Wednesday, 7 November 2012

A curious incident

Corroding sample!
Yesterday's ICP samples are coming along nicely. I'm happy to say nothing had exploded when I went to check on them this afternoon. Today I also started boxing up the last few samples from medieval Riga, and came across this oddity ---->. All of the samples from Riga were waterlogged, and something strange has happened to this one; you can see all that orange spreading across the bottom of the sample? It is some sort of iron staining, and is very clearly seeping from the sides of the tin into the fine grained clay floor part of the sample. There were also lots of little salt crystals all over the surface, and you can see to the centre right a small area at the top of the metal tin has corroded! I'm wondering if there is a metal object in the block somewhere that is degrading?

A few of the other samples had unfortunately started growing mould. In future I will learn to wear a mask when unwrapping waterlogged samples! Hopefully this was just on the surface and will not have impacted the sediments where the slide cut will be made, though I'll be on the lookout for their little fungal hypae.

Slices from the blocks, ready to be cut to slide size

The blocks from September have now set and are in the process of being cut to size ready for mounting on slides. Absolutely beautiful preservation of wood, charcoal, dung (of course) and floors. The slides themselves should be ready by Xmas, I'll have to resist spending the holidays with ny nose down the microscope....

Tuesday, 6 November 2012

Elementary, said he

80s-tastic plasma lamp (Wikipedia)
Today I was mostly preparing lake sediment core samples for ICP. ICP stands for Inductively Coupled Plasma, and comes either in the MS (mass spectrometry) or OES (optical emission spectrometry) variety. Both are methods for measuring the different elements present in a sample. The ICP bit is a 'torch' containing a gas, typically argon, that is ionised by heating it via electromagnetic induction (remember those weird glowing globe plasma lamps?). Still confused? This means there is a coil wrapped around the torch, which produces a very strong electromagnetic field when turned on. The argon gas is ''lit' by an electrostatic spark, and the gas becomes ionised.
The sample (dissolved in liquid) is sprayed into the argon flame, and also becomes ionised. As different components in the sample become ionised they gave off a characteristic energy. The MS and OES parts is the bit that detects the elements present. MS does this by measuring their atomic mass, whilst OES measures the wavelengths of energy emitted by the elements. Each element produces a characteristic wavelength, so we can identify them on this basis.

Preparing samples from Lake Nineris, Latvia

And why do we want to measure elements you may ask? ICP is another one of those lab methods that has been developed for forensic and environmental applications then adopted by archaeologists. It is used routinely to look for metal toxicity in human blood and tissue samples, and to identify levels of pollutants in water. In archaeology, the idea is the same, but we are looking at the past history of such 'contamination' events. In the lake cores we have from the Baltic for the Ecology of Crusading project, we are measuring a range of elements in the sediments which can be linked to human activity. The lakes are located close to crusader castle sites, and pilot studies have linked signals such as increased lead to the castle construction. Other signals can be linked to activities such as landscape clearance.

Dissolving sediments in HNO3
Before we can analyse the samples, they have to be prepared for the ICP machine. This involves grinding/sieving them to less than 20mm and dissolving them in concentrated nitric acid. Grinding up and weighing packets of soil takes longer than you'd think. They'll sit in the acid overnight, then will be heated for 9 hours tomorrow (checking at intervals for explosions and whatnot). The resulting solution is then filtered, diluted and injected into the ICP.

So far I estimate there are about 60 samples per lake core, and maybe 15 lake cores. Eep.

Monday, 5 November 2012

Riga slides in progress

Riga town samples set in resin, ready for cutting into slides
Another week of lab work in Reading this week for the Ecology of Crusading project. The samples I boxed up in September for resin impregnation are now ready for cutting, and hopefully some of them will be finished into slides by the end of the week. They look like they've set really well; I was a little worried there would be problems as some of them were quite damp with lots of 'manure' layers, which can sometimes distort when dried or interfere with resin curing. I boxed up a few more samples from medieval Riga (Latvia), and 3 from Święta góra (Poland) today. It's going to be a tremendous amount of microscope work to get them all analysed, but I'm looking forward to it!

I also started preparing some lake core samples for ICP analysis today, which was delayed due to some minor hiccups which will hopefully be corrected tomorrow. That should take the rest of the week, with the Association for Environmental Archaeology conference starting on friday. I will be heading to the keynote, and really hope I can make some of the talks on saturday; lots of presentations on Near Eastern projects that I would love to hear the latest results from, including the Central Zagros Archaeological Project that I worked on in 2008 and 2009.