Using Eddy Current Technology in Conservation Science (Podcast 8)

Download Episode 8 as an mp3 or subscribe via iTunes.

Curtis Desselles and Houren Zhu build and test eddy current analyzers. Photo courtesy of Jason Church.

Kevin Ammons: Welcome to the Preservation Technology Podcast. I am Kevin Ammons and today we join NCPTT’s Jason Church and Curtis Desselles to learn about developments and low cost metallurgical analysis.

Jason Church: So Curtis thank you for coming in and talking to us today.

Curtis Desselles: It’s my pleasure.

Jason Church: I have seen a lot of buzz lately about NCPTT and eddy currents so I want to talk to you today about what they are and how NCPTT got involved with them and get a little inside to how they are being used. So the first thing I was wanting to know today is how did NCPTT get involved in eddy currents?

Blythe McCarthy investigated eddy currents in conservation science in an NCPTT grant in 1999. Photo courtesy of Blythe McCarthy.

Curtis Desselles: Well back in 1999 a grant was given to Blythe McCarthy from the Freer Gallery and she tested different eddy current analyzers that were on the market and to see if they were feasible for Cultural Material and she found they were great potential so I read the article myself and Mary Striegel had gave me the article and I found it to be very interesting. It had a lot of potential and I thought that I could build one for off the shelf parts for a really economical cost.

Jason Church: So how much do currently units that are currently available on the market what is the price range that they run in?

Testing the eddy current probe on aluminum foil. Photo by Jason Church.

Curtis Desselles: Well they start about just for the probe it is about $1,500 dollars per probe and then the machine itself can run you from $25,000 dollars all the way up to $50,000 dollars.

Jason Church: So when you say you were looking to try to develop one that was somewhat economical what is the price range that you are talking about?

Curtis Desselles: Well it is definitely below $500 dollars and I could probably do one for less than 200 dollars.

When a flaw is introduced to the conductive material, the eddy currents are disrupted. Image courtesy of Curtis Desselles.

Jason Church: Well that is quit the stretch. So what are eddy currents?

Curtis Desselles: Eddy currents are analogous to a flowing stream. Let’s just imagine that a stream flows and there are no rocks in it and so it just flows really smooth and then imagine if we have rocks in it you always see these little currents flowing around the rocks those are called Eddies. So in a magnetic field is like the water and the rocks are like the metal. So just imagine when the magnetic field hits the metal. These little eddies form in a circle and the magnetic field direction is opposite in the middle than that of the magnetic field of the magnet or the electromagnet.

Jason Church: How does this work? Why would we be interested in this?

Curtis Desselles: Well any defects in metal or any kind of contour changes you would be able to pick that up and what we do is we subject the metal to a magnetic field by an alternating current and according to which ever frequency and it produces these little opposite currents in the metal. Now when we put the electromagnet or whatever the sensory coil close to the metal. There is a change in voltage and that is what we can measure. As we scan the object we can measure the change of voltage in there so we can know where all the discontinuities or the defects are at.

Jason Church: So this is a nondestructive test?

Curtis Desselles: Yes. It is completely nondestructive and as we know you know magnetic fields are used all over the place in MRIs in hospitals so basically we can do the same thing.

Jason Church: So I know these are used a lot in industry to look at micro cracking and stress cracking I know aerospace industries use these a lot also they do thickness coating testing in industry. You know here at NCPTT we look at cultural resources so how could eddy currents be use to look at cultural resources?

Curtis Desselles: Well let I’m going to give you several examples. One example would be if a metal is corroded over and there is some mark underneath it and you didn’t want to destroy the object by removing the coating cause you could remove whatever mark it was you could scan the object and penetrate and get the information by the penetration of the eddy currents and therefore you could nondestructively see what the makers mark or hallmark underneath the corrosion. That is one method and another method would be to just analyze and the metal you could analyze cause each metal has a signature that is unique to itself and reaction of the voltage drop of the eddy currents.

Jason Church: Sounds like that would be really useful for underwater archaeology.

Curtis Desselles: Underwater archaeology yes. Corrosion even objects that are in the museum itself that have been there for a while that we didn’t want to destroy but we don’t want to you know we want to find some kind of nondestructive method of finding out all the details that we have missed.

Jason Church: What about hallmarks or even serial numbers for forensics that are really worn through.

Eddy current testing on numanistics, silver coins in this photograph. Photo by Jason Church.

Curtis Desselles: Now that is a great possibility because right now they use acids and chemicals methods to allow like the FBI at Quantico they look at serial numbers that people have tried to file off and you can file off a serial number but it is never completely gone. So eddy currents would nondestructively it wouldn’t destroy the evidence so you would have a great evidentiary chain so that would be good for law enforcements not only just cultural materials but law enforcement as well.
Jason Church: Okay. Yea I know one of the really important things in hallmarking is to date them and come up with the manufactory or smith that made them.

Curtis Desselles: Like the knives we have several knives in Los Adaes one of the sixteenth century forts in the area and we could look at some of the makers marks because that was very important. People always put their mark on the cultural objects that they made.

Jason Church: Now I know Blythe McCarthy had looked at a lot of guilding methods so it could be used for platings?

Blocks of different guilding thicknesses are used in eddy current testing. Photo by Jason Church.

Curtis Desselles: Right the Eddy Current penetration is based on the fact that the higher the frequency of the wave the more resolution but the less depth and vice versa so yes we could look at the coatings like guildings and surfaces like that are coated with even plastics you could look at the thickness of it.

Jason Church: I know this year we had NCPTT hosted lecture and workshop on eddy currents at the 2009 AIC conference that was held in Los Angeles. What other kind of things would you like to do with eddy currents as far as open to the public?

Curtis Desselles: Well I would like to make an eddy current microscope to where we could try different frequencies variable frequencies and go deeper and deeper and deeper and like get a layer like a cat scan so we would do one layer and go a little deeper and get another layer and another layer and build a 3D image just like a CAT scan or a MRI. Or just a plain old eddy current microscope. The beauty of this method is that it can be portable and cost effective.

Jason Church: And of course it is nondestructive.

Curtis Desselles: And it is nondestructive.

Jason Church: That is a really important thing for conservation and the museum world. Where else can people go to find out more information is it just solely in house or do you have stuff out there?

Léon Foucault discovered eddy currents in 1851. Image courtesy of Wikipedia.

Curtis Desselles: Well we are in the process of setting up a web site where we will have tutorials. Right now I have tutorials available that I can send by email. I have a complete tutorial on how to build the instrument the part list and the software that was written that I wrote for it. It is available right now free of charge and I would like to we are in the process of a website where all this will be centrally located .

Jason Church: So we are saying not only is it economical but the software and the plans are available to people in conservation to be able to build their own unit.

Curtis Desselles: Exactly at no charge and you know if they have any questions if they don’t have the technical ability to build it. We can help them in some way for instance help them with some circuit boards. It is not a difficult procedure but it does take a little bit of skill in soldering, drilling, and cutting.

Jason Church: Well this sounds good is this something that you look forward to in the future of making more that’s available to the public?

Curtis Desselles: I do I would like to take this method a little further because it is a very old technology and to me bringing redirecting old technology and using new technology to make it better is a worthy gold. So there is no use in reinventing the wheel so you just take what you have already and make it better with the newer technology that we have.

iPhone graphing tool with eddy current probe. NCPTT has scheduled an eddy current iPhone application for 2010. Photo courtesy of Jason Church.

Jason Church: Technology transfers has always been an important thing at the National Center for Preservation Technology and Training.

Curtis Desselles: And that is good that our mission coincides with what I like to do.

Jason Church: Well thank you very much Curtis.

Curtis Desselles: Well thank you.

Jason Church: I look forward to looking at more eddy currents in the future and checking out that website.

Curtis Desselles: Well thank you and thank you for having me.

Kevin Ammons: That was Jason Church and Curtis Desselles. If you would like to learn more about this project visit our podcast show notes at the National Center for Preservation Technology and Training. That’s ncptt.nps.gov. Until next time goodbye everybody.

Produced by Jeff Guin and Bethany Frank