Author: Cory Rogge

A standard method in many conservation science labs is the use of Meth-Prep II (a methanolic solution of trifluoromethylphenyl trimethyl ammonium hydroxide) for the transesterification of triglycerides to methyl esters and esterification of free fatty acids (FAs). However, this method gives you the total fatty acid content of the sample, including free fatty acids, their oxidation products, fatty acids bound in metal soaps, and intact triglycerides. If you want to assess the extent of hydrolysis of an oil paint film, which might be related to water sensitivity, you need to use another method. What Joy has chosen to do is to modify the Meth-Prep II solution to prevent transesterification. She takes 400 μL of Meth-Prep II, evaporates it at 50 ^oC under nitrogen to remove the methanol, rinses it with toluene, and then adds 500 μL of toluene and 100 μL of t-butanol. T-butanol does not permit transesterification, but the methylation of free alcohol groups can still occur. So a sample can be analyzed with this modified solution- only the free fatty acids and fatty acids in metal soaps (non-glycerides or NG) will be methylated, and thus only they will quantified by GC-MS. Then the same sample can be re-treated with the normal Meth-Prep II solution to determine the fatty acids in triglycerides.
As with any quantitative GC-MS method there are potential issues. The sample needs to be well homogenized to ensure extraction and reaction. The efficacy in methylation of fatty acids soaps is not known, and so those species may not react fully and be underrepresented in the chromatograms. She also sees partial evaporation of her internal reference when the sample is retreated with unmodified Meth-Prep II and so that must be accounted for.
With all of these caveats in mind, they began to look at reference paints, including Bellini tube paints and handmade paints in cold-pressed linseed oil. For the new handmade paints the fatty acid profiles of the NG and the total FAs look similar. But pigments do make a difference- for yellow ochre and red iron oxide paints, the NG accounted for 50-57% of the total FAs, but for malachite and lead white paints the NG fraction was much lower, approximately 12%. This might be due to the formation of metal soaps and poor extraction of fatty acids from those complexes.
For water sensitive Winsor and Newton oils she found 17-37% NGs, although the high levels of palmitate present in cadmium paints may indicate the addition of palmitate soaps. However, there was nothing obvious about the fatty acid profiles that seemed to relate to the water sensitivity. Likewise, a water sensitive cadmium paint in a 1961 Jack Youngerman painting, while also having elevated palmitate levels suggesting palmitate soap addition, had nothing else unusual about the binder. To address this issue cadmium yellow and a chrome yellow paint were mixed with linseed oil, applied onto canvas and glass and artificially aged. The resulting cadmium yellow paint film was very water sensitive, while the chrome yellow film was not. However, there was no major difference between the NG ratios of the two films, and so something else must be rendering the paint sensitive. Furthermore, that sensitivity is not related to the palmitate soaps found in the commercial paints. So there’s more work to be done to determine what is going on, but in the meantime beware cadmium paints!
She had the opportunity to analyze a sample from MOMA’s Water Lilies by Monet, which has a very water sensitive surface. The total FA content of the samples were very low, indicating a very medium poor paint, consistent with Monet’s working method of removing media from his paints. The NG content depended strongly on the sample location and pigments present. The pink-orange paint contained no stearate or palmitate, while other samples contained very high palmitate in the NG fraction. Again, the origin of these differences cannot be explained at this time- but might be due to soap formation and incomplete methylation.
Additional objects tested with this methodology included a variety of Italian paintings ranging from 1595-1758 in date, a Byzantine painting, a Fayum portrait, and a Jackson Pollock painting. The Italian paintings had higher NG fractions than was observed for the tube paints and modern paintings, 22-66% of the total fatty acids, but the NG fraction did not correlate with the age of the material- so extent of hydrolysis cannot be used to determine the age of an object. One possible explanation for this is that paintings this old have likely been cleaned several times and this may have impacted the amount of free fatty acids. The Byzantine painting had a higher NG content of 40-90% and the high palmitate and stearate amounts suggested the formation of metal soaps. Joy found that the modified Meth-Prep method simplified the analysis of beeswax and resulted in less overlapped and confusing chromatograms. In 19^th century beeswax she found palmitate and stearate, but in a sample of unpigmented wax from a Roman tomb, she detected neither of those species, implying that they had all evaporated with time. However, in the pigmented Fayum portrait both species were seen, which might be due to the formation of metal soaps, which would help preserve those species, or to the modification of the wax- a contentious subject in the art history field. The white sample from the Pollock painting contained no fatty acids at all, so they evaporated the Meth-Prep solution and reanalyzed it using a method developed for proteins and determined that it was actually a casein paint.
So this seems to be a promising method, and one that could fit easily into a laboratories work flow. Obviously there is more work to be done- particularly on determining efficacy of extraction of fatty acids from soaps and trying to determine how certain pigments create water sensitive films without altering the fatty acid ratios…
 
 
 

“Darling, forever is a long, long time, and time has a way of changing things.”
—Big Mama, The Fox and the Hound.
Kristen chose that quote to begin her talk, and indeed it is true of all objects in cultural heritage institutions. Her particular institution, the Walt Disney Animation Research Library is home to over 60 million objects, including animation drawings, pencil story sketches, watercolors, maquettes, and animation cels, the latter of which make up approximately 10% of the collection. Animation cells are transparent sheets of material, usually celluloid (nitrocellulose), cellulose acetate, or polyester that have been inked on one side, and then painted with gum based media on the other. These materials, particularly cellulose nitrate and cellulose acetate, are highly prone to degradation, so the library partnered with the Getty Conservation Institute to investigate the material nature of the cels and determine if the environmental conditions in the library (62-65 ^oF, 50% RH) and the microclimate packaging used were sufficient to protect these objects.
Phase one of the project involved investigation of the material nature of the animation cels, addressing the questions: what is the base polymer of the cel, what plasticizers are present and in what quantity?  They initially used a portable FTIR and non-destructively analyzed 200 cells dating from 1937-2003, and then chose 80 of those items to do more in-depth, microdestructive GC-MS analysis on. In their initial survey they found cellulose nitrate, cellulose diacetate, cellulose triacetate, and polyester as the base support material. Surprisingly, the material used for a single production would not necessarily be the same; it seemed that the artists used whatever was at hand. Due to the limits of the talk time, Kristen chose to focus on the acetate films for the rest of her talk.
The cellulose acetate cels were found to contain 11 different plasticizers, 6 different types in the diacetate and 5 in the triacetate. These included triphenyl phosphate and a variety of phthalates including dimethyl phthalate (DMP) and bis(2-methoxyethyl) phthalate (DMEP). The plasticizers accounted for 12-19% by weight of the diacetate films and 8-14% by weight of the triacetate films. The diacetate films had an acetyl content of 35-41%, while that of the triacetate films was 43-45%. They are not sure if the range is due to partial hydrolysis or variability in manufacturing. There was no correlation between acetyl content and production year. However, it did appear as if the older films might be a bit more yellow, but more work needs to be done to determine if this is a viable observation.
Phase two was assessing the environmental conditions, or as Kristen said quoting Mufasa from The Lion King, “Everything you see exists together in a delicate balance.” Since this portion was focusing on cellulose acetate cels, Kristen referenced Image Permanence Institute guide for the storage of acetate film (https://www.imagepermanenceinstitute.org/webfm_send/299) and pointed out that the library’s storage conditions were outside of the optimal range recommended by the IPI (50 ^oF or lower, 20-50% RH). However, the cels are composite materials and it is unknown how the gum based paints would react to lower temperatures and what kind of mechanical stresses might be induced by those conditions, so they feel that the chosen conditions are a good compromise. However, belief isn’t the same thing as actual data, so they performed a survey using AD strips, looking at four vaults and 8 different locations. The strips were assessed after 8 and 96 hours and only one location, next to the Bambi collection, was off-gassing acid. They are planning on investigating that collection further to zero in on the problem.
The library also stores and exhibits the cels in passé-partout packages that include a window mat, the cel (usually hinged with Japanese paper and methyl cellulose), matboard, and a sheet of Artsorb conditioned to 50% RH. They include a RH strip to be able to easily determine if the Artsorb is working. This was supposed to be best working practice but after an exhibition of cels at the Grand Palais, they noticed microcracks and paint delamination and became worried that the package was concentrating off-gassed materials and accelerating the decay of the cels. To try and determine what was happening they extracted gas from a sealed package and analyzed the entrapped air by GC-MS. No acetate (at least above 800 ppb) was detected, suggesting that hydrolysis of the cellulose acetate was not occurring.  AD strips were also inserted into packages and no color change noted. Thus they decided that neither the package, nor the exhibition conditions was causing hydrolysis of the acetate groups. They did find adsorbed camphor in a package containing cellulose nitrate, so scavenging of plasticizers might be an issue. Thus, they think that their environmental conditions are generally good and that the passé-partout packages are not the issue. They are not sure what caused the observed deterioration; whether it was mechanical damage caused by temperature changes, vibration during shipping, etc. and are continuing to look into this. They are also planning to begin looking at the ink and paints to better understand the complete nature of the cels.
For more information on this project see the Getty Newsletter:
http://www.getty.edu/conservation/publications_resources/newsletters/29_1/animation.html