Behind me what do you see? Do you see an oven? Well, no, we're not cooking on it, it's not really an oven.
Do you see a kiln? No, we're not hardening clay in it either.
Though the inside of that is baked to ceramic hardness.
No, that is a furnace. I call it a furnace because it's being used to work glass.
And experimental archaeology is being carried out here at Jarrow Hall, in Tyne and Wear.
Now there is a problem in archaeology. And that is what on earth were the Anglo-Saxons
using in England between the 7th and the 9th century to make glass out of?
Yes, I know, I imagine you've probably wondered about that too.
Well, it seems that there was a glass shortage in this period.
Now there was a time earlier when the Romans were running things,
and the Empire was big, and there was an economy of scale.
You could produce things like glass in vast quantities somewhere where it was easy to do so,
because you had easy access to the ... correct raw materials,
and then export it in bulk for people to do whatever they wanted to do with it somewhere else.
And it seems that in Roman times the key ingredient was from Egypt,
and this was called, what was it called? Novo? Natron, thank you, it's called 'natron'.
And natron is a naturally occurring form of sodium carbonate.
And you find this in salt water delta areas near the mouth of the Nile where
the water has evaporated away leaving behind crystals, some of which are this very good nitron. Nitron? Natron!
I was so close, it's a new word to me.
I've learned three new words today: natron, cullet and moil. And we'll come to those other two in a minute.
Now, they really were producing this in staggeringly large quantities.
And there is no way to illustrate this better than to introduce you to
the staggering spectacle that is the Glass Slab of Beth She'arim, in Galilee in what is today Israel.
When this was found in 1956 it was assumed to be concrete because, well, that's what it looks like.
But in 1963 they analysed it and found that it was in fact glass.
And at that time it was the biggest piece of glass in the world.
Its exact date is disputed, mainstream thought says 4th century AD,
but there is an idea that this is early Islamic rather than late Roman.
Anyway, it weighs a mind-blowing 9 tons.
The ancient Egyptians had furnaces that made about 5 pounds of glass at a time.
Nine tons! Enough to make 60,000 bottles.
It was heated in a massive furnace with fire alongside it,
which created a volume of hot gas trapped above it,
and held it at molten temperature for about 5 to 10 days.
Using an estimated 20 tons of firewood.
By crikey.
So in the ancient world they really were able to create these huge resources,
and then it was cheaper to ... export them around the Empire.
And so that's what the late Romans in Britain were using to make glass.
But unfortunately the Roman Empire rather famously collapsed.
Not exactly in 410, but after 410 things generally went a bit pear-shaped.
And by the time the Byzantines are busy fighting the Arabs off,
the supply of new raw glass coming into England has pretty much dwindled to nothing.
So it seems that between the 6th and 9th century
the Anglo-Saxons were having to recycle glass over and over and over again.
In fact, archaeologists have been dating and analysing glass based on an assumption
which is that, "Ah, the later it is then the more times it's been recycled
and so ... maybe we can tell something useful about this."
But can we actually? Is this based on good scientific data?
What happens to glass of the period when it's heated up in a correct, authentic furnace like this?
As it's re-melted and recycled over and over again, how does its composition actually change?
So the thing to do is: science!
Do some experimental archaeology, make an authentic kiln.
This is not necessarily 100% authentic, we don't have very good evidence for this sort of thing.
It's just clay and sticks and so forth, so these sort of things tend not to survive,
and tend not to be written about in any great detail.
But, as far as we can tell, this is what the furnaces would have looked like.
And you stick a load of glass in, made very precisely to the chemical composition
of the raw glasses that they would have had to work with.
These have been ... heated in a modern electric emission-less kiln
which doesn't introduce any new contaminations.
And then put it in there, heat it up, keep it at the heat for quite some while,
and then have a glass blower come in, work with it, rate it, say how workable he thinks it is.
Keep samples of it, analyse those for their chemical composition.
Repeat: recycle and recycle. And this goes over and over again.
And it takes a very long time, because you have to keep the glass hot for something like 12 hours. ...
With a piece of steel you bring it up to working temperature then you start bashing it with a hammer, that's fine.
But with glass it's not like that.
Because you get a load of broken, smashed up glass, and it's got air gaps in between it.
And those air gaps, of course, when the glass becomes molten become bubbles.
And the bubbles take ages to rise to the top because it's so very viscous.
At first it's like thick toffee, and even when you've got it pretty hot it's still pretty viscous.
So you have to keep it very hot for about 12 hours before you can work it.
So that's the case today, that was the case back then.
So glass workers presumably had to work round the clock,
and they would've had family and friends stoking their furnaces throughout the night
if they wanted to work in daylight hours, which I imagine most of time they would.
And so it was a laborious process, and so that's what's happening here.
And so they are throwing bits of wood in every, well,
when they're trying to get it up to full firing temperature, about once every seven minutes.
You have to refuel this thing for hour upon joyous hour, drinking tea and eating cake in the meantime.
But boy, does this thing put out soot!
Here you see glass of differing colours.
The chemical makeups of the ingredients heated were all the same,
but different bits of glass have been oxidized in the furnace by differing amounts.
If the manganese in the glass gets oxidized a lot it goes pinky-purple,
and a little bit less and the small amounts of pink filter out the yellow, making it blue.
With less filtered yellow it looks green.
Note here a piece that straddled the borderline in the furnace between the green and the blue areas.
And here we have some of the products of this furnace, which you'll notice are all a pretty consistent colour.
Now these are called 'canes', it's just a rod that's been pulled out of the molten glass.
These are the ends of the canes,
which have been deliberately pinched into this shape so it's easy to break them up again.
And these were a palm cup, which is a type of Anglo-Saxon glass vessel from the period.
But they'd not been annealed. By annealed I mean allowed to cool really, really slowly,
actually in a heated environment.
And because they weren't annealed, as they cooled they snapped because
... it shrank too quickly for it to cope with the changes of stress.
And another thing we have in here is a 'moil', this is a 'moil'.
And you may be able to see a couple of tiny little dark marks in it.
These are contaminations that have been introduced by contact with some of the steel tools,
the ends of the rods and the blow pipes that are used in glass working.
And the glass is so hot that when it touches a piece of steel it can create 'scale',
iron oxides forming on the outside of that steel which then contaminate the glass.
And you end up with lots of moils, which are sometimes so contaminated
it's just not worth using them again to make more glass things, so these get thrown away.
And sometimes archaeologists find big piles of moils.
Right, well what do I have here? I have a crucible. "Why is it broken, did you drop it Lloyd?"
No I didn't. This was broken deliberately because it was too big to get into the furnace.
Now at the bottom of this crucible you might be able to see there's some solidified glass.
And that has formed a pool at the bottom.
And there are also little legs of it, little dribbles, running down from the rim here.
But on the upper sides of this crucible there is a sheen forming,
Is this a glaze? Is this glass? Well, not really, this is you might call it an ash glaze,
that has formed in the kiln because the potash becomes like a vapour.
It's in the air in the kiln, ... I suppose I shouldn't call it air, in the gases in the kiln.
And these condense on the outside of the crucible and form this sheen.
And this is not just something that makes your crucible look pretty,
it's also getting into the glass, we think.
So as the glass gets recycled, and the soda in it,
the sodium ... carbonate in it, is being driven out by the heat
the workable quality of the glass goes down, because that's the flux if you like.
But if it's being replaced by potash from the kiln,
then actually you end up with a glass that remains workable for quite some while.
And the glass blowers who have been coming here and testing the products of this furnace have been quite impressed.
They've said that with modern glass you might get only one or two recycles out of it.
... This stuff you can recycle it perhaps five times, and it's still pretty decent.
So it seems that there are some advantages of primitive, ... or at least more ancient, technologies.
You can see inside, the flames in a sort of vortex of movement.
Right now it's way below molten glass temperature, but even so you can see that everything in there,
the crucible, its contents, the inside walls of the furnace, is a bright orange.
To give you an idea of how well this can burn wood,
we just added one small bit of wood to the back of the furnace and left the top off.
And this is what we had a few seconds later.
The top of a furnace like this, or more usually a much smaller one,
could be used like a Bunsen burner for heating up the ends of canes of glass,
which could then be turned into beads or the like.
It doesn't take many seconds to get it soft enough to bend.
Here we have four wires, each of which has a thermocouple temperature measuring device on the end,
and these feed into this gadget which gives readings of the various temperatures.
And you can see perhaps scrawled in the side clay of the furnace here (which is about 8 inches thick) A B C D.
And the crucial ones are B and C because
it's around this sort of height inside the furnace that the glass will be worked.
So this is the bit that you have to get up to about 1,070 degrees or so
for the glass to be at the ideal working temperature.
Now the idea was that I would show you a glass worker working the glass.
However, thanks to the train in front of mine breaking down, I was two hours late and missed him.
Here are some of his tools: snips there, a bench, a 'marver' (a flat platform for rolling soft glass upon).
It is thought that medieval glass workers may have had smaller things, just a board strapped to the leg.
Now, you may think that because we're using olde worlde technology and we're putting
lovely authenti-woods in like ash and alder and birch and so forth,
that there'd be a lovely, delicious wood smoke coming off this.
But I regret to inform you that the smoke smells more like coal smoke.
I think it's because it's so hot in there that this wood is being very, very thoroughly burned.
... The smoke is a very fine soot and, yeah, it smells like coal.
Here [Victoria] opens the warming hole, through which the iron sticks,
the 'punty' rods or 'punties' as they are known, are poked to be heated up ready for use.
The main door for access to the crucible is just made of daub, which must be a pretty amazing insulator
because there were bits of straw visible on the outside of the furnace that weren't even singed.
But I couldn't shortchange you and leave you with no pictures of glass working in progress,
so here's some footage I shot a couple of years ago in the pretty town of Visby during its medieval festival.
Yes, the oven is clearly modern, but the glass blower has made an effort and is wearing a period tunic.
And the basic techniques have not changed much since ancient times.
You can see how the bench is used with the craftsman sitting between the two rails.
There's not actually much huffing and puffing with the blowing bit.
In the main one blows a bit of comparatively cool air into the tube, and then sticks a thumb over the end.
And the trapped air then heats up and expands to do the work.
The hot glass sticks to hot things and not to cold things, which is pretty convenient
for getting a 'gather' of glass on the end of a heated blowpipe,
attaching bits of glass to each other, and for shaping them with cool steel tools.
Many sources will tell you that the Romans invented either glass making or glass blowing. This is false.
But the Romans did colossally increase the amount of glass vessels being made,
and made glass cheap and plentiful enough, and transparent enough,
to make glass windows on high status buildings practicable from about the end of the 1st century BC.
I'm here with Victoria Lucas, who has designed this experiment.
So you're into experimental archaeology in general, is that your thing?
- Yes, so a very large part of what I do, and part of my PhD, is experimental archaeology.
And I'm actually a member of the experimental archaeology
research group at Newcastle University, which we call EXARN.
And we've kind of had a number of different projects we've had running,
so including my glass furnace, we've had a glass bead working furnace, copper smelting.
We have someone who looks at Paleolithic ... stone axes.
- Ooh right. - So she's done all sorts of things with chopping
various bits of wood, or digging ground with them.
In fact she even hit a couple of pig skulls with some axes.
- Oh, brilliant. So you get a virgin bit of stone, so it's an axe that's never been used,
and you use it for a particular thing, chopping down a tree or whatever,
and then you look at the wear patterns on it and compare that with actual excavated examples.
- Yes with archaeological examples, and looking at them you can tell the difference between
one that's been used for a tree, or one that's been used for a pig. - Right.
- Trying to understand the way in which these artifacts were used.
Yeah, so we've kind of had a fairly wide range of projects, although mostly focusing on fire because fire is good.
- I notice that mainly your thing is heating stuff up and melting stuff.
- Yes, I mean I think experimental archaeology probably tends to attract pyromaniacs in general. ...
- Okay, but there is a downside to this level of pyromania in your case,
because what are you going to have to do all night?
- Yes. So I'm going to have to be here all night, in fact all night for the next five nights, keeping the furnace going.
It actually is a lot more fuel-efficient to keep the furnace hot
rather than to let it cool down and then try to reheat it.
- At the moment it's about 800, 900 degrees centigrade, something like that? - Yeah.
- And you're putting in a log every, what, 20 minutes or something like that?
- Yeah, we feed 20 minutes at the moment. Then when we switch to the period where
we'll actually be trying to get it up to glass working temperature,
we'll have to switch to every 7 minutes of charging. - Right.
- And if you're trying to actually bring it up to temperature from cold,
you're really looking at maybe even a slightly higher frequency than that.
So it's really, really hungry on the fuel consumption. - And that would take something like, to go
from cold to using temperature would be something like five hours.
- Yes, 3 to 5 hours. - So we can imagine that that would be the case back in the day then.
In the ancient and medieval worlds a glass worker,
even if he wasn't going to work all through the night,
might get his son or a slave or something, to keep it stoked with a small amount of wood
just to keep that temperature up. - I mean it's particularly important in my case
because I only have as much wood as my budget, because the university ...
- So they don't allow you just to chop stuff down? - No, I mean, ... - I mean you could burn that fence.
- ... it's got to be seasoned, it's got to have been dried out sufficiently.
In fact we had a batch of dodgy wood that came from a different company the last time we ran it.
And that was pretty much a disaster. It almost, even with that fire at 1,000 degrees,
poorly seasoned wood was almost putting that fire out.
It was really struggling to cope with it. - Because it was too damp?
- Yeah, so it really demonstrates that you need properly seasoned wood to run this.
- Right. So even they did allow you to hack stuff down that would be no use to you? - No.
- Right, and you were telling me that a lot of glass workers were itinerant.
- Yes. - So they would turn up somewhere and they wouldn't have brought wood with them.
- Presumably not. - So they'd have to buy local seasoned wood, so it'd be doubly precious to them.
- Yes, exactly yes. - Alright, well there you go, you see.
You learn these things when you talk to experimental archaeologists. Science!
[ What would the ancients have thought of the idea of glass being used for doors? ]
Lindybeige!
[ "What? Don't be daft! The two things we want from doors are strength and opacity. No one will ever want a glass door!" ]
[ "You're more likely to get customers for a glass blindfold!" ]