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What would a glass furnace look like?
Hi all.
So as the title says, does anybody know what a prehistoric or early Medieval glass furnace would look like?
I have a site in Ireland where we have a small tear shaped building, with massive stone walls, about 2.5m across internally. Most of the floor space is filled with a rectangular stone lined box that is full of bright orange burnt sandy soil with lots and lots of charcoal. This material is baked hard. There is an entrance into the building at the south east that could also be acting as a flue (?).
The reason I ask about the glass furnace is we have found two small glass beads on the site and I was wondering if they could have been made in the building. We have excavated a very similar building several meters to the north but this did not have any sort of fire box in the centre and had a wider entrance located at the north west. They are contemporary structures and obviously relate to some sort of manufacturing/processing activity. I would read it as a multi stage process where 'process A' is occurring in the hut to the north and 'process B' is occurring in the hut to the south. There are probably other buildings nearby so there could well be 'process C, D E' etc
The site may well be late Bronze Age but I suppose it could be Iron Age or even Early Med, charcoal samples are being sent to Queens next week...

So any suggestions? Any references for early glass manufacture? Any tell tale signs of glass manufacture?

[Image: furnace1.jpg]

[Image: furnace2.jpg]
Tricky one.. have to confess... never come across one before .. perhaps others have.

here are guidelines on how to investigate

would say I would be surprised to find BA glass making where you are. But am often wrong :face-huh:
Thanks for that, I'll have a look through that tonight, it looks really useful. We found a few pieces of stuff that looked very like that clinker stuff in the flue/entrance so that may support the idea that it is a glass furnace...

Yeah the date...well I think that is why we were drawing a blank looking for info. There are some mentions of glass beads from Late Bronze Age and Iron Age sites (almost all high status sites ie Rathgall Hillfort and Emain Macha) and even some hints at manufacturing but nothing conclusive. Typically the sort of beads we've found would be identified as Early Medieval and that may well be the case here....on the other hand the site was covered by quite a build up of peat, to a similar depth as some of the proven Bronze Age sites on Slievemore, and is related to pre-bog field walls some of which at least were built in the Middle Bronze Age. In addition we have a small but quite nice flint and chert assemblage from the site which wouldn't be what I would expect from an Early Med site...One solution may be that the terrace was originally created in the Bronze Age but was reoccupied in the Early Med which would be a neat solution but doesn't necessarily fit with the depth of the peat. Then again peat formation and growth can be highly varied when you start looking at a very local scale, so maybe that is misleading us.

Anyway, damn cool site whatever the date. Any help is gratefully received as I'm pretty much peeing in the dark here Big Grin
An interesting looking site that you have there. I've never excavated a glass furnace and have no experience in Irish archaeology, so I'm probably not the best person to comment on this one! I was wondering if you were sure of your interpretation that the structures "obviously relate to some sort of manufacturing/processing activity"? Could it be a small ?'tailed' ring cairn instead of a building/furnace. From the photos the general form of the structure looks 'cairn-like' with a possible central cist in the interior. I take it there is no evidence for burial/funeral rite, but I would not see this as a problem as 'empty' cairns and cists are not unknown. I'm sure I've seen reports of cairns that have evidence of burning as well.

From the photos and description to me it looks more like a small ring cairn with a cist to me, perhaps of Late Neolithic or Early Bronze Age date? If so it would be interesting to see if there is any evidence for pre-cairn activity (post holes/pits, etc underneath the structure as they can sometimes have earlier activity/phases associated with them). I'd be interested in seeing what this one turns into if you have time to post any updates.
Looks like a cool site, although are you sure its a furnace? I have never dug in Ireland or have any experience of glass furnaces but it seems a little strange to me that you do not appear to have burning other than within the stone box. I agree tmsarch that it does look rather like a cist, and if you are loking at manufacturing on any great scale there would be more debris, more waste over a wider area. Does the rest of the base area of the feature show signs of heating? It seems like a very localised fire that maybe items such as beads could have been put into?

Just throwing ideas out there - have no idea really x

But i did find an article on Irish prehistoric glass making on jstore but unfortunatley dont have access, its called "GLass and the manufacture of prehistoric and other early glass beads: technical background and theory which i think would be worth a look or maybe you could contact these guys
Thanks for that, yes i'll keep updating this if there is interest. What I think I'll do is post up more photos so the sequence of deposits inside the building can be seen as it's all rather interesting. It's certainly an unusual site and there's
This is probably unrelated as you haven't mention large quantities of burnt stone but in north Wales there is one type of site where we find huge quantities of crushed white quartz which we cant explain - burnt mounds! And stone lined troughs....
Drink is all I can think of!

That said... sweat lodge as well... stone trough? burning?

seriously though... it is an option... and I have seen these stone troughs recently at a burnt mound site in SCotland. with burning inside as well!
Funnily enough i was talking about that with some Archaeologists from Wisconsin a few weeks back! In Ireland we have huge numbers of burnt stone mounds, horrible soggy bloody things lol. However it's almost exclusively sandstone that's used. the mountain we are working on is quartzite and shist with huge quartz seems and there is no sandstone nearby so we did have a think about that when the guys from the US said they had Burnt Mound equivalents over there using quartz. I didn't know Welsh ones used quartz! That must look amazing. Irish Burnt Mounds tend to have about 80 to 90% stone in a sooty black soil, and that's not the sort of quantities we're talking about here, the quartz isn't occurring in that sort of density. How dense do you find your burnt quartz mounds? Perhaps the quartz could be used as pot boilers and this would lead to the sort of density we are finding, but I'm not sure.

Oh, and I should mention that the only deposits that aren't producing large quantities of the quartz are the burnt fill of the stone box and the immediately overlying layer of burnt material!

Yeah Fulacht Beer....ummm sooty lol
Yeah the sweat lodge argument was where i was thinking with this one - particularly in Ireland where the structural and historical evidence is so much more compelling than with us

Check out this website for some striking similar structures and

and Barfield, L.H. 1991. ‘Hot stones: hot food or hot baths? Burnt mounds and hot stone technology. Sandwell Metropolitan Borough Council.

Barfield, L.H and Hodder, M.A . 1987. ‘Burnt mounds as saunas and the prehistory of bathing? Antiquity 61.

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