Jou nal o Cul u al He i age 74 (2025) 195–203
Con en s lis s a ailable a ScienceDi ec
Jou nal o Cul u al He i age
jou nal homepage: www.else ie .com/loca e/culhe
VSI: TechnoHe i age2024
A mul i-analy ical app oach o he iden ifica ion o su ace whi ening
phenomena in con empo a y oil pain ing and i s applica ion o me al
soaps
E ika Ta ilon e
a
, Oska González-Mendia
a , ∗,Ila ia Cos an ini
b
,Kepa Cas o
b
,
I xaso Magu egui
a
a
Depa men o Pain ing, Uni e si y o he Basque Coun y UPV/EHU, Leioa, Bizkaia, Spain
b
IBeA Resea ch G oup, Uni e si y o he Basque Coun y UPV/EHU, Leioa, Bizkaia, Spain
a i c l e i n o
A icle his o y:
Recei ed 20 Feb ua y 2025
Accep ed 10 June 2025
A ailable online 27 June 2025
Keywo ds:
Whi e haze
Me al soaps
Oil pain ing
Mul i-analy ical s udy
a b s a c
This esea ch p esen s a mul i-analy ical app oach o he cha ac e iza ion o he deg ada ion p oduc s
ha o m whi e hazes on pain su aces. The su ace o Un i led (1971), an oil pain ing by San os Iñu ie a
(1950–2023), one o he mos no ewo hy con empo a y Basque a is s, was chosen as a ep esen a i e
example. The appea ance o su ace whi ening phenomena is a majo issue in oil pain ing conse a ion
and hei iden ifica ion ep esen s a challenge since hei o igin can be e y di e se. In o de o cha -
ac e ize he whi e subs ance o e Un i led (1971) and unde s and i s o ma ion, a global examina ion
was pe o med, including adiog aphy and UV induced fluo escence, ollowed by he analysis o di e -
en ypes o mic osamples acco ding o he diagnos ic echnique applied: pain agmen s, c oss-sec ions
o su ace sc apings. The me hodology included he applica ion o se e al echniques on he a o emen-
ioned samples: Digi al Mic oscopy (DM), Op ical Mic oscopy (OM), Scanning Elec on Mic oscopy-Ene gy
Dispe si e Spec oscopy (SEM-EDX), X- ay Compu ed Tomog aphy (CT), mic o X-Ray Fluo escence (μ-
ED-XRF), Raman Spec oscopy, A enua ed To al Reflec ion-Fou ie T ans o m In a ed spec oscopy (ATR-
FTIR), X- ay Di ac ion (XRD), Gas Ch oma og aphy-Mass Spec ome y (GC–MS) and X- ay Pho oelec on
Spec oscopy (XPS). The p oposed app oach esul ed in he iden ifica ion o lead soaps (palmi a e and
s ea a e) as he p ima y cause o he whi ening o he pain ing su ace. Addi ionally, i p o ided in o -
ma ion o he ma e ials and echniques employed by he a is and e ealed ha he lead sou ce was
p obably a lead d ie . Thus, his wo k is a s ep o wa d in o de o deal wi h one o he mos significan
p oblems cu en ly being add essed in con empo a y a conse a ion, p o iding ele an in o ma ion o
he efficien emo al o he di e en whi ening phenomena.
©2025 The Au ho s. Published by Else ie Masson SAS. This is an open access a icle unde he CC
BY-NC license ( h p://c ea i ecommons.o g/licenses/by-nc/4.0/ )
1. In oduc ion
Oil pain ings a e suscep ible o a ious al e a ions due o he
in e ac ion be ween hei chemical componen s and he influence
o ex e nal ac o s. One o hese al e a ions consis s on he appea -
ance o whi ish subs ances co e ing pa o o he en i e su ace
o he pain ing. These subs ances may ha e di e en mo pholo-
gies and can ha e a numbe o causes. Al hough i is possible o
find hei sou ce in biological agen s such as mic oo ganisms [ 1 , 2 ],
o mechanical p ocesses like mic o-c acks on he su ace, usually
complex in e nal and physico-chemical ageing mechanisms seem
o be in ol ed in he o ma ion p ocess. One o hem is he c ys-
∗Co esponding au ho .
E-mail add ess: oska [email p o ec ed] (O. González-Mendia) .
alliza ion o sal s, namely ca bona es, chlo ides, ace a es, oxala es,
lac a es o sul a es, he mos equen being magnesium sul a e
hep ahyd a e, known as epsomi e [ 3 ]. Ano he possible cause is he
mig a ion o ee a y acids gene a ed by he hyd olysis o iglyc-
e ides p esen in he binde o in we ing and ex ending addi i es
such as s ea a es o beeswax [ 4 , 5 ].
Among hese p ocesses, me al soap o ma ion is ega ded as
he p ima y phenomenon associa ed wi h su ace whi e hazes, cu -
en ly gene a ing he g ea es conce n in he conse a ion o oil
pain ings. In ac , he widesp ead p esence o hese soaps in bo h
adi ional and con empo a y wo ks o a no only esul s in sub-
s an ial modifica ions o hei aes he ic quali ies bu is also associ-
a ed wi h conse a ion issues. Fo ins ance, he o ma ion o ag-
g ega es wi hin he pain laye s can gene a e ensions and lead
o delamina ion and loss o cohesion [ 6 ]. Me al soaps can also
h ps://doi.o g/10.1016/j.culhe .2025.06.007
1296-2074/© 2025 The Au ho s. Published by Else ie Masson SAS. This is an open access a icle unde he CC BY-NC license
(
h p://c ea i ecommons.o g/licenses/by-nc/4.0/ )
E. Ta ilon e, O. González-Mendia, I. Cos an ini e al. Jou nal o Cul u al He i age 74 (2025) 195–203
o m la ge clus e s, b eak he su ace and appea in he o m o
p o usions [ 7 , 8 ]. The o ma ion o me al soaps, desc ibed ex en-
si ely in he li e a u e, occu s when ee a y acids eac wi h he
me al ca ions o some pigmen s o addi i es, o ming me al ca -
boxyla es [ 9–11 ]. In addi ion, agen s such as high ela i e humidi y
and empe a u e le els, ei he main ained o e ime o wi h cyclic
a ia ions, seem o accen ua e he p ocess [ 12 , 13 ]. The mos e-
quen ly obse ed me al soaps in oil pain s ha e been lead and zinc
s ea a es and palmi a es, bu ca boxyla es o o he elemen s such
as calcium, coppe , cadmium, cobal , i on o in ha e also been
iden ified [ 9 ].
Conside ing all he a o emen ioned, he iden ifica ion o me al
soaps is o i al impo ance o unde s anding hei o ma ion
p ocesses, which is essen ial o he p ope conse a ion o oil
pain ing. The iden ifica ion has been usually ca ied ou by means
o spec oscopic echniques such as Fou ie -T ans o m In a ed
(FTIR) o Raman [ 14 , 15 ] and X-Ray Di ac ion (XRD) [ 16 ]. Gas
Ch oma og aphy-Mass Spec ome y (GC–MS) is pa icula ly sui -
able o iden i ying he o ganic ac ion [ 17 ]. Fu he mo e, elemen-
al analysis echniques such as X-Ray Fluo escence (XRF) o Scan-
ning Elec on Mic oscopy-Ene gy Dispe si e Spec oscopy (SEM-
EDX) a e use ul o ob aining in o ma ion on he na u e o he
me al ion, as well as o he ino ganic subs ances [ 18–20 ]. SEM-EDX
is especially help ul in s udying he s a ig aphic dis ibu ion o he
soaps a g ea e magnifica ion [ 20 ] and i is complemen a y o he
in o ma ion ob ained by Digi al Mic oscopy (DM) and Op ical Mi-
c oscopy (OM) [ 21 , 22 ].
In his wo k, all hese analy ical echniques ha e been com-
bined o s udy a con empo a y oil pain ing signed by he Basque
a is San os Iñu ie a (1950–2023), owned by he O begozo Foun-
da ion. This is one o he mos significan p i a e con empo a y
a collec ions in he Basque Coun y and ga he s ep esen a i e
a wo ks om a key pe iod whe e social and poli ical changes
acili a ed he in oduc ion o new ma e ials and me hods ha
now exhibi specific al e a ions. Un i led (1971) is an un a nished
oil on can as ha exhibi s a whi ish, homogeneous b i le sub-
s ance ha co e s pa o he su ace, c ea ing a haze ha mod-
ifies he appea ance o he pain ing. The mul i-analy ical app oach
included also wo echniques no widely applied in pain analysis.
On he one hand, X-Ray Pho oelec on Spec oscopy (XPS) [ 23 , 24 ]
was used o ob ain su ace in o ma ion and, on he o he hand,
X-Ray Compu ed Tomog aphy (CT) was employed o ob ain h ee-
dimensional in o ma ion o a pain agmen and imp o e he un-
de s anding o he in e nal s uc u e [ 25 , 26 ].
2. Resea ch aim
This wo k aims o be he mos comple e analy ical s udy o da e
o he iden ifica ion o su ace whi ening phenomena on pain -
ings. Fo his objec i e, a comp ehensi e mul i-analy ical app oach
including gene al examina ion (UV induced fluo escence and a-
diog aphy), di e en mic oscopic echniques (digi al, op ical, op i-
cal wi h UV, elec onic), CT, SEM-EDX, μ-ED-XRF, Raman, ATR-FTIR,
XRD, GC–MS and XPS was applied o he analysis o a ep esen a-
i e con empo a y oil pain ing co e ed by a whi e haze.
3. Ma e ial and me hods
3.1. A wo k and sampling
Sc apings o a eas wi h and wi hou whi e haze and mic o-
agmen s we e collec ed om he uppe le ed a ea o Un i-
led ( Fig. 1 ) using a scalpel and a s e eomic oscope (de ailed in-
o ma ion on he numbe o samples and echniques applied o
each is p o ided in Table S1). Fo c oss-sec ions p epa a ion wo
pain agmen s we e embedded in apid cold-cu ing bicompo-
nen esin Techno i ® 4004 (He aeus Kulze GmbH), cu c oss-
wise using a S ue s Mini om and we polished wi h silica ab asi e
pape (P40 0 0 g ades), using a S ue s polishing holde . The em-
bedded samples we e u he polished using Mic oClo h (Buehle )
wi h polyc ys alline Diamond suspension Me aDiTM Sup eme 1F
μm (Buehle ).
3.2. Analy ical me hods
3.2.1. Gene al examina ion and digi al mic oscopy
The pain ing was pho og aphed unde isible and UV ligh
(370 nm) wi h a Canon EOS 750D came a. Radiog aphy was ob-
ained using a TX-50/100 x- ay sys em (Radiologia S.A.) ope a ing
a 50 kV and 50 mA o 0.3 s. The digi al image cap u e de ices
we e eusable pho os imulable phospho pla es (35.4 ×43 cm)
ha , a e being adia ed, we e scanned by means o a compu e ’s
imaging so wa e ia he FCR P imax T2 scanne (Fujifilm). DM im-
ages we e ob ained wi h a 1.3 megapixel Dino-li e AM4917MZT in-
s umen wi h a magnifica ion ange om 10 o 220x.
3.2.2. Op ical mic oscopy
Nikon Eclipse Ci-Pol ligh mic oscope equipped wi h DS-Fi3 dig-
i al came a was used o analyze he c oss-sec ions unde isible
ligh and a Nikon Eclipse 50 Digi al equipped wi h a Sigh 10 cam-
e a o analyze hem by ul a iole fluo escence.
3.2.3. Scanning elec on mic oscopy-ene gy dispe si e X- ay
spec oscopy
SEM-EDX o c oss-sec ions and pain agmen su aces was
pe o med on JEOL JSM 7600F scanning mic oscope (JEOL L d). The
ins umen o e s a esolu ion in seconda y elec ons o 1.2 nm a
30 kV and 3 nm a 1 kV and a esolu ion o 3 nm in backsca e ed
elec ons a 15 kV. To a oid he cha ging o he non-conduc ing
polyme su ace, abou 20 nm ca bon laye was deposi ed on o
samples in a Q150T ES coa e (Quo um Technologies). Seconda y
elec on images (SEI) and back-sca e ed elec on (BSE) images
we e aken a 20 kV accele a ion ol age. EDX was pe o med us-
ing an Az ec Ul im max de ec o (Ox o d Ins umen s). Elemen al
mappings we e ob ained using a s ep size o 0.15–0.75 μm.
3.2.4. Compu ed omog aphy
CT measu emen s o a pain agmen we e collec ed on
an EasyTom XL 160/230 equipmen om RX Solu ion Company
(Me ología Sa iki S.A.). The double ube and de ec ion sys em
was configu ed wi h he nano ube wi h middle spo size (80KV,
150 mA) and CCD (exposu e ime 1.5 s and nºa e age ames 15)
op ions. The scanning omog aphy wi hou fil e s was selec ed in
con inuous plus e e ence mode a e black, gain and bean align-
men . The scanning ime was mo e o less 9 h wi h a oxel size
o 0.7 mm, 1408 p ojec ions o e 360 °and 1304 slices. The scan-
ning size was 1.0 heigh and 1.3 mm diame e wi h final geome -
ical magnifica ion o 29.54. The da a se s we e measu ed and e-
cons uc ed by XAc so wa e om RX Solu ion. The econs uc ion
olume was ob ained a e spo and geome y co ec ions and ing
fil e applica ion. Sca e and beam ha dening we e e alua ed in
o de o imp o e he final olume econs uc ion omog aphy.
3.2.5. μ-X- ay fluo escence
A High Resolu ion μ-ED-XRF M4 To nado spec ome e (B uke
Nano GmbH) was used o eco d elemen al maps o c oss-sec ions
and pain agmen su aces. Measu emen s we e ca ied ou unde
acuum (20 mba ), wi h an X- ay hodium anode p o iding a ol -
age o 50 kV and a maximum cu en o 600 mA. The ins umen
was equipped wi h a silicon de ec o wi h an achie able esolu ion
o 145 eV (Mn K αline). The indi idual spec a we e eco ded o
196
E. Ta ilon e, O. González-Mendia, I. Cos an ini e al. Jou nal o Cul u al He i age 74 (2025) 195–203
Fig. 1. Un i led (1971) by San os Iñu e a. Oil on can as (114 ×146 cm).
5 ms wi h a epe i ion o 5 scans and a dis ance be ween he spo s
o 20 μm. All maps we e made conside ing he K αline o he
co esponding elemen , excep o lead, o which he L αline was
used. Con ol o he ins umen , as well as p ocessing and edi ing
o he maps, we e ca ied ou wi h he M4 To nado so wa e.
3.2.6. Raman spec oscopy
The Raman analyses o c oss-sec ions we e ca ied ou by
means o an InVia Renishaw con ocal mic o Raman spec ome-
e (Renishaw plc.) coupled o a DMLM Leica mic oscope p o ided
wi h 5 ×, 20 ×, 50 ×, 50 ×(long dis ance) and 100 ×lenses using
532 and 785 nm exci a ion lase s. The lase s we e se a low powe
(no > 1 mW a he sample) in o de o a oid he mal pho odecom-
posi ion o ans o ma ion. Da a acquisi ion was ca ied ou using
he Wi e 4.2 so wa e package (Renishaw plc.). Spec a we e ac-
qui ed be ween 10 0 and 320 0 cm−1 (a e age 1 cm−1 o spec al
esolu ion) and se e al scans we e accumula ed o each spec um
in o de o imp o e he SNR (10–20 s, 5–100 accumula ions).
3.2.7. A enua ed o al eflec ance—Fou ie ans o m in a ed
spec oscopy
A Nicole Summi X (The mo) spec ome e was used o analyze
efflo escence and pain sc apings in ATR mode wi h Diamond c ys-
al ( wo eplica es o each one). Ai was used as e e ence. Spec a
we e acqui ed o 32 scans wi h 4 cm−1
esolu ions in a 40 0 0–40 0
cm−1 ange. The spec a we e collec ed and e alua ed wi h Omnic
so wa e.
3.2.8. X- ay di ac ion
The X- ay powde di ac ion pa e ns o efflo escence and pain
sc apings ( wo eplica es o each one) we e collec ed by using a
PHILIPS X’PERT PRO au oma ic di ac ome e ope a ing a 40 kV
and 40 mA, in he a- he a configu a ion, seconda y monoch oma-
o wi h Cu-K α adia ion ( λ= 1.5418 ˚
A) and a PIXcel solid s a e
de ec o (ac i e leng h in 2 θ3.347 °). Da a we e collec ed om 5 o
80 °2 θ(s ep size = 0.026 and ime pe s ep =10 0 0s) scan speed
0.007 °/s, a RT. A fixed di e gence and an isca e ing sli gi ing
a cons an olume o sample illumina ion we e used. P elimina y
iden ifica ion o he ini ial phases was e alua ed using he Pow-
de Di ac ion File (PDF2) da abase. PANaly ical X´
Pe High Sco e
p og am was used o he iden ifica ion p ocedu e.
3.2.9. Gas ch oma og aphy—mass spec ome y
GC–MS analysis was ca ied ou ollowing he me hod p o-
posed by La Nasa e al . [ 17 ] which allows o analyze mix-
u es o ee a y acids and me al soaps. All he eagen s
necessa y o he analysis we e pu chased om Sigma-Ald ich:
Isooc ane ( o analysis), idecanoic acid (analy ical s anda d),
N,O-bis( ime hylsilyl) ifluo oace amide (BSTFA) con aining 1 %
ime hylchlo osilane and 1,1,1,3,3,3-hexame hyldisilazane (HMDS).
A ound 500 μg o efflo escence sc apings we e de i a ized ol-
lowing wo di e en p ocedu es. On he one hand, 5 μL o ide-
canoic acid solu ion in ace one (4 μg/g) we e added as IS and a e
d ying unde ni ogen flow 20 μL o BSTFA (1 % TMCS) and 150 μL
o isooc ane we e added. This mix u e was hea ed a 78 °C o
81 min and was used o he analysis o ee a y acids and ca -
boxila es. On he o he hand, 5 μL o idecanoic acid solu ion we e
added as IS and a e d ying unde ni ogen flow 20 μL o HMDS
and 150 μL o isooc ane we e added. This mix u e was hea ed a
60 °C o 30 min and was used o he selec i e analysis o ee
a y acids.
Bo h mix u es we e analyzed in an Agilen gas ch oma og aph
model HP 6890 and HP 5973 mass spec ome e . Samples we e in-
jec ed in spli less mode a 280 °C. GC sepa a ion was pe o med
on a used silica capilla y column HP-5MS (J&W Scien ific, Agi-
len Technologies, s a iona y phase 5 % diphenyl-95 % dime hyl-
polysiloxane, 30 m leng h, 0.25 mm i.d., 0.25 mm film hickness).
Ch oma og aphic condi ions we e: ini ial empe a u e 80 °C, 2 min
iso he mal, 20 °C/min up o 280 °C, 10 min iso he mal. MS pa am-
e e s we e: elec on impac ioniza ion (EI, 70 eV) in posi i e mode;
ion sou ce empe a u e 230 °C; in e ace empe a u e 280 °C. Ch o-
ma og ams we e acqui ed in ull scan ( ange 50–700 m /z ) and he
197
E. Ta ilon e, O. González-Mendia, I. Cos an ini e al. Jou nal o Cul u al He i age 74 (2025) 195–203
Fig. 2. De ail o Un i led (1971): Vis image (a), UV induced fluo escence (b), adiog-
aphy (c).
ollowing ions we e ex ac ed: palmi ic acid m/z 313, s ea ic acid
m/z 341 and idecanoic acid m/z 271. The injec ion olume was
2 μL. A Supelco 37 componen FAME Mix (Sigma-Ald ich) s anda d
was un be o e he analysis.
3.2.10. X- ay pho oelec on spec oscopy
XPS expe imen s we e eco ded wi h a Ve sap obe III AD Phys-
ical Elec onics (ULVAC) sys em equipped wi h a monoch oma ic
AlK α adia ion sou ce (1486.7 eV). An ini ial analysis was ca ied
ou in o de o de e mine he elemen s p esen in he su ace o a
pain agmen (wide scan: s ep ene gy 0.2 eV, pass ene gy 224 eV)
ollowed by a mo e exhaus i e analysis o he iden ified elemen s
(de ail scan: s ep ene gy 0.05 eV, pass ene gy 27 eV, ime pe
s ep 20 ms) wi h an elec on ake-o angle o 45 °The spec om-
e e had p e iously been calib a ed by using he Ag(3d5/2) line a
368.26 eV. The spec a we e fi ed wi h he Casa XPS 2.3.26 so -
wa e, which models he con ibu ions, a e backg ound sub ac-
ion (Shi ley).
4. Resul s
4.1. Gene al examina ion
The whi e haze o e he ed pain o he a wo k can be clea ly
obse ed in he de ail pho og aphy in Fig. 2a . Unde UV ligh , he
efflo escence exhibi s a bluish fluo escence ha is e en no iceable
in a eas whe e i is no isible o he naked eye ( Fig. 2b ). I is well
known ha zinc and lead me al soaps can exhibi he e ogeneous
luminescence [ 27 ]. When s udying he same pain ing a ea wi h X-
ays, no e idence o he efflo escence was obse ed ( Fig. 2c ). This
is no su p ising, since me al soaps migh o e a di e en esponse
depending on he hickness o he laye and he con as wi h he
es o he laye s o he pain ing. Indeed, Noble [ 9 ] poin s ou ha
he agg ega es can appea as ei he da k o ligh depending on he
abso bance o he laye in which hey a e o med.
4.2. Mic oscopic analysis
The digi al mic oscope image shows he aspec o he whi ish
efflo escence ( Fig. 3a ). I is impo an o no e ha a e olling o e
he efflo escence up o 30 imes a co on swab dipped in deion-
ized wa e , i emained unal e ed, which indica es a lipophilic na-
u e. When a close iew o he su ace was ob ained hanks o
SEM images, a ema kable di e ence was obse ed be ween he
haze ich a ea ( Fig. 3b ) and he pain su ace ( Fig. 3c ). The efflo es-
cence mo phology shows i egula pla es homogeneous in size (1
o 5 μm wide and a ound 0.1 μm hick) ha esemble he shape
o he efflo escence iden ified by Wingel e al. [ 21 ] as zinc soaps
in a pain ing by Edwin Aus in Abbey.
The efflo escence could also be obse ed by OM in he op laye
o he c oss-sec ion wi h a hickness o a ound 15 μm ( Fig. 4a ).
Unde UV illumina ion ( Fig. 4b ) he haze exhibi ed a fluo escence
in ag eemen wi h he esul s ob ained in he gene al analysis.
These c oss-sec ions images a e also use ul o unde s and he pain
laye s uc u e o his a ea o he a wo k ha seems o be o med
om bo om o op by: a whi e p epa a o y laye , a yellow-o ange
laye (pain -laye 1), a pink laye (pain -laye 2) and a pu ple laye
(pain -laye 3). This s uc u e can be be e obse ed in he c oss-
sec ion o Fig. S1. The BSE image ( Fig. 4c ) shows ha he p epa-
a ion laye is qui e homogeneous, while he pain laye s a e he -
e ogeneous and ich in high a omic mass pa icula e subs ances,
especially in he uppe po ion o he op pain laye , which could
be associa ed wi h he p esence o a hea y elemen such as lead.
The su ace efflo escence can be also obse ed in BSE and appea s
in a da ke one han he hea y elemen con aining subs ances bu
b igh e han he binding medium and some o he o he pigmen
pa icles. Tha migh be due o he highe o ganic con en o he
me al soaps in compa ison wi h he hea y elemen con aining sub-
s ance [ 28 ].
4.3. Compu ed omog aphy
Tomog aphic econs uc ion was pe o med based on a egion-
g owing algo i hm ha c ea es a ep esen a i e 3D image o he
en i e olume. The p ocedu e consis s o selec ing a se o “seed
oxels” co esponding o he ma e ial o p ope y o in e es and
g owing egions om hese seeds [ 26 ]. In his way, an easy seg-
men a ion o ma e ials wi h di e en abso p ion can be ob ained.
Since he aim was dis inguishing me al soaps con aining a oms
wi h high a omic weigh (i.e. high g eyscale alue), a e ca e ully
s udying he g eyscale his og am a h eshold (iso alue o 25,154)
was se in a semiau oma ic way. The esul ob ained a e fil e ing
is shown in Fig. 5 . As in he BSE image, in he CT i can be ob-
se ed how pa icles con aining highe a omic weigh elemen s a e
dis ibu ed wi hin he pain laye s. The ad an age o omog aphy is
ha h ee-dimensional in o ma ion is ob ained, which can be used
o be e unde s and he dis ibu ion o hese pa icles h oughou
he di e en pain laye s. Fo ins ance, in his case a pa icula ly
in e es ing egion o pa icle agg ega e is iden ified in he op le
co ne o he sample ha migh be ela ed o he o ma ion o he
haze. Un o una ely, a leas in his scena io, he echnique was no
use ul o dis inguish he efflo escence i sel , p obably due o he
low di e ence in a enua ion coefficien s compa ed wi h he un-
al e ed pain su ace and he ac ha pa icles in he pain laye s
a e hea ie han he efflo escence. All he a emp s made o dis in-
guish he efflo escence using he g eyscale his og am we e unsuc-
cess ul.
4.4. Elemen al analysis
SEM-EDX allowed he iden ifica ion o he elemen al composi-
ion o he efflo escence, he high a omic weigh pa icles p e i-
ously de ec ed by BSE and omog aphy, and he es o he laye s
in he pain ing. Bo h he efflo escence and he hea y elemen con-
aining pa icles we e ich in lead, as co esponds o a sys em in
which led soaps a e o med. In Fig. 6b i can be obse ed ha he
p epa a o y laye does no con ain lead, while all he pain laye s
do, being he uppe laye he iches in his elemen . Fu he mo e,
an a ea especially ich in lead can be obse ed in he uppe mos
laye o he c oss-sec ion ha is associa ed wi h he efflo escence.
The elemen al mapping o calcium ( Fig. 6c ) and zinc ( Fig. 6d ) a e
also shown since hey a e common coun e -ions in me al soaps.
In his case, he p esence o hese ions in he efflo escence laye
is e y sca ce, bu calcium was abundan in pain -laye 1 and 2
and zinc in he p epa a o y laye and in he pain -laye 2. Indeed,
hese elemen al mappings oge he wi h he mappings o o he el-
emen s (Fig. S2) p o ided a p elimina y unde s anding o he ma-
e ials used in he pain ing, as i was u he confi med by Ra-
198
E. Ta ilon e, O. González-Mendia, I. Cos an ini e al. Jou nal o Cul u al He i age 74 (2025) 195–203
Fig. 3. Image ob ained wi h DM (x50) o he su ace o a mic osample o he pain ing ich in whi e efflo escence (a). SEI o an a ea whe e he whi e haze is p esen (b),
and an a ea whe e he whi e haze is no p esen (c).
Fig. 4. C oss-sec ion showing he laye dis ibu ion (0: p epa a o y laye /1: pain -laye 1/2: pain -laye 2/3: pain -laye 3/4: whi ish efflo escence) (a), unde epi–illumina ion
(b), in UV-induced fluo escence mode (c) and BSE image (d).
Fig. 5. Nano omog aphy o a pain sample.
man and XRD analysis: p epa a o y laye was o med by zinc and
i anium whi e, pain -laye 1 was o med by an i on con aining
ea h-pigmen , calcium ca bona e was used as a fille in he pain s
Fig. 6. SEM images om c oss-sec ion: BSE (a)/EDX mappings o Pb (b), Ca (c) and
Zn (d).
and ba i e could be ound in bo h he p epa a ion and pain ing
laye s.
Elemen al analysis o su ace agmen s was also ca ied ou
in o de o ob ain mo e in o ma ion abou he efflo escence and
o s udy he dis ibu ion in a di e en axis. This su ace analysis
199
E. Ta ilon e, O. González-Mendia, I. Cos an ini e al. Jou nal o Cul u al He i age 74 (2025) 195–203
Fig. 7. Mic oscopic image o a mic osample su ace (a) and μ-ED-XRF mappings o
Pb (b), Ca (c) and Zn (d).
Fig. 8. Raman spec a ob ained om he whi e haze a ea showing a cha ac e is ic
spec um o me al soaps (a) and lead con aining pa icles ound in he pain laye s
iden ified as li ha ge (b). The bands ma ked wi h R co espond o he embedding
esin (785 nm lase exci a ion).
was pe o med by SEM-EDX and μ-ED-XRF and, al hough he o -
me o e s a be e spa ial esolu ion, he images ob ained wi h he
la e allowed a sui able in e p e a ion in a as e way wi hou e-
qui ing he coa ing o he sample. In he μ-ED-XRF maps in Fig. 7 ,
i can be obse ed ha he efflo escence is ich in lead ( Fig. 7b )
and ha he mapping o his elemen ma ches o a la ge ex en
he dis ibu ion o he haze. Again, no significan p esence o cal-
cium ( Fig. 7c ) o zinc ( Fig. 7d ) can be de ec ed in he efflo escence.
Fu he mo e, hese mappings we e use ul o iden i y a eas o he
pain ing ha we e no a ec ed by his phenomenon.
4.5. Raman analysis
The Raman spec um ob ained om he uppe mos laye o he
c oss-sec ion ( Fig. 8a ) exhibi ed he cha ac e is ic bands o me al
soaps (Raman bands a 890, 925, 1064, 1002, 1131, 1296, 1416,
and 1440 cm−1
) plus signals om he esin in which he sample
was embedded (Raman bands a 360, 482, 600, and 812 cm−1
).
The cha ac e is ic bands could be a ibu ed mainly o palmi a es
o s ea a es o a wide ange o me allic ions [ 15 , 29 , 30 ]. Fu he -
mo e, Raman spec oscopy allowed us o iden i y he lead con ain-
ing pa icles ound in he pain laye s as lead (II) oxide li ha ge ( α-
PbO, main Raman bands a 147 cm−1
) ( Fig. 8b ). Al hough he mos
common sou ce o lead soaps a e pigmen s such as lead whi e,
ch ome yellow o lead and in yellow [ 31 ], none o hese pigmen s
was iden ified in he samples in addi ion o li ha ge. Ne e heless,
he o ma ion o lead soaps has also been a ibu ed o o he lead
con aining subs ances such as oxides used as d ie s [ 32 ]. Hence,
he mos p obable sou ce o lead in ol ed in he o ma ion o he
me al soap is a lead-based d ying agen .
Besides he in o ma ion ega ding he efflo escence and he
lead con aining subs ance, Raman images (ob ained conside ing
he band wi h he highes in ensi y o each compound) o e ed
a mo e h oughou iew o he pain ing echnique used by San os
Iñu e a. In his way, pigmen s used in he pain laye s we e iden-
ified as α-goe hi e ( α-FeO(OH), Raman band 385 cm−1
) in pain -
laye 1, dioxazine iole (PV23, Raman band 1205 cm−1
) in pain -
laye 2, a mix u e o naph hol ed AS (PR146, Raman band 1582
cm−1
) and ph halo g een (PG7, Raman band 748 cm−1
) in pain -
laye 3. Fu he mo e he pigmen s and fille s in e ed om ele-
men al analysis we e confi med: ba i e (BaSO4
, Raman band 987
cm−1
), calci e (CaCO3
, Raman band 1086 cm−1
), qua z (SiO2
, Ra-
man band 465 cm−1
) and i anium oxide, mainly u ile (TiO2
, Ra-
man band 612 cm−1
) wi h aces o ana ase (TiO2
, Raman band 142
cm−1
). See Fig. S3 o mo e in o ma ion.
4.6. Analysis o he efflo escence by ATR-FTIR
The ATR-FTIR spec a ob ained om he efflo escence and pain
sc apings exhibi ed a ma ked di e ence in he ange 180 0–120 0
cm−1 ( Fig. 9 ). Indeed, his is he a ea in which s e ching p ofiles
o oil pain and me al soaps a e expec ed o di e [ 29 ]. Unlike
Raman spec oscopy, FTIR allows o dis inguish he me al ca ions
based on he COO−asymme ic s e ching band be ween 1600 and
1500 cm−1 while he leng h o he ca bon chain is mo e difficul
o elucida e. Acco ding o he bands in 1538 and 1511 cm−1 and
he e e ence spec a a ailable in he li e a u e [ 10 , 14 , 29 ] he e -
flo escence seems o be o med mainly by lead palmi a e and/o
s ea a e. Disc imina ing be ween hese species is a difficul ask
since he only di e ence is he se ies o CH2
p og ession bands
be ween 1350 and 1175 cm−1 whe e s ea a e shows an addi ional
band [ 14 , 33 ]. The bands obse ed in ha egion o he efflo es-
cence a e complex and sugges ha he soap is no o med by a
single species bu by a mix u e o s ea a e and palmi a e, a phe-
nomenon al eady obse ed by o he au ho s [ 33 ].
4.7. Analysis o he efflo escence by XRD
As expec ed, di ac ion pa e ns o pain ing and whi e haze
sc apings (Fig. S4) showed many simila i ies since sampling o he
efflo escence ine i ably in ol ed also he sampling o pa icles o
he su ounding pain ing. Ne e heless, he e we e significan di -
e ences, especially a low angles. In his way, some o he di e -
ences could be a ibu ed o palmi ic and s ea ic ee a y acids,
bu he mos ema kable di e ences all wi hin he ange o lead
soaps. Howe e , he D-spacings did no ma ch ei he wi h lead
palmi a e o wi h lead s ea a e, bu lied be ween hem ( Fig. 10 ).
These in e media e D-spacings we e also obse ed by Sal adó e al.
[ 33 ] and a ibu ed o he o ma ion o a mix u e o lead palmi a e
and lead s ea a e. Indeed, hese esul s ag ee wi h he obse ed by
ATR-FTIR.
200
E. Ta ilon e, O. González-Mendia, I. Cos an ini e al. Jou nal o Cul u al He i age 74 (2025) 195–203
Fig. 9. ATR-FTIR spec a o sc apings om an a ea ich in whi e haze ( ed) and om a pain sample wi hou whi e haze (blue). Signals a 1538 and 1511 cm−1
co espond
o he asymme ic COO−s e ching bands o a lead soap. The black squa e shows a de ail o he CH2
p og ession bands o he alipha ic chain.
Fig. 10. Magnifica ion o he di ac ion pa e n a low angles o e lapped wi h he
expec ed signals o lead palmi a e (blue) and lead s ea a e (g een).
XRD analysis also confi med he p esence o some compounds
in he pain ing, p e iously iden ified by Raman spec oscopy:
goe hi e, dioxazine iole , naph hol ed AS, ba i e, calci e, u ile,
zinc oxide, qua z and lead oxide (Fig. S4). Lead oxide showed
a e agonal c ys al sys em confi ming he p esence o li ha ge,
which again sugges s i s use as d ie in he oil pain [ 32 ].
4.8. Analysis o he efflo escence by GC–MS
GC–MS analysis was ca ied ou in o de o ob ain in o ma-
ion abou he o ganic ac ion o he efflo escence. On he one
hand, when ee a y acids and ca boxyla es we e analyzed o-
ge he , he mos abundan species we e palmi ic and s ea ic acids
ollowed, o a lesse ex en , by azelaic and sube ic acids. This is in
ac he composi ion expec ed o a cu ed oil pain film [ 34 ]. On
he o he hand, when ee acid and ca boxyla e o ms we e dis-
inguished, he la e ones p o ed o be p edominan (3.1 a io o
palmi a e/palmi ic acid and 4.1 a io o s ea a e/s ea ic acid, see
Fig. S5). These findings indica e ha he efflo escence is mainly a
mix u e o lead palmi a e and lead s ea a e, wi h mino amoun s
o he co esponding ee a y acids, which also ag ees wi h he
esul s ob ained by FTIR and XRD.
4.9. Analysis o he efflo escence by XPS
The XPS analysis e ealed he p esence o ca bon, oxygen and
lead in a pain agmen su ace pa ially co e ed by he whi e
haze (Table S2). Al hough he amoun o lead was highe when ex-
amining a eas ich in haze, i is impo an o no e ha his me al
was also de ec ed in appa en ly haze ee a eas. Al hough XPS is
no a echnique o choice o he cha ac e iza ion o me al soaps,
we ob ained some esul s ha dese e a en ion and should be u -
he in es iga ed. Fi s ly, he binding ene gies o oxygen su e ed
a significan dec ease in he efflo escence (Fig. S6). This migh be
explained by he ac ha oxygen is in a ca boxyla e coo dina ed
wi h a lead (II) ion while in oil pain ing is mainly in es e o ee
acid o m. E en i me al soaps ha e no been widely s udied by
XPS, hese esul s could be compa ed wi h hose ones ob ained
by Ca alho [ 24 ] o cuppe palmi a e. Secondly, and mo e su p is-
ingly, he signal o lead in he efflo escence showed a significan
con ibu ion o Pb(0) (Fig. S7). In he ex emely unlikely case ha
me allic lead was p esen in he pain su ace i should ha e been
de ec ed by XRD, so his finding is p obably an a e ac o he XPS
analysis. Indeed o he au ho s ha e epo ed Pb(0) o ma ion in
pain analysis unde high-in ensi y X- ay beams [ 35 ] o in o he
lead con aining compounds du ing XPS analysis [ 36 ]. This a e ac
was only de ec ed in he efflo escence analysis and no in he e -
flo escence ee pain ing, he e o e, lead soaps migh be p one o
pho olysis unde hese analy ical condi ions.
5. Conclusions
This s udy demons a es he efficacy o employing a mul i-
analy ical app oach o cha ac e ize and unde s and su ace whi en-
ing phenomena o ma ion in oil pain ings. Fu he mo e, i p o ides
a de ailed insigh in o he u ili y o each echnique and he spe-
cific ype o in o ma ion i o e s o a s udy case in which me al
soaps we e iden ified. Elemen al analysis by SEM-EDX and μ-ED-
XRF acili a ed he s udy o he dis ibu ion o lead ions in c oss-
sec ions and pain agmen su aces, and showed ha lead de i es
om pain laye s. Rega ding he cha ac e iza ion o he me al soap,
XRD, Raman and ATR-FTIR p o ed o be use ul and complemen a y
echniques. ATR-FTIR should be especially add essed since, using
201
E. Ta ilon e, O. González-Mendia, I. Cos an ini e al. Jou nal o Cul u al He i age 74 (2025) 195–203
a ela i e simple ins umen a ion, bo h he me al ion and he o -
ganic chain could be elucida ed, while Raman spec oscopy e eals
cons ains in iden i ying he coun e -ion. Ne e heless, he cha -
ac e iza ion o he o ganic ac ion is no always easible by ATR-
FTIR when dealing wi h complex samples as i is he case s udy in
which signals o di e en compounds can o e lap. In hose si ua-
ions, he applica ion o GC–MS demons a ed o be a alid s a -
egy, especially conside ing ha ca boxyla es and ee acid o ms
can be dis inguished ollowing a wo-s ep de i a iza ion p ocedu e.
XPS analysis did no o e addi ional in o ma ion abou me al soaps
bu he shi obse ed in binding ene gies when compa ing he e -
flo escence and he oil pain spec a opens new esea ch possibil-
i ies. Simila ly, CT image ob ained om he analyzed pain ag-
men did no eflec he dis ibu ion o he haze bu was use ul o
ace pa icles con aining a oms wi h high a omic weigh in h ee-
dimensions. I is impo an o highligh ha e en i all he poin
analysis echniques equi e sampling, only GC–MS is des uc i e,
and he e o e, wi h a ca e ully planned analy ical wo kflow, sam-
ple collec ion can be minimized while maximizing analy ical ou -
comes.
Wi h espec o he me al soap o ma ion p ocess in San os
Iñu e a’s Un i led (1971), in o ma ion ob ained om SEM-EDX and
Raman images sugges ha he lead sou ce is li ha ge used as a
d ie and no a lead pigmen . Lead migh ha e in e ac ed wi h
s ea ic and palmi ic acids om oil o c ea e lead s ea a e and
palmi a e. The manual addi ion o he d ie could ha e con ibu ed
o an une en dis ibu ion o he whi ish haze. The mul i-analy ical
app oach also e ealed he laye s uc u e o he a wo k o med
by a p epa a ion laye ich in zinc and i anium whi e and h ee
laye s con aining, om bo om o op, goe hi e, dioxazine iole
and a mix u e o ph halo g een and naph hol ed AS, oge he wi h
fille s such as calcium ca bona e, ba i e o qua z. In his way, his
wo k o e s aluable insigh s in o he a is ’s echnique and, o he
bes o ou knowledge; i is he fi s s udy o his kind conduc ed
on a pain ing by San os Iñu ie a.
The s a egy applied in his s udy is ans e able o he analysis
and conse a ion o oil pain ings expe iencing o he deg ada ion
phenomena such as sal o ma ion o a y acids efflo escence. In
ac , due o he flexibili y o he mul i-analy ical amewo k, ech-
niques ha do no o e conclusi e esul s o me al soaps migh
o e mo e aluable in o ma ion o o he whi ening phenomena
as in he case o XPS o sal analysis o GC–MS o a y acid de-
e mina ion.
I is necessa y o bea in mind ha all hese su ace whi en-
ing subs ances no only a ec he aes he ics o he a wo ks, bu
also ep esen a conse a ion p oblem. Despi e ecen ad ances in
his field, much emains o be in es iga ed abou he unde lying
causes and he mechanisms ha gene a e hem. In his way, his
wo k con ibu es o he de elopmen o be e conse a ion s a e-
gies o p ese ing oil-pain ings and he consequen esea ch o
e ec i e solu ions o he long- e m p ese a ion o a .
Acknowledgmen s
This wo k was suppo ed by unding om he Basque Go -
e nmen o he FARMARTEM consolida ed g oup (g an numbe
1673/22 ). Also, we would hank o echnical and human suppo
(Luis Ba olomé, Se gio Fe nández, Ai o La añaga, Belén Sánchez
and Lei e San elices) p o ided by SGIke o UPV/EHU and Eu opean
unding (ERDF and ESF). Likewise, we a e g a e ul o he O begozo
Founda ion o p o iding he a wo k o s udy.
Supplemen a y ma e ials
Supplemen a y ma e ial associa ed wi h his a icle can be
ound, in he online e sion, a doi:10.1016/j.culhe .2025.06.007 .
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