Rehydroxylation Dating

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I referred to this news story as being poten­tially the archae­olo­gical story of the dec­ade on twit­ter. Potentially is a good weasel word, but if Rehydroxylation Dating can be inde­pend­ently veri­fied then it could be a more import­ant form of dat­ing than radiocar­bon dat­ing. The reason it’s so excit­ing is that this method will allow archae­olo­gists to date pot­tery. A couple of warn­ings before I start. I am not a mater­i­als sci­ent­ist so it’s pos­sible that if some­thing seems odd that’s me mess­ing up the descrip­tion. The other is I am not on the research team — I’ve merely emailed some questions.


Saxon Pottery
Late Saxon Pottery, but how late? Photo (cc) Wessex Archaeology.

Pottery and other ceram­ics make up most of the data that you’ll find on an archae­olo­gical site. Unfortunately there hasn’t been an easy way to dir­ectly date it. The most com­mon way is by style. Pot types and tech­no­logy come into and out of fash­ion. Terra sigil­lata, Samian Ware, is par­tic­u­larly good for this as styles turned over rap­idly. However, that no help if all you have is a frag­ment of cruddy Iron Age pot. Another method would be by asso­ci­ation with organic mater­ial. If you find some grain in the same strata, you can date that and by asso­ci­ation when mater­ial was depos­ited in that strata. There are some prob­lems. Radiocarbon will give you a range of dates rather than one date. This range can be quite wide and it’s prone to con­tam­in­a­tion. What would be use­ful would be a way of dat­ing ceram­ics dir­ectly. You can do this with ther­mo­lu­mines­ence, which uses nat­ural radio­activ­ity to give a date, but it’s com­plex and dif­fi­cult so it’s rarely used. A team mainly based at Manchester University have announced that they can date ceramic mater­i­als, such as pot­tery, tile and brick, through a pro­cess called rehyd­roxyla­tion. It seems to be sim­pler than both ther­mo­lu­mines­ence and radiocar­bon dat­ing and much harder to acci­dent­ally con­tam­in­ate. There are some impress­ive addi­tional uses for the method which could make a lot of excav­ated mater­ial a lot more use­ful.

How does it work?

The idea has developed from a known prob­lem in archi­tec­ture — kin­etic expan­sion. Molecules in clay have sites which react with water, H2O, to take on hydroxyl groups (OH). When you fire clay to make a pot or a brick, you drive out these hydroxyl groups. Once you have your fired ceramic it starts react­ing with water vapour in the atmo­sphere to take on hydroxyl groups again. The longer you leave it, the more OH the ceramic absorbs. Wilson et al looked at this in a paper from 2003, Kinetics of Moisture Expansion in Fired Clay Ceramics: A (Time)¼ Law (DOI: 10.1103/PhysRevLett.90.125503). They found that rehyd­roxyla­tion of ceram­ics fol­lowed a power law, and that it was lin­ear with respect to (time)¼.

powerlaw
An equa­tion y=x¼ which flat­tens as you move right.
Plot by WolframAlpha.

In every­day terms it means that equal amounts of mass are taken up on a ratio of 1, 16, 81, 256… So if it takes a day (or a week, or a month) for a ceramic to increase by 1 gramme of mass then it will have increased by 2 grammes from its start weight after 16 days (weeks, months etc), 3 grammes after 81 days and so on. This gives you a graph a bit like the one to your right, which is a pig to read use­fully, which is why Wilson et al plot against (time)¼ because that gives you a nice straight line.

After in their paper on kin­etic expan­sion the authors men­tioned the pos­sib­il­ity of archae­olo­gical dat­ing. Now they have a technique.

You take a sample around 3g to 5g in mass and heat it to 105ºC. You leave it here for a while to make sure the sample is dry and you’re not weigh­ing any excess water. When you’re sat­is­fied it’s dry, you weigh it to get the aged weight. Then you stick it in an oven at around 500ºC. This drives out the hydroxyl groups. You leave it to bake for four hours. When it’s done you stick it on a microbal­ance in an envir­on­ment with con­stant tem­per­at­ure and humid­ity. The microbal­ance is highly accur­ate, meas­ur­ing the weight down to +/- 0.1 μg. That’s 0.0000001 grammes (or around 0.000000004 ounces if you prefer). After two to four days you have stable weight gain and from the meas­ure­ments you can extra­pol­ate how long it would take for the sample gain its lost weight back.

That might sound simple, but then a lot of the really clever ideas are.

But what about errors?

One of the prob­lems with the write up and press release is that dates were quoted without errors. This makes sci­ent­ists won­der because errors are part of life when it comes to record­ing data. If you don’t know how big your errors are then it’s hard to tell how accur­ate you are. The paper has errors on the dates but these are quoted to one stand­ard devi­ation. The example they give is a brick from the King Charles II build­ing. This was a built between 1664–1669 and Wren later remod­elled it in the 1690s. The date they got was 1691, so that would seem to con­firm that the brick came from the remod­el­ling phase. However the stand­ard devi­ation is +/- 22 years. That means that there’s around a 66% prob­ab­il­ity that the cor­rect date falls between 1669 and 1713. If you want a 95% cer­tainty, which is what archae­olo­gist should be doing with radiocar­bon dates, then you have to go to two stand­ard devi­ations. There’s a 95% prob­ab­il­ity that the brick dates between 1647 and 1735. That looks less impress­ive, but it’s not all bad news. The error comes simply from the num­ber of samples, five in this case, and the res­ult­ing aver­age. If you need more accur­acy then it should be pos­sible with more samples. Nonetheless this is some­thing to watch for. Any archae­olo­gist who sees a chro­no­logy they’ve built their career on over­turned is quite reas­on­ably going to ask about the errors.

The other prob­lem is that the dat­ing requires you to know the mean (aver­age) tem­per­at­ure for the ceramic since it was fired. Current research sug­gests that this could have quite an effect on dates. I don’t think you’re going to be able to get pots fired yes­ter­day with appar­ent dates of cen­tur­ies. To get that kind of effect the 2003 paper says you’d need to stick your pot­tery in an auto­clave set to 186ºC for a few hours. But over long peri­ods of time get­ting the tem­per­at­ure wrong might well put your tem­per­at­ure out by quite a bit. So far the exact effect is unknown, but they note that regional weather records seem to be pro­du­cing plaus­ible dates.

The tem­per­at­ure prob­lem also works in reverse. Medieval bricks were giv­ing a date of around 70 years. That’s because that’s when the Germans bombed the house they were in, effect­ively re-firing the bricks and reset­ting the internal clock. It’s not unusual for houses to have a marked destruc­tion layer when you excav­ate. In fact ash lay­ers can provide use­ful bound­ar­ies in strata. This could mean that dates from pot­tery in such build­ings will be from the time of destruc­tion, not the time of use. For example I’d expect all ceram­ics from Herculaneum to appear to date from AD 79.

Isn’t bury­ing a tile at the bot­tom of the sea going to be a problem?

The first objec­tion that I thought of was for mar­ine archae­ology. If this method is all about react­ing with water, isn’t going to be a prob­lem if you’re try­ing to date a tile which has sat at the bot­tom of the sea for a couple of thou­sand years? Bizarrely being immersed in the sea might give more accur­ate res­ults. It’s a mat­ter of the scale at which the reac­tion works.

Imagine look­ing at a tile through a micro­scope. You’ll see all sorts of folds, bumps and even pores lead­ing fur­ther into the tile. It’s clear these will be able to fill up with water if you dunk the tile. Wilson et al have been think­ing about this and you’d be right. Water would get in and react. Yet what they’ve found that even in the air this effect only takes a few hours to hap­pen. After that all the eas­ily access­ible reac­tion sites have reacted and the effect is over. The rehyd­rox­il­a­tion they are meas­ure hap­pens at a much smal­ler scale, the nanoscale.

If you could look at clay so the extent that you could exam­ine molecules, you’d see it was a tangle of lat­tice struc­tures and sheets all packed together and twis­ted around each other. This is where the hydroxyla­tion Wilson et al are meas­ur­ing occurs and at this scale excess water on the out­side of the tile is irrel­ev­ant. It’s the tem­per­at­ure of the struc­ture that alters how quickly hydroxyl rad­ic­als are incor­por­ated into the clay.

What I am won­der­ing, and a mar­ine sci­ent­ist could tell me I’m very wrong, is that on the seabed ceram­ics are going to be pro­tec­ted from vari­ations in tem­per­at­ure to a greater extent than on land due to thermal lag. If this is the case, and we can know that water at a depth X will have a mean tem­per­at­ure Y, then the sea water may actu­ally provide a more stable envir­on­ment for the reac­tion and so it could be mod­elled more accurately.

Bonus fea­tures

This tech­nique could be used on all pot­tery excav­ated from now on, but I doubt it will. If you’re dig­ging a site which is occu­pied for cen­tur­ies, then know­ing that this bit of pot was fired on this date won’t add a lot to your know­ledge about the site as a whole. You’d be bet­ter off spend­ing your budget on some­thing else. Still, for cer­tain key con­texts, this tech­nique offers a way to quickly date key pot sherds. The great advant­age this tech­nique has is that it seems hard to con­tam­in­ate. If you know where a sherd came from you don’t have to have it dated imme­di­ately. If you’re writ­ing up a report months later you could still send the sherd to be dated if you had records. Curating organic mater­ial without con­tam­in­a­tion on the off-chance you’d want a radiocar­bon date would be much more expensive.

It also allows you to date any other pot­tery which has already been dated. So when did the Minoan civil­isa­tion fall? Rehydroxylation dat­ing of pots dug up by Sir Arthur Evans could be dated today to give answers. Suddenly all this mater­ial that museums have been hold­ing onto, in case some new tech­nique is inven­ted, is a lot more use­ful — because this is exactly that kind of tech­nique. The next obvi­ous hurdle is that not every cur­ator is going to be happy with requests like: “Can we smash a bit of this pot please?” to get a sample for dating.

As far as dat­ing goes it might effect­ively have no limit. Obviously the reac­tion gets pro­gress­ively slower as rehyd­roxyla­tion pro­ceeds, but it seems ceram­ics have a large capa­city to absorb water. Wilson et al cite research that sug­gests ceram­ics could add 1% to 2% of their mass through reac­tions with water, which would mean the method could date mater­ial as old as ten thou­sand years. As far as we know, that’s when most ceram­ics start being used. There’s some palaeo­lithic art which may be older, but the chances of being allowed to drill five grammes out of that is so small that it’s not worth both­er­ing about.

So what will it turn upside down?

When radiocar­bon dat­ing was adop­ted it had a dra­matic effect on dat­ing. The Neolithic was moved for­ward and back by a thou­sand years or more as people dis­covered that car­bon dates needed to be cal­ib­rated. The dates are more settled now as there’s a few ways of inde­pend­ently dat­ing mater­ial. Dendrochronology has been par­tic­u­larly help­ful, so I’d be sur­prised if rehyd­roxyla­tion dat­ing sud­denly proves the Neolithic is a thou­sand years older than every­one thought. What it could do is upset some pot­tery sequences that have gone unques­tioned and unex­amined for a few dec­ades. It’s been so long since some typo­lo­gical stud­ies that no-one has gone back to check how sound some assump­tions are. I’ve a hunch that it could shift dates for Geometric and Archaic pot­tery in the Mediterranean and upset some sequences there. There are hints that the Dark Ages really aren’t as eco­nom­ic­ally quiet as some his­tor­i­ans like to think. I’m sure I can come with plenty of wild spec­u­la­tion, but there is one obvi­ous place where I think rehyd­roxyla­tion dat­ing could have an imme­di­ate effect.

I men­tioned quite a while ago that there’s a major debate going on about the set­tle­ment of Easter Island. It rests on how reli­able you think some radiocar­bon dates from coral are. They date from around AD 800, which is when Fenley and think Easter Island was settled. Hunt and Lipo in con­trast think that the island was settled in AD 1200. Looking at what is being found else­where in the Pacific, and at the prob­lems in radiocar­bon dat­ing mar­ine mater­i­als I think that Hunt and Lipo are prob­ably right. What rehyd­roxyla­tion dat­ing offers is the chance to bring new data to the argu­ment. The pro­cess would be simple. Get pot­tery which every­one agrees dates from the earli­est archae­olo­gical con­texts and date it. The error bars are cur­rently wide, but with the dis­agree­ment being around 400 years it should be good enough for now. If the pot­tery can be dated to before or after AD 1000 then we have a win­ner, with the exact details to be pinned down by fur­ther study.

That would be an excel­lent intro­duc­tion for this new tech­nique and would cer­tainly make a lot of archae­olo­gists pay very close attention. Well 10/10 for wild spec­u­la­tion. There’s a very good reason why you can­not date Easter Island pot­tery in the com­ments below.

The newly pub­lished paper is:
Wilson, M.A. Carter, M.A. Hall, C. Hoff. W.D. Ince, C. Savage, S.D. McKay, B. and Betts, I.M. 2009 ‘Dating fired-clay ceram­ics using long-term power law rehyd­roxyla­tion kin­et­ics’, Proceedings of the Royal Society A. doi:10.1098/rspa.2009.0117

At the time of me pub­lish­ing this, the doi is not work­ing. There’s noth­ing I can do about that, but I you can down­load the paper from the Royal Society’s own web­site. The link worked for me, but I don’t know if this is sup­posed to be a secret page.

Thanks to Moira Wilson at the University of Manchester’s School of Mechanical, Aerospace and Civil Engineering for answer­ing ques­tions that I emailed and cor­rect­ing an embar­rass­ing mis­take I made on the (time)¼ law and to Alex Mack at the University of Leicester’s Centre for Interdisciplinary Science for track­ing down the Royal Society link.

8 thoughts on “Rehydroxylation Dating

  1. I under­stood from the press reports that the pro­cess doesn’t work on glazed or painted ceram­ics, which would also cut down on its use­ful­ness. You don’t men­tion this lim­it­a­tion, so per­haps there has been fur­ther clarification.

    Visit Zenobia’s web­site at Empress of the East

  2. Dan Milton

    Easter Island pot­tery? I thought the Polynesians lost the art of ceram­ics when they moved to clay-poor islands in the years BC.

  3. Excellent sum­mery: Dating the pots them­selves may sat­isfy the ceram­icists, whose sys­tems have been under strain from dendro and other dat­ing, (eg. Aegean /Thera.)

    If the big vari­able is tem­per­at­ure, could this be used as a com­par­at­ive dat­ing tech­nique for samples from the same site or area?
    BTW:
    “Pottery and other ceram­ics make up most of the data that you’ll find on an archae­olo­gical site”
    Naughty, naughty, there is already a unhealthy and dis­pro­por­tion­ate interest in what people in the past kept their food in; — don’t for­get the evid­ence for struc­tures and the built envir­on­ment, which folk cre­ated to keep their pots in.

  4. The poten­tial for my line of work is great — depend­ing on errors and costs!
    The late antique depos­its I’m deal­ing within syria where we have some dice burn­ing destruc­tion depos­its so potentally dat­ing end depos­its as well as the date of man­u­fac­ture.
    con­trast Geoff Carter’s com­ments above:

    Naughty, naughty, there is already a unhealthy and dis­pro­por­tion­ate interest in what people in the past kept their food in”

    with those on BAJR http://​www​.bajr​.org/​B​A​J​R​F​o​r​u​m​/​t​o​p​i​c​.​a​s​p​?​T​O​P​I​C​_​I​D​=​2​325
    “Freedom from the tyranny of finds spe­cial­ists at last!”

    the lat­ter in my exper­i­ence is the more widely held view — the finds are only there to date when the dirt was depos­ited
    sigh

  5. I was not try­ing to pick a fight with ceram­icists, as a spe­cial­ist in build­ings and struc­tures I am out­numbered hun­dreds to one, which was my point; it is easy to loose sight of the import­ance of under­stand­ing archae­olo­gical con­texts, it is not just about dat­ing them.
    My com­ments must be seen in the con­text of
    “Pottery and other ceram­ics make up most of the data that you’ll find on an archae­olo­gical site”,
    which is blantent ceram­icism, simply point­ing this out does not make me an anti-ceramicist, — some of my best friends have been from that background.

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