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Article

Lead Coupon Reactivity to Organic Acids, Aldehydes, and Esters in an Oddy Test Environment

by
Kylie Blake
1,
Hanna Konicki
1,
Michael J. Samide
1,* and
Gregory D. Smith
2
1
Department of Chemistry and Biochemistry, Butler University, Indianapolis, IN 46208, USA
2
Indianapolis Museum of Art at Newfields, Conservation Science Laboratory, Indianapolis, IN 46208, USA
*
Author to whom correspondence should be addressed.
Heritage 2025, 8(5), 158; https://doi.org/10.3390/heritage8050158
Submission received: 27 March 2025 / Revised: 23 April 2025 / Accepted: 27 April 2025 / Published: 30 April 2025
(This article belongs to the Special Issue Conservation and Restoration of Metal Artifacts)
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Abstract

:
The Oddy test is an accelerated metal corrosion test used extensively by cultural institutions to determine the suitability of a material for use in museums. Alternatively, the use of gas chromatography-mass spectrometry (GC-MS) to directly identify volatile organic compounds (VOCs) from construction materials is growing in popularity because of its comprehensiveness and speed. Interpreting the reactivity of these potential pollutants, however, relies on ‘chemical intuition’ based on observed functional groups since the reactivity of only a handful of common VOCs has been studied intensively with regard to artworks. While short chain organic acids are known to be deleterious to some metals, polymers, and other culturally relevant materials, the common observation of lower volatility acids as well as their complementary aldehydes and esters in these offgassing experiments do not have clear indicators of their potential for artwork damage. In this work, the lead coupon, known to be a sensitive indicator of damaging organic acids, was exposed to known concentrations of a homologous series of organic acids, aldehydes, and esters from C2 to C18. Analysis of the coupon surface by infrared and Raman spectroscopies, and of the headspace within an Oddy jar by GC-MS, provides insights into the corrosion processes of these potential pollutants. Humidity was identified as a necessary component for corrosion to occur, and very volatile and semi-volatile compounds up to C9 created the corresponding lead carboxylate on the coupon surface in addition to lead carbonate. For higher order acids, and to a far lesser extent the esters and aldehydes, a high concentration of the VOC was necessary to induce small amounts of corrosion. In some instances, the gas phase chemistry of the reactor was particularly complex, suggesting mixtures of pollutants may prove more problematic to artist materials than single offgassed species.

1. Introduction

Materials used in museum construction and artwork storage and display must be vetted to determine their potential deleterious impact on cultural heritage objects. The Oddy test is an accelerated metal corrosion test used to determine the suitability of these materials for use in museums and historic environments [1]. While several variants and refinements have been introduced throughout the years [2,3,4,5,6], the general premise remains the same; three metal coupons (Cu, Pb, and Ag) are exposed in a sealed container to the emissions from the material being tested for 28 days at elevated temperature and relative humidity. Upon completion of the test, the metal coupons are inspected visually for evidence of corrosion versus a control coupon. If significant corrosion (tarnish, discoloration, or crystalline salts, for example) is observed in the sample set of coupons, the material undergoing testing is deemed unsuitable for use. Despite its widespread use, the Oddy test requires expensive consumable materials, is labor and time-consuming, and generates heavy metal waste. Furthermore, because of the wide number of variations in testing protocols, the results from this test can be open to differences in interpretation [6].
Efforts to find new methods of material suitability testing that employ chromatographic separation have been reported in the literature [7,8,9,10,11,12,13,14,15,16,17,18,19,20]. These methods generally examine the volatile organic compounds (VOCs) and some inorganic emissions from a material and can be divided into two primary techniques: Solid-Phase Microextraction (SPME) and Thermal Desorption (TD or DTD). SPME has the advantage that sampling can occur inside any enclosed space, whether a headspace vial containing a museum construction material or casework containing an art object. SPME is limited by a stationary phase adsorption equilibrium step that could preferentially concentrate one class of pollutant over another. TD (or DTD) removes the equilibration step as all emitted VOCs are collected and trapped at cryogenic temperatures prior to analysis. However, samples must fit into the TD system, making this technique impossible for testing inside environments, such as a museum case. Regardless of the method, once the VOCs have been identified, the potential reactivity of those chemicals toward cultural heritage materials can be inferred.
Linking the identity of a VOC to potential corrosion on a metal coupon or another type of degradation of a cultural heritage material requires an advanced understanding of chemical reactivity and has been referred to as “chemical intuition” [17]. This intuition requires an understanding of the reactivity of different classes of compounds with the metal coupons found in the Oddy test, as well as other VOCs present in the warm and humid environment inside the Oddy jar. A test material that emits a cocktail of VOCs could increase the likelihood of cross-reactions prior to interaction with the metal coupons [21,22]. All of these variables make it necessary to link the VOCs detected via chromatography to the actual processes happening inside the Oddy jar.
Organic acids are compounds known to corrode metals, catalyze degradation reactions, and discolor pigments [23,24,25]. Volatile organic acids, such as acetic and propionic acid, are known pollutants in museum environments, and have been extensively studied [20,26,27,28]. It remains unclear whether organic acids with lower solubility and volatility, such as nonanoic and stearic acid, change the acidity in the atmosphere and promote corrosion of the lead coupon and, therefore, pose a threat to cultural heritage objects. Furthermore, other classes of compounds such as aldehydes and esters, which can oxidize or hydrolyze under certain conditions to produce organic acids, have not been specifically studied to determine their likelihood to damage cultural materials.
In this work, a series of organic acids with different volatility and solubility in water have been employed as single-chemical pollutants in an Oddy jar environment. The lead Oddy test coupon was studied exclusively in this work because it is known to be highly sensitive to organic acids. Detection and identification of the corrosion products formed by exposure of various organic acids to the lead coupon under various test conditions can inform a mechanism of reactivity and provide greater definition to “chemical intuition”. Furthermore, an analogous series of ester and aldehyde homologs have been studied as single-chemical pollutants under the same test conditions. Corrosion products on the lead coupon from exposure to aldehydes and esters, as well as the chemical profile of the Oddy jar atmosphere due to the reactivity of these chemicals, can lead to an understanding of how chemical reactions inside the testing environment form pollutant molecules. By understanding how organic acids, esters, and aldehydes behave in an Oddy test, one can understand better how these classes of compounds impact corrosion on an Oddy coupon, and by analogy, damage cultural heritage objects. Therefore, when a VOC emitted from a test material is detected using a GCMS method, it can be evaluated based on the classification of the chemical and flagged as a known pollutant so that suitable alternative materials can be identified and used.

2. Experimental

Chemicals—Lead metal (J.T. Baker, Avantor, Phillipsburg, NJ, USA, 99.994%) was polished using a 3200 Micromesh cloth immediately prior to use. Chemicals were used as received: lead(II) carbonate (Sigma-Aldrich, St. Louis, MO, USA, ACS grade), lead(II) acetate trihydrate (Jansen Chimica, Johnson & Johnson, New Brunswick, NJ, USA, >99%), acetic acid (Fisher Scientific, Waltham, MA, USA, ACS Grade), propionic acid (Fisher Scientific, Waltham, MA, USA, 99%), butyric acid (TCI, Portland, OR, USA, 99%), valeric acid (TCI, Portland, OR, USA, 98%), hexanoic acid (TCI, Portland, OR, USA, 98%), heptanoic acid (Sigma Aldrich, St. Louis, MO, USA, 96%), nonanoic acid (Eastman, Kingsport, TN, USA, technical), lauric acid (Sigma-Aldrich, St. Louis, MO, USA, 98%), palmitic acid (Oakwood, Estill, SC, USA, 99%), stearic acid (Fisher Scientific, Waltham, MA, USA,, 95%), ethyl acetate (Sigma-Aldrich, St. Louis, MO, USA, 99.7%), ethyl propionate (Eastman, Kingsport, TN, USA, 98%), ethyl valerate (Sigma-Aldrich, St. Louis, MO, USA, 98%), ethyl heptanoate (Sigma-Aldrich, St. Louis, MO, USA, 98%), ethyl nonanoate (Sigma-Aldrich, St. Louis, MO, USA, 97%), isopropyl laurate (Combi-Blocks, San Diego, CA, USA, 98%), acetaldehyde (Sigma-Aldrich, St. Louis, MO, USA, 40% in water), propionaldehyde (Sigma-Aldrich, St. Louis, MO, USA, 99%), butyraldehyde (Sigma-Aldrich, St. Louis, MO, USA, 99%), valeraldehyde (TCI, Portland, OR, USA, 95%), hexanal (TCI, Portland, OR, USA, 98%), heptanal (Sigma-Aldrich, St. Louis, MO, USA, 95%), and nonanal (Combi-Blocks, San Diego, Ca, USA, 96%). Water for the Oddy test was obtained from a Nalco Water DI Express® (Naperville, IL, USA) dual-bed deionization system. Oddy jars were purged with Ultra High Purity (99.9999%) argon or Ultra Zero Grade Air (AWG, Indianapolis, IN, USA)for specific experiments as detailed in Table 1.
Synthesis of lead(II) nonanoate—A commercial reference sample of the metal carboxylate was not available, so the compound was synthesized in the lab. A 1.1 mol excess of nonanoic acid was added dropwise to an aqueous solution of sodium hydroxide, after which the pH was measured with a test strip to ensure that no hydroxide remained. A waxy precipitate formed upon the addition of an aqueous solution of lead(II) nitrate. The solid was isolated via centrifugation and washed three times with deionized water. Once dry, the lead(II) nonanoate reference material was characterized using infrared and Raman spectroscopy and compared to the literature [29].
Performing the Oddy Test—Figure 1A shows the system adapted from the Metropolitan Museum of Art “3-in-1” Oddy test [30] used to perform accelerated corrosion testing. In general, 500 µL of water in a 700 µL sample tube was placed inside a 100 mL Oddy jar. A stainless-steel coupon holder was positioned in the mouth of the jar, and lead coupons were suspended in the jar for study. Pollutant chemicals were spiked directly into each jar by adding a known volume of the pure compound. Jars were sealed with Viton O-rings and tightened using 4 N-m of force. Pollutant amounts in Table 1 and Table 2 are expressed as the mmol of chemical added per jar volume (mM) because different volume reaction jars were used in these studies.
Three modifications were made to accelerate the testing procedure and ensure that the pollutant and not the jar was responsible for any detected corrosion. First, control jars were run in series with sample jars, so that a jar must pass control with no detectable corrosion (visible inspection and Raman analysis) on the lead coupon before the jar system (lid, O-ring, and re-polished lead coupon) could be used with a pollutant source. This is different from the published procedure where control and sample jars are run in parallel using different jar systems, and a side-by-side comparison of coupons is needed for analysis and evaluation. Second, all tests were performed at both the traditional 60 °C temperature for 4 weeks and an elevated 80 °C temperature for 1 week. A comparison of the two temperature/time combinations is presented in the discussion. Finally, jars only contained the lead coupon during testing (as opposed to the traditional three-coupon system consisting of lead, copper, and silver), as lead is known to be very sensitive to organic acids.
Analysis of the Lead Coupon—In addition to visual inspection of the coupon, all samples of lead removed from the Oddy jar were subject to analysis by Raman spectroscopy using a Metrohm BW-Tek i-Raman + (Plainsboro, NJ, USA) spectrometer equipped with a 495 mW 785 nm laser. The system was coupled with a Metrohm BW-Tek microscope equipped with 20× and 50× objectives. The system was controlled using Metrohm BWSpec software (ver. 4.15_10) and all spectra were acquired using the 50× objective with 20% laser power (34.9 mW measured with a ThorLabs PM100D (Newton, NJ, USA) meter at the laser focus), 2 s integration, and 10 co-adds at a 4.5 cm−1 resolution. Corrosion products were also analyzed via infrared spectroscopy using a Thermo Scientific Nicolet iS10 FT-IR spectrometer (Waltham, MA, USA) equipped with a Smart iTR ATR stage with a ZnSe crystal. Data was collected (16 co-adds at a 4 cm−1 resolution) and processed using Thermo Scientific Omnic software (v. 7.1a).
Kinetics Measurements—The rate of formation of lead(II) oxide on the lead coupon was measured following a modified literature procedure [29]. Three polished lead coupons of known mass (±0.00001 g) were suspended in an Oddy test jar placed in an 80 °C oven. At specific time points, the coupons were removed from the jar, weighed, returned to the jar, and the jar was returned to the oven. This step ensured oxygen was replenished in the jar between each measurement. A parabolic rate law was used to assess the data, and a plot of (Δmass/area2)2 vs. time was used to find the rate constant.
Gas Chromatography—Mass Spectrometry—Headspace analysis inside the Oddy jar was performed using a custom Oddy jar fitted with an Ace#7 threaded glass adapter sealed with a green GC inlet septum (Figure 1B). Additional headspace analyses were performed using 20 mL headspace vials (Restek, Bellefonte, PA, USA, 23082) sealed with screw-thread caps (Restek, Bellefonte, PA, USA, 23092). Oddy test volumes were proportionally scaled by a factor of 5 for analyses carried out in the smaller headspace vials to maintain the same reactor:water ratio (200:1), but the general procedure remained the same. During and after the Oddy analysis, a 300 µL volume of the gas inside the hot Oddy jar was removed by a 500μL Hamilton gas-tight syringe and injected into a Thermo Scientific 3100 gas chromatograph (Waltham, MA, USA) equipped with a Frontier Lab Micro Jet Cryo-trap (Quantum Analytics, Houston, TX, USA, MJT-1035E). Separation was achieved with a Phenomenex (Torrance, CA, USA) Zebron ZB-50 high polarity capillary column (30 m × 0.25 mm × 0.15 µm) following a standard thermal program (40 °C for 5 min; 10 °C/min ramp to 250 °C; hold at 250 °C for 6.5 min), and detection was performed using a Thermo Scientific ISQ single quadrupole mass spectrometer (Waltham, MA, USA). Compound identification was performed using Xcalibur software (v. 4.5.445.18) and the NIST2014 mass spectral library.

3. Results and Discussion

Control coupons—Lead metal naturally undergoes oxidation in air to form a thin surface coating of lead(II) oxide (PbO) on the coupon, which appears dull gray with some streaks of red (litharge) or yellow (massicot). Because of the potential importance of this thin oxide layer in further corrosion of the metal coupon, the rate of this reaction at 80 °C and 100% relative humidity was studied in quadruplicate, and kinetics results are shown in Figure 2. The data reveal a parabolic rate law, and the initial rate of formation of PbO in the Oddy environment is 5.7 µg/cm2/h. After 14 days, the reaction appears to slow to under 2 µg/cm2/h. Once the coating of PbO is sufficiently thick, the access of molecular oxygen to the bare metal surface is apparently hindered, and the reaction slows. This rate change due to coating thickness is like that found by Tyagi [31], though those data were collected at temperatures greater than 80 °C. The measured rate at 80 °C suggests that PbO forms slower than the visual appearance of the white acid corrosion products (observed visually), supporting the idea that the organic acid can also react directly with the lead metal to form corrosion.
Lead coupons undergo a polishing step prior to use to remove most of this surface coating of PbO and reveal the lead metal. Figure 3 shows a photograph of four lead coupons related to the present work, and the insets show each coupon under 20× magnification with diffuse illumination. Coupon A presents a freshly polished lead surface where the bulk of PbO has been removed, and polishing marks are clearly seen in the lead. However, some PbO remains and is visible on both images of Coupon A. Coupon B shows the lead coupon after removal from a control Oddy jar when no pollutant was present. The coupon has a dull gray appearance and has lost distinct polishing marks. The presence of PbO can be observed as orange streaks in the magnified image. No other evidence of corrosion is observed on a successful control coupon. Coupon C shows lead after exposure to 6.2 mM acetic acid for 1 week at 80 °C and 100% relative humidity. The fine, white crystalline corrosion product consists of lead(II) carbonate and lead(II) acetate as determined by vibrational spectroscopy, and the polishing marks are obscured or eroded by the corrosion. For comparison, Coupon D shows lead after exposure to 4.3 mM nonanoic acid for 1 week at 80 °C and 100% relative humidity. The surface corrosion has a waxy, flaky consistency that was identified by vibrational spectroscopy as primarily lead(II) nonanoate, though a small amount of lead(II) carbonate is sometimes observed. In all Oddy test experiments, the jar system must pass a control (Coupon A to Coupon B) before the reused lead coupon is exposed to a pollutant. If a control test shows any detectable corrosion products from visual inspection or from spectroscopic detection other than the small amount of PbO normally present on a control coupon, the lid and O-ring are rejected and cannot be used for further studies.
Volatile Organic Acids—Organic acids and aldehydes generated in an Oddy test, along with other acidic gases (e.g., SOx, NOx, HCl, etc.), are readily evidenced by the lead coupon [30]. Acetic acid, as a very volatile and reactive acid, rapidly corrodes the lead coupon forming both lead acetate and lead carbonate. Acetic acid as a museum pollutant has been well documented [23,24,25], and the mechanism for the corrosion of lead by acetic acid has been studied [26,27,28]. Other organic acids have received far less attention in the conservation field.
Table 1 lists the results of corrosion testing for the lead coupon in this work after exposure to a homologous series of organic acids in an Oddy jar, starting with acetic acid. In the presence of both higher (6.2 mM) and lower (0.28 mM) concentrations of acetic acid in an Oddy test environment (room air, 100% RH, 60 or 80 °C), lead(II) acetate and lead(II) carbonate were detected using Raman spectroscopy (Figure 4). When the Oddy jar was purged with argon, but still contained water, only lead(II) acetate was detected, suggesting the carbonate forms as a result of the presence of air. In the argon environment, no carbon dioxide is present inside the jar to allow the formation of the carbonate corrosion product. When water was removed by purging the Oddy jar with dry air (−50 °C dew point, 1 ppm CO2), neither of the original corrosion products were detected, and the lead coupon remained pristine. When dry air was used to purge the jar, but anhydrous sodium carbonate (a constant source of CO2 generation at elevated temperatures such as these [32]) was present along with the acetic acid pollutant, no corrosion products were detected. This confirms that water (humidity) is necessary inside the jar for corrosion reactions to occur.
Table 1. Reactivity of organic acids toward lead in Oddy Tests.
Table 1. Reactivity of organic acids toward lead in Oddy Tests.
Pollutant ChemicalConcentration(Condition)Corrosion Detected
80 °C for 7 Days60 °C for 28 Days
Acetic Acid (C2)6.2 mM(humid air)Pb(OAc)2 and PbCO3Pb(OAc)2 and PbCO3
0.28 mM(humid air)Pb(OAc)2 and PbCO3Pb(OAc)2 and PbCO3
6.2 mM(dry air + Na2CO3)None Detected---
6.2 mM(humid argon)Pb(OAc)2---
Propionic Acid (C3)0.30 mMPb(OPr)2Pb(OPr)2
Butanoic Acid (C4)0.30 mMPb(OBu)2 and PbCO3Pb(OBu)2
Valeric Acid (C5)0.30 mMPb(OVal)2Pb(OVal)2
Hexanoic Acid (C6)0.30 mMPb(OHex)2Pb(OHex)2
Heptanoic Acid (C7)0.30 mMPb(OHep)2Pb(OHep)2 and PbCO3
Nonanoic acid (C9)4.3 mMPb(ONon)2Pb(ONon)2
0.54 mMPb(ONon)2Pb(ONon)2
Lauric acid (C12)0.40 mMPb(OLau)2 and PbCO3Pb(OLau)2 and PbCO3
Palmitic acid (C15)3.8 mMNone DetectedTrace PbCO3
0.36 mMNDTrace PbCO3
Stearic Acid (C18)3.8 mMNDTrace PbCO3
0.32 mMNDND
Figure 5 depicts a mechanism for how acetic acid could form corrosion products through reaction with the lead coupon. At 100% RH and room temperature (21 °C), water forms a thin film (reportedly at least 10 monolayers thick) on the surface of the lead coupon [33]. In the proposed mechanism, acetic acid establishes an equilibrium, partitioning between the air, the Oddy test water reservoir, the glass surfaces, and the aqueous layer on the metal coupon. The hydronium ion (H3O+) in the hydration layer on the metal reacts with a surface coating of lead(II) oxide to form lead(II) ion (Figure 5A). Alternatively, H3O+ can react with lead metal directly in a redox process to form lead(II) ion (Figure 5B). Once the lead(II) ion is formed and the aqueous layer becomes saturated with acetate ion, lead(II) acetate trihydrate solid can precipitate, which is observed as a white solid on the surface of the lead coupon. Carbon dioxide can also establish an equilibrium in the water layer, forming carbonic acid. This multi-step process could be accelerated in the presence of catalytic amounts of hydronium ions, since control jars containing the lead coupon in the absence of acetic acid showed no visible lead carbonate corrosion. Once formed, the carbonic acid in solution can react with lead metal or lead oxide to form lead carbonate (Figure 5C).
To examine this generalized mechanism further, multiple volatile and soluble acids were used as single-chemical pollutants spiked into the Oddy jar, as described in Table 1. A homologous series of organic acids from propionic acid to dodecanoic acid all showed evidence of the corresponding lead(II) carboxylate salt corrosion. Figure 6 presents the infrared spectra, and Figure 7 shows the Raman spectra for the lead corrosion product formed upon exposure to nonanoic acid (C9), along with the corresponding spectrum of a lead(II) nonanoate reference. In both cases, the spectra for the reference compound are nearly identical to the corrosion product, indicating that lead(II) nonanoate is formed on the surface of the lead coupon. The shift in the infrared spectrum for the carbonyl peak at 1700 cm−1 for nonanoic acid to 1500 cm−1 for nonanoate anion is consistent with coordination to a metal center, as shown previously [34]. An analysis of corrosion products formed during exposure of the lead coupon to the other organic acids studied in this work produced Raman and infrared spectra similar to those found in the literature for the corresponding lead(II) carboxylate salts [29].
Non-volatile organic acids—Table 1 also lists the corrosion products detected when the lead coupon was exposed to palmitic acid and stearic acid, which have vanishingly small vapor pressures (9 × 10−3 and 2 × 10−3 mmHg, respectively) at 80 °C. Increasing the chain length of the organic acid decreases the volatility and solubility of these acids in water. When concentrations are high, lead(II) carbonate is detected for these long-chain organic acids with a 28-day exposure at 60 °C, but no corrosion products are observed for the 7-day exposures at 80 °C. Furthermore, when concentrations are low, only palmitic acid exposure results in the formation of lead(II) carbonate when the jar was heated for 28 days at 60 °C. When the pollutant cannot migrate through the jar to encounter the hydrated lead surface due to low volatility, it is possible that only a small amount of carbonic acid formed in water near the non-volatile acid (Figure 5C) is available for reaction. This carbonic acid can then travel throughout the jar to the lead coupon and cause carbonate corrosion on the lead coupon surface. With longer exposure times, the carbonic acid concentration in the jar increases, and the lead surface becomes a sink for this CO2. When the chain length is very long (stearic acid) and the concentration of the pollutant is small (<3.2 mM), no corrosion products were detected. This indicates that the hydronium ion concentration is similar to that present in a control jar, meaning stearic acid is no longer acting as a pollutant, and carbonic acid formation does not occur.
Organic Aldehydes and Esters—There are several sources of volatile organic esters and aldehydes in heritage environments including modern artistic materials, construction materials and furnishings, perfumes and scents added to enhance an exhibition, cleaning products, and even museum guests [35]. Aldehydes in an oxygen-rich environment can undergo oxidative degradation that proceeds through a radical intermediate [36]. Table 2 shows corrosion data along with chromatographic data for Oddy jars spiked with a single chemical pollutant aldehyde homologous series. For each aldehyde, the Oddy test was performed with and without the lead coupon.
Table 2. Reactivity of aldehydes in the Oddy jar environment at 80 °C for 7 days. ‘SIC’ indicates a GC peak only detectable in selected ion mode at the m/z ion indicated.
Table 2. Reactivity of aldehydes in the Oddy jar environment at 80 °C for 7 days. ‘SIC’ indicates a GC peak only detectable in selected ion mode at the m/z ion indicated.
Pollutant Chemical (mM)Corrosion on Coupon aVOCs Detected in Presenceof Lead Coupon bVOC Detected in Absenceof Lead Coupon
Acetaldehyde(1.5 mM)PbCO3Pb(OAc)2Dihydrofuran2,4,6-trimethyl-1,3,5-trioxaneHexadienalOctatrienalAcetic acid
Propionaldehyde(1.4 mM)PbCO3Pb(OPr)22,4-dimethylfuranEthanol2-ethyl-2-butenal2,4,6-triethyl-1,3,5-trioxanePropionic acidDiethyl ketone2-ethyl-2-butenal
Butyraldehyde(1.4 mM)PbCO3Pb(OBu)2PropanolDipropyl ketone2-ethyl-2-hexenal2,4,6-tripropyl-1,3,5-trioxaneButyric acidDipropyl ketone2-ethyl-2-hexenal2,4,6-tripropyl-1,3,5-trioxane
Valeraldehyde(1.5 mM)PbCO3Pb(OHex)21-ButanolValeric acid (SIC = m/z 73)Dibutyl ketoneValeric acidDibutyl ketonePentanoic anhydride
Hexanal(1.5 mM)PbCO3Pb(OHex)21-Pentanol (SIC = m/z 70)Hexanoic acid (SIC = m/z 87)Dipentyl ketoneHexanoic acidDipentyl ketone
Heptanal(1.4 mM)PbCO3Pb(OHep)21-HexanolHeptanoic acid (SIC = m/z 87)Dihexyl ketoneHeptanoic acidDihexyl ketone
Nonanal c(1.5 mM)PbCO3Pb(ONon)21-OctanolNonanoic acid (SIC = m/z 129)Dioctyl ketoneNonanoic acidDioctyl ketone
a Lead corrosion products on coupons from Oddy jars incubated at 60 °C for 28 days were identical to those reported for 80 °C trials. b A similar suite of reaction products was detected in samples run at 60 °C for 28 days. c GC-MS data for nonanal is shown in Figure 8.
Figure 8 shows chromatograms obtained for the Oddy jar headspace environment spiked with nonanal (C9) before reaction, after reaction in the absence of lead, and after reaction in the presence of lead. Product distributions inside the jars varied significantly under these conditions. When the lead coupon was absent, the primary VOC detected in the jar was nonanoic acid with minor amounts of dioctyl ketone. However, the presence of the lead coupon changes the mechanism of reaction for the aldehyde. In this case, the lead coupon acts as a sink for the carboxylic acid, forming both lead(II) carbonate and lead(II) nonanoate (See Figure 5). The Oddy jar environment reflects this since nonanoic acid is only detected in trace amounts due to its depletion by reaction with the lead and removal from the jar’s atmosphere. Interestingly, the jar then contained significant amounts of octanol, and the amount of dioctyl ketone increased by a factor of 20. A published mechanism for the formation of carboxylic acid (B), the alcohol (C), and the dialkyl ketone (D) is summarized in Figure 9 [36]. In the absence of lead, the acyl radical can react with oxygen and abstract a hydrogen to make the peracid, which can then undergo rearrangement and form the carboxylic acid. When the lead coupon is present, nonanoic acid still forms, but it is possible that the alkyl radical is stabilized by the lead coupon to favor the formation of the alcohol and dialkyl ketone. These results show how the Oddy jar environment can promote different complex mechanistic pathways for the reaction of pollutant molecules and may likely impact the results of an Oddy test and material suitability determination.
Esters are known to undergo hydrolysis in the presence of catalytic amounts of acid or base to produce the corresponding organic acid along with an alcohol as shown in Figure 10.
When the concentration of the ethyl ester pollutant in the Oddy jar is low (1.5 mM), only trace amounts of ethanol were observed using GCMS. The presence of the anticipated organic acid in the ester-spiked Oddy jar was difficult to confirm. The hydrolysis reaction that produces the organic acid is an equilibrium, and given the very small amounts of alcohol detected in all trials, the concentration of the organic acid is also expected to be extremely low. Because the acid co-elutes with the ester in the GCMS analysis used to probe the reactor headspace, and because both the acid and the ester share many common ion fragments, it was impossible to identify the acid even with a selected-ion chromatogram (SIC). While Raman spectroscopy did reveal a very small signal at 1050 cm−1, corresponding to the formation of lead(II) carbonate in the presence of various esters, no substantial coverage of corrosion product on the coupon was observed. This evidence suggests that esters are not acting as a significant pollutant in the Oddy test when present in low concentration, possibly due to low rates of conversion to the corresponding acid.
When the concentration of ethyl ester homologs is high (10 mM), the GCMS data reveal the presence of ethanol in the Oddy jar. Ethanol is the primary VOC reaction product detected when ethyl acetate is the pollutant, and the lead coupon from this jar was coated with a white crystalline product identified as lead(II) acetate by Raman spectroscopy. As the chain length of the ester increased (using ethyl propanoate, ethyl butyrate, ethyl valerate, ethyl heptanoate and ethyl nonanoate), the amount of ethanol detected was significantly smaller than that seen for ethyl acetate, but the corresponding lead(II) carboxylate corrosion was still detected on the coupon. This evidence demonstrates that, when esters are present in higher concentrations in the Oddy jar environment, hydrolysis can occur to differing extents based on the identity of the ester. Furthermore, the organic acid produced by the hydrolysis reaction will corrode the lead coupon.

4. Conclusions

Determination of material suitability for use in heritage and historic environments is most often accomplished using the Oddy test, but instrumental methods can provide a way to determine more quickly the specific VOCs emitted from the material. Whether these VOCs are in fact damaging to artists’ materials requires chemical intuition and further confirmation. In this work, organic acids, esters, and aldehydes have been added as single-chemical pollutants to the Oddy jar, and corrosion of the lead has been examined. Volatile organic acids have been shown to cause corrosion by the formation of lead carbonate or lead carboxylate. After long exposures of 1 to 4 weeks, non-volatile organic acids induce trace amounts of corrosion on the lead coupon, and at low concentration (<3.2 mM) may not be considered a pollutant.
Aldehydes have been shown here to undergo oxidative degradation inside the Oddy jar, likely through a radical intermediate, and products of this reaction can act as pollutant molecules, forming lead carboxylate salts. Organic esters in the Oddy jar also show evidence of hydrolysis, but only when present in high concentrations (>10 mM) are corrosion products observed.
It is possible that the primary mechanism for corrosion proceeds when an organic acid (present or produced by side reactions) ionizes on the hydrated surface of the lead coupon to produce hydronium ion (H3O+). This, in turn, can react with the lead metal or the naturally forming lead oxide to produce lead(II) ions. Importantly, dry reaction atmospheres showed no corrosion of the lead, even with highly volatile organic acids known to be aggressive pollutants under ambient conditions. This observation supports the use of desiccants in display and storage containers to avoid the pollution-induced corrosion of lead or possibly other metal artifacts.
The adage “the dose makes the poison” is also at play in Oddy test reactors. The amount of the potential pollutant remains a factor, as evidenced by the reaction products identified for non-volatile acids and esters present in different concentrations. In addition, these experiments suggest that the presence of multiple organic compounds in the Oddy test environment increases the complexity of reactivity. For instance, if both an ester and organic acid are present in the Oddy jar, the acid could catalyze the hydrolysis of the ester to produce more organic acids, a subject currently under investigation by the authors. Given the complexities of the potential reactivities of pollutant molecules, detection of a volatile (C9 or smaller) organic acid, ester, or aldehyde in instrument-based material suitability testing should flag a material as a potential threat to objects in a museum environment and warrant further study, including perhaps a full Oddy test to ensure the safety of heritage collections when exposed to materials that emit these compounds. Conversely, non-volatile acids, aldehydes, and esters (like stearic or palmitic acids) are unlikely to cause significant damage to metal artworks unless present at high concentrations under dire conditions of extreme heat and high relative humidity.

Author Contributions

Conceptualization, M.J.S. and G.D.S.; methodology, K.B., H.K., M.J.S. and G.D.S.; investigation, K.B., H.K. and M.J.S.; data curation, M.J.S. and G.D.S.; writing—original draft preparation, M.J.S. and G.D.S.; writing—review and editing, K.B., H.K., M.J.S. and G.D.S.; supervision, M.J.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photographs of Oddy reaction jars. (A) Adaptation of the Met 3-in-1 system. (B) Oddy jar modified with an Ace #7 glass adapter for sampling the atmosphere inside the jar.
Figure 1. Photographs of Oddy reaction jars. (A) Adaptation of the Met 3-in-1 system. (B) Oddy jar modified with an Ace #7 glass adapter for sampling the atmosphere inside the jar.
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Figure 2. Kinetics plot showing the rate of formation of lead(II) oxide film on a lead Oddy coupon at 80 °C and 100% RH. The rate of formation initially proceeds at a rate of 5.7 µg/cm2/h (blue markers). After 320 h, the rate of formation slows to under 2 µg/cm2/h (orange markers). Each point is an average of four trials.
Figure 2. Kinetics plot showing the rate of formation of lead(II) oxide film on a lead Oddy coupon at 80 °C and 100% RH. The rate of formation initially proceeds at a rate of 5.7 µg/cm2/h (blue markers). After 320 h, the rate of formation slows to under 2 µg/cm2/h (orange markers). Each point is an average of four trials.
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Figure 3. Photograph of lead Oddy coupons. Inset shows the coupon under 20× magnification with diffuse illumination. (A) freshly polished lead, (B) control coupon after incubation in jar without pollutant, (C) coupon after exposure to acetic acid, (D) coupon after exposure to nonanoic acid.
Figure 3. Photograph of lead Oddy coupons. Inset shows the coupon under 20× magnification with diffuse illumination. (A) freshly polished lead, (B) control coupon after incubation in jar without pollutant, (C) coupon after exposure to acetic acid, (D) coupon after exposure to nonanoic acid.
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Figure 4. Raman spectra of lead(II) carbonate (PbCO3, purple box), lead(II) acetate trihydrate (Pb(OAc)2·3H2O, tan boxes), and corrosion products on a lead coupon exposed to 6.2 mM acetic acid in an Oddy jar environment (80 °C and 100% RH).
Figure 4. Raman spectra of lead(II) carbonate (PbCO3, purple box), lead(II) acetate trihydrate (Pb(OAc)2·3H2O, tan boxes), and corrosion products on a lead coupon exposed to 6.2 mM acetic acid in an Oddy jar environment (80 °C and 100% RH).
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Figure 5. Schematic of a proposed mechanism for the creation of corrosion products when a lead coupon is exposed to acetic acid in an Oddy jar environment. The rows separate metallic lead (gray), the hydrated surface layer (blue), and the reactor air atmosphere (white). (A) reaction of acid with PbO; (B) reaction of acid with Pb; (C) reaction of acid with CO2 and then Pb.
Figure 5. Schematic of a proposed mechanism for the creation of corrosion products when a lead coupon is exposed to acetic acid in an Oddy jar environment. The rows separate metallic lead (gray), the hydrated surface layer (blue), and the reactor air atmosphere (white). (A) reaction of acid with PbO; (B) reaction of acid with Pb; (C) reaction of acid with CO2 and then Pb.
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Figure 6. Infrared spectra of lead(II) nonanoate reference compound (blue, lower) and the corrosion product on the lead coupon after exposure to nonanoic acid in the Oddy jar (orange, middle). A reference spectrum for nonanoic acid (green, top) illustrates a shift in the carbonyl peak, indicating the formation of the carboxylate salt.
Figure 6. Infrared spectra of lead(II) nonanoate reference compound (blue, lower) and the corrosion product on the lead coupon after exposure to nonanoic acid in the Oddy jar (orange, middle). A reference spectrum for nonanoic acid (green, top) illustrates a shift in the carbonyl peak, indicating the formation of the carboxylate salt.
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Figure 7. Raman spectra of lead(II) nonanoate (Pb(ONon)2) reference compound (blue) and the corrosion product on the lead coupon after exposure to nonanoic acid in an Oddy jar environment (orange).
Figure 7. Raman spectra of lead(II) nonanoate (Pb(ONon)2) reference compound (blue) and the corrosion product on the lead coupon after exposure to nonanoic acid in an Oddy jar environment (orange).
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Figure 8. Chromatograms for the GC-MS analysis of the atmosphere inside an Oddy jar spiked with 1.5 mM nonanal before reaction (orange, top), after reaction without lead present in the system (teal, middle), and after reaction with lead present in the system (blue, lower). Minor peaks likely correspond to reaction products related to other mechanisms [36], but they provided poor library matches. Peak area for dioctyl ketone was 8.7 × 107 with lead and 4.8 × 106 without lead, indicating a significant change in the reaction inside the Oddy jar.
Figure 8. Chromatograms for the GC-MS analysis of the atmosphere inside an Oddy jar spiked with 1.5 mM nonanal before reaction (orange, top), after reaction without lead present in the system (teal, middle), and after reaction with lead present in the system (blue, lower). Minor peaks likely correspond to reaction products related to other mechanisms [36], but they provided poor library matches. Peak area for dioctyl ketone was 8.7 × 107 with lead and 4.8 × 106 without lead, indicating a significant change in the reaction inside the Oddy jar.
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Figure 9. A reaction mechanism showing the reaction pathways possible for aldehydes in the Oddy Jar [36].
Figure 9. A reaction mechanism showing the reaction pathways possible for aldehydes in the Oddy Jar [36].
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Figure 10. Hydrolysis of an ester to form a carboxylic acid and alcohol.
Figure 10. Hydrolysis of an ester to form a carboxylic acid and alcohol.
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MDPI and ACS Style

Blake, K.; Konicki, H.; Samide, M.J.; Smith, G.D. Lead Coupon Reactivity to Organic Acids, Aldehydes, and Esters in an Oddy Test Environment. Heritage 2025, 8, 158. https://doi.org/10.3390/heritage8050158

AMA Style

Blake K, Konicki H, Samide MJ, Smith GD. Lead Coupon Reactivity to Organic Acids, Aldehydes, and Esters in an Oddy Test Environment. Heritage. 2025; 8(5):158. https://doi.org/10.3390/heritage8050158

Chicago/Turabian Style

Blake, Kylie, Hanna Konicki, Michael J. Samide, and Gregory D. Smith. 2025. "Lead Coupon Reactivity to Organic Acids, Aldehydes, and Esters in an Oddy Test Environment" Heritage 8, no. 5: 158. https://doi.org/10.3390/heritage8050158

APA Style

Blake, K., Konicki, H., Samide, M. J., & Smith, G. D. (2025). Lead Coupon Reactivity to Organic Acids, Aldehydes, and Esters in an Oddy Test Environment. Heritage, 8(5), 158. https://doi.org/10.3390/heritage8050158

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