Ancient technology: Saxon glass-working experiment

behind me what do you see do you see an oven well now we're not cooking on it is not really an oven do you see a kiln no we're not hardening clay in it either though the inside of that is is baked two ceramic hardness now 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 with 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 at 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 roof there the correct raw materials and then exported in bulk for people to do whatever they wanted to do with it somewhere else and it seems that in Roman times the 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 nine nile where the the water has evaporated way leaving behind crystals some of which are this very good night Ron night Ron Natron I was so close it's a new word to me I've learned three new words today Natron cullet and oil that will come to those other two in a minute now they reworking 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 shale rim 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 analyzed it and found it was in fact glass and at that time it was the biggest piece of glass in the world it's exact date is disputed mainstream thought sees 4th century AD but there is an idea that this is early Islamic rather than late Roman anyway it weighs a mind-blowing nine tons the ancient Egyptians had furnaces that made about five pounds of glass at a time nine tons enough to make sixty thousand 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 five to ten days using an estimated 20 tons of firewood by cracky so in the ancient world they really were able to create these huge resources and then it was cheaper to import to export them around the the Empire and so that's what the the late Romans in in 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 analyzing glass based on an assumption which is that the later it is then more times it's been recycled and so you 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 it's heated up in a correct authentic furnace like this as its remelted 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 a hundred percent 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 to work with these have been hilted and heated in a modern electric emission list kiln which doesn't introduce any new can terminations 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 analyze 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 you can't 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 like thick toffee and even when you 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 house which 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 mean time but boy does this thing put out soot here you see glass of differing colors 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 screen 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 color 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've 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 they it 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 Moyles which are sometimes so contaminated it's just not worth using them again to make more glass things so these gets thrown away and sometimes archaeologists find big piles of Moyles 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 some solidified glass and that has formed a pool at the bottom and there are also little legs of it little little dribbles running down from the the rim here but on the besides of this crucible there is a there's a sheen forming is this a glaze at this class well not really this is you might call it an ash glaze has formed in the kiln because the the potash becomes like a vapor it's in the air in the in the kiln in the exposition called 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 make dirt your crucible look pretty it's also getting into the glass we think so as the glass gets recycled and the this soda in it the sodium 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 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 these 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 and 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 thelike 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 scroll in the side clay of the of the furnace here which is about eight inches thick ABCD 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 1070 degrees also 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 Mines breaking down I was two hours late and missed him here are some of his tools snips there a bench a marva 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 older World II technology and we're putting lovely or fenty 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 this smoke is a very fine foot and yeah it smells like coal here Valerie opens the warming hole through which the iron sticks the punty rods or Pontes 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 dorbz which must be a pretty amazing insulator because there were bits of straw visible on the outside of the furnace that weren't even see lead 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 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 craftsmen 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 glassblowing 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 due in part PhD is experimental archaeology and I'm actually a member of experimental archaeology research group at Newcastle University which we called Exxon and we've kind of had a number of different projects we've had running so including my glass furnace we have a glass speed working furnace copper smelting I've had someone who looks at Paleolithic axe stone axes alright 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 pigs goals with some axe Oh brilliant so you get a virgin bit of a stone and that is an axe has never been used and you use it for a particular thing chopping young tree or whatever then you look at the wear patterns on it and compare that with actually looking at can you tell the difference between one that's been used for a tree or one that's been used for a pig right understand the way in which these artifacts were used yes so even we've kind of had a fairly wide range of projects although mostly focusing on fire because right I notice it says mainly your thing is it's heating stuff oh yeah and melting stuff yes I think I think experimental archeology probably tends to retract pyromaniacs in general okay but there is a downside to this level of pyromania and your case because what are you going to have to do all night yes so I'm gonna 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 then try to reheat it at the moment it's about eight nine hundred degrees centigrade something like that and you're putting in a log every what 20 minutes or something then when we switch to the period where we'll actually be trying to get it out to glass working temperature we'll have to switch every seven 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 and that would take something like to go from cold to to using temperature would be something like five 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 his Sun or a slave or something to to keep it stoked with a small amount of wood just to keep that temperature up in my case because I only have as much wood as my stuff down in fact we had a batch of dodgy woods that came from a different company the last time we ran it was pretty much a disaster it almost even with that fire in a thousand degrees right Paulie seasoned wood who is almost putting that fire out it was really struggling to cope with it so because it's too damp yeah you need a properly seasoned wood to run this right so even they did allow you to hack stuff down that would be no use to 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 they'd have to buy local seasoned wood so it'd be doubly precious to them all right well there you go you see you learn these things when you talk to experimental archaeologists science [Music]

Channel: Lindybeige

Views: 275,545

Keywords: anglo, saxon, anglo-saxon, archaeology, archeology, experimental, science, test, glass, working, blowing, craft, ancient, medieval, technology, roman, furnace, firewood, kiln, oven, burn, heat, visby, glaze, crucible, natron, marver, moil, moyle, cullet, jarrow hall, experiment

Id: Sr17NtDxm-c

Length: 17min 11sec (1031 seconds)

Published: Fri Aug 09 2019

Reddit Comments

[u/Grinny_Smile]

Love Lindy. Long live the Lord of the Beige!

[u/Papa_pierogi]

Finally, I couldn’t stop thinking about how the saxons made glass from the 7th century. I could hardly sleep. But now my question is put to rest at last!