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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.

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[5] S. Charten, R.P. Evcrshcd. P.W. Blinkhom, V. Denham . A rchatomctry 37 (199 5)

113.

[6) R.P. Eversheo, K-1. Amor, f. Collistcr, G. E&ton, S. Charta Analyst 119 [IW) 909.

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