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Jnumal OP
Analytical and Applied Pyrolysis
m Pmkm
ELSEWER
40 41 11997)167-N
Formation of long-chain ketones in archaeological
pottery vessels by pyrolysis of acyl lipids
A.M. Raver., P.F.
van Bergen,
A.W. Stott, S.N. Dudd, R.P. Evershed
Orgo&G rm hc n l b t ry Un i t. Sc hm l o f C ke n t i st t y , Un i n r si ty o f B rt it a f . C m tac k i C hc . B ri rt d .
BSS
ITS, UK
Rmivcd
I8 October 19%; acceptd
I3 February 1997
Sludicsof organic residues preserved in ungkazcdarchaeological pottery have rwcak d the
prescna of homologous series of long-chain ketones con Gting 29-35 carbon a toms. The
C,,. C,, and C,, ketones arc parfidarly abundant and exhibit a distioct moo&nodal
distribution. The presence of long-chain
ketones in
potshads is usually ascribed to the
absorption of epicuticular waxes into the
pottery
fabric during
the
cwking of kafy
vegetables. However, compo und specific stabk carbon isotope (SW) analyses of the
individual lipids present
in the potsherd extracts,
in combination with &tailed structural
information, indicates that these ketones do not derive from plant waxes. Isotopic and
structural analysis of the fatty acids, which always cD_ocFurwith the kctmu suggest
hat
a
precursor-product relationsh$ exists. Micro-scale pyrolysis of a range of Bee fatty acids and
triacylglycerols in the presence of v arious inorganic matrices was umkrtakcn in exploring the
possibility of an abiologicml route to the fonaatitin of the
ketones. Ikpcnding on the
pyrolysis conditions crnployed. substantial yields of long rnict-c~hain
etones were
formed
which were structurally and isotopically congruent to those observed in tbc ancient pt-
sherds. The ketones are formed by ketonic *xylation (a type of
had
to head
condensation reaction). probably involving fatty acid metal salts as intcrtu4iatca, the
metal
being provided by the i norgani c atrix. Apart from the abundant long
mid-chain
ketones
various other products such as ntcthylketoncs, methyl esters, allwrcs. alk-l-mcs and
homologous series of minor ketones arc formed P Psecondary pyrolysis products. These fatter
products are not found in the pottery pr aMy due to kss vigorous
tbcrmd ronditioru
achieved during the original use of the vessel compared with those attained ia the laboratory
pyrolysis experiments. Evidence for this comes from
the
formation of tbc fatty acid methyl
esters which are only
produced
under the most forcing of pyrolysis conditions. 6 1997
Elsetier Science B.V.
Cormponding aulhor.
Ol6S-2370~97/517.00 1997 ElscvicrScience
B.V. Ali righIs mwwd.
PU 50165.237O(Y7)00036-3
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268
A.M. Raccrr et al . /1. Anal . A& yrdysis 40-41 ( l99 ?) L57-28.5
Keywords: Micro-scak pyrolysis; Ketonic decarbanylatiun; Mid-chair, ketones; Degraded
animal fat; Potsherds: Archaeology
Pottery is amongst one of the most cor:lmon classes of art&cl recovered from
archaeological excavations 111.Unglazed vessels were commonly used in the past
for food processing or storing foodstuffs and during their lifetime they could
absorb substantial quantities of liquid or liquified commodities that came into
contact with them. The protection afforded by absorption within the fired clay
matrix of the vessels results in remnants, i.e., organic redducs, of the origirlal vessel
contents surviving over many millennia [2,3]. Studies of lipids exrractdd from
unglazed archaeological pottery have revealed
a wide
range of commodities associ-
ated with the use of vessels in the past, including animal fats [4], beeswax [?I. and
leafy vegetables [4.6].Cegraded animal fats occur most commonly in potsherd.; and
are characterised by the presence of high abundances of saturated fatty scids.
particularly Cta:. and C18:,,, ogether with mono-, di- and triacylplycerols. The
variety of acyl lipids results from hydrolysis by enzymatic or abiologictildecay upon
vessel use or burial. In some instances the distributions of compounds, revealed
throug l gas chromatographic (GC) analyses of lipid extracts, suggest the presence
of a single commodity, e.g,, degraded animal fat, However, mixtures of components
do occur with reasonable frequency. These mixtures reflect the range of commodi-
ties that will have been processed in the individual vessels for example degraded
animal fat/plant epicuticular waxes [q and animal fat/beeswax [S].
Investigations of the lipid extracts of a number of potsherds from a variety of
geographical locations in the UK and mainland Europe have reveal& the presence
of a very distinct distribution of lang mid-chain ke ones comprising predominantly
31, 33 and 35 carbons atoms and which arc’present m the relative proportions ctl,
1:2:1. An example of one such distribution is shown in Fig, I. This distribution
always co-occurs with other lipid species characteristic of partially degraded
(hydrolysed) animal fats, i.e., fatty acid, mono- and diacylglycerols. In addition to
the major saluraled components, Cs3 and CJ5 monounsaturated ketones have also
been identified in these lipid extracts. The origin of long mid-chain ketones in
organic residues of archaeological potsherds has previously been assigned to the
incorporation of ketones present in cpicuticular leaf waxes [8,9] during the cooking
process [4]. However, the following features of the ketones shown in Fig.
I
are
inconsistent with them deriving from epicuticular leaf waxes:
(i) Mixtures of ketones are rare as leaf waxes ale usually dominated by only one
homologue in the range of C,,-
Cg3 9], For example, nonacosan-15one is found in
Bra~icu oleraceu (cabbage) and hcntriacontan-I&one in Rl l i unr orrunr(leek),
(ii) CJ5 ketones rarely occur in plants,
(iii) The ca.
I
:2:
1
ratio ui C3,:CnS:C35etones seen in many archaeological vessels
has yet to be seen in any modem plant,
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R.41. Rrrun rt CT .J+ Anal. Appl. Pplpsir 40-d (1997) 2667-285 2f?9
(iv) The Cj3 ketone from the extract shown in Fig. 1 is tritriacontau-lti-one, an
asymmetrical ketone+ whereas those found in epicuticular waxes arc typicaIly
syInmctrical,
(v) The apparent absence of alkanes or alkands of the same carbon numlx~ as
the ketones is inconsistent with the biosynthetic pathway of ketones in
plants [81.
Additional evidence for the mixture of ketones having a source other than plant
epicuticular leaf wax came from compound specify stak carbon isotope (a “C)
measurements using isotope ratio monitoring gas chromatogrupby masJ sp~tromc-
try (inn-GCMS) [IO] of the individual lipids present in several potsherd extracts.
The 6 W values for the
qvj6: lipids extracted
from three vessels of contrasting
ketone abundance kre shown in Fig. 2 (note that whik the abundance of the
ketones varies, the distribution is distinct). Isotope fractionation during lipid
biosynthesis in plants leads to a relatively high depktion of the ‘?I in mid-chain
ketones (and otherlmng-chain alkyl compounds) produced by C, plants [I I- 15’J
-Xii wet&&$&&t ia the potsherd extracts did not exhibit the same degree of
depletion in 13Cand instead exhibited 613C values lying outside the range for the
analogous compounds biosynthesised by Cj plants [16,17J.
These apparent anomalies in stable isotope composition, rtructure and distinctive
distribution pattern led us to consider an alternative origiu for these ketones. The
co-occurrence of the ketones and fatty acids, together with their similarities in 5’?I
values [ 17,lSl (Fig. 2) implied that these two compound classes are closely related,
possibly as chcmieal precursors and products.
Thermal decarboxylation of fatty acid salts at tcmpwatures in e~~cessf 3W’C
has been reported as a route for the preparation of symmetrical aliphatic kttones
[Is-23). The process, known as ketonic decarboxylation, proceeds as follows:
Fig I, High tcmpwtuwgas chromJtoynm f II otal ipid eMact fmm a middk Saxon lpstih Ware
cooking vwc) (FLXIS). Key: ndkates fatty ti with C
t, 01 c,,, c,*,, referring to tctwkanoic,
hcxadcc~oic and octrrdccanoic acid, respectively; IS is intcruat stnaduj (n-tarattimtanck K
indicatu mid-chain ketonu with the prscding nunber indi&og the total number of’ carbon atoms
DAG we diacylglyw&: TAG arc triacylglyrrrds. Vute that the both Ctoo and C,,, fatty acids arc
Off-SC .
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270
L
I
C
110
I.26 )
IAi.aq
..__. _._
C
100
(- 27. 2)
I
33K
a
(-266)
.
+-IL
0
10
Tma mnuea)
Fig. 2. Partial gus chromatograms 0; total lipid extracts of three early bronze age cooking vessels rom
SI VeiMlinglbcrg. Austria, showing Ihe variation in relative nbuaduncc of the long mid-chain kcrcnes
lu. b and CL Key: ndicrtn futty acids with C,, .,,, C,,,O,
etc. indicating saturated fatty acids and C,,,
indicating B monounssurMed fuuy acid; K refers to long mid-chain ketones with the prcctxiing number
indicating the total number of c&on atoms; IS is internal standard (Sr-cholcs~anrj. Values in
pnrenthesis refer to 6°C values of individual compunds.
RCO,H I- R’CO,H -I- MO + RCO ,MOC (O)R’ + H l
RCO,M OC(O)R’ 4 RC(O)R’ + MCI -I-CC&
The overall reaction is shown for a divalcnt metal oxide (M O): if two fatty acids
are present, then crossed and self-conden;ed products can be expected. Ceram ic
cooking vessels contain me tals known to eficiently catalyse the fatty acid ketonic
dxarboxylation [20-231, the metals being indigenous to the source clay of the
fabricated vessel [24,25]. The presence of such metals h as been confirmed by
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electron probe micro analyses of the specific potsherd samples in which ketones
have been observed [I 71.
This paper reports the resuhs of a series of micro-scale pyrolysis experiments
involving heating free fatty acids and triacylglycerols
in the
presence of various
inorganic matrices an d metal oxides. These experiments constitute an essential part
of our investigations aimed at establishing the origins of mixtures of mid-cha in
ketones in archaeological ceramics. Some preliminary findings of the experiments
described in this
paper are referred to briefly in an earlier report [la].
2. Mater and methods
Three ceramic fabrics were used
in
this investigation, earIy Bronze age (AU SP)
and middle Saxon (IPSWICH ) coarse wares and a modern Gred clay @I.Mm T)
(suffixed num bers in Fig. 2 refer to speciGc sample codes. consistent in all analyses
involving the she&). The Austrian pottery originated from the early Bronze age
[ca. jooo BP) tittlement at St Veit-Klinglhr&, SKI Saltberg, Austria and was
provided by the
Departmenl
of Archaeology, Ssutham pton University [26]. The
sherds selected for analysis were from the body of vwsels interpreted as ‘cooking
vessels’ based on their form an d dimen sions. Ipswich Ware (ca. 650-35 0 AD ) is
post Rom an pottery produced on an industrial scale that has been recovcrcd from
many archaeological sites in eastern Englan d [27]. The particular sherd used in this
study came from Flixborough, Hum berside (FL?CCb imulation pottery consisted of
a clay-based fabric taken from a modern replica vessel [mixture of I137 Keu ptr
marl (Sreffordshire) and sand (3:l ratio) and fired at goo9q. Sherds were stored at
amb ient temperature away from direct sunlight or sources of beat.
The CItiO, C18:0, Cltl:, fatty acids and triacylglycerols ( >99? purity) wetr
obtained from Sigma Chemical wnpny and
the
metal
salts
fro.m BDH Chemicals
Ltd.
2-2. rgmic residue
un a v J is
The preparation and extraction of shards is reported elszwherc [J]. Briefly,
ground pottery was extracted with chloroform/methanol (2:l v/v) by ultra9onica-
tion lo obtain a total lipid extract (TLE), Prior to GC and GC /MS analyses the
TLEs were dcrivatised, using BSTFA (with added 1% (v,‘v) trimethykhlorosilane).
to trimethylsilyl (TMS) derivatives.
All matrices (e.g., clay
fabrics
from AUSP, 1PSWICH and SIMPOT; Table I)
used for pyrolysis experiments w ere ground and s&cu t exlracrcd. D ried matrix
material and reactants (e.g., fatty acid and metal o.xidc) were thoroughIy homoge-
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nized. Quartz glass tubes 150 mm x 1.5 m m id.. 3 mm o.d., 4 mm) were filled,
approximately, to a quarter with the reaction mixture which was then covered with
analytical glass beads (BDH, GLC grade). The glass beads were placed on top of
the reaction mixture to condense material otherwise lost by vaporisation at etevated
telnperatures. The base of the tube was heated with a micro-burner for
approxi-
mately IO-20 s. Heating was terminated when a condensate was seen to form on
the glass beads or shortly afterwards. Tubes were cooled to ambient and externally
cleaned with chioroform/methanol {Z: v/v). Tubes were then crushed and extracted
with chloroform/methanol (2:
v/v; S
ml) by ultrasonication. Following centrifuga-
tion (3OQOpm, 10 min), the supematant was filtered (glass wool plug and a PTFE
2 pm filter cartridge) and then reduced in volume under a stream of nitrogen. l’h~
matrix was re-extracted and the extract obtained added to the first extract.
The different pyrolysis experiments carried out are listed in Table 1. The
temperature of the clay matrix during heating was obtained with a K type
thermocouple packed into a tube with thr same reactants. In the ‘mild’ pyrolysis the
clay matrix was heated to between 350 and 45O”C,while in the ‘vigorous’ pyrolysis,
ternpm:ures as high as 800*C were reached by the matrix. Since matrix tempcra-
ture was dirticult to control precisely due to the mode of heating employed, i.e.
micra-burner, the appearance of the condensate on the glass beads was used to
monitor the progress of the pyrolysis and temperatures being reached in individual
experiments.
2.4. G/c GC/MS
irm-GCjh4S
6C analyses were carried out using a Hewlett-Packard 5890 Series II gas
chromatograph fitted with an on-column injector (injector temp. XX). Com-
pounds were separated on a CP-Sil SCB column (WCOT fused silica, IOU%
dimethyl polysiloxane, 50 m x 0.32 mm i.d., 0.12 pm film thickness) with a
temperature programme of 50- 150°C at approximately 10°C min-I, to 300°C at
approximately Y’C
min - ‘,
then isothermal for 20
min.
Helium was used as carrier
gas. Alternatively, a DB-I column (J and W, Durabond FSOT, 100% dimethyl
polysiloxane. 15 m x 0.32 mm i.d., C.1 pm film thickness} was used with a
temperature programme of SO-200°C at 7°C min-‘, to 3OOOCt 10°C min-‘. to
3SO*C t 20°C min-
’
then isothermal for I5 min. Hydrcgen was used as carrier
gas.
Compounds
were detected using a flame ionisation detector. The chror,;ato-
graphic data were acquired and processed using HPCHEM windows.
GC/MS analyses were carried out using a Carlo Erba HRGC 5160 MegaSeries
GC. with on-column injection. connected to a Finnigarl MAT 4500 quadrupole ms
(transfer line heated to 2 35O”C),Electron
ionisation
I70 eV, 300 PA) was utiliscd
with a total scan cycle time
of 1 s.
Data were analysed using Finnigan MAT
INCOS software, Peak identificarions were based on comparisons with mass spectra
and retention times of authentic ccmpounds.
Stable carbon isotopic ratio measurements were performed on a Varian 3400 CC
(SPI injector; with the same CP-Sil 5CB column and column conditions as for GC
analysis), coupled via a combustion unit (CuO/Pt combustion reactor, SSO’C) o a
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Finnigan MAT D&a-S stable isotope ratio mass spcctromLtcr (electron ionisation,
100 cV, 1 mA). All data wcrc processed using Finn&an
MAT software he
fatty
acids wcrc measured as TMS derivatives and corrected for st;~bk isotopic contrib\L-
tions from TMS carbons by tht? use of an off-line calibrated fatty acid standard
[ I7,28,29].
Off%nc pyrolysis of various fatty acids and triacylglycerols in the presence of
clay matrices and metal salts was used to simulate the physio&crnical conditions
required to promote kctonic dacarboxylation in ancient pQtmy ytssels through
heating. The results from the range of experitrrmts performed are summa&cd in
Table 1. The temperature ranges used arc behcvcd to bc a ~NXI
simulation
of the
wood or charcoal fires probably used for heating vessels in antiquity. It should be
noted that all experiments were performed under anhydrous conditions.
All pyrolysis expcrimcnts involving fatty acids resulted in the formation of loog
midchain ketones, rbe composition of which &p&cd on the nature of the
precursor fatty acids (Table 1) A though t he composition of the major pyro lys is
products, i.e., ketones, was very consiste;?? across tttc range of cxpcrimcnts per-
fanned, the distribution of minor products varied somewhat depending upon the
reaction conditions and/or rcagcnt combination employed.
y r o l y s i s of hexadecanoic acid in the p_resence f on% 1 fued day matrix (AUSP)
productd only low yields of hccntriacontan-l&one (31 K). The yield was not
improved by varying the ratio of mixing or temperature [t?j. In contrast, pyrolysij
of fatty acids in tbc presence of metal
salt done, e.g.,
CaO, produc& higher yields
of the long mid-chain ketones, topthcr with various other minor lower molecular
weight products (Table I). Signihatntly, the major products obsevcd in both cases
were the same as the major ketones seen in the extracts of archaeological potteq.
More significantly still, wcrc the results o f experiments in which the fatty acids were
heated in the prescncc of both metal salt and matrix to yield a SigniGcantlyhigher
abundance of ketones compared with tbosc reactions performed in the pnscncc of
metal salt alone (rig. 3(a)).
To investigate the possible effects of the matrix on the extent of reaction, four
powdered matrices (Table 1) were used: (I) a modern fired clay vessel (SIMPOT);
(2) a pre-cxtractcd sherd of
Austrian pottery which had been shown to contain a
high abundance of ketoria (AUSP); (3) a pm-extracted sherd oi lpswich Ware that
had been shown to contain large amounts of fatty acids but no ketones IIP-
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276
A.M. Ko wn ef al . l .1. AIN/. Appl. Pjw l~& 40-41 ( IW?) 267-285
SWJCHJ, and 4) borosilicate ground glass (Ground Glass). GC and GC/MS
analyses of the pyrolysis products showed that no significant differences existed in
the composition of the pyrolysis products when different matrix materials were
used.
In
addition to the calcium
oxide used in
the
preliminary
experiments, several
other metal oxides and salts, having different reactivities, were used to inveaigale
their effectiveness
in promoting
ketonic decarboxylation (Table
I).
The objective
wa;; to enhance the natural population of available metal ions by effectively doping
the matrix. Early work on ketonic decarboxylation has shown that group 2 and
transition metals were more efficient as catalyst in the reaction [19-231 then group
i
1 S h n ~ l r U o n r x p s r ime n l
G , , and Gmo + CW
u l f J o f o u s
i
z
1QK
u I
31K
17K
A_Ll
8 K fd i _:_
.
i
L
(b)
3CK
.i
F ig . j . ? & a l g a s c h r u ma t o g r a ms o f p y r o l y s i s p r c k l u c t s f u r me d u p o n [ a ) m i l d u n d ( b ) v i g o r o u s
- -?-
s imu iar ion expe l&Hs us in g hexadccae~~c and oc tadecano ic ac id in the resence o f 60 ; AUSP mat r ix
Aw added in the case o f the mi ld exper iment . Key : I in i i ca ies sa tura ted fa t t y ac ids & , , C I IEw e tc . ;
C , ,. , Me. c tc in d ica tes the me:ky l w le r o f the pur t iu& r fa t t y ac id : K re fe rs to lon g n id -ch u in
ketones
with the paling number
i nd ica tm s the to ta l num ber o f carbon a tom s ; 0 refe rs to a lkan- l6w1~s and
a lkan- l&ones ; 17 K and I9 K are methy i kc lones w i th 17 and 19 carbon a toms ; lg : I K and 2011 K l i t? :
unsatura ted e thy l kc tona w i th 18 and 20 carbcm a toms :
a r e c o n t a m in a n t s . F o r a d d i t i o n a l i n f o r ma t i o n
see text and Table I .
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A.M. Ruwa ct ul. 11. Am . Appl. Pytolysk 40-41 (1997) 267-285 217
1 metals [22], The li,;c of alumina or silii togctbcr with hcx&zanoic and
octadccanoic acids yicldcd C,,, Ca3and C, kctt?ues in ow obun&nce. Pyrolysis of
fatty acids in the prcscnce of calcium carbonate, iron oxide or magn&urn oxide
with a modem lircd clay matrix (SIMIWTj, yicldcd ketones in similarly high
abundance to that obtained for calcium oxide (Fig. 3(a)).
The mults show that mixtures of ketones, with the chtr.stic distributions
Seen in some archaeological ~cssds, arc formed readily by pyrolysis of fatty acids in
the prcscncc of a ceramic matrix and a metal oxide or salt (Fig. 3(a) v-s Fig. 2). The
distribution of pyrolysis products appars to bc indqendcnt of tbc nature of the
ceramic matrix used, illthough the matrix itself plays a role in tbc reaction siacc the
formation of kctoncs occurs more readily in the pruencc of a Eramic matrix. The
exact role of the matrix is unclesr but it may scrvc to trap mokcuks in the region
of rhc vessel wall when: bigb tempraturc gas phase reactions can LOX. Wiiut
the muirix ihe reaction appears to proceed in the melt at lowcr temperatures @low
the boiling point of the mixture).
At mild pyrolysis tcmpcralurcs 135%45WC) kttoncs wcrc pro&cd in high
abundance when metal salts or oxides were mixed with the matrix (Fig. 3@)). Apart
from the spcciGc long mid-chain ketones om products wcrc obacrvcd. Tbc
pyrolysatcs prod& under more vigorous heating conditions ( 2 8WC) sbowcd a
lower abundance of fatty acids and m proportions of products ocher than
the chmcteristic ketones {Fig. 3(b)). Tbcsc additional products comprised s&s of
homologous saturated and unsaturated kctoncs, alkanea alkcxs and fatty acids of
shorter carbon chain length than the precursor acid(s). Methyl esters of tbc
precursor fatty acids were also pro&cd under vigorous conditions. Thcsc addi-
tional products wcrc not obscrvcd in tbc lipid extracts of tbc a&t pttcry.
GC analysis of the pyrolysis products of C,,, and C,,, in the prcscn~ of Mg?D
and SIMPOT (Fig. 4) showed
that
apart from unreactcd fatty acids, three abundant
ketones were produced upon pyrolysis. Thcsc wcrc pcntatriacontan-1% (35
K ,
tritriacontan-16-one (33 K) and hcntriacontan-16-one (31 K). In addition, two
rclatlrely abundant methytkeetoncs I17 K and 19 K) were produced, togcthcr with
smaller amo unts of an homologous seti of allan-l&ones and alken-16-ones
(carbon range 17-33) and alkan-1800ncs and alkcn-IS-ones (carbon range 19-35).
The unconventional numbering of the kcEctom osition has been adopted to cmpha-
sisc the structural relationship to the precursor fatty acids. In addition, methyl
cstcrs of the precursor fatty acids, alkancs and alkcncs were produced in iow
abunda ncc.
Tbe homologous scrics of keloncs wen produud in approximately equal conccn-
tralion, apart from the methylketones which wcrc prcscnt in much
grcakr
abun-
dance (Fig. 4). Investigation of the homologous s&s of ketones by GClMS
revealed
that their structures
clearly retied the nature of the precurso
r fatty acid
subjected to pyrolysis. For exam ple, pyroI;rsis of octadecenoic acid
yielded only
alktn-M-o nes (Table 1). The pyrolysis of C,,, and C IIrU fatty acids together
yielded two families of struclurally related kcconcs, rramcty, alkan-t&ones and
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A.M. Rawn tv ul. /J. And . App l. P.vro/):ris 40-41 1997) 267-28s
1Gi R
a
17K
E $
15 ?
Fig. 4. Purtial gas chromiUogram of products formed upon mild pyrolysis of hexudecanoic und
oct~decunoic ucid in the presena of MgO and SIMPOT matrix. Key: I indicutcs fatty acids; K refers
to long mid-chain ketones with the preceding
number ndicating
he total
numberor carbonutom~;0
refers to
rlkan4Conc-s
nd alkan-l&ones: efers to n-rlkrrne and n-trlkenex with Ilr:lR indicating
hexadec-l-enc. I7 K and I9 K are methyl ketones with 17
and 19
carbon utoms; Values in parenthesis
r&t ID fi’%Y vAes of individual compounds. For additional information see text rnd ruble 1.
alkan-IS-ones. Although no unsaturated fatty acids were present in the reaction
mixture. a complementary
series
of mono-unsaturated ketones is present amongst
the reaction products
(Fig. 4).
The series of unsaturated ketones have the double-
bond in the alkyt carbon chain that varies in length to give the homologous series,
Hites
and
Biemann [23], pyrolyzed calcium decanoate at temperatures ranging
from
4W’C
to 6MPC and observed formation of an homologous series of ketones
based on alkan-IO-ones. They reported no unsaturated ketones but did observe
alkanes and alkenes. In contras t to the results obtained for the calcium salt, they
showed that pyrolysis of the symmetrical ketor.c, tonadccan-lo-one, under the
same conditions did not produce a significant homologous series of ketones. This
fintiing supports the conclusion that the shorter chain length ketones were not
produced by pyrolysis of the primary condensation product, i.e. mid-chain ketone.
Our findings indicate that the structures of all the keto nes produ ced can he
accounted for on the basis of the precursor fatty acids. No ketones were observed
other than those formed by condensation of
the precursor fatty acid(s) (C,,,, and
C,,J.
thus indicating
that the minor ketones must have been produced during a
pyrolysis reaction involving a pre cursor fatty acid rather than b y further
degrada-
tion of the primary condensation products.
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A.M. htvn ct al. /J. And. Appl Pynd~s~ 40-4 ft997) 267-28 5
279
Hitcs and Bicmann [23] also studied the formation of the shorter chain ketoucs
with the aid of calcium hoate mally deuterated at C-8, C-9 and C-10 and
found that the methyl, ethyl and butyt groups of the altan-It?+= origiuated from
the carbons closest
to the carbony group in the salt. II mechan ism proposes
free radii reactions involving combinatious of wcyt and alkyl radicals. They
proposed that the atkyl radicals can further disproportionate to produce alkancs
and alkencs or produce tower carbon nut&r radi& by
a conccrtd process
of
hydrogen rearrangement and B -scission. They argued that the proecsa would have
to be conccrtcd since lower carbon number radicals bearing the dcutcratd carbons
arc not produced. The lower carbon numb er radicaki then react with the acyt
radical to produce the lower carbon
number
kdoncs, and rcwt with otbcr
alkyl
radical spccics to produce dkanes. Howcvcr, tberc are inconaistc&cs in their
proposed mechanism. Firstly, alkanes aud alkcncs of carboa chaiu kugths longer
than the initiR1 atty acid wcrc not
present,These productswould be cxpcctd
to
arise by recombination of the initial alkyl radical with other high carbon number
atkyl radicals. Secondly, ketone formation by
free rad ical combination, would not
explain the rcportcd diffcrcnces in reaction products when difpert
metals wtrc
utilised [tOJO] or diff- in the origin of the ketone carbonyl crvbon in a
crossed condensation using hrbcllcd oompounds [3l]. Hites and Bitmann [23]
explained the high abundance of mcthyt kctoucs by invoking the stcpwise dcgrada-
tion of the rtlkyl radicals to methyl radicals which then combine with acyt radio&
However, this dots not explain why the minor ketonesarc formed in roughly qual
abundance (see Fig. 4).
The pyrolysis of C16:. and CltIOacid in the
presence
of Mg0 and modern
simulation pattcry matrix (Fig. 4) yickkd shorter chain saturatedad unsaturated
fatty acids
in roughly qual abundance. WC
suegcst
that thest shorter chain
carboxylicacids arc‘mopped up’ by the precursor fatty acids present as salts
of the
metal, allowing them to eondcnsc to form the shorter chain (C,, to C,) asymmct-
rical kctoncs in roughly qua1 quantities (Fii. 7). It is suggcstcd that the prominent
methylketones were formed by a six centre ring rearrnngcmcnt of the primary
ketone product (Fig. 5 and Fig. 6) Methyl kctouee were not observed by Hitcs and
Bicmann [23] in their pyrolysis of ketones, so it is suggested &at this dazomposi-
tion/Esttp,ngcment is catalyscd by the prcscncc ofthemctaloxideuscdhlour
experiments. This low energy rcarrsngcment (Fig. 5) would explain the relatively
high concentrations of the methytketonts. This type of rearm-at mechan ism
would also explain the specific atkene distribution observ ed in Fig. 4. Hcncc,
pcntatciacontan-18.one and hentriacontan-l one would yield Cl6 and C,, slkencs
Fig. 5. Pmpnd low energy six cmtrc rearm-t for tbc fomution o f spcdic m ethyl ketones and
alk-l-ems (Fig. 6).
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280 A.M. Rarm cl aI. / J . A nd . A p p l . Pyo f y s i s 40 - 41 ( 1997 ) 267 - 285
Starting
Components
1 ycfM ,&C-~ 1
Self-eonksation
I
Prhnrry
Products C&,.
( 3W
LL-
Secondary
Products
i
(ir:iR)
17K)
WW
(1B:lR)
Ah
Fig. 6. Proposed pilthways
teading o the formation of the
main pyrolysis products
(Fig. 51.
respectively, while tritriacontan-l&one
woul
yield both these latter alkene:; (Fig.
6). Hence,
only products of the major ketones would be detectable.
On-line Curie point pyrolysis of the sodium salt of the C,,,:, acid has been carried
out
by Hartgers et al. [32]. The authors reported only alkenes and alkanes up to C,,
with anomalously high amounts of the C,, and C,, alk-I-enes. and the C,, alkan e,
which were attributed to the specific decomposition reactions of the fatty ac id salt.
These products were also observed in higher concentrations than other alkanes and
alkencs in Fig. 4 and could a rise through decomp osition of a me tal salt of the
analogous fatty acid precursors (Fig. 7).
A mechanism for the pyrolytic degradation of fatty acids by radical decom posi-
tion, involving H transfers and /I scission, was proposed by Hartgers et al. [32]. The
decomposition produced homologous series of aikenes and alkancs together w ith
corresponding shorter chain length fatty acid salts (Fig. 7). Where the radical
decomp osition of the fatty acid yielded an alkene the complementary product
would be saturated, alternatively the formation of an alkane would give rise to the
complementary unsaturated fatty acid salt. These shorter chain saturated and
unsaturated fatty acid salts would be available to react with one of the precursor
fatty acids thus yielding the homologous series of saturated and unsa turated
ketones (Fig. 7). Methyl esters of the precursor fatty acids were found in the
extracts from fhe vigorous pyrolysis of fatty acids.
It is
assumed that under such
conditions higher populations of radicals are produced, of which the methyl
radicals are the most reactive. These are assumed to either attack the fatty acids or
the salt directly, in order to produce methyl esters.
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A.hf. WCR er al. /J. A nd. A ppl. Pyrolysis M -41 (M7) 267- U
281
3.2. Pyro is experiments oj triacyigIjxerols
Since the major lipid components of fresh animal fats are triacylglycerols
(TAGS), a series of pyrolysis experiments was performed using these
compounds as
substrates_ Pyrolysis of tripalmitin, tristearin and mixtures of trip in and
tristearin in the presence of calcium oxide alone. and in combination with powdered
ceramics showd that more vigorous pyrolysis conditions were rquircd to gcneratc
ketones from TAGS than from free fatty acids (Table 1). The pyrolysis ptoducts of
tripalmitin (Fig. S(a)) produced, primarily, the mid-chain CJ, ketone (31 K) and a
C,, methyl ketone (17 K), whik the main products of the pyrolysis of trWarin
(Fig. 8(c)) were a mid-chain CJ5 ketone (35 K), a C,9 mcthylkctonc (19 K) together
with a CJ, ketone (33 K). Pyrolysis of a mixture of the two TAGS -ted all
three C3,, C,a and Cl5 ketones in high abundance (Fig. 8(b)). In addition to the
abundant ketones, minor products comprising homologous s&s of ketones,
alkanes and alk-l-enes were produe& These were the same series as obsewed in
the pyrolysis
experiments involving fa fatty acids as
substrates
(Fig. 4). The
mechanismfor the formation of the pyrolysisproducts is beEwed to be analogous
to that for the free fatty acids, except that the precursor fatty acid salt has to be
generated from TAGS prior to the free radical reactions &ng. When tri-
Fig. 7. Proposed pathways leading IO the formation of the minor py rolysis products.
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282
A.M. Raven et al. /.f. And . App l. Pyrolys is 40-41 1997) 267-285
u
77
a
$
C,l.oMe
7
51
K
r:
:K
‘2
:2DK
z
.
’ cm
33K
10
20
kw
(minutss)
Fig. 8. Partial gas chromatogram of products
formed
by vigorous pyrolysis of triacylglywulb usiug (a)
tripalmitin: (b) mixture of tripalmitin and iri~tearin and (c) tristearin in the presence of CaO and AUSP
matrix. Key: ndicates fatty acids; K
refers to long mid-chain ketones with the preceding number
indicating the total number of carbon atoms; 0 refers to alkan-Idones and alkan. 18.ones; 0 refers to
n-alkanes and n-alkencs with MAR indicating hexadec-l-me.
palmitin and tristearin were pyrolysed together the CJ3 ketone w as produced in
greater abu ndant than the Cjl and C35 ketones. An analogous pattern of pyrolysis
products was also seen for the reaction of C,,:, and C ,,:. fatty acids (Fig. 3(a) and
Fig. 4). High temperature GC o the products generated during vigorous pyrolysis
showed trace amo unts of TAGS remained after heating. Interestingly, diacylglyc-
erols (DA Gs) were present amongst the products but no monoacylglycerols
MAGs). This pattern contrasts markedly with that produced under mild pyrolysis
conditions where the starting TA GS persisl in high abundance and the principa’
products are DA Gs, M AG s and, interestingly, methylketones. The reason for the
formation of the latter during the mild pyrolysis of TAGS is unclear.
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A.M. Rawn er al. /J. And . Appl ydysis 40-41 1997) 267-28s 183
3.3.
Total i pid exmcts of archaeologi cal omry
Fig. 2 shows typical gas chromatograms of the total lipid extracts recovered from
potsherds containing long mid-chain ketones. The major ketones identifi8d in aH
c8Bscs ere pentatriacontan-l&one (35 K), tritriacontan-&one (33 K) a8d hentria-
contan-16-one (31 K). Minor components, which were only detected in those
samples containing abundant ketones, included 8 C, even carbon llumber ketone
(34 K), identified as solely tettatriacontan-l?-one, C,, ketone8 (32 K) as mainly
dotriacontan-Idone, but with some coelution of dotriacontan-l5-one, and C,
ketones (30 K) being a mixture of triaconlan-&one and t&conlan-G-one with the
latter being the dominant component (Fig. 2(a)). The C, ketones (29 K) were
identified as mainly nonaoosan-M-one with 8 tmce amount of nonacosan-12-one
present. Unsaturated long mid-chain ketones were identified BSpentalriaconten-i8-
one (35:l K) and tritriaconten-l&one (33:l K) with the unsatruation in the 18
carbon chain substituent of the carbonyl group. Apart from the ketones a number
of fatty acids were also preSent including the ma.r fatty 8cids, C,,, and CH:. and
minor fatty acids, (&, C,,, C,,.* C18:,,CIW and Cm
As mentioned in the introduction the 6 t3C values of the nutin ketones present in
the archaeological potsherds were found to be simihu to those of the co-~~~&g
fatty acids (Fig. 2; [ 17,181).Bzed on compoSitional and isotopic data obtained
from the simulation experiments (Fig. 4), the pmsenoc of these chamcterkztic
mixtures of long symmetrical and ;isymmetrical mid-chain ketones in archaeological
pottery can be explained completely on the basis of the thermally induad ketonic
decarboxylation of the fatty a&IS also present.
The ketone8 found in archaeological pottery 8re probably not formed in gnat
abundaace during a single use of a vessel but gradually accumulate with continued
USC. he first stage in ketone production is the formation of the metal s8. tand &e
metaI requL.l
for salt formation
may derive from the clay fabric or. be Introduced
a5 components of commoditits tig proccssbd in the vessehi,
While the relative proportions of the individual fatty acids may vary somewhat
between different extracts, the distribuiion of the ketones is fairly robust aad can
always be recognised sc long as the C
6a and CllR fatty acids are major comp~
nents of the pyrolysed fat. By the very nature of condensation reactions between
two closely related precursors. a close to binomial distribution of reaction products
is entirely expzcted.
Methyl ketones were not found in any of the pottery samples an8Iyr& which
may indicate that the vessels had been subjected to kss vigorous he8ting during
their ust in antiquity. The influence o water on the pyrolysis of acyl lipid8 and ths
formation of long-chain ketones is king considered in our current work sirice this
would have been involved in any co&rig process although in the wall of a ‘fat
sealed’ vessel, enhydrous conditions might exist.
It has been shown that the cIay matrix itself can catalyse the kctonic decarboxy-
Iation reaction. It h8S been known for many ytars that clays are an active matrix
due to their natural acidity and structure [33].Most clays betong to the Montmoril-
lonite group and have a general structure comprising of sheets of aluminosiiicates
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284
A.M. Ram et al. /J. Anal. Appt. Pyrolysis 40-41 1997)
267-285
sandwiching water and cations, As a result of firing the clay, the layers collapse and
the clay becomes a ridged porous ceramic. Some pores are of molecular dimensions
which allows some steric selectivity of molecules but also produces it ‘cage effect’
where-by reaction products formed between the layers may be trapped due to their
size 1341.This &Id have a concentrating effect on any modified lipid generated
within the vessel wall. In this way appreciable concentrations of ketones produced
may accumulate over time with repeated use of vessels. Hence, these ketones
may
be o f
value as indicators for heat treatment of vessels in antiquity possibly
providing a chemical basis on which to draw distinctions between vessels used for
cooking rather than storage.
4. Conclusions
The work present& here provides an explanation for the previously unknown
origin of specific long mid-chain ketones found in a large number of potsherds
recovered from archaeological sites throughout Europe. It has been shown that
these ketones can be farmed by ketonic decarboxylation from fatty acyl lipids (free
fatty acids and triacylirlycerols)which are absorbed
in c lay f&b& of potsherds .
Apart
from I*jng mid-chain ketones, various secondary pyrolysis products
are
produced during
the
pyrolysis experiments. The main secondary products
are
methylketones which are formed through six member ring rearrangements of the
primary ketones. In addition, homologous series of short chain saturated and
unsaturated fatty aci,is, alkenes, alkanes, alken-l&ones, alkan-l&ones and/or
alken-l&&ones, lkan-l8-ones are produced. The shorter chain fatty acids, alkanes
and alkenes are formed by radial decomposition of fatty acid metal salts whereas
the alkanones and alkenones originate from the ketonic decarboxylation of the
precursor
fatty acid with some of the shorter chain fatty acids. The
secondary
pyrolysis products are not detected in extracts from archaeological pottery indicat-
ing that during the original use of the vessel less vigorous thermal conditions were
achieved compared with those obtained in the laboratory pyrolysis experiments.
The presence of these ketones may therefore provide an indication as to the mode
of use of the vessel.
Acknowledgements
We thank the NERC for financial support for mass spectrometty facilities
(GR3/2951, GR3/3758 and FG6/36/01) and research grant (GR319543).Mr Jim
Carter
and Andrew Gledhill are thanked
for invaluable technical assistance.
Professor Stephen Sherman (Institute of Archecology, UCL), Mr Paul W.
Blinkhorn (Northampton&ire Archaeology Unit), I\qrs Varian Reeve and Mr
Robert Pert-in (English Heritage Central
Archaeology
Service) are thanked for
samples and advice.
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1993, p_ 247.
[3] R.P. Evershed, World Archacvl. 25 il993) 7 4.
[4] R.P. Evershed, C. Heron, J.L. Goad, Analyst 115 (19%) 1339.
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