Bisulca Et Al-2012-Deterioration of Fossil Resins and Implications for Amber Taphonomy-AM MUS NOV

8/12/2019 Bisulca Et Al-2012-Deterioration of Fossil Resins and Implications for Amber Taphonomy-AM MUS NOV

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2 AMERICAN MUSEUM NOVIAES NO. 3734

represent three chemical subclasses o ossil resins, and each o the resins reacted differently tothe various aging conditions, with New Jersey amber particularly unstable. Based on these results,amber collections should be stored in an environment with stable humidity, relatively low heat,and minimal exposure to light. Anoxic sealing and storage, and particularly embedding ambersamples in a high-grade epoxy, may be beneficial, and urther investigation is indicated.

INRODUCION

Te Division o Invertebrate Zoology at the American Museum o Natural History housesa scientifically unique collection o ossilierous amber, including material ranging rom theEarly Cretaceous (145110 Ma) to as recent as the Holocene. Ancient arthropods and otherorganisms preserved in over 10,000 amber samples are studied by researchers rom around the

globe. Significantly, the collection contains 420 holotypes, and the number o important speci-mens rom various deposits increases yearly.

Despite the exceptional preservation o ossil resins and their organismal inclusions indeposits throughout the world, any amber or copal samples that are removed rom anoxicsediments will begin to deteriorate over time. Tis degradation can maniest in one or more othe ollowing ways: darkening o the ambers surace; crazing, consisting o the developmento a network o fine surace cracks; and, in the most serious examples, racturing and crackingmore deeply within or even through the piece (fig. 1). A crazed or darkened surace compro-mises visibility, and can also directly affect the preservation o any inclusions, setting the stage

or internal cracking and deterioration. A significant crack can easily penetrate or obliterate aninclusion. Moreover, i an organism is close to the surace, repolishing may be difficult orimpossible. In severe cases, oxidative damage can infiltrate amber via fine cracks (fig. 1BE),affecting the integrity o the entire piece and any inclusions it contains. o whatever extentdeterioration takes place, it puts important paleobiological data at risk (in such areas as tax-onomy, systematics, taphonomy, and biogeography) in both studied and unstudied specimens.

Accelerated-aging tests involving light (UV/Vis), relative humidity (RH), and temperaturehave widely been used to assess the long-term stability o synthetic polymers such as plastics.Measurements are taken to assess oxidation, color change, or loss o mechanical strength todetermine the mechanism o deterioration or to estimate the rate. Since amber is a highlycross-linked organic polymer, essentially a natural plastic, many o the testing parametersdeveloped or plastics are directly applicable.

Previous studies o deterioration in amber have used accelerated aging to address similarconservation issues. Williams et al. (1990) exposed Dominican amber to an atmosphere con-taining ormic acid, acetic acid, or hydrogen sulfide, as well as to uctuating relative humidityand seven months o UV and natural light. Crazing and exoliation o the surace occurredollowing chemical exposure, and also afer sudden decreases in RH (rom 45% RH to below35% RH). Light caused no visible deterioration but was ound to cause surace oxidation basedon Fourier transorm inrared spectroscopy (FIR) analysis (Waddington and Fenn, 1988;

Williams et al., 1990). Te ossil resin samples in the present study were subjected to a combi-


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2012 BISULCA E AL.: DEERIORAION OF AMBER 3

FIGURE 1. Historical deterioration o amber pieces in the AMNH collections:A.Crazing o several old Balticamber specimens. B. Old Baltic amber piece with insect exhibiting darkening. C. Same specimen aferrepolishing. D.Darkening o several old Baltic amber specimens. E.Same specimens afer repolishing. F.Old Baltic amber piece showing overall deterioration including crazing and large crack through the insectinclusion. G.Same piece afer embedding in a high-grade epoxy (Epoek 301-2).


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nation o UV and visible light (UV/Vis) at different levels o relative humidity, as well as tohigh temperatures.

ypically, museum amber collections are kept in closed cabinets, usually subject to somenatural humidity uctuations. Te AMNH collection is stored in gasketed steel cabinets, andeach amber piece is placed in a riction-fitting plastic box. Fossilierous pieces o amber aretypically exposed to intervals o intense light when a specimen is studied microscopically, usingtransmitted/reected fiber-optic light or as much as several hours per day or several days oreven weeks. UV light is known to degrade polymers through photooxidation, or chain scis-sioning (as well as some oxidation through the production o heat), physically maniesting asembrittlement and discoloration. Te susceptibility o plastics/organics to oxidative degrada-tion depends on many actors, but largely on a materials affinity or oxygen and the overallpermeability o the polymer structure (Strong, 1996). Generally, that part o the light spectrumapproaching and entering the UV region is more likely to cause damage to amber, since mostossil resins absorb light in this region (Searle, 1994; Marin et al., 1994). Oxidation o amberis believed to occur primarily in the exocyclic methylene groups (Shashoua, 2002; Tickett,1993; Williams et al., 1990). However, because ossil resins rom different deposits are knownto contain different types and amounts o unctional groupsunsaturated carbonyls, esters,and others (Lambert and Frye, 1982)reactions such as oxidation or changes in unsaturationthat occur during aging might be expected to show species-specific changes in absorption

ABLE 1. Summary o ossil resins used in present study.

Location Age Classa/ Origin Appearance Characteristic deterioration

Myanmar (Burma) 97100 Ma Ib / Conierae:Metasequoia?

Yellow or dark orangeto russet

Apparently very durable /homogeneousbbut canbreak along calciteintrusions

New Jersey 91 Ma Ib / Conierae:Cupressaceae

Clear yellow, yelloworange, opaque yellowor red

Friable / brittlesubjectto crazing, cracking. Deepred pieces orm needlelikecracksc

Baltic 45 Ma Ia / Conierae:Pinaceae orSciadopityaceae

Clear or opaque yellowor yellow orange toivorylike

Considered durable, butsubject to darkening andsurficial crazing over time

Dominican Rep. 1719 Ma Ic / Angiospermae:Hymenaea

Generally clear yellow,seldom opaque,oc. blue tinted or blue

Some crazing noted overtime

Zanzibar, E. Arica 1000 ybp Ic / Angiospermae:Hymenaea

Clear yellow ypical polygonal crazingo surace over relativelyshort time

aChemical resin classes based on Anderson et al., 1992, 1995, 2006.b

Grimaldi et al., 2002.cGrimaldi et al., 1989.


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ABLE 2. Accelerated aging tests / ossil resin samples used.

est (no.) Agent RH (%)Presence

o O2Samples by

type o resin

Additionalepoxy-coated

samples,Dominicanand copal

Additionalepoxy-coated,

New Jerseysamples otals

1 UV/Vis 0 2 1 11

2 UV/Vis 50 2 1 113 UV/Vis 0 anoxic 2 1 11

4 UV/Vis 50 anoxic 2 2 1 13

5 UV/Vis 2070 2 2 1 13

6 dark 0 2 1 11

7 dark 50 2 2 1 13

8 dark 0 anoxic 2 1 11

9 dark 50 anoxic 2 1 13

10 dark 2070 2 2 1 13

11 heat 1015 4 2 2 24

OALS 120 10 12 144

or each kind o amber. I so, the stability and types o deterioration o various ossil resinsshould differ accordingly.

o test this, samples rom five distinct Cretaceous, ertiary, and Quaternary deposits werestudied (table 1). Burmese amber is mid-Cretaceous (ca. 100 Ma) in age, uppermost Albian toearliest Cenomanian (Grimaldi et al., 2002; Cruickshank and Ko, 2003; Guanghai Shi, personalcommun., 2011); New Jersey amber is slightly younger, uronian aged (ca. 91 Ma) (Grimaldiet al., 2000). Baltic amber is dated to the Middle Eocene (ca. 45 Ma) (Engel, 2001), whileDominican amber is Miocene (ca. 1719 Ma) (Grimaldi, 1996; Iturralde-Vinent and MacPhee,1996). Carbon 14 dating by Grimaldi (unpubl.) indicates that Zanzibar copal is Recent (Holo-cene), ormed less than 1000 ybp.

MAERIALS AND MEHODSA total o 11 testing conditions were prepared (table 2), and these treatments were

maintained and monitored or one year. Sealed microchambers were constructed out o aUV-transparent plastic (Acrysol SUV, 0.125" [3.175 mm] thick cell-cast acrylic, withapproximately 70% transmission at 280 nm, Spartech Corp.) and barrier film (Escal, Mitsubi-shi Corp.) (fig. 2A). Relative humidity was maintained using preconditioned silica gel, and wasmonitored with card hygrometers. Anoxic environments were created with oxygen absorbers(Agelessand RP System, Mitsubishi), and were monitored with oxygen indicators (AgelessEye). o mitigate potential effects o pollutants or off-gassing o microchamber materials, a

pollutant control sheet (Scavengel, Art Preservation Services) was placed within each cham-ber. A ull-spectrum uorescent light (Lie Lite, Light Energy Source, Inc.) was used or light


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exposure, because it has a similar spectral range to that o natural sunlight with a componento 6% UV-A and 1.2% UV-B (fig. 2B). UV-C light was not used or exposure, because suchhigh-energy light (wavelengths below 295 nm) may cause reactions that would not normallyoccur under natural conditions (Wypych and Faulkner, 1999). Te light output was 64008000 lux, as measured by a YF-179 Digital Light Meter (recording all light rom 320730 nm).UV levels were rom 0.40 to 0.51 W/m2 throughout the bulb, as measured by a PreservationEquipment UV 300 (peak reading at 360, 200400 nm range). While under illumination, all

samples were rotated periodically to ensure uniorm exposure. Elevated temperature wasachieved by a Fisher Scientific Programmable Isotemp Oven with orced-air heating at 45 C.As a rule o thumb in polymer studies, each 10 C effectively doubles the reaction rate, but attemperatures too high (approaching the glass transition temperature, g, which is as low as50 C or copal [Howie, 1995]), deterioration will occur that would not normally take place(Halliwell, 1992).

S P: Small, thin uniorm slices (1.5 cm 1.25 cm 3 mm) o amberrom each deposit were cut and polished. Where possible, these slices were produced rom oneor at most two large pieces o amber, in order to minimize variation within amber types. An

additional set o samples rom New Jersey amber, Dominican amber, and Zanzibar copal werecoated with a thin layer o Buehler EpoxiCure Resin (bisphenol-A epoxy resin with N-butylglycidyl ether and a mixture o amine hardeners), an epoxy routinely used at the AMNH tostrengthen and preserve riable ambers during and afer preparation o organismal inclusions(Nascimbene and Silverstein, 2000). Te samples were placed in special trays while undergoingaging experiments (fig. 2C).

A: Beore and ater aging, each sample was digitally photographed using aMicrOpticsfiber-optic ash unit with an Infinitylens (www.microptics-usa.com). Colorchange in the amber was measured using an UltraScan XE spectrophotometer (Hunter Labs)with D65 illumination (daylight source) and sphere (diffuse/8) optical geometry. Te instru-ment was calibrated with white tile and black card standards (Hunter Labs). Samples were

FIGURE 2. Experimental setup. A.ransparent UV/Vis light-transmitting plastic structures o microcham-bers, constructed out o Acrysol SUV. B.Complete microchambers with amber samples in the UV/Visaging setup. C.rays to hold amber samples while undergoing aging experiments.


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attached to a semimicrocell holder (10 20mm cuvette), and measurements were made in

transmission mode rom 360 to 720 nm at 10nm intervals. Final spectra were the average othree measurements to account or minor di-erences o thickness throughout the ambersamples. In order to address whether aging canresult in a visually perceptible color change,absorbance data were converted to CIELabcolor space values using Universal(Ultrascan)sofware (Hunter Labs, 1985). CIELab is a sys-tem that was designed by the French Commis-sion Internationale de lEclairage (CIE) in1976 to approximate human visual perceptiono color. In CIELab, color is defined by threeparameters, or coordinates: L is a measure olightness (black = 0, reerence white = 100);a* is a measure o redness, and ranges romgreen (a) to red (+a), while b* is a measure o yellowness ranging rom blue (b) to yellow(+b) (fig. 3). Te change in each o these parameters, E, is calculated beore and afer agingto determine color change (E = [(L)2+ (a)2+ (b)2]).

RESULS AND DISCUSSION

Te deterioration o amber was maniest in three distinct ways: as surace crazing, as adecrease in absorbance in the UV region o the spectrum, and as yellowing or darkening.

C: Tis type o damage was observed only when uctuating humidity and lightexposure were combined. In visual examination o all amber samples, the most pronouncedcrazing occurred in New Jersey amber (fig. 4AC), which also showed a significant change inspectrophotometric absorbance.

Crazing is characterized by a dense network o fine surace cracks, generally associatedwith mechanical stresses applied to glassy or semicrystalline polymers. When these stresses areassociated with solvents, they are reerred to as solvent crazing (Strong, 1996; Tickett et al.,1995). Fluctuating humidity will cause many polymers to expand and contract as moisturecontent changes (Halliwell, 1992), thereby inducing crazing. Obvious examples are varnishesapplied to wood or paintings. Crazing has commonly been observed in amber stored ordecades in museum collections, as well as on ancient European amber artiacts, and has some-times led to the ormation o more serious internal cracking and surace aking. Ambers lowpermeability initially limits such stresses to the materials surace. However, based on variouspolymer studies, craze ormation will progress deeper into the material over time, ultimatelyresulting in complete exoliation o the surace and crazing along any internal ractures. I an

+b

+L

-L

+a

-a -b

FIGURE 3. Model o CIELab Color Space: 3-Drepresentation o perceptible color change.


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internal racture in amber reaches an arthropod or other inclusion, it can cause the entiresurace o the inclusion to darken, craze, and ultimately be obliterated (fig. 1FG). Te durationo an amber pieces exposure to any particular humidity regime plays a critical role, since thepolymer needs enough time to reach equilibrium with ambient conditions. Changes in RH mayoccur too quickly or a material to respond. Tus, test conditions in this study, in which ossilresin samples were exposed to abrupt RH changes at two-week intervals, were intentionallydevised to enhance the stresses that can occur. Under these specific parameters, even Burmeseamber showed some crazing (fig. 4DF). Unstable conditions could occur in museum collec-tions that do not have humidity controls, especially in temperate regions.

In the present study, it is notable that crazing did not occur in samples exposed to uctuat-ing humidity in the absence o light. Tis indicates that the oxidative effect o light plays asignificant and possibly necessary role in crazing, riability, and embrittlement.

A L F R: Spectrophotometric scans o the five ossil resinsmade beore testing show characteristic curves in absorbance spectra, which are each distinctin intensity at the lower wavelengths (fig. 5). Peak absorbance or both the New Jersey andBurmese ambers occurs at significantly higher wavelengths than or the other ossil resins, lyingurther within the UV-B region (385 nm or New Jersey; 380 nm or Burmese).

Spectrophotometric absorbance values were converted to the CIELab color space system to

assess color differences. Each ossil resin type studied exhibited specific characteristics in color

FIGURE 4. Crazing afer exposure to a combination o UV/Vis and uctuating humidity. A. New Jerseysample prior to treatment (size o all samples: 1.5 cm 1.25 cm 3 mm). B.New Jersey sample afer treat-ment. C.Detail o crazing on New Jersey sample. D.Burmese sample prior to treatment. E.Burmese sampleafer treatment. F.Detail o crazing on Burmese sample.


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variability within individual samplesprior to aging. Copal was ound to

have the most uniorm coloration,and was overall the lightest, whileNew Jersey and Burmese ambers hadthe greatest variation in color (table3). Because significant color variationexisted within individual amber slices,averages between scans or each piecewere used to compensate or anyslight differences. However, this alsoresulted in a large standard deviationamong measurements, particularlyin the UV region. Consequently, theCIELab color data were more difficultto interpret. As such, only samplesthat were uniorm in their colorationand did not contain cracks, inclu-sions, or air bubbles were assessed or CIELab color change ollowing the aging tests.

Te change in CIELab color values or uniorm samples beore and afer aging is depictedin table 4, as well as the total difference, shown as E. For industrial materials like many plas-

tics, in which samples tend to have uniorm initial coloration, differences typically becomevisually perceptible when E is greater than 5.

Y: Significant color change in the orm o yellowing was ound only with expo-sure to elevated temperatures, and occurred or all ambers tested with the exception o Bur-mese amber (fig. 6). Yellowing is caused by an increase in absorbance in the blue region o thespectrum. Tis effect was most pronounced or Baltic amber (fig. 7A, B), with Dominican andcopal samples also exhibiting significant yellowing, but to a lesser extent. An example o thelack o discernable yellowing in Burmese amber can be seen in figure 7C, D. While heat-agingin this study represents extreme conditions, these results are consistent with the observation

that amber darkens with age, as routinely occurs in old collections o Baltic amber.

400 450 500 550 600 650 7000

0.5

1

1.5

2

2.5

3

WAVELENGTHS(NM)

ABSORBANCE

CopalDominicanBaltic

New JerseyBurmese

FIGURE 5. Distinct absorbance spectra (averaged) or eacho the five ossil resins beore aging.

ABLE 3. Average CIELab values or ossil resin types prior to aging.

Fossil resin L (lightness) a (redness) b (yellowness)

Burmese amber 72.30 10.58 0.04 3.28 33.02 5.86

New Jersey amber 70.50 15.48 3.33 1.81 48.62 14.95

Baltic amber 88.26 3.37 4.34 0.89 22.68 4.12

Dominican amber 84.56 6.25 2.72 1.45 28.88 11.56

Zanzibar copal 83.82 7.09 0.17 0.40 9.30 3.66


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ABLE 4. CIELab color change in samples with uniorm coloration.

Fossil resin (no.) RH Light L a b E

Burmese amber 2 0 UV/Vis 0.94 0.91 1.10 1.74 50 UV/Vis 2.06 1.11 0.59 2.4

5 ux UV/Vis 0.24 0.32 0.18 0.4

11 0 dark 6.69 0.09 2.08 7.0

13 50 dark 3.19 0.75 1.32 3.5

15 0 dark anoxic 14.56 0.42 0.11 14.6

17 50 dark anoxic 4.22 0.49 3.27 5.4

19 ux dark 0.87 0.16 0.81 1.2

24 heat 4.32 0.91 0.52 4.4

New Jersey amber 1 0 UV/Vis 3.61 1.43 3.65 5.3

3 50 UV/Vis 2.25 1.56 4.23 5.0

5 ux UV/Vis 11.27 1.11 2.11 11.5

11 0 dark 5.72 0.17 2.51 6.2

14 50 dark 4.07 0.91 15.62 16.2

15 0 dark anoxic 0.86 0.21 3.13 3.3

18 50 dark anoxic 6.30 0.27 2.62 6.8

20 ux dark 6.01 0.71 7.01 9.3

22 heat 3.46 2.00 7.43 8.4

Baltic amber 1 0 UV/Vis 0.71 1.64 1.30 2.2

4 50 UV/Vis 0.41 0.15 4.04 4.1

6 ux Uv/Vis 1.51 0.30 5.05 5.3

11 0 dark 1.15 1.07 2.68 3.113 50 dark 1.52 0.97 3.84 4.2

16 0 dark anoxic 3.48 1.38 2.38 4.4

18 50 dark anoxic 3.08 0.63 6.18 6.9

19 ux dark 4.93 0.20 1.82 5.3

24 heat 0.74 1.04 25.57 25.6

Dominican amber 1 0 UV/Vis 2.83 1.58 4.02 5.2

3 50 UV/Vis 1.47 1.52 6.13 6.5

5 ux UV/Vis 1.31 2.13 9.02 9.4

12 0 dark 3.72 1.08 7.49 8.4

14 50 dark 0.21 0.18 1.60 1.6

17 0 dark anoxic 0.78 0.81 1.69 2.020 ux dark 3.39 0.30 0.89 3.5

23 heat 4.41 1.08 15.98 16.6

Zanzibar copal 2 0 UV/Vis 0.90 0.17 0.30 1.0

3 50 UV/Vis 0.92 1.33 2.73 3.2

6 ux UV/Vis 3.44 1.07 4.03 5.4

12 0 dark 1.05 1.31 0.98 1.9

14 50 dark 0.31 1.05 0.19 1.1

16 0 dark anoxic 2.81 0.70 0.80 3.0

18 50 dark anoxic 0.80 0.85 1.11 1.6

20 ux dark 0.90 1.16 2.36 2.8

22 heat 3.48 3.41 11.46 12.5


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C UV A: Te most significant change in absorbance spectra duringaging occurred in the UV region that was monitored (360400 nm). All our ambers testedshowed a decrease in UV absorbance afer exposure to heat, as well as light and/or uctuatinghumidity. Baltic amber appeared the most stable overall, and showed little significant changein absorption, except when exposed to elevated temperature or light with uctuating humidity.Dominican amber appeared somewhat less stable than Baltic, exhibiting significant absorbancechanges rom elevated temperature as well as exposure to light (UV/Vis). New Jersey amberappeared to be the most unstable o the ossil resins, showing a significant decrease in absorp-tion in all aging conditions except dark anoxic.

Qualitative assessment o absorption spectra beore and afer aging demonstrates that thelargest absorbance changes recorded were in the New Jersey samples, ollowed by the Burmese

samples. Tis was expected, as both New Jersey and Burmese ambers absorb more light in theUV portion o the spectrum, indicating a greater susceptibility to UV deterioration (Strong,1996). Interestingly, copal samples showed an increase in absorbance in the UV range withexposure to heat, light, and uctuating humidity, which probably reects the extent o cross-linking in resins that have barely matured.

A general trend noted is that the greatest differences in UV absorbance were seen withlight exposure, particularly when combined with uctuating humidity. However, in almost alltests involving RH, such changes in UV absorbance were less pronounced in dark-aged sampleso comparable humidity, which is evidence that exposure to light (visible and ultraviolet) plays

a significant role in the deterioration o ossil resins. In act, dark-aged samples ofen showedlittle change in absorption regardless o humidity.

-15 -10 -5 0 5 10 15 20 25 30 35

-10

-5

0

5

10

b (yellowing)

L

(change

in

lightness) Burmese

BalticDominicanCopal

New Jersey

FIGURE 6. Yellowing o the five resins ollowing heat aging, based on change in CIELab values.


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H: Te trends or the effect o humidity itsel are more difficult to interpret. Quali-tatively, there is a trend or increased deterioration with the presence o moisture, in both lightand dark-aged samples. Te role o moisture in deterioration is expected o organic materialsand has been ound in accelerated aging tests o Baltic amber (Shashoua, 2002). However, the

differences between aging at 0% and at 50% in this study were not conclusive, and no optimalhumidity level was determined. In other studies, both excessively low and excessively highhumidity (above 50%) have been linked to the deterioration o particular amber types. Forexample, low relative humidity is detrimental to Dominican amber (Williams et al., 1990), whileBaltic amber is adversely affected by both high and excessively low humidity (Shashoua, 2002).

No studies have determined the optimal storage humidity or different ossil resins, andthere are numerous and ofen divergent storage protocols at various institutions, rom high tolow humidity, with and without anoxic conditions. Te results o this study and others takentogether suggest that a stable RH environment is critical. Collection policy should also be takeninto considerationRH uctuations imposed by requent removal o specimens rom themicroclimate(s) o that collections storage acility during research could be more detrimental

FIGURE 7. Images showing effects o heat on amber samples. A.Baltic sample prior to treatment. B.Yellow-ing o Baltic sample due to heat exposure. C.Burmese sample prior to treatment. D.No discernible yellowingo Burmese sample afer heat exposure.


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to the amber (particularly in conjunction with periods o exposure to intense light) than long-term storage at a humidity level that does not match the museums ambient conditions.

Further study is indicated to address any interactions between uctuating temperature andhumidity that may apply to stored collections (specifically to what extent crazing or otherdeterioration can be inuenced by such interactions). Tis would, however, require the use ospecial environmentally controlled chambers.

A S: It is notable that or New Jersey amber, significant deterioration orchange was avoided only under anoxic conditions, and that, overall, the five ossil resins showedthe least change in absorbance when this treatment was imposed. However, urther study iswarranted regarding the benefits o anoxic storage, since oxygen-ree conditions were notmaintained during light exposure (anoxic environments in the UV/Vis tests ailed over thecourse o the study).

M F F R: Te general increase in ragility and suscep-tibility to degradation in older ambers is probably due to several actors. First, these resins arephysically and chemically more mature. Ragazzi et al. (2003) ound a significant correlation(r = 0.7, p < 0.01) between the age o amber deposits and the thermal behavior (i.e., tempera-ture o dissociation) o the ambers. Te oldest ambers they studied, rom riassic and Creta-ceous deposits, generally had the highest thermogravimetric values, while ertiary ambers andHolocene copals showed lower values. Te age o various ossil resins likewise is correlated withhardness (Nascimbene et al., unpubl.). Age is thereore a significant variable in the polymeriza-tion, cross-linking, and isomerization (collectively, the maturation) o ossil resins. Tis may

help explain the general trend we ound between UV absorbance and amber age, in which themore mature, older resins tested show a higher absorbance, while the youngest ones show theleast (see fig. 5).

New Jersey amber was produced by conierous trees o the cedar amily Cupressaceae(Anderson, 2006; Grimaldi et al., 2000). According to the molecular classification o naturalresins proposed by Anderson et al. (1992) and Anderson and Crelling (1995) (see table 5), andbased on a recent study by Anderson (2006), New Jersey amber is designated as Class Ib. ClassIa Baltic amber (given the Ia designation because it was the first amber studied) contains thecross-linking agent succinic acid, and is rom a conierous tree probably o the amily Pinaceae.

Class Ia and Ib ambers are labdanoid diterpenes having a regular stereochemistry. Class Icossil resins, in contrast, are labdanoids with an enantio configuration. Tese include, but arenot restricted to, relatively recent resins derived rom broadlea trees o the genus Hymenaea(Leguminosae) (Nascimbene et al., 2010). Species o Hymenaeaproduced the ambers rom theMiocene o Mexico and the Dominican Republic, as well as the copals rom Colombia andeastern Arica. Although the tree that produced the mid-Cretaceous Burmese amber is as yetundetermined, it is certainly conierous and likely taxodiaceous/cupressaceous. Recent chemi-cal testing by Anderson reveals that, like the New Jersey amber, the Burmese amber is Class Ib(Anderson, personal commun.).

Baltic amber may be naturally durable because o extensive cross-linking with succinic acid(and the apparent durability o Burmese amber may also be due to a strong cross-linking agent,


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as yet unidentified). Geological age and molecular composition together generally appear tohave the greatest effect on how amber matures. However, depositional actors and geologicalprocesses may also play a significant role.

M A S: One actor when considering relative humidityor storage is the degree o moisture in sediments where amber or copal is ound. Tere ismounting evidence that most i not all amber deposits are significantly preserved by anoxicconditions due largely to the presence o moisture or at least part o their history (Gomez et

al., 2002; Grimaldi et al., 2000; Iturralde-Vinent, 2001). Te amber-bearing sediments were inmany cases initially deposited in a nearshore marine environment, such as a delta or estuary.In act, New Jersey amber occurs in damp to wet lenses o sof lignite overlain by unconsoli-dated layers o sand and clay (Grimaldi et al., 2000). Tough the sediments are acidic andsulurous, the clay unctions as a chemical buffer as well as a physical barrier to atmosphericexposure, protecting especially deep layers with stable moisture. It is interesting to note thatCopper Age Baltic amber jewelry that was buried in peat bogs with corpses is remarkably wellpreserved, ar better than that o later Roman and Middle Age pieces that were exposed toatmospheric conditions in tombs (Grimaldi, 1996). Just as taphonomic conditions like thepresence o moisture appear to play a major role in the preservation o amber, similar condi-tions can presumably be mimicked in collections o amber.

ABLE 5. Chemical classification o ossil resins (Anderson and Crelling, 1995).

Class Description Examples

I Most known ambers: macromolecular structuresderived rom polymers o labdanoid diterpenes,including labdatriene carboxylic acids, alcohols,and hydrocarbons

Ia Regular configuration, normally includingcommunic acid, communol, and succinic acid

Baltic amber

Ib Regular configuration, ofen including communicacid, communol, and biormene. Succinic acidnot present

New Jersey and Burmese ambers

Ic Enantio configuration, including ozic acid, ozol,

and enantio biormenes

Dominican amber, Zanzibar copal

II Derived rom polymers o bicyclic sesquiter-penoid hydrocarbons, such as cadinene andrelated isomers

(Lower Eocene) Indian and Arkansas ambers

III Natural ossil polystyrene (ertiary) German and New Jersey resins (rare)

IV Nonpolymeric, including sesquiterpenoids basedon the Cedrane carbon skeleton

Moravian amber

V Nonpolymeric diterpenoid carboxylic acid,including abietane, pimarane and iso-pimaranecarbon skeletons

Retinites in European brown coal


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C E A: In this study, close visual examination o the suraces oamber samples that were coated with epoxy did not reveal any noticeable deterioration ollow-

ing accelerated aging. However, the surace o the epoxy finish itsel did show some degradationafer three months exposure to UV light (yellowing, minor crazing), similar to that o uncoatedamber pieces subjected to the same conditions. In this case, one advantage o sealing (coatingor embedding) amber samples is that any compromised epoxy can be ground and polishedurther or even removed completely. Notably, long-term observation and regular examinationo embedded AMNH amber pieces suggest that an epoxy seal successully protects such speci-mens rom exposure over time to uctuating humidity or the ull oxidative effects o light andheat, especially i specimens are stored in a dark, stable collections environment.

Although no longer a common practice, it should be mentioned that storing ossil resinspecimens in solutions like mineral oil is highly discouraged, because the oil will interact withthe amber, and will also make urther preparation difficult or impossible. Class II ambers (likeIndian amber or amber rom Arkansas) are particularly susceptible.

A C E: Te need to protect amber specimens rom deterio-ration applies not only to paleontological collections, but also to stored archaeological collectionscontaining amber artiacts, as well as to excavated amber samples awaiting preparation. Unpre-pared samples are ofen stored in less than ideal conditions over several years beore they areeventually processed. Also, amber pieces in exhibitions (whether paleontological or archaeologi-cal) are ofen subject to inadequate conservation, and some exhibitions are permanent. As anexample, a long-term exhibition in the Museo Geominero in Madrid housed numerous Domini-

can amber pieces with insects, and or the first several years the specimens were kept in poorconditions leading to significant deterioration (crazing, darkening, etc.). Fortunately, necessarychanges were finally instituted with respect to temperature, RH, and UV light, and the displaycase containing the amber was henceorth environmentally monitored (Baeza et al. 2007).

CONCLUSIONS

Results o the present study reveal that amber is best housed in closed storage or sealedcontainers to minimize exposure to light, with maintenance o stable humidity. Tough this

was not tested, it is assumed that storage in cooler temperatures (e.g., ca. 21 C) is also benefi-cial. For all ossil resins tested, the least deterioration took place in dark storage with stablehumidity. Although an optimal humidity level was not determined, our results indicate thatstable RH is critically important. Similar results were obtained in other studies using Baltic andDominican amber, but which also suggested an optimal humidity in the range o 35%45%.Tere appears to be considerable variation in the effects o humidity among amber types, andNew Jersey amber in particular is the most susceptible to damage rom exposure to light, heat,and uctuating humidity, even in mild conditions.

Te need to seal amber specimens to protect them rom damaging exposure was known asearly as the mid-19th century, beginning with the practice o embedding small pieces o Balticamber containing ossils in a natural modern resin (Canada balsam), stored within glass wells.


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Amber preserved this way in several European museums shows no crazing or darkening. Oneimportant disadvantage o the technique, though, is that urther preparation o the piece is very

difficult to impossible without damaging the amber; and unortunately, without subsequent prepa-ration, details o the inclusions are usually compromised or viewing by modern systematists. Also,embedding amber pieces in Canada balsam is highly impractical or collections with thousands opieces, and or the larger (ca. >2 cm diameter) pieces that may actually be the most valuable ones.

For ossil collections, the present method o choice is to embed amber pieces in a high-grade epoxy (Corral et al., 1999; Nascimbene and Silverstein, 2000; Hoffeins, 2001), which ismore efficient than using balsam and also allows preparation to be done multiple times. Teepoxy strengthens and clarifies riable amber specimens, enabling the close preparationrequired to view details o inclusions, and also preserves the amber pieces or long-term study.Epoxies, however, have limitations similar to amber (Down, 1984, 1986), and some o themyellow over time even when only minimally exposed to light and heat. As noted earlier, theamber samples that were coated with the Buehler epoxy or this study showed some suracedeterioration o the epoxy (yellowing, minor crazing) within three months o accelerated UVaging; however, no visible changes were seen to the epoxy-coated amber itsel.

Te conservation effects o the various epoxies used or embedding or coating amber speci-mens is a subject or urther investigation.

ACKNOWLEDGMENS

Te authors wish to thank reviewers Enrique Pealver and Janet Waddington or theircomprehensive input and commentary, by which the paper was significantly improved. C.B.was unded by a Natural Science Collections Conservation Grant. Te authors also extend theirspecial appreciation to Robert G. Goelet, Chairman Emeritus, Board o rustees, AMNH, orunding the work o P.C.N. We thank Steve Turston, graphics specialist in the Division oInvertebrate Zoology, AMNH, or his valuable assistance with the figures, as well as ormerScientific Assistant am Nguyen, Division o Invertebrate Zoology, AMNH, or photomicro-graphs o the samples tested. In addition, C.B. would like to acknowledge Ken Wendt atHunterLabs or use o equipment and valuable advice.

REFERENCES

Anderson, K.B. 1995. Te nature and ate o natural resins in the geosphere. Part V. New evidence con-cerning the structure, composition, and maturation o Class I (polylabdanoid) resinites. InK.B.Anderson and J.C. Crelling (editors), Amber, resinite, and ossil resins: 105129. Washington, DC:American Chemical Society Symposium Series 617.

Anderson, K.B. 2006. Te nature and ate o natural resins in the geosphere. XII. Investigation o C-ringaromatic diterpenoids in Raritan amber by Pyrolysis-GC-Matrix Isolation FIR-MS. Geochemicalransactions 7: 19.

Anderson, K.B., and J.C. Crelling. 1995. Introduction. InK. B. Anderson and J. C. Crelling (editors),Amber, resinite, and ossil resins: 110. Washington, DC: American Chemical Society Symposium


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Series 617.Anderson, K.B., R.E. Winens, and R.E. Botto. 1992. Te nature and ate o natural resins in the bio-

sphereII. Identification, classification, and nomenclature o resinites. Organic Geochemistry 18:829841.Baeza, E., et al. 2007. Proyecto de conservacin preventiva y restauracin de las colecciones de mbar

del Museo Geominero (Instituto Geolgico y Minero de Espaa). Actas del II Congresodel GrupoEspaol del IIC: 361370.

Corral, J.C., R. Lpez Del Valle, and J. Alonso. 1999. El mbar cretcico de lava (Cuenca Vasco-Can-tbrica, Norte de Espaa). Su colecta y preparacin. Estudios del Museo de Ciencias Naturales delava 14: 721.

Cruickshank, R.D., and K. Ko. 2003. Geology o an amber locality in the Hukawng Valley, northernMyanmar. Journal o Asian Earth Sciences 21: 441445.

Down, J. 1984. Te yellowing o epoxy resin adhesives: report on natural dark aging. Studies in Conser-vation 29: 6376.

Down, J. 1986. Te yellowing o epoxy resin adhesives: report on high-intensity light aging. Studies inConservation 31: 159170.

Engel, M.S. 2001. A monograph o the Baltic amber bees and evolution o the Apoidea (Hymenoptera).Bulletin o the American Museum o Natural History 259: 1192.

Gomez, B., X. Martnez-Delcls, M. Bamord, and M. Philippe. 2002. aphonomy and paleoecology oplant remains rom the oldest Arican Early Cretaceous amber locality. Lethaia 35: 300308.

Grimaldi, D. 1996. Amber: window to the past. New York: Abrams/American Museum o Natural History.Grimaldi, D., C. Beck, and J. Boon. 1989. Occurrence, chemical characteristics, and paleontology o the

ossil resins rom New Jersey. American Museum Novitates 2948: 128.

Grimaldi, D., J.A. Lillegraven, .P. Wampler, D. Bookwalter, and A. Shedrinsky. 2000. Amber rom UpperCretaceous through Paleocene strata o the Hanna Basin, Wyoming, with evidence or source andtaphonomy o ossil resins. Rocky Mountain Geology 35: 163204.

Grimaldi, D., A. Shedrinsky, and .P. Wampler. 2000. A remarkable deposit o ossilierous amber romthe Upper Cretaceous (uronian) o New Jersey. InD. Grimaldi (editor), Studies o ossils in amber,with particular reerence to the Cretaceous o New Jersey, 93102. Leiden: Backhuys.

Grimaldi, D., M.S. Engel, and P.C. Nascimbene. 2002. Fossilierous Cretaceous amber rom Myanmar(Burma): its rediscovery, biotic diversity, and paleontological significance. American Museum Novi-tates 3361: 172.

Halliwell, S.M., 1992. Weathering o polymers. Rapra Review Reports 5 (5): 1119.

Hoffeins, H.W. 2001. On the preparation and conservation o amber inclusions in artificial resin. PolishJournal o Entomology 70: 215219.

Howie, F. 1995. Aspects o conservation o ossil resins and lignitic material. InC. Collins (editor), Careand conservation o paleontological material: 4752 London: Butterworth-Heinmann.

Hunter Labs. 1985. Applications Notes 6: 12.Iturralde-Vinent, M. 2001. Geology o the amber-bearing deposits o the Greater Antilles. Caribbean

Journal o Science 37: 127.Iturralde-Vinent, M.A., and R.D.E. MacPhee. 1996. Age and paleogeographical origin o Dominican

amber. Science 273: 18501852.Lambert, J.B., and J.S. Frye. 1982. Carbon unctionalities in amber. Science 217: 5557.

Marin, J.W., J.A. Lechner, and R.N.Varner. 1994. Quantitative characterization o photodegradationeffects o polymeric materials exposed in weathering environments. InW.D. Ketola and D. Gross-


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APPENDIX

F S F R

Fossil resin Sample (no.) reatment Fossil resin Sample (no.) reatment

BurmeseBurmeseBurmeseBurmese

1278

UV with 0% RH


349

10

UV 50% RH

BurmeseBurmese

56

UV 20%70% RH

BurmeseBurmese

1112

Dark 0% RH

BurmeseBurmese

1314

Dark 50% RH

BurmeseBurmese 1516 Dark anoxic / 0% RH

BurmeseBurmese

1718

Dark anoxic / 50% RH

BurmeseBurmese

1920

Dark 20%70% RH


21222324

Accelerated temp.

New JerseyNew JerseyNew JerseyNew Jersey

1278

UV 0% RH

man (editors), Accelerated and outdoor durability testing o organic materials, ASM Special ech-nical Publication 1202. Philadelphia: American Society or esting and Materials.

Nascimbene, P., and H. Silverstein. 2000. Te preparation o ragile Cretaceous amber or conservationand study o organismal inclusions. InD. Grimaldi (editor), Studies o ossils in amber, with par-ticular reerence to the Cretaceous o New Jersey, 93102. Leiden: Backhuys.

Nascimbene, P.C., et al. 2010. Physicochemical comparisons and implications o new amber depositsrom the Lower Eocene o India and the mid Cretaceous o Ethiopia. Abstracts: FossilsX3: insects,arthropods, amber. Capital Normal University, Beijing, China, August 2025, 2010: 155.

Ragazzi, E., G. Roghi, A. Giaretta, and P. Gianolla. 2003. Classification o amber based on thermalanalysis. Termochimica Acta 404: 4354.

Searle, N.D. 1994. Effect o light source emission on durability testing. InW.D. Ketola and D. Grossman(editors), Accelerated and outdoor durability testing o organic materials, ASM Special echnicalPublication 1202. Philadelphia: American Society or esting and Materials.

Shashoua, Y. 2002. Degradation and inhibitive conservation o Baltic amber in museum collections.Ph.D. dissertation, Department o Conservation, National Museum o Denmark, Copenhagen.

Strong, A.B. 1996. Plastics: materials and processing. Englewood Cliffs, NJ: Prentice Hall.Tickett, D. 1993. Te inuence o solvents on the analysis o amber. Conservation science in the UK:

preprints o the meeting held in Glasgow, May 1993. London.Tickett, D., P. Cruiskshank, and C. Ward. 1995. Te conservation o amber. Studies in Conservation 40:

217226.Waddington, J., and J. Fenn. 1988. Preventive conservation o amber: some preliminary investigations.

Collection Forum 4/2: 2531.Williams, R.S., J. Waddington, and J. Fenn. 1990. Inrared spectroscopic analysis o Central and South

American amber exposed to air pollutants, biocides, light and moisture. Collection Forum 6: 6575.Wypych, G., and . Faulkner. 1999. Basic parameters in weathering studies. InG. Wypych (editor),

Weathering o plastics: testing to mirror real lie perormance: 113. Norwich, NY: Plastics DesignLibrary.


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New Jersey

New JerseyNew JerseyNew Jersey

3

49

10

UV 50% RH

New JerseyNew Jersey

56

UV 20%70% RH


1112

Dark 0% RH


1314

Dark 50% RH


1516

Dark anoxic / 0% RH


1718



1920

Dark 20%70% RH

New JerseyNew JerseyNew JerseyNew Jersey

21222324

Accelerated temp.

BalticBalticBalticBaltic

1278

UV 0% RH


349

10

UV 50% RH

BalticBaltic

56

UV 20%70% RH

BalticBaltic

1112

Dark 0% RH

BalticBaltic

1314

Dark 50% RH

BalticBaltic 1516 Dark anoxic / 0% RH

BalticBaltic

1718


BalticBaltic

1920

Dark 20%70% RH


21222324

Accelerated temp.

Fossil resin Sample (no.) reatment Fossil resin Sample (no.) reatment

Dominican

DominicanDominican

1

78

UV 0% RH

DominicanDominicanDominicanDominican

349

10

UV 50% RH

DominicanDominican

56

UV 20%70% RH

DominicanDominican

1112

Dark 0% RH

Dominican

Dominican

13

14

Dark 50% RH

Dominican 15 Dark anoxic / 0% RH

DominicanDominican

1718


DominicanDominican

1920

Dark 20%70% RH

DominicanDominicanDominican

222324

Accelerated temp.

CopalCopal

CopalCopal

12

78

UV 0% RH

CopalCopalCopalCopal

349

10

UV 50% RH

CopalCopal

56

UV 20%70% RH

CopalCopal

1112

Dark 0% RH

Copal

Copal

13

14

Dark 50% RH

CopalCopal

1516

Dark anoxic / 0% RH

CopalCopal

1718


CopalCopal

1920

Dark 20%70% RH

CopalCopalCopal

212223

Accelerated temp.


21/21

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