Last updated: July 19, 2023
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Podcast 014: Technologies for Drying Archeological Wood from Shipwrecks
James Barry, National Park Service
Conservation Science
Jeff Guin: Eric, welcome to the podcast. How did you get involved with the science of conservation?
Eric Schindelholz: Hi Jeff. Thanks for having me.
Eric Schindelholz: First off, I became aware of conservation as an undergraduate archeology student and went on my first excavation in Israel and met a conservator for the first time and realized there is more to archeology than digging up stuff. There is also the preservation aspect. So the science of preservation along with the interest in archeology is kind of what steered me into conservation to begin with.
U.S.S. Monitor
Eric Schindelholz: As an undergrad, I took some chemistry courses and more archeology courses, and then went on to obtain a Master’s in conservation. My first job out of conservation school for my Master’s was on a shipwreck project called the U.S.S. Monitor. Just a little background on the U.S.S. Monitor project: It was, at that time, one of the largest conservation projects dealing with iron shipwrecks in the world. There’s about 250 tons of material from this Civil War shipwreck that had been recovered.
Eric Schindelholz: Our team was in charge of preserving. So that lent to some major challenges in dealing with the volume of the material that we had, and also the variety of different types of material. We had lots of iron, but we also had lots of waterlogged wood and organic artifacts. So it’s kind of through this project where we started working a lot with material scientists and other scientists outside the field of conservation to start doing some research and design where it could tackle these challenges that aren’t usually brought up in the normal conservation lab. So that’s kind of how I became interested and started in the science of conservation.
NCPTT Preservation Technology and Training Grant
Jeff Guin:So that project led into the research that we’re talking about today, which is the NCPTT grant. Is that correct?
Eric Schindelholz: That is correct. During that project, I mentioned that we had a number of these wood artifacts. We had gun tools that they used to maintain and fire the guns with. We also had some personal effects, furniture details and pieces, all of which were fairly degraded and covered with rust. Some of which also had iron pieces attached to them. So these were complicated artifacts and we had a lot of them.
Eric Schindelholz:So then our task was to then find sort of the best treatment method that we could use for these artifacts, and we looked at some of the typical types of conservation treatments, but we also looked into this fairly new conservation treatment that had been recently developed, and that was in the 90’s, by St. Andrew’s University called supercritical drying.
What is supercritical drying?
Jeff Guin:OK, well explain what supercritical drying is.
Eric Schindelholz:Supercritical drying is basically, well, the method that we used in this study, is basically soaking a waterlogged organic artifact, in our case wood, in a series of organic solvents like acetone and ethanol to replace the water in the wood with the solvent. And then taking that solvent-soaked artifact and placing it in kind of loosely described as a “pressure cooker” and pumping in carbon dioxide, bringing the heat and pressure up in this vessel to create supercritical carbon dioxide.
Eric Schindelholz:Supercritical carbon dioxide is a form of matter, or supercritical fluid actually is a form of matter, that is neither a liquid nor gas, but displays kind of the properties of both liquids and gases. And so we are all familiar with our three most common states of matter, which would be solid, liquid and gas. So there are a couple of other types of these phases, one of them being the supercritical.
Eric Schindelholz: So at this elevated temperature and pressure, you have this fluid that has the solvent properties of a liquid, but the diffusivity of a gas—meaning, when we put the solvent-soaked artifact in our loosely-termed “pressure cooker” and remove that solvent with the carbon dioxide, it does it so very quickly because it is like a gas. But it also does it without liquid surface tension. And liquid surface tension is pretty important when drying any type of artifact.
Eric Schindelholz:You can find this phenomenon very simply at your home with the drying of a sponge. The liquid-surface tension is basically the force put on an object the by a drying liquid, which causes stress on it and in the form of a sponge, which has a very weak cell structure (very similar to water-logged wood), that sponge shrinks up.
Eric Schindelholz:So back to case-and-point here in our pressure cooker with the supercritical carbon dioxide and by removing all this liquid solvent and replacing it with this gas-like substance that doesn’t have a liquid-surface tension, you’re avoiding that shrinkage from occurring. And after you go through this process, which can take a number of hours to a few days, what happens is you just depressurize the vessel and you are left with this artifact filled with carbon dioxide. And we know at room temperature and at standard pressure that carbon dioxide is a gas. So you are essentially left with just a piece of wood filled with air.
What's the history of supercritical drying?
Jeff Guin: Tell us about the history of this method, or this concept, for drying.
Eric Schindelholz: Supercritical fluids in general have been around for more than a hundred years, and they were first observed in the 1800s. And by the mid-1900s, they were being used in a lot of industrial processes. Some processes using aerospace to dry different types of materials and as solvents, and now-a-days they are used quite a bit in industry and commercial applications down to even dry cleaning your clothes.
Eric Schindelholz: If you ever go to one of these “green” drycleaners, what they are using is basically supercritical carbon dioxide. So as a whole supercritical fluids have been around for quite a while. The folks, as I mentioned earlier from St. Andrew’s University, adapted a kind of a method and developed their own spin on the drying method using organic solvents for waterlogged wood. What we did then was to evaluate their method against common types of treatment methods for waterlogged wood.
Findings of the project
Jeff Guin: What came out of the project? What were your findings?
Eric Schindelholz:Well our research was concerned with evaluating the process that those at St. Andrews University developed against polyethylene glycol and freeze-drying process, which is just essentially putting a synthetic wax in the wood and freeze-drying it–much in the way that some taxidermy is done nowadays and that also prevents shrinkage from occurring.
Eric Schindelholz: And we also evaluated this method against just plain air drying. And what were looking for was the shrinkage of the wood. The goal was to have the least amount of shrinkage possible, and also is any cracking occurred and some of the qualitative kind of aesthetic appearance of the wood as well.
Eric Schindelholz: What we found were many of the things we suspected or hypothesized. First, that the waterlogged wood that we used and just let sit out on the lab bench and air dry shrunk up like a sponge — no surprise there. But also the current popular method of filling this thing up with synthetic wax or polyethylene glycol by freeze-drying gave very good results and little shrinkage.
Eric Schindelholz: Some amount of shrinkage occurred from the supercritical process we were using. But really it is fairly negligible compared to some other types of treatment techniques out there and also to just air drying in general. So we had fairly good results.
Eric Schindelholz:We had some pieces of wood where the degradation was very high and the supercritical fluid wasn’t able to prevent some of the shrinkage that occurred on that wood and also caused cracking. That was an unexpected result that we had.
Eric Schindelholz: We found that the supercritical process presented us with good results, but kind of on par with the current technique of synthetic wax and freeze-drying.
Eric Schindelholz: There are some advantages to this technique, and this is why we kind of looked at it, and it can still be applied in many cases, and that it doesn’t leave material in the wood. You are left with the natural piece of wood at the end, instead of a piece of wood that was soaked with synthetic wax or other materials. It is also much faster than current techniques. The freeze-drying technique that I had explained can take on the order of a few months to a few years depending on the type of artifact, whereas the process of using supercritical fluid cuts that time down to a few days to a few weeks. So from a cost-wise perspective and time and resources, supercritical fluid could be more advantageous for large projects than the traditional techniques.
Other applications for supercritical drying?
Jeff Guin:Are there other applications for supercritical drying?
Eric Schindelholz:There are other applications for supercritical drying, not just with wood, but also with other types of waterlogged artifacts. And that has been explored by St. Andrew’s University and Clemson University, who were some of our co-investigators on the project, are using this type of technique to dry waterlogged corks from their Hunley submarine shipwreck.
Eric Schindelholz: So there are different things that can be done with drying, but also supercritical fluids in general can be applied to the field of conservation in many different ways. I had mentioned that commonly now you find these drycleaners that use supercritical carbon dioxide to clean things.
Eric Schindelholz: This could be applied to museum textiles, probably with fairly good results, and to other fairly fragile materials such as cleaning feathers, which is obviously a problem in conservation. So there are a number of different applications out there. It just takes the time and the money to really look into this.
Does supercritical drying require specialized equipment?
Jeff Guin: You mentioned the pressure-cooker concept. Does it require specialized equipment or a dedicated laboratory to execute this type of method for drying?
Eric Schindelholz: Supercritical fluids for drying require fairly high pressures, sometimes on the order of scuba tank pressures, so thousands of psi. Ours was below 1,000 psi, but at the same time, you need to have specially constructed vessels that are well engineered and safety checked because as you can imagine, with such high pressures, when something goes wrong you may end up with a lack of a laboratory or artifact or personal effects afterward.
Eric Schindelholz: So, it does require some specialized equipment, but not necessarily specialized laboratories. I mean, just like a freeze dryer that someone can be specialized on or someone trained on, so can a supercritical fluid chamber.
Where can supercritical drying be performed?
Jeff Guin: Where currently can supercritical drying be performed?
Eric Schindelholz:There are only a few laboratories in the world that are really looking into cupercritical drying. I had mentioned St. Andrew’s University. They had a supercritical chamber, which they carried out waterlogged wood conservation.
Eric Schindelholz: I’m not sure what the status is at this moment. Also Clemson University, one of our co-investigators, has a supercritical chamber and are working on developing different techniques using this type of methodology. I know there are a few others … but I’m not sure off the top of my head where they are.
Future trends for supercritical drying
Jeff Guin: So where do you see this research going in the future?
Eric Schindelholz: I think we’ve now established from this research and the research at St. Andrews that this is a fairly viable technique. We carried out our experiments on a very small scale. Our chamber that we used was on the order of a few liters in capacity. At this point, I think there has been enough research that has been conducted so that one could scale up the process and maybe start treating larger artifacts and look at some of the effects that we have observed such as this cracking phenomena, and trying to figure out some of the mechanisms behind that and figure out some of the optimum treatment methods that are out there.
Jeff Guin: Eric, thanks for being on the podcast.
Eric Schindelholz: Thank you.
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