perhaps someone should ask the Viatnamese if napalm caused similar effects to some of their stone buildings!
I think that the problem lies in concentrating the heat, even in modern pottery kilns it doesn't melt the clay only the glaze which has a lower melting point. I'm sure we have all seen the "reconstructions" on tv where they try to make Bronze/Iron Age tools, the heat required is considerable yet I haven't seen much vitrification of the oven interior.
I once used salvaged bricks from a demolished brickworks oven, they were well baked! but showed little sign of a glassy finish. I think that "vitrified" brick has an extra finish applied to the exterior.

Are there any sites down south showing this? Does it depend on rock type? So many questions-so few answers.

Love it! :-)

B BrigantesNation
BrigantesNation May 13, 2002 19:25:41
Replying to jimit

Thanks for the info T it was really interesting but I'm with Jim on the concentration of heat. All but a very few combustable materials do not burn to a high enough temperature even with compressed air.

However I can say that Wincobank in Sheffield shows an even heat based vitrification over the entire circumference of the inner rampart. I think this is a twin rampart hill fort and interestingly the out rampart shows no sign of vitrification (a later addition perhaps). I have a couple of samples and will try to get a good picture (black rock just looks like black rock without serious lighting).

Here's a description of the evidence at Wincobank:

I was told about the vitrification by my father in law, who said he had picked up a piece of rock almost like glass when he visited the fort 7 years ago.

When I visited the first bit of rampart I looked at had indeed been fired. I've made lots of fires in my time and the top layer of rock reminded me of rock which had been in an intense fire for several days - we used to have the the biggest bonfire around and I was chief fire lighter! I'd call this type of rock example A, I took a chunk of it. As I walked round the inner rampart (this fort looks like it has two ramparts) I could see that this heated rock evenly and totally covered the entire radius of the rampart and formed a layer which was 3-4 ft wide.

The burnt effect was graduated, with those rocks on the outside of this layer apparently reaching a cooler heat than those in the middle of this layer. Possibly showing that the rocks which were originally on the top of the rampart reached a higher temperature.

As I walked along the rampart I could see many areas where the rock had almost melted and had certainly fused with other rock I also got a sample of this kind of rock (Example B).

Looking at the samples, example B seems to be several pieces of rock which have bonded together, they all show surface bubbling with bubble diameters of between 1 and 5 mm. Rather than being reddish like example A, example B is much blacker and very black in places.