An Overview of Oriental Lacquer Art and Chemistry of the Original High-Tech Coating Donald M. Snyder Basic Research, Armstrong World Industries, 2500 Columbia Avenue, Lancaster, PA 17604 Traditional arts and crafts that have been developed over hundreds or thousands of years are often based on elements of surprisingly sophisticated science and technology. The empirically derived knowledge behind such diverse areas as ultra-high-carbon Damascus steels ( I ) , the unique tonal qualities of ancient Chinese temple bells (2),the decorative gold working of pre-Columbian South America (3), and the secrets of the classicalviolin makers of Italy (4),just toname a few, have only recently begun to be well characterized and understood. One such topic of particular interest t o an organic chemist working in the field of polymer research is the ancient art of Orientallacquer. Even some of the earliest accounts of European travelers in the Far East took notice of the unusual physical properties of Chinese and Japanese lacquers as protective coatings on furniture. Further reading on the subject brings to light other interesting points such as the question, "What does the ancient art of Oriental lacquerwork have in common with a case of poison ivy?"The answer to this puzzle is an interesting illustration of the interrelationship of structure and function that is the basis of much of organic chemistry and that connects what may appear to he totally unrelated phenomena. This article is intended to relate some of this information in more detail, along with a brief summary of the current state of knowledge of this unusual material. Before we proceed, i t is first necessary t o point out the distinction between the term "lacquer" as i t applies to the Oriental decorative arts and its current usage in the chemical industry. The modern term "lacquer" is used to designate a wide range of coatings that share the common characteristic of drying by evaporation of the solvent medium in which the resin is dissolved, with no oxidation, polymerization, or other chemical change taking place (5).In contrast, the true Oriental lacquer of China and Japan is a natural product, the resinous sap of theRhus vernicifera tree, which is gathered by tapping the hark in a manner similar to the collection of natural rubber. After purification and processing, the lacquer can be used directly as a liquid coating. I t is the subsequent hardening process of the true lacquer that distinguishes i t from the synthetic variety in that solvent evaporation is not the primary drying mechanism but rather an oxidation-induced polymerization that cures the film. As a result, the final coating is extremely hard but not excessively brittle, is capable of taking a polish brilliant enough to rival even glazed porcelain, and is orders of magnitude more durable and solvent-resistant than the synthetic imitations. Even today i t is a little-appreciated fact that this ancient coating material has physical properties that were unrivaled until the advent of high-performance resins such as epoxies and polyurethanes. I t is generally acknowledged that the use of lacquer as a protective, and later decorative, coating originated in China, most likely prior to the fifth century B.C. (6,7).I t was being used as an artistic medium by the latter half of the second century B.C., and the techniques for its use were continuously refined throughout the Han, T'ang, Sung, Yuan, Ming, and Chingperiods. The use of lacquer spread in the eighthor
ninth century A.D. to Japan, where the a r t reached its zenith. T o the Chinese culture, with its artistic emphasis on poetry, painting, and calligraphy, the use of lacquer was regarded primarily as a craft. The Japanese, however, saw in this material an artistic medium of great flexibility and developed many new techniques to utilize its full potential. They rapidly outstripped their Chinese teachers, and by the fifteenth century the flow of knowledge was reversed, with Chinese artists traveling to Japan to study the secrets of its lacquer masters. Such was the durability and beauty of lacquer that during this long period almost any item that could be lacquered was, from desks and other furniture pieces to sword scabbards, saddles, temple pillars, door pulls, boxes, and countless other items both formal and personal. T o appreciate fully the labor involved in making such items, one should understand that producing a fine lacquerware piece required more than artistic talent: it also called for the most tedious. repetitious, painstaking craftsmanship. The sequence of coatine a wooden surface will illustrate the general process. An iiitial coating of a special lacquer called seshime is used to seal the pores and surface of the prepared wood, which must be dry and well-seasoned. A layer of regular lacquer is applied and allowed to harden completely. I t is then polished by hand, rubbing with succeedingly finer grades of abrasive, from powdered charcoal to hartshorn, until the surface is mirror smooth. Another layer of lacquer is applied, and the process is repeated as many times as necessary for the particular technique being done. For a simple surface, even a thin lacquer layer may comprise 20 to 30 individually applied coats-while to produce a thickness of lacquer suitable for carving may require 200 to 300 coats (8). Depending on the artistic technique being used, the lacquer could be either clear or colored with dyes and pigments. For example, the thick carved red lacquer of primarily Chinese origin was heavily pigmented with cinnabar. Once the base layer or "ground was prepared, the colored lacquers could he coated as designs on the base to produce lacquer paintings; but this artistic form is relatively rare in
The Cover The three pieces of aientaf lacquer on the cover are ail items from the personal collection of Kurt Herberts and were repraduced from his bwk Oriental Lacquer-Art and Technique [Abrams: New Y&. 19641. The background piece is the cover of a late 18th-century Japanese writing box with black lacquer enerin (1.75 X 9.6 X 8.25 in.). The design is done in the techniques of togidashi, himmkie. takamakie, and okibirsms. with gold and silver inlays, painted lacquer. and wrinkled silver dust. The mid olece is an eariv 19th-centurv Japaneoe low case inro with decaralian of rocks, peonles. and ualerfa I done m hmmkteand takamakleon a gold ground and flowers o m gold lea1 (3 9 X 1 9 X 1 0 n ) The red plscs is an early terncentury Japanese low-case inro with netsuke. done in the 17th.century Chinese style of carved red lacquer (2.6 X 2.25 X 0.75 in.).
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Volume 66
Number 12 December 1969
977
Japan, where the classic lacquer masters preferred to work in the "makie" or "sprinkled-picture" technique. In this form, designs done in clear lacquer are dusted &th gold or silver powder before curing and then covered over with more coats of clear lacquer. A solid layer of the metal dust was often used as the background rather than the traditional black base..eivine rise t o the well-known kinii (eold .- eround). u ginji (silver ground), and shibuichi-ji (mixed silver-cha;: coal eround) forms. Rather than snrinkline-the nowder even. ly, a variation known as nashiji (pear-ground) was done by scatterine the metal dust irreeularlv - to eive a texture similar t o the sk& of the Japanese p&. In addition to these techniques, many other decorative effects were developed, such as engraving the lacquer and filling in the lines with metal powders and inlaying with tortoise shell, mother-of-pearl, ivory, gold or silver flakes, finely divided eggshells, semiprecious stones, and other exotic materials (9). In Japan, some of the most outstanding examples of decorative lacquer art are to he found on small boxes. The most well-known of these are the writing sets, ceremonial tea sets, and inro, the small medicine boxes that were once hung from the sash of traditional Japanese costumes and that are so highly prized today by Western collectors. Some hits of the unusual chemistry involved in lacquer curina.become apparent when studying .. . .. references in old a r t publications on the collection, processing, and use of Oriental lacquer. Although the lacquer of China and Japan is best known, other areas of the Far East do produce similar materials from related plant sources such as the Rhus succedanea in Vietnam and both Melanorrhoen usitata and laccif~ra from the Laos-Thailand-Cambodia area (10, 11). For the sakeof simplicity, we will restrict this discussion to the more thoroughly investigated Japanese material. The raw sap from which lacquer is prepared is isolated by making lateral incisions in the trunks of mature lac trees, which are known as ch'i-shu in Chinese and urushino-ki in Japanese. This is usually done between the months of June and iiovember, with the yield of sap being about 250g per tree. It is interesting t o no& that ~apanesefurnituremakers even have preferences for particular months to collect sap. They generally regard the mid-July-to-August sap as best for final finish lacquer, while the September sap is believed t o be most suited for the base finish (12). Small branches of the tree are also broken off and soaked in water for 10 days to isolate the special seshime lacquer. The raw trunk sap is strained to remove mechanical impurities such as hark and dirt and
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then stirred in shallow pans over low heat to evaporate the excess wnter. The thirkened lacauercan then be useddirectly for coating. At all stages durLg this process the sap and finished lacquer must he handled with meat care. as contact with the skin may produce an intense allergic reaction. Susceptible persons have been known to develop severe inflammation 'imply from being exposed to the ;apom from uncured lacquer. An ancient remedy for the skin irritation and blisteringof lacquer poisoning specifies covering the afflicted area with the raw flesh of a river crah. While this may appear bizarre a t first, it will later be shown to have a sound chemical basis. Another unusual feature of the lacquering process is that the liauid cures t o its hardened state most kfficient~ya t moderate temperatures in the presence of air a t relative humidities of meater than 80%. In fact. traditional lacquer workers waul; often store their piecks, hetween coats, in riverside caves to have them cure in the shortest possible time. I t should also he noted that once the lacquer cures, the hardened coating is physiologically inert, even to the point of being suitable for use on food dishes. The striking similarity of lacquer poisoning to the effects of poison ivy is not surprising, as the Rhus vernicifera tree is a member of the family anacardiaceae, which numbers among its other members the familiar poison oak, poison sumac, and poison ivy (Toxicodendron radicans) (13). In fact the Japanese term for lacquer, urushi, has been used to name the class of compound that is the principal active constituent of both lacquer sap and poison ivy, urushiol(14). As shown in Figure 1, this material is a mixture of several closely related derivatives of catechol in which the ClsC,, side chains differ in location and number of double bonds (14). Urushiol from Rhus vernicifera is primarily catechol derivative IV, in which the side chain is an 8(a,11(E), 13(Z)-pentadecyltriene (16). The exact composition of the urushiol mixture found in Oriental lacquer depends on the plant source from which i t is isolated. The lacquer from sources in the tropics tends to turn black on exposure to air, while the temperate-zone sources in Japan, China, and Korea yield a lacquer that remains relatively clear when cured. The traditional black lacquer of Japan thus relies on pigmenting with various types of carbon black. The typical composition of raw sap from the Japanese lac tree is 20 to 25% water (reduced to approximately 3% after processing), 65 to 70% substituted catecbol derivatives, 8% mixed rarbohvdrates. 2% nitroeencontaining plant gums, and less than 1%laccase enzymei17, 18). The last item listed, the laccase enzyme, is present in very small quantities; but it is vitally important, as the normal lacquer curing process will not occur in its absence. As a general class of biocatalyst, i t is ap-quinol-02-oxidoreductase enzyme with a redox potential of 415 mV a t 25 OC and a pH = 7.0. The overall structure of the enzyme is a copper glycoprotein with a molecular weight of about 120,000 and a com~ositionof 45% suears and 55% common amino acids with four Cu2+ions/mole&le (19). When one considers the presence of the unsaturated side chains in the urushiol struckue, i t would not seem unreasonable t o make the initial hypothesis that the curing reaction responsible for lacquer hardening is directly analogous to the oxygen-induced curing of natural dryine oils, such as tung oil. Tung oil is a trigljceride that contains a high percentage of eleostearic acid (9,11,13-octadecanetrienoicacid (20,21). On exposure to atmospheric oxygen, the oil forms a cross-linked network through the triene double bonds by initial formation of allvlic hvdroneroxides. While i t is temptingto rationalize the cure mechanism in lacquer as a similar cross-linkine throueh the urushiol triene segment, a number of facts strongly suggest that something more complex is involved. T o beein with, it is known that the laccase enzyme consumes 0 2 very rapidly in the lacquer, strongly suppressing the allylic hydroperoxide formation that is necessary for initiating the tung-oil-type cross-link-
ing reaction. Studies on the catechol nucleus itself have shown that electrochemical oxidation results in formation of the o-henzoauinone structure shown in ea 1 and that this leads todimerization with eventual formation of low-molecular-weight polymer of complex structure (22).
Further evidence implicating an oxidation process is seen in some chemically related are& Oxidative coupling of phenolic materials in decaying plant tissue is resp(,nsible for the formation of comnlix Doivmers termed "humic acids" in soils (23). The laccase enzyme is also believed to be respousible for catalyzing the oxidative polymerization of phenolic flavans such as catechin, to complex polymers generally reof heferred to as "tannins" (24).Oxidative oolvmerization . . nolic systems is also not k i q u e to natural systems. ~ e n e r a l Electric's family of Noryl polyphenylene oxide plastics is based on oxidative polymerization of 2,6-dimethylphenol using a copper-amine catalyst (25). ~aianedeworkerswho hive been studying lacquer curing haveadvanced the hypothesis that thecross-linkingreaction of urushiol is initiated by the laccase enzyme oxidizing the catechol unit to the semiquinone radical by a one-electron process (26). The semiquinone radical can then react further by a number of competing pathways. The complexity of the nrocess is illustrated bv the fact that durine the removal of kater from the raw sap;approximately 20% of the urushiol is converted to a mixture of dimers and olieomers from which a t least 20 distinct compounds have beenisolated and identified (27). Model studies using 3-pentadecylcatechol indicate that the semiquinone radical can readily disproportionate to the corresponding o-henzoquinone as shown in eq 2 (28).
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As a result. i t is believed that the reactive s ~ e c i eDresent s in the lacquer curing sequence are the parent (&oxidized) urushiol. the semiauinone radical. and the o-benzoauinoue. Based on the structures of the c ~ " ~ l products in~ identified
in the partially oxidized lacquer, i t is theorized that the semiquinone radical can react with the urushiol by an aromatic free-radical substitution to form biphenyl-type dimers. Further studies on model svstems have indicated that ~ - the o-benzoquinone intermediate may react with the allylic sites on the triene side chain of urushiol by dehydrogenation to give either a radical or a cation that can then add to the aromatic nucleus of another urushiol molecule (29).A generaloutline of the possible reaction pathways is summarized in Figure 2. As the reaction scheme shows, the coupline oroducts retain an active catechol ring for further oxidatrooh and subsequent coupling to produce apolymer via a step-growth process. Since the triene side chain contains at least two allylic sites, the potential t o form a three-dimensional crosslinked network is also apparent. However, because of the difficulty in analyzing the cured polymer, it is not yet known whether these reactions are representative of the subsequent steps leading to the cross-linked solid. More structural determination on the cured lacquer will he necessary before definitive conclusions can be drawn. The proposed mechanism accounts for many of the unique points noted so far regarding the lacquer curingprocess. The requirement for oxygen is explained bv the action of the lacease enzyme catalizing the formationof the semiquinone radical and o-quinone intermediates. This also tends to explain why lacquer must he applied as multiple thin coats rather than as a few thick coats. The laccase enzyme consumes oxygen very rapidly, and thus, oxygen must continuously diffuse into the film to allow curing to continue. A thick coating would cure fastest a t the surface and thus tend to hinder more oxyen from diffusing down into the lower layer, resulting in incomplete cure a t the bottom anddegrading the mechanical properties of the film. The fact that the laccase enzyme is essentially an amiue-com~lexedcomer .. catalyst even suggests a rational basis for the previously mentioned river-crab remedy for lacquer ~oisonine.The oxygen-carrying protein in crib blood;~ n i t hemoglobin but hemocyanin, a copper-based system, and its deactivating effect on the catechol portion of urushiol suggests that the hemocyanin copper can also catalyze the coupling reaction in the presence of oxveen. ~ l t h o u g hi t is bo&d that many of the questions raised during development of this topic have been addressed, i t is equally clear that many other puzzling aspects remain to be explored. Since studies have shown that the catechol portion of the molecule is the active site responsible for the allergic reaction (30), the toxicoloeical inertness of the cured film stands in striking contrast t o the allergic potential of the catechol units in their lower molecular weight form. And, if the catechol hydroxyls were not consumed in the curing reaction, the cross-linked network would simply represent a polymer-hound catechol system, which modelstudies have shown to be oxidatively unstahle (31). The nature of the transformations, if any, on the catechol nucleus during the final stages of hardening are still in question. The differences between the stem lacquer and the branch-derived seshime lacquer have also not yet been investigated. The fact that the seshime lacquer cures more slowly and gives a harder, more brittle material is prohably related to both the composition of the urushiol from the peripheral branches and to the fact that the isolation process also extracts tannins and other water-soluble bark substances into the finished lacquer. The effect of these materials on the urushiol cross-linking remains to he determined. In this article I have attempted to show that while the use 'of Oriental lacquer dates back thousands of years, a scientificunderstandingof thiscomplex materialisonly nowemerging. Heneath the smoothly polished surface of a lacquer coating lies a wealth of chemistry that is yet to be explored. Investigation into this most ancient of coating materials will undoubtedly continue to reap a rirh harvest of interesrine chemistry. ~
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Volume 66
Number 12
December 1989
979
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Literature Clted
16. 17. 18. 19. 20. 21.
1. Shelby,O. D.; Wsdsworth, J. Sci.Am. 1985,252(2J. 112 2. Shen. S.Sci. Am. 1987.256141.104.
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