The Dating Game

Intro: 

Hi! Welcome to the metal detecting show Episode number 30. My name is Ciaran and I have been metal detecting for nearly 30 years. This week I talk to you about the dating game no this is not a chat about how to pick up a partner who metal detects but how the scientific community would go about dating a find.  So, let's get On with the Show. 

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Hey before we start I want to Thank you for listening to the show and I hope you enjoy the show this week. If you want to give me feedback or interact with the show, please reach out to me on twitter @detectingthe or Instagram @themetaldetectingpodcast or If you want to pop me an email to ciaran@themetaldetectingshow.com  

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Ok firstly on to my adventures in metal detecting well this week I was in catch up mode and got out twice. One to my what I would call a relic type beach where everything is older than 20 years and battered from the beach but this beach is where I tend to find my oldest and most interesting finds and this time was no different with several pre euro coins and relics including old pigeon rings and pins even some of the rubbish was interesting with old style pull tabs and labels popping out. This hunt if you are following me on instagram is the hunt that I had spent two hours chatting to the camera and showing finds to a camera that was off, I had forgot to turn on the camera in my excitement to get out and believe me that was painful.

the next day I hit a popular beach looking for a more modern find but unfortunately I was repeatedly interrupted by an old man who was looking for fairy rings so lost a good 30 mins there chit chatting but further interruptions came included the local police to Check on a car that had nothing to do with me so I was destined to be limited but finds were what you would expect some modern coins, toys and fresh air.

I was then I vowed to commit to getting back on the fields as soon as possible and took the long way home preving on a few fields along the way. Let me know if you do that go out of your way just to look longingly at a field in the hope the farmer will come over to you and say to you all she needs now is a good detecting do you know of anybody that could sort her out. 

Anyways enough of that…..

Also this week I got around to something that I was thinking about for a few weeks and that was an experiment in the sublimation of corrosion from the surface of a coin. So to give you an example you might be familiar with.

Did you ever see those videos of the guys using high power lasers to remove surface rust from steel panels, Well this works by a process called sublimation where the rust is heated up so fast that it goes straight from solid to gaseous form bypassing liquid leaving a pristine surface on the steel. Seeing as I have a Laser lying about my shed I thought it would be a cool experiment to see if I could sublimate off some of the surface patina from an old 2p. 

now when I posted this on instagram I got a few DM’s of people asking why did I ruin the coin and just to put it on the record it was a 2p from 1975 even though it looked old it was a super common and damaged on the beach beyond any value, anyway to continue . 

In theory this shouldn’t work as the temperatures required for sublimation to occur are way beyond the capability of my little laser. So after 5 passes the laser did do something it looks like it altered the patina to be more of a char and over the last few days is had been flaking off to leave a clear surface so watch out for my instagram update in a few days but maybe the next experiment will be with 10 pass’s but I think I might be onto something and I may need a more powerful laser as I had suspected in the first place or maybe ill just stick to the angle grinder. [joke sound]

So this week I want to talk to you about how the scientific community go about dating finds and hopefully I can relate this to metal detecting but remember im not a scientist but a general joe whos google foo is above average. So here goes. 

Most dating techniques fall into two modes one being absolute dating and the other being relative dating no not that type of relative dating relax there deliverence [Banjo Sound]. 

Starting with Absolute dating this is a dating methodology that is chemical in nature and is quiet expensive and beyond the means of most metal detectorist the techniques include but not limited to Radio Carbon Dating which ill chat about later during the tech time out this week and Thermoluminence dating which is normally reserved for ceramics and sediments that looks at the crystalline minerals that have formed due ot the minerals were heated. Absolute is considered scientifically accurate but we are going to focus on relative dating which is considered less trustworthy than absolute dating. Stratigraphy is the primary method of Relative dating and that is the study of the formation of strata in the soil due to stratification which essentially means that as over time soil will form layers with the top most layer the humus layer being the newest layer normally forming within 12 months this layer is made up of leaf litter and decomposed organic matter the next layer below that would be the topsoil layer which is made up of humus and minerals in various degrees but did you know that it takes approx 100 years for an inch of topsoil to form. It is in these two layers that we as metal detectorist normally operate. 

However there are other layers including the Eluvian layer which is just below the topsoil followed by subsoil and finally bedrock. If an object is found in any of these layers they can be assigned a relative date for example if an object is dropped within the last year it is safe to assume it will be found in the homous layer and you can assume any find in the layer has arrived there in the last 12 months just to be clear this doesn’t actually age the object it the objects age is determined by its manufacturing date it just tells you when it was dropped.. 

Now as any will tell you we normally operate in the topsoil layer and Humos layer but did you know that wHen a person drops something onto the ground that it actually doesn't sink into the ground but actually if left alone the ground grows over it. So this doesn’t really help us all we know is that if we find some thing in the homus layer it was dropped within the year and if its deeper it will have been dropped from any period between 1 – 1000 years ago. 

Now before you all break your necks i'm going to say yes of course plowing and flooding will change that rate of decent into the soil in fact plowing will serve to keep a n object in the top soil and possible bring up an object due to Granulation which is the reason your big corn flakes sit at the top of the bow while the smaller ones fall to the bottom but that is for another episode of the podcast. 

So how does this help you in dating a find well remember it can only give you a relative idea to when the find was dropped what you must do is create a cohort of finds including rubbish from the same layer and the same site. So if you find 10 finds on the surface you group them in your <1 year pile, group your 1 inch down finds together and on and on until you get to a point where you don’t care about the relative date so maybe 10 inchs. 

Then reviewing each pile you try to deduce the date of at least one find you might find an item specific to a period of time like a bit of broken pottery or clay pipe allowing you to age that particular strata of layer within the soil. For example roman pottery will have fine red pottery with a glossy red slip and can be dated to before 410 AD but it is relativly accurate to assume that all finds from that layer could have potentially come from this era. 

Now this is what archeologist’s call context of finds to each other but as you can see it is not very accurate and next to useless for a metal detector. Now add a plow to this scenario and this makes the possibility of building a picture of the find context to the relative date an almost impossibility. 

So why am I talking about this well next time your in a argument about the context of a finds being ruined by metal detectorist you can say well I found this 6 inches down in topsoil that has been plowed for the last 50 years and thus the relative date of the strata is not a factor on this site. 

Up next is this weeks tech time out where I talk about an absolute dating method we have all heard of. One that fundamentally changed the dating of finds and that is radio carbon dating. 

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Time for this weeks tech time out this week ill want to give a high level view of Radio Carbon Dating some times referred to as just carbon dating and how it works. Carbon dating was developed in the 1940 at the university of  Chicago by willard libby who received the Nobel price in chemistry in 1960 for his work on the subject.

So how does it work well all carbon based lifeforms including us eat plants that have in them Carbon 14. which is the radioactive isotope of Carbon and is created in the atmosphere when cosmic rays collide with the neutrons of N14 altering it to become Carbon 14. While alive we and other carbon based animals constantly exchange this C14 in our bodies so as it is constantly replenished. However when the life form dies this exchange stops and the C14 that is in the body is never exchanged out again. Now because C14 is radioactive it suffers from radioactive decay and C14’s radioactive half-life is about 5730 years. Now since the 1960 scientists are measuring the C14 of plants and atmospheric data from many samples and have been able to build up a calibration curve of the amount of C14 a body should have depending on when it died. the further back you go the less C14 is in the sample.   

So when we die we have a finite amount of C14 in our bodies and this amount halves based on it’s half life of 5730 year so theoretically if I died 5730 years ago I would have half the amount of C14 present in my body compared to the day I died. So by measuring the amount of Carbon 14 in a sample and comparing it to the calibration curve we are able to determine the absolute date of the sample to within 30 years but only accurate to approx 50,000 years ago. Now I don’t expect this interest to be of use to any metal detectorist but I do hope its interesting and that’s it for this weeks tech time out.

Wrap Up 

That’s it for this week’s  I hope you liked this episode of the metal detecting show podcast.

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Get out there eyes down and Happy Hunting.