Lessons Learned: You Cannot Treat Dry Wood Like Wet Wood

by Tessa de Alarcon

Documentation of conservation treatments undertaken in the lab is a very important part of what we in the conservation department do at the museum. One of the main reasons we document our treatments is so that conservators in the future don’t have to try to figure out what was done to an object. Instead those future conservators can read our reports and start off knowing the treatment history.  

An array of treatment reports, and log book entries, from the conservation department at the Penn Museum

We are also sometimes those future conservators, looking back at previous treatments. This means that not only can we see an object’s treatment history, we have the opportunity to evaluate and learn from the decisions made by conservators decades ago. Some of these treatments were successful, and some were not. Now we know to avoid those treatments that clearly did not work.

I recently completed a treatment on an object that is a good example of this process. E11151 is a carved wooden figure from Nubia. It was treated before I worked on it by a conservator in the 1970’s. The photo below is how the object looked when it entered the lab at the end of last year.

Before treatment image of E11151 taken in 2019

In the case of this object, it was noted in the 1970’s as having a slightly powdery surface. The conservator conducting the treatment decided to apply a consolidation method that is frequently used on waterlogged wood: immersion in a solution of PEG (polyethylene glycol). PEG treatments are still done today and are very effective at consolidating and stabilizing waterlogged wood before it dries out. But because of this treatment and others done at the museum around this same time period, we have learned that consolidation with PEG is not effective on dry wood, even if it once was waterlogged. E11151 was dry wood and did not come from a waterlogged context.

E11205, E11187, E11203, E11151
E11151 is on the far right.
This publication photo was taken before the object was treated in the 1970’s.

The photo above is a publication photo of the object before it was treated in the 1970’s. While there is no photography with the 1970’s treatment report, there is a sketch which suggests that the object looked similar to this photo when it entered the lab at that time. As you can see there are quite a few differences in the object’s appearance as it entered the lab in 2019 and how it looked before it was treated in the 1970’s.

Before treatment photo of E11151. The red arrow is pointing to an area of thick, white, and hard high molecular weight PEG and the blue arrow is pointing to soft and sticky low molecular weight PEG

Two different molecular weights of PEG were used on this object, a hard high molecular weight one that was used to consolidate the powdery wood and looked white on the surface where it was very thick, and a low molecular weight one that was used to join pieces of the object together and left the surface tacky and sticky. The sticky PEG also trapped a lot of dirt and dust on the surface of the object.

After treatment detail of the top of E11151

The detail above shows the top of the head of the object. The report from the 1970’s states that during treatment the object began to crack and fragment. PEG is typically dissolved in water or ethanol. Both solvents were used in the PEG treatment of this object. These two solvents can be mixed together and during a normal PEG treatment the wet wood starts with PEG in water and then moves to PEG in ethanol: this helps start the drying process. As the waterlogged wood is also already wet, the PEG can penetrate fully into the swollen wood. However, in the case of dry wood, these solvents (ethanol typically has some water in it) introduce moisture into the object, and it starts to swell as wood is very responsive to moisture. It then has to dry out again after the treatment, causing the wood to shrink. This is stressful for the object. This stress is what likely caused the cracking documented in the report and visible in the image above.

E11151, after treatment photo from the treatment completed in 2020

Here you can see what the object looks like now that I have finished my treatment. Consolidation is a very permanent and tricky to reverse treatment, even when adhesives that remain soluble, like PEG, are used. There is currently no way to remove the PEG from this object. All I have done is reduce the PEG on the surface by cleaning it with ethanol. I also used the previous documentation to figure out where small detached fragments went so that it looks more like it did originally. While the treatment that was undertaken in the 1970’s seems to be over-treatment as it caused new problems, some more severe than the problems the object had to begin with, I do also want to recognize that it is because of past experiments like this one that I know not to use PEG on dry wood.

Because wholesale consolidation is a fairly permanent and risky treatment, I think long and hard when I choose a consolidant. I also remain aware that there is a chance that someday, some future conservator, will deem some of my treatments mistakes as well. Hopefully my mistakes will be ones that they can learn from too.

To learn more about PEG treatments for waterlogged organic materials, check out these links:

Ellen Carrlee Conservation blog: High Molecular Weight PEG for basketry

Queen Anne’s Revenge blog: Waterlogged wood – Gotta bulk it up!

Cardiff University Conservation: Newport Ship Workshop