Degradation of Japanese Lacquer under Wavelength Sensitivity of Light Radiation

doi:10.4236/msa.2011.210203

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Materials Sciences and Applicatio ns, 2011, 2, 1507-1515

doi:10.4236/msa.2011.210203 Published Online October 2011 (http://www.SciRP.org/journal/msa)

1507

Degradation of Japanese Lacquer under

Wavelength Sensitivity of Light Radiation

Kosuke Nakagoshi, Kunio Yoshizumi*

Kyoritsu Women’s University, Tokyo, Japan.

Email: *[email protected]

Received June 24th, 2011; revised July 24th, 2011; accepted August 8th, 2011.

ABSTRACT

Degradation of Japanese lacquer caused by light irradiation was examined at various wavelengths. By exposing lac-

quer specimens to a narrow monochromatic light band isolated from dispersed polychromatic light emitted by a Xe

lamp source, the wavelength sensitivity characteristics of lacquer degradation could be determined on the basis of ra-

diant energy. Tame-Urushi (brown) lacquer displayed peak degradation maxima at 220 and 315 nm. A broad shoulder

peak was also observed in UVA. For Shu-Urushi (cinnabar) lacquer, in addition to peaks in the UVA-UVB range, a

large degree of degradation was observed following exposure to light in the visible range. Ao-Urushi (green) lacquer

showed similar characteristics, although it was less prone to degradation. Similarly, Shin-Urushi (black) lacquer

showed little change in response to light, although UV light caused limited degradation. These results indicate that

along with the damage caused by UVA and UVB, visible light in the range 510 - 650 nm may also have a significant

degradation effect. Our results provide experimental evidence that Japanese lacquer responds differently to light of

various wavelengths and that specific wavelengths, including visible light, can cause significant degradation.

Keywords: Japanese Lacquer, Degradation, Wavelength Sensitivity, Monochromatic Light, Spectral Reflectance,

Visible Light

1. Introduction

Japanese lacquer, Urushi, is a natural polymer, which is

used to decorate many types of crafts [1]. Raw lacquer is

an oil-in-water emulsion that is traditionally collected as

sap from specific Urushi trees [2]. Unlike synthetic

paints that dry by solvent evaporation, Urushi dries by

polymerization in moist air. As a result, the molecules

join together and form a hard, lustrous, and durable

coating [1-3].

Despite the legendary stability provided by its non-

repeating polymeric cross-linked structure, the lacquer

surface is susceptible to damage by light and color

photobleaching [1,3]. Moreover, this damage by light is

not reversible. Therefore, restoration of Urushi crafts

damaged by light is of great concern [4].

The chemical pathways by which synthetic polymers

undergo photodegradation have been fairly well investi-

gated [5-10]. UV radiation triggers many concurrent

chemical processes in polymers exposed to it and results

in various modes of damage [11]. Moreover, polymer

stability has been investigated widely to examine the

properties [12-16]. However, research on Urushi lacquer

degradation has been minimal, possibly because of lim-

ited information about the process [3].

Assessing the damage caused to materials because of

light exposure requires an understanding of their spectral

sensitivity. Spectral sensitivity data for polymers are

typically generated using a source of monochromatic

radiation or a filtered Xe white light source [6,7,11,17].

In the field of biology, results on wavelength sensitivity

have been published [18-20].

In this study, we focused on the wavelength sensitivity

of Japanese lacquer to degradation. Wavelength of light

irradiation is one of the most important factors in the

degradation process [6-9,17]. Ultraviolet radiation is

known to strongly promote degradation of materials [11].

In addition, visible light is considered to contribute to

lacquer degradation in some cases [21]. However, few

studies have focused on the dependency of degradation

of materials on the wavelength of irradiation. For in-

stance, dyestuffs are strongly influenced by light irradia-

tion [22-32]; however, there is little information on

whether specific wavelengths of light are more damaging

Degradation of Japanese Lacquer under Wavelength Sensitivity of Light Radiation

1508

than others.

In the present study, the degradation characteristics of

Japanese lacquer containing pigmented color sources

were investigated by performing a wavelength sensitivity

analysis on a radiant energy basis. The lacquer samples

were first exposed to monochromatic light, and the

wavelength dependency of degradation was determined

on the basis of action spectra. The results are expected to

provide crucial information about the conservation of

traditional Japanese lacquer crafts in museums and art

galleries as well as an increased understanding of natural

polymer sciences.

2. Experimental

2.1. Materials

The lacquer material obtained from the sap of Urushi

trees was traditionally used to coat wood substrates. The

primary component of lacquer is Urushiol, the chemical

structure of which is shown in Figure 1. Raw lacquer

comprises the following major components: 60% - 65%

Urushiol (pyrocatechol derivatives), 20% - 25% water,

8% carbohydrates, and 2% glycoproteins. Lacca, one of

the glycoproteins, acts as a catalyst in the presence of

oxygen and moisture to cause polymerization of the

Urushiol molecules. For colorization, pigments were

added to the primitive lacquer. Four different colors were

examined in this study, prepared by adding cinnabar,

copper oxide, and iron oxide to about 1% of primitive

lacquer on a weight basis. The first sample, called Tame-

Urushi, is natural dark lustrous brown without any added

pigment. The second one, called Shu-Urushi, is prepared

by adding cinnabar and mercuric sulfide. The third one,

Ao-Urushi, is prepared by adding copper oxide and is

green. The fourth one, Shin-Urushi, is prepared by add-

ing iron oxide and is black.

2.2. Exposure to Light Sources

The samples were irradiated with monochromatic light

by using a JASCO CRM-FD spectroirradiator (Figure

2).

The spectroirradiator was equipped with a 300 W Xe

Figure 1. Chemical structure of Urushiol.

Figure 2. Schematic diagram of the spectro-irradiator: 1.

Xenon arc lamp; 2. elliptical sphere mirror; 3. mirror; 4.

slit; 5. mirror; 6. mirror; 7. diffraction grating; 8. mirror; 9.

sample holder.

arc lamp with an elliptical half-sphere mirror to collect

emitted light. Radiation from this source was converted

into monochromatic light by using a diffraction lattice

grate with 1200 lines/mm. The wavelength dispersion

was about 2 nm·mm−1 and the slit was set to 2 mm, re-

sulting in an irradiation range of approximately 4 nm for

each irradiation wavelength. The specimens were posi-

tioned in a sample holder and exposed to monochromatic

radiations spaced at 16 nm intervals within the 208 - 650

nm wavelength range. For each wavelength, the light

intensity in W/m2/nm was periodically measured using a

photometer. The photometer consisted of a thermopile

detector attached to the spectroirradiator. Light exposures

were conducted at temperature and relative humidity

ranging from 20˚C to 25˚C and from 50% to 70%, re-

spectively. These systems were also used in previous

experiments published elsewhere [33-35].

2.3. Evaluation of Fading

Color change of the specimen was measured using a

Minolta Model CM-3700d color analyzer with a 4  7

mm2 viewing aperture. The amount of fading was evalu-

ated in terms of color difference and was calculated using

the following formula proposed by the CIE Committee in

1976:

ΔE = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2,

where ΔL* is the lightness–darkness difference, Δa* is

the redness–greenness difference, and Δb* is the yel-

lowness-blueness difference.

The reflectance spectra of the lacquer specimens were

also measured by the above color analyzer.

2.4. Determination of Radiant Wavelength

Sensitivity

The accumulated energy (J/m2/nm) was calculated in

light intensity (W/m2/nm) by exposure time for each ex-

Degradation of Japanese Lacquer under Wavelength Sensitivity of Light Radiation1509

posure wavelength, because the light source is not ex-

pected to radiate at the same intensity for each wave-

length. For a specimen, the relationship between the ac-

cumulated radiant energy and the color difference was

examined in a time series experiment at each exposure

wavelength. Then, a smooth curve was drawn to model

the representative fading rates. Measured color changes

recorded at a specified radiant energy was then deter-

mined from the curve to obtain wavelength sensitivity of

the lacquer at each wavelength.

3. Results and Discussion

3.1. Color Features of Pigmented Japanese

Lacquer

Figure 3 shows the reflectance spectra of the lacquer

specimens: (1) Tame-Urushi (brown); (2) Shu-Urushi (cin-

nabar); (3) Ao-Urushi (green); and (4) Shin-Urushi (black).

In Figure 3, panel (1), the reflectance begins to in-

crease at wavelengths around 560 nm and continues to

increase, although all reflectance levels are less than 10%

within the visible light range. Such low reflectance im-

plies a dark brown color.

In Figure 3, panel (2), the reflectance increases con-

siderably from approximately 6% at a wavelength 590

nm to approximately 20% at 600 nm. Further, it contin-

Figure 3. Reflectance spectra of (1) Tame-Urushi; (2) Shu-

Urusi; (3) Ao-Urushi; and (4) Shin-Urushi.

ues to increase gradually to approximately 30% at a wave-

length 700 nm. These characteristics clearly indicate a

bright red color.

In Figure 3, panel (3), the reflectance peaks at wave-

lengths within the range of 460 - 610 nm, indicating a

bluish-green color. However, the reflectance level is only

6% - 8%, signifying a dark tone.

Figure 3, panel (4) shows that the reflectance appears

flat across the visible light range with low reflectance

level at approximately 5%. This is a specific characteris-

tic of black.

3.2. Fading Characteristics of Japanese Lacquer

under Monochromatic Light

As described in the experimental section, the degradation

results shown in Figures 4-7 were obtained on speci-

mens exposed to a narrow radiation band isolated from

the dispersed polychromatic light. In this case, the

strength of irradiance depends on the light wavelength

because the source does not emit wavelengths of equal

intensities. We can compensate for this by varying the

exposure time at each wavelength in order to maintain a

constant irradiance.

Figure 4 shows the fading characteristics of Tame-

Urushi following exposure to monochromatic light irra-

diation at wavelengths 373, 451, and 613 nm. These

wavelengths were selected as representatives of the over-

all light exposures that cause fading. Each curve shows

the variation in the specimen color under a continuous

monochromatic radiation at a constant wavelength. Fad-

ing levels are increased with increasing accumulated ra-

diant energy. Instead of a linear fading rate, the resulting

fading rate was found to be curved. The color following

irradiation at 373 nm is most considerably faded, repre-

senting a color difference of about 10. The next most

severe fading is seen following irradiation at 451 nm.

The least fading is observed under irradiation at 613 nm,

representing a color difference of about 2.

Note that in addition to variation in color due to

chemical changes of the material, fading measured here

incorporates dull appearance that occurs as a result of

very fine cracks on the surface of the lacquer following

irradiation. In this study, overall light degradation at the

specimen surface is evaluated by fading measured as

difference in color.

These results imply that the energy contained in both

ultraviolet and visible light is sufficient to break the mo-

lecular bonds of the Urushi polymer. At the same time,

because ultraviolet light contains more energy, it is un-

derstandable that it causes more damage to lacquer.

Figure 5 shows the fading characteristics of Shu-

Urushi following irradiation with monochromatic light at

373, 451, and 613 nm. Similar to the fading shown in

Degradation of Japanese Lacquer under Wavelength Sensitivity of Light Radiation

1510

Figure 4. Fading character of Tame-Urushi under (1) 373;

(2) 451; and (3) 613 nm of monochromatic light irradiation.

Figure 4, fading for Shu-Urushi lacquer was highest at

373 nm, moderate at 451 nm, and least at 613 nm. How-

ever, the degradation following irradiation in the visible

range was more remarkable than for Tame-Urushi.

Figure 6 shows the fading characteristics of Ao-

Urushi following irradiation with monochromatic light at

373, 451, and 613 nm. On the whole, the degradation

features of Ao-Urushi were similar to those of Shu-

Urushi, although Ao-Urushi seems to be more resistant to

fading in general and is more colorfast than Shu-Urushi.

Figure 7 shows the fading characteristics of Shin-

Urushi following monochromatic light irradiation at 373,

451, and 613 nm. The fading is lower compared to that of

other lacquers discussed in this study. Shin-Urushi seems

to be quite colorfast and resistant to light degradation.

Figure 5. Fading character of Syu-Urushi under (1) 373; (2)

451; and (3) 613 nm of monochromatic light irradiation.

However, even at a low level, the fading level is most

remarkable at 373 nm, which suggests that ultraviolet

light exposure induces more lacquer degradation. In this

study, degradation of lacquer was methodically evaluated,

characterizing changes in color in terms of reflectance at

the material surface. In fact, some degradation products

may be manifest as small molecules that might be easily

lost at the surface of the material. These molecules have

not been previously reported [1,3]. Moreover, water

plays an important role in lacquer degradation. Water,

beginning as one of the main components of the sap,

contributes to polymerization of lacquer. Up to 3% of the

water is considered to remain bound in the final lacquer

coating, even after drying, as part of hydrophilic com-

pounds [1]. This water loss may lead to internal stress

Degradation of Japanese Lacquer under Wavelength Sensitivity of Light Radiation1511

Figure 6. Fading character of Ao-Urushi under (1) 373; (2)

451; and (3) 613 nm of monochromatic light irradiation.

and possible cracking, causing degradation of lacquer.

3.3. Wavelength Sensitivity

The wavelength sensitivity characteristic, also known as

an action spectrum [13,28,36], for the degradation of

Tame-Urushi is shown in Figure 8. In this study, the

fading characteristics of the lacquer coated specimens

were determined when the accumulated radiant energy

reached 5 MJ/m2/nm for each wavelength, as shown in

Figures 4-7.

In Figure 8, two intense peaks were observed at 220

and 315 nm in the UVB-UVC range for Tame-Urushi.

Moreover, a large and broad shoulder peak was observed.

This suggests that UVA radiation has a significant deg-

radation effect, whereas visible light causes lesser degree

Figure 7. Fading character of Shin-Urushi under (1) 373;

(2) 451; and (3) 613 nm of monochromatic light irradiation.

of degradation.

Figure 9 shows the wavelength sensitivity characteris-

tics for the degradation of Shu-Urushi under a radiant

energy of 5 MJ/m2/nm at each wavelength. A large peak

is shown at 220 nm in UVC. In addition, a combined

large and broad peak is observed in the UVA-UVB range.

This indicates that the lacquer is strongly degraded under

UVA sunlight irradiation. Moreover, for Shu-Urushi, the

degradation peak is observed in the green-red light range

of 510 - 650 nm. Because the intensity of sunlight is

maximal around this wavelength, it is expected that

Shu-Urushi would be especially prone to degradation by

accumulating sunlight intensity during daily usage. This

agrees with a previous report by Umney, which suggests

that the lift time for red lacquer is eight months and three

Degradation of Japanese Lacquer under Wavelength Sensitivity of Light Radiation

1512

200300 400500 600700

Radiant wavelength, nm

Color difference

Figure 8. Wavelength sensitivity characteristics for the fad-

ing of Tame-Urushi under a radiant energy of 5 MJ/m2/nm

at each wavelength.

200 300400 500600 700

Radiant wavelen

th, nm

Color diff eren c e

Figure 9. Wavelength sensitivity characteristics for the fad-

ing of Shuurushi under a radiant energy of 5 MJ/m2/nm at

each wavelength.

and a half years for black lacquer under the display lights

in museums [3]. This is further supported by the result in

Figure 9.

Figure 10 shows wavelength sensitivity characteristics

for the degradation of Ao-Urushi under a radiant energy

of 5 MJ/m2/nm at each wavelength. The peak is observed

at 315 nm and a shoulder at around 250 nm in the

UVB-UVC range. A broad shoulder peak also exists in

the UVA range. Moreover, a peak at around 600 nm in

the visible light range is observed as a result of degrada-

tion. However, the levels of color difference are lower

compared to previous Tame-Urushi and Shu-Urushi spe-

cimens. Ao-Urushi is considered to degrade relatively

faster compared to Tame-Urushi and Shu-Urushi speci-

mens.

Figure 11 shows wavelength sensitivity characteristics

for the degradation of Shin-Urushi under a radiant energy

of 5 MJ/m2/nm at each wavelength. The levels of color

difference are very low at all wavelengths compared to

other lacquer specimens in this study. This implies that

Shin-Urushi may be inert to visible light. However, a

noticeable change is observed in the UV range. The deg-

radation caused by UV irradiation can be recognized in

Figure 11, although the level is very low. While Shin-

Urushi is resistant to visible light, UV light can still

cause degradation of the polymer. This observation is

200 300400 500600 700

Radiant wavelength, nm

Color difference

Figure 10. Wavelength sensitivity characteristics for the

fading of Ao-Urushi under a radiant energy of 5 MJ/m2/nm

at each wavelength.

Degradation of Japanese Lacquer under Wavelength Sensitivity of Light Radiation1513

200 300 400500 600 700

Radiant wavelength, nm

Color difference

Figure 11. Wavelength sensitivity characteristics for the fad-

ing of Shin-Urushi under a radiant energy of 5 MJ/m2/nm

at each wavelength.

consistent with warnings that lacquer should not be ex-

posed to light below 400 nm [3]. This appears to be gen-

erally accepted and related to the relationship E = hν.

However, our results indicate other aspects of the prob-

lem, as previously described. Visible light also has a

considerable impact on the degradation of some lacquers.

The structure of lacquer crafts is complex, and a com-

bination of many factors influences their degradation.

Photoreactions are generally related to specific wave-

lengths, which are designated by the energy gap, as ex-

plained by quantum chemistry. In our study, pigmented

lacquers had different wavelength sensitivity to degrada-

tion. Therefore, those pigments are expected to transfer

their absorbed irradiation to characteristic energy gaps in

the micro mechanism of the degradation process.

4. Conclusions

In this study, the degradation characteristics of pig-

mented Japanese lacquer were investigated by wave-

length sensitivity analysis on the basis of radiant energy.

Specimens were exposed to monochromatic light and the

degradation characteristics were compiled as action

spectra. We obtained the following results:

Tame-Urushi (brown) displayed peak degradation ma-

xima at 220 and 315 nm. A wide shoulder peak was also

observed in UVA. As for Shu-Urushi (cinnabar), in addi-

tion to the peaks in UVA-UVB, a strong peak was ob-

served in the visible light range. The results indicate that

not only UVA and UVB but also visible light of wave-

length 510 - 650 nm has a significant degradation effect

for this lacquer. Ao-Urushi (green) showed similar char-

acteristics, with lower degree of degradation. Shin-Urushi

(black) was not seriously damaged by light although the

UV exposure caused small degree of degradation.

This study provides experimental evidence that Japa-

nese lacquer responds differently to light of various

wavelengths and that specific wavelengths, including

those in the visible range, can cause significant degrada-

tion. Further research is necessary to completely under-

stand the nature of Japanese lacquer. However, this study

provided insight into new aspects of the degradation

characteristics of Japanese lacquer.

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