文化財における金属周辺木材の白色現象：
特徴と発生機構
Whitening phenomenon of wood adjacent to metal components of cultural properties:
Characterisation and occurrence mechanism

周 怡杉ZHOU Yishan
1. Introduction3) develop suitable preventive conservation strategies

The whitening phenomenon of wood could adversely

correspondaffect the appearance and aesthetic value, and cause

phenomenon. To realise this objective, the following

worries about the durability of the historic wooden

studies were conducted in the research. First of all, thecultural properties

phenomenon was characterised by scientific case studies.

todifferent

typesofthewhitening

Generally, the whitening phenomenon of wood

On the base of this result, a hypothesis of the occurrence

occurred in historic wooden architectures would likely bemechanism was proposed. The hypothesised mechanism

attributed to the external factors: White-rot fungi1, UV

and the related affecting factor were evidenced and2and visible irradiations . While in recent researches of the

improved through model experiments with fresh wood.

whitening phenomenon occurred in the wood in contact
with base-stones, the whitening phenomenon of wood has

2. Characterisation for the whitening phenomenon of3

been ascribed to inorganic compound, especially gypsum .

wood adjacent to metal: in the case of the Confucian

On the other hand, the whitening phenomenon of wood isTemple of the Kodokan of Mito Domain

also observed in the area adjacent to metal components inthe case of historic architectures (Fig.1).
The metal components of cultural properties areirreplaceable parts for the consideration of preserving theintegrity of original materials, as well as the historic andaesthetic values. On the view of preventative conservation,
it is essential 1) to distinguish the whitening phenomenonof wood adjacent to metal components from the othertypes of the whitening phenomenon, 2) figure out theoccurrence mechanism, conditions and affecting factors,a)b)

Fig. 1 The whitening phenomenon of wood adjacent to metalcomponents, in the case of Japanese historic architectures.
学位論文梗概集 2018

Fig. 2 a) Plan of the Kodokan(“弘道館修理工事報告書”1963)
and the photograph of the Confucian Temple((施設紹介. 弘道館 公 園 . http://www.koen.pref.ibaraki.jp/park/kodokan04.html.
[Accessed 27 Sep., 2015]); b) the 2 different types of whitening
phenomenon of wood in the Confucian Temple.

This part of research attempted to distinguish theconstituentsin“non-particle-attached”

whiteningwhitening phenomenon of wood adjacent to the metal

phenomenon of wood once adjacent to copper

component from the other types of the whitening

components.

phenomenon at different locations in the case of theThe connection between Cu content and whitenessConfucian Temple of the Kodokan of Mito Domain,degree has been confirmed in the case of vertical wooden

through in situ elemental analysis, colourimetricwindow frames in situ. In contrast, in the case of themeasurements and microscopic observation (Fig.2).

Nageshi, wood with significant Cu content while lower

As the results, it is proved that:whiteness degree is also verified.

XRF analysis could be an effective method to figure

In conclusion, the existence of Cu in the wood will notout the elemental characteristics of wooden constructions

necessarily lead to whitening colouration change of wood.

with the different types of whitening phenomenon.

It is worth to figure out how the Cu content affects wood

Results of elemental distribution investigation for exterior

tissues, and if the affection would result in theside present that the whitening phenomenon in wooden“non-particle-attached” type and the “particle-attached”

constructions lower than 70 cm height is predominantly

type of whitening phenomenon in Cu affected wood.

affected by Ca, Fe, Cl, S elements; the whitening
phenomenon of wood adjacent to copper components is

3. Characterisation for the whitening phenomenon of

predominantly affected by Cu element (Fig.3).

wood adjacent to metal: in the case of the Old

No matter in “Ca affected” or “Cu affected” whitening

Iwasaki-ke Suehiro-bettei Villa

phenomenon of wood, both the “non-particle-attached”

On the results of the previous investigation, further field

type and the “particle-attached” type of whitening

investigation for the Old Iwasaki-ke Suehiro-bettei Villa

phenomenon are observed.

were conducted to identify the chemical composition ofThe FTIR analysis for the Nageshi samples confirms

the additional whitish particles among wood tissues, andthe simultaneous chemical changes of wood structural

figure out the chemical features of wood with theFig. 3 The wood with whitening phenomenon adjacent to copper components(radical section of wood) in the case of the Confucian Templeof the Kodokan of Mito Domain: a) location of the whitening points adjacent to metal (5-W,9-W) and reference points distant frommetal(5-R,9-R), and the microscopic photographs of which; b) the staked column graph of I(S), I(Cl), I(Ca), I(Fe), I(Cu) (detected intensity ofelement) obtained by XRF analysis with L* values (whiteness degree in the 1976 CIE L*a*b* colourimetric system).

Summaries of Academic Theses 2018

The existence of oxalate ion content is confirmed inwood. In the wood adjacent to the Cu and Cu-Zn alloy
components, the oxalate ion may generate from theCu/Zn

affecteddegradationofwood

structural

constituents, subsequently combined with the metal ion toform the additional oxalate particles.
Overall,it could

propose that the whitening

Fig. 4 The Old Iwasaki-ke Suehiro-bettei Villa

phenomenon of wood adjacent to metal may relate to thewhitening phenomenon for the better understanding.

chemical changes of wood structural constituents for theAs the results, the scientific investigation confirms thecase of “non-particle-attached” whitening phenomenon,

various surface features of wood with similar whitening

both chemical changes and formation of certain inorganic

phenomenon affected by different types of metal.

compounds

inwoodtissuesforthecaseof“particle-attached” whitening phenomenon.

A “non-particle-attached” whitening point is confirmedin wood adjacent to Cu-Zn alloy components at interior
location. In this case, a simultaneously degradation of

4. Quantification of ion content for the whitening

wood structural constituents is verified with FTIR

phenomenon of wood adjacent to metal: withanalysis.

nondestructive investigation method

Besides, additional different inorganic compounds are

On the results of the field investigation, it is speculated

identified in the other investigated whitening points, nothat the formation of the metal oxalate compounds could

matter the locations and the functions of the wood

be ascribed to the combination of mobile zinc and oxalate

components:

ions in wood. The quantitation of zinc and oxalate ions inwood with the whitening phenomenon may display• Jarosite [KFe3(SO4)2(OH)6] in the vicinity of ironsignificantnails;characteristics

andprovidevaluable

information to discuss the occurrence mechanism for this• Zinc oxalate hydrate (ZnC2O4•2H2O) in thetype of whitening phenomenon. In order to realisevicinity of Cu-Zn alloy components;

scientific investigations for more extensive cases without• Moolooite (CuC2O4•xH2O) in the vicinity ofdamages in sampling procedures, a new nondestructive

copper nails.

method using filter paper was developed exclusively for

The SEM-EPMA observation presents the existingcultural properties.

state of inorganic compounds may be fine particles
among wood tissues (Fig.5).

a)b)Fig. 5 The whitening points adjacent to metal (3-1, 3-2) and reference point distant to metal (3-R) in the case of the Old Iwasaki-ke
Suehiro-bettei Villa, a) the spectra of XRD analysis, CS = calcium sulfite, OA = oxalic acid, ZO= zinc oxalate dehydrate (ZnC2O4•2H2O), c)
the SEM-BSE image of the 3-2 point with attached particles; the brighter tone of particles represent greater average atomic numbers thanwhich in wood tissues, Tr: tracheid, Bp: bordered pit.
学位論文梗概集 2018Fig. 6 The stacked column graphs of anion content and cation content in the case of samples from No. 1, 3, 4, 5 investigate location
which are points adjacent to Cu-Zn alloy components(1-W, 3-1,3-2, 4-1, 4-2, 5-1, 5-2) and the corresponding reference
points(1-R-Int, 1-R, 3-R, 4-1-R, 4-2-R, 5-1-R, 5-2-R).

The method was designed with following procedures:1) Prepare clean filter papers: filter paper pieces
2of corrosion, catalyse the chemical changes of woodchemical components; 2) the oxalic acid is generated aswith size as 10×10 mm ;the product of the chemical changes; 3) the combination

2) Put prepared filter papers on the surfaces of woodof oxalate ion and metal ions in wood would result insamples and drop 5ul distilled water on the filter

precipitation of metal oxalate compound particles amongpaper;

wood tissues 4 . The degradation of wood structural

3) Retrieve the naturally dried filter papers, and thenconstituents may result in the “non-particle-attached” typesoak each piece into 5 ml distilled water for 48 hs;

whitening phenomenon of wood; the growth and4) Analyzing the water extraction of filter papers byaccumulation of the particles will ultimately result in theIC analysis.

“particle-attached” type whitening phenomenon of wood.

The results of the practice on the Old Iwasaki-ke

However, the exact formation mechanism of theSuehiro-bettei Villa displays the most significant C2O422+

additional inorganic compounds in wood has not beenand Zn ion contents in the whitening wood adjacent tospecified yet, and it is not clear how would the formation

the metal component of higher locations(Fig. 6). The

processes affect the surface colouration, internaleaves may shelter these locations from some influences ofproperties, durability and values of wooden culturalenvironmental factors to ensure the accumulation of ion

properties.

contents and the metal oxalate precipitations.

Therefore, a model experiment was performed withartificial degradation of metal affected fresh wood, aimed

5. Artificial degradation of Cu and Zn affected wood:to provide more basic information of whitening

occurrence mechanism of the phenomenon

phenomenon adjacent to the copper alloyed component

The scientific investigations of previous sections

for future conservation activities. The Cu and Zn elementscharacterised the surface features of the whitening

are the most general affecting metal elements of copper

phenomenon of wood adjacent to metal components.

alloyed components in the case of cultural properties.

Based on these results, a hypothesis is proposed toHence, these two elements were selected in this

explain the occurrence of the phenomenon adjacent to Cu

experiment. The changes of the fresh wood samples wereand Cu alloyed components: 1) the metal ions in wood,

evaluated through monitoring: 1) the formation andwhich migrate from metal components during the process

growth process of oxalate particles, 2) whitening

Summaries of Academic Theses 2018

colouration change of the surface 3) chemical changes; 4)IC analysis for anion contents and High-performancethe mechanical performance.

liquid（1）Experiment designmonosaccharide contents with wood meal samples.

chromatography(HPLC)

analysisforIn this part of the research, affection of Cu and Zn content

For the wood plate samples, the Colorimetrictoward wood, and the formation of metal oxalate

measurements, SEM-EPMA analysis was performed tocompounds in which were focused on. It was decided tomonitor the colorimetric changes and formation process

perform artificial degradation of wood samples withof metal oxalate particles. The mechanical properties

different Cu/Zn contents under the same condition ofwere evaluated by flexural vibration test under 25°C and560°C and RH 90%(relative humidity) , and evaluate theRH 60% condition.

degradation performance every 30 days.

（4）Results

The recent Sugi and Keyaki were prepared as plate andThewood meal samples for monitoring changes of chemical

performances of wood is evidenced after the 90 days

and mechanical features respectively.

exposure under the 60°C and RH 90% conditions.

（2）Materials

affectionof

metalcontent

ondegradation

Relatively higher decomposition product contents

The healthy recent wood of Sugi (softwood, Japanese(formate and oxalate ion content, monosaccharide

cedar) and Keyaki (hard wood, Japanese Zelkova) werecontent) and more significant spectral changes of FTIR

cut into rectangular plates and crushed into meal withanalysis are confirmed in the case of the samples withparticle sizes of 177-250 um for evaluating different

Zn/Cu content. For the case with the same kind of metal

properties.

content, higher content present more significant affection

The wood plate and meal samples affected by Cu/Zn

on the degradation of wood. The Cu element always

with different contents were prepared by soaking woodpresents more significant affections than Zn element

plates respectively in ZnCl2 and CuCl2 solutions with 3

under the same level of content in wood.

different levels of concentrations.

The high correlation coefficients of oxalate ion content（3）Methods

with different monosaccharide contents are evidenced

The chemical changes were evaluated in the combination

dependingof FTIR analysis and water extraction analysis included

monosaccharide

onwoodspecies

contents

are(Table1).

The

generatedfrom

Table 1 The Pearson correlation
coefficient(r) of oxalate ion content withmonosaccharide contents.

Fig. 7 The SEM-BSE image of Keyaki samples with 2.53% Cu content. The scatteredparticles with brighter tone are observed among wood tissues after 90 days exposure.

Fig. 8 The high magnification SEM-BSE images of exposed samples with the highest Cu content. The sizes of particles
increased with the increase of exposure time.

学位論文梗概集 2018

polysaccharide in wood 6 , including cellulose andwood cell wall structural constituents is verified. For thehemicellulose constituents. The decays of bands assigned“particle-attached” type whitening phenomenon, the fineto cellulose and hemicellulose are also verified in FTIR

particles are identified as Jarosite [KFe3(SO4)2(OH)6] inanalysis, along with the appearance of bands belong to thethe vicinity of iron nails; Zinc oxalate hydrate

metal oxalate compound, in the samples with high Cu(ZnC2O4•2H2O) in the vicinity of Cu-Zn alloy

content. It is implicated the oxalate content in wood may

components; Moolooite (CuC2O4•xH2O) in the vicinity of

result from the depolymerisation of polysaccharide

copper nails.

content in wood.

The existence of Zn/Cu content could accelerate theAmounts of additional particles among wood tissues ofdegradation of wood chemical constituents, whichsamples with high Cu content are confirmed with theproduce oxalic acid in wood. Fine particles of metal

SEM observation. On the results of FTIR analysis, EPMA

oxalate compound would occur under high Cu content inqualitative and mapping analysis, it is speculated that thewood. However, the degradation with products of oxalic

Cu oxalate compounds is exist as fine particles amongacid and the occurrence of metal oxalate particles may notwood tissues (Fig. 7). Under the observation with highdirectly lead to whitening colouration change of wood.

magnification, the particles appear as aggregations offlake-like crystals with the sizes increased with theincrease of exposure time (Fig. 8).

On the results of this research, the following
prospective are proposed:1) The accumulated oxalate content in wood could

The more significant decrease in mechanical properties

be utilised to evaluate the degradation degree ofis also confirmed in the case of samples with high Cu

woodcontent (Fig. 9). However no visible whitening

understanding of the correlation between oxalate

nondestructively,

onthefurther

colouration changes could be confirmed, whether inand wood degradation;

particle formed or non-particle formed samples.

2) It is necessary to develop the strategies forpreventing migration of metal content into wood

0.025

and removing the metal content off the wood,

K-Zn 1.90%
K-Cu 2.53%
K-Blank

especially under the preservation environmenttanδ0.020

with uncontrollable humidity conditions.
Acknowledgements
We would like to acknowledge to members of the Kodokan Office, Mito
City, Ibaraki Prefecture, and Mr. Gotou of the Agency of Education ofIbaraki Prefecture, members of Board of Education of City of Tomisato,
Chiba Prefecture, especially Mr. Toshiyuki HAYASHIDA, for providing
warm-hearted assistances and kind supports in this research.

0.015

0.010

0.005
0306090

Exposure days
Fig. 9 The tanδ values obtained by the Flexural vibration test ofKeyaki samples with 0%, 1.90% Zn, 2.53% Cu content. The
increase of tanδ may imply the occurrences of some internal
decomposition or breakdown of wood compositions.

6. Conclusion
The 2 patterns of whitening phenomenon adjacent tometal are confirmed both in the Confucian Temple of theKodokan and the Old Iwasaki-ke Suehiro-bettei Villa,
which

are

“non-particle-attached”

typeand“particle-attached” type. In “non-particle-attached” type

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whitening phenomenon, simultaneous decay of the three

Summaries of Academic Theses 2018