Warning: This post is somewhat technical, kind of lengthy, and it has an example of early recycling near the end.
A former professor of mine recently called with some technical questions about the Conservation
Department’s new x-ray equipment. Instead of responding to him via e-mail I thought I would provide a kind of open response here. And why not, right? It just might be interesting to you and perhaps also to my colleagues in the conservation world. Besides, in my mind, the IMA’s blog seems to be the perfect place to discuss x-radiography considering the previous non sequiturs of Cheesecake and Redbuds.
I’m a little worried, though, because I doubt many of the folks upstairs realize that we have the capacity to produced x-rays down here in the basement and this might make them a bit nervous. I think it’s fair to say that most people have a fear of x-rays for good reason, because they have the potential to change humans on the atomic level, and that’s unnatural at best; but, trust me, we’ve taken a lot of safety precautions to make sure that all of the x-rays that we produce stay in the room that they are produced in. The room is an enclosed space that is lined all the way around with an 1/8 of an inch of lead. We use a Geiger counter to confirm that this room is successfully containing the energy.
Now I’m not going to take the time to fully explain what an x-ray is or how an x-ray tube actually produces x-rays. If you’re interested, go here to learn what x-rays are or go here for a brief discussion of how they are produced (the links will introduce you to such things as anode, cathode, colliding electrons, KV, MA, and so on). I will say that an x-ray is a form of electromagnetic radiation that carries a relatively large amount of energy and momentum. Depending on an x-ray tube’s capabilities, x-rays can penetrate through various objects – from canvas paintings to this covered bronze vessel with handle (60.24, called a “You”) from the Western Zhou Dynasty (c 900 BCE) and even through large pieces of stone. Our “you” (pronounced: yo) is a traditional type of Chinese ritual vessels; you can go here to find out more about the other types of Chinese ritual vessels.
In the radiograph above of this Chinese vessel (notice I removed the lid), you can “see through” it because the x-rays have penetrated all the way through the copper alloy from which this vessel is made (it’s about 3/8 of an inch thick). You should be able to see that this vessel was actually broken into many pieces and then the fragments were re-assembled at some point (Jim Robinson, the IMA’s Jane Weldon Myers Curator of Asian Art, believes this was likely re-assembled shortly after it was recovered from burial. Side note: you can go here to see a video of Jim and the director talking about the new Maya Lin installation, Above and Below.)
X-ray technology has been used frequently in the field of conservation for many years. In fact the IMA purchased its first x-ray tube more than 30 years ago; but last year that tube finally broke down. We were all pretty bummed about that until, out of the blue, the IMA was given a gift from an anonymous donor that allowed us to purchase a bigger and better x-ray tube and shift to a Computed Radiography system. I really have no idea who this donor is, or these donors are, but I’d like to say thanks. Seriously, thanks a lot! I know that the whole conservation department has been energized by this new piece of equipment.
So, the old system that we had produced radiographs just fine but we had to use x-ray film to capture the images – if you want to get a sense of what this film was, think of large piece of black & white film (14” x 17”) and you’re in the right ballpark. Though you can make high quality and clear images using film, the process is time consuming, messy, and, in a way, a thing of the past. Plus to view film radiographs they need to be lit from the back (or they can be scanned as a digital file, but you then loose some of the resolution). There are plenty of examples of analog technology that already are or soon will be digital.
As I mentioned, the digital technology we now use to capture images is called computed radiography (CR). While CR requires the same kind of tube to generate x-rays, one of the main differences between CR and film is that instead of shooting on film, we now shoot onto reusable phosphor imaging plates that are scanned and then viewed on our computer. So, instead of developing film in a series of chemical baths, we simply stick this “plates” in a scanner (here’s a spec sheet on our imaging plates and here’s a spec sheet on our scanner). If you don’t mind hearing high-pitched noises, you can even watch one of these plates go through the scanner at this link (and, no, I don’t need to be reminded how geeky it is that I made a youtuber of this process; clearly I realize I’m a bit geeky, but oh well). Needless to say, this process is much easier and faster than developing film by hand.
With this new system we now have the capacity to do things we’ve never done before – plus we’re kind of proud to be among the first institutions to use CR in a museum conservation department. CR has been used widely in the medical and industrial fields for some time now but it’s relatively new to the museum world. When I say that we can do new things with this system this is because with our new x-ray tube we can produce x-rays of higher energy. Our old tube could only produce x-rays up to 110KV but now our new one can go to 200KV. Here’s the spec sheet on our new tube.
With our old tube I didn’t have the capacity to generate strong enough x-rays to penetrate through the Chinese vessel pictured above. Previously to do something like this we would have to collaborate with nearby industries to use their x-ray equipment. For example, in 1981 the IMA worked with Detroit Diesel-Allison to produce radiographs and other technical analysis of this vessel and others like it for the out-of-print catalogue Beauty and Tranquility: The Eli Lilly Collection of Chinese Art. Of course collaboration is always a good thing, and we’re always looking for new partners with which to collaborate, but in this case it’s much faster to be able to do this work in our own lab.
Since this object was radiographed in 1981 I have an excellent film-based example to which I can compare the new digital file that I created with the CR system. This is important because one of the main questions about the difference between film-based radiography and CR revolves around the idea of the quality of the image, its resolution. That is to say, can the CR system make an image that is of the same quality as what used to be made with film? The other part of this question is if CR can produce high-quality images when higher KVs are used.
I think so, but it may be a bit difficult for me to prove it here because, of course, I can’t really show you the film-based radiograph over the internet and the monitor I used to view CR files is a high-resolution monitor (here’s a spec sheet on our new monitor). Clearly, the best way to do this kind of comparison would be with me down in the department’s analytical room. Since we can’t do a side-by-side comparison, I’ll explain to you an example of what our CR system can do.
Jim Robinson mentioned to me one day that this vessel had an interesting example of early recycling. I’ll let Yutaka Mino and Jim explain what they observed about bottom of the You in their catalogue back in 1981:
“The exterior of the base has a grid design of raised lines, and the chaplets in the bottom are randomly placed. Two chaplets are of particular note. One is under part of the inscription, showing that the characters were not cast with the vessel. The other interesting chaplet has a fine spiral pattern of leiwen on it. This means that an old, possible broken or unsuccessfully cast vessel was salvaged and used in the manufacture of this object. The practice of recycling old bronzes is manifested in other bronzes.”
I think you can see these chaplets clearly in the radiographs I created (here’s a spec sheet on the software we use). Have a look: can you find the three chaplets in this image? In case you’re wondering, chaplets are used in the casting process to act as a kind of spacer between the inner and outer molds. In this image the chaplets appear kind of like islands in the rest of the copper alloy. These chaplets were inserted into the mold before the liquid metal was poured in.
Look closer, can you see the chaplet with the spiral pattern on it? Can you also see the grid design of raised lines that Yutaka and Jim were talking about? Pretty cool, huh?
So, finally, I think that CR produces images that are of equal or higher resolution than film radiography, and it’s faster and easier to use. Plus, the images that are created are easier to share, and even potentially be blogged about. If you’ve read this whole post, thanks and congratulations; I really didn’t mean for it to be this long, it just sort of happened.
If you have questions, post them in the comments and I’ll answer them the best I can.
The digital radiographs I produced were shot at these settings.
Radiograph from the side 170 KV 3 MAs 1.7 minutes
Radiograph top 125 KV 3 MAs 1.4 minutes
Radiograph of lid only 110 KV 3 MAs 1.1 Minutes
Here’s an interesting article from JAIC:
“Technical Examination of the Classical Bronze Horse from the Metropolitan Museum of Art”
From the California State University Northridge:
“Piece Mold, Lost Wax & Composite Casting Techniques of the Chinese Bronze Age”
Filed under: Conservation