Robert Walker Department of Metallurgy & Materials Technology University of Surrey Guildford, Surrey
Corrosion and Preservation of Bronze Artifacts
Articles made of copper and copper alloys, especially hronze and brass, have been used for several thousand years. Native copper was used at lirst and then as knwvledne increased, copper w s smelted from pure ores. Bronzes first came into use sometime around the beginning of the third millennium B.C. The addition of tin to copper to produce hronze may originally have been accidental due to the melting of copper ores (1) containing tin as an impurity, hut it was soon found to be beneficial and it was added deliberately. One main advantage of adding tin to copper is to increase the strength of the cast objects. The alloy can also he further hardened bv hammeringtogive h ~ t u . t(kdsor r wt.apms than d~tainedfrom copper. Thus mld-u,orkingcopper tu giw n reductiun iri7o'i inrrc~asesthe hardness 121 from about XI to l3U H V . whereas for copper contaming 8% tin the increase is much greater, being up to 250 HV. Many articles were prohahly cast and then hammered in the cold condition giving the distmctive, distorted, cored-structure that has been found in microstmrtnres of artifirk 1.7) ~.,.
Alloys In the past considerable emphasis has been laid on the correct selection of alloys because the properties of an article deoend upon the com~osition.A 4th centurv B.C. Greek stele talilet ipt,&ied the n,mpc,sition of bronze (4fi,r drum fittings a? 1 I pnrts copper 1 port tin; the iml~ortanceof adhering to rht,specification in terms of service perfarmance and eronomics w s indicared together with tests tochrck theauulitv. . . Karly Chine>e meti~ll~~rgists 1.51piwe cupper:tin ratios d 5:l for cauldrons and bells, 4: l for axes, % I rur ha1hcrt.i and SDeari. 2:l for swords and knives, 3:2 for erasing knives and arrows, and 1:1 for mirrors and speculum. (They did not differentiate between tin and lead so that the figure for tin probably included lead.) Much later Leonardo d a Vinci ( 6 ) laid down comoosition limits for tin and lead in hronze: bronzes of the Tuscan Renaissance period for statuary and artillery uses had a characteristic analvsis of 88% comer. .. . 9% tin and 2% lead. while hells were castwith 33% tin to improve the sonority. Bronze The specialized properties of bronze, notably the good castahility, fine appearance and good resistance to corrosion, made it oarticularlv suitable for s c u l ~ t u r e sin Greek and Roman times. The "~olossusof ~ h o d e s ,a statue of Helios which is thought to have been about 40 m high and erected in 280 B.C., was believed ( I ) to he hronze with 10.8% tin and 10.2% lead. The Romans (7) also used bronze for coins and engineering purposes including pumps, stopcocks and valves for water supplies. for the doors and tiles of the dome of the .. I'anthron as well as fur furniture, \.essels, ornaments and mirrors. From 600 A.D. church hells were made or hronze inrluding the (;re?[ Rell of the Krrmlin made in 173:1 weighing I95 X IWkyand (i.9 m in dinmeter 171. The nmposititm ut the firit hrcm~cswas cuntrdled by the compusitim oithc.ores in t l ~ elocnlitv. The use or hrmze de~ c n d e dupon the ave~lahilitvo l t i n 171. In E r w t the develbpment or the Bronze Age was delayed hecause'their copper came from Sinai where there was no tin, and it was only later when Syria was dominated that tin became available and bronze was made. The Sumerians (8) reverted from bronze to copper for the period 3200 B.C.-2700 B.C. due to the failure of tin supplies and the same thing happened in Troy (9).The
Greeks were probably the first to intentionally add lead to statuarv hronzes either to improve the fluiditv of the molten alloy (7; or for the fin(:patina which it develop! ( 5 , . Accordin:: t(, Bear7i the composition of the brunze in antique sMtlles can be divided according to period into three (10): 1) Graeco-Etmscan, 600 to 300 B.C., with 83-918
14% lead.
copper, 7-12s tin,
2, Roman, B,C,to 300A,D.,with 64-79%copper,5-11% 10-27% lead. 3) Tuscan Renaissance, 1300 to 1700 A.D., with 85-928 copper,
fLl3%tin. 1-28
l e d
Bronzes may he classified as either having a low tin content and being. sinele or with a hieher tin content and two - phase . phnic. Examples of single phase hronles inclnde Greek statues with 6.1%tin and Athmian coins w i t h :I.:!-5.9% tin I I ) . while two phase alloys were used for ancient British swords (G-18% tin), early Chinese and Korean mirrors (2630% tin), battle axe heads (16.6% tin), and ladles (19% or 23% tin), (I1). The composition of the alloy affects hoth the physical and mechanical properties while the corrosion risiitance of a two phase alloy is usually lower than that of a single phase alloy. The bronzes produced in ancient times often contained impurities. The original rornpuiition uf these I~nmzeshai been studied Iwcnuse of this knuuledpe may indicate tht: technolag). usttd in their production. It is also important hecousr the prrwnce of impuritieitan affect both the microstructureand the corrosion properties. Thus the hieh lead content in manv Etruscan bronzes (11) indicated a deiiherate addition, probahlv to improve the fluidity, althoueh it has also been suegested that it was used t o give a fine patina. ~ubstantial amounts of iron (12) up to 20%, in pre-Iron Age copper and hronze artifacts is considered to be due to iron impurities in the copper ore and to the deliberate addition of iron oxide to the furnace charee to act as a flux. The oresence of this amount of iron would certainly have a marked effect on the corrosion behavior. Scran metal was freouentlv resmelted and gave rise to variations in the composition and properties of the final material. The composition of the metal a t the surface of a bronze may he very different from that in the interior. Youne (13) has found that a h n z e containing G5"bcopper and :Witin in the centrr changed toabout YSOivopuer and 5% tin n t thesurfare. and he attr'huted this to a process of "parting" or selective leeching during corrosion. Surface enrichment may also occur in the manufacturing process (14) due to (I) selective oxidation during reheating, (2) mechanical displacement of a lower meltine-. m i n t ohasehurine..high ,. temnerature working. ~. .when a mdren eutectic may be squeezed tu the surface: (31macrosegrrgation arising irum the original solidification process. Not surprisingly the casting techniques of ancient hroners were not always wrv eood and inclus~onswere often uresent. Slow cooling of the &&en metal can give the extensi'e coring and intercrystalline porosity found in many objects. Incomplete deoxidation of the metal during casting leads to the formation and entrapment of oxide skins in the surface. This feature may he enhanced by the presence of harmful impurities and excessive turbulence during the pouring of the molten metal. These defects which may he internal andlor external have a detrimental effect on the properties of the articles.
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Volume 57, Number 4. April 1980 / 277
Corrosion of Copper
Copper is a relatively noble metal and has a fairly good corrosion resistance. It is, however, thermodynamically unstable so it combines with other elements to form more stable comnounds such as CODDer .. oxide. Manv of the corrosion products are similar in composition to naturally-occurring minerals and the mineralo~icalt e r m i n o l o ~is often used. This aspect oirurrosiun has t~eenextensively studied t y Getten$ (1.j).The thermudynnmic stability of metnllic compounds is indicated by the appropriate enthalpy (heat of foimation). Thus copper may initially form cuprous oxide (enthalpy -40 Kcallmol) which is then converted to a more stable form such as a hydroxide (-107 Kcallmol) or a carbonate (-142 Kcall mol). The rate of change depends upon kinetic factors such as the composition of the environment and the nature and solubility of the products. When untreated copper is exposed to air it tarnishes in a few weeks and becomes dull, due to the formation of oxide films. For a copper roof to turn green may take 50to 100years in Dure air. hut this nrocess occurs much more auicklv in polluted atmospheres. This acceleration is due to the sulfur dioxide in the air heinedissolved in rain, uxidized and reacting with the copper to form sulfates. The e x x t composition of t h i corrosion ~ r o d u c t ds e ~ e n d upon s the atmosphere so that the patina may he or pkrmit further Eorrosion leading to deep and ~enetratinpattack. of many articles, it is necessary to ~ u r i the n ~ melt and cast the metal and then allow it to cool. This exposure of copper to air at high temperatures gives black cupric oxide (CuO), also called tenorite (15). This oxide may be removed in suhseauent processes such as fettline and finishine the cold, cast object. ~ k cuprous d oxide (~u~0)"or cuprite (15j is the natural oxidation nroduct formed when conner reacts with air a t ambient temperatures; many corrodedartifacts of comer or bronze consist mainlv of this comnound. I t is interesting to note that the conversion of copper metal to cuprite involves an increase in volume of about 70% (16). hut the original shape of the article is retained, because some of the comer ions migrate outward and are deposited as low densitv ha& salts at the outer surface. These oxides are intermediate corrosion products and may he converted to basic salts. Thus they are present in ancient structures between any existing uncorroded metal and the outer basic salts. Carhon dioxide in the air dissolves in rain, dew, or moist soil to give carbonic acid which attacks copper. Basic or mixed salts are formed. Green malachite (15) (CuCOnCu(0H)z) is the more abundant formation, hut blue azurite ( 2 ~ u C 0 3 Cu(0H)z) also occurs. The initial products are partially soluble and the final corrosion products can form some distance away from the site of corrosion. Colloidal gels sometimes occur and may give handed structures which can he seen when examining cross-sections of the objects. Collins has ohserved hotryoidal malachite up to 1.2 cm thick with crystals of cuprite and a crust of azurite on Chinese hronzes that have corroded underground (17). In polluted atmospheres containing hydrogen sulfide, copper may form chalcocite CuzS, or covellite, CuS (15). The latter is especially abundant on copper and bronze recovered from wrecks of wooden shins. because sulfate-reducine hacteria (Desulphouibrio) are & e n found in the decaying&ood, and thev can convert sulfates in water to sulfides accordinato the equation. Sod2- + 4H2 S2- + 4H20 These bacteria also survive underground so that buried structures may show similar corrosion products. As already mentioned certain hronzes contain bigh concentrations of iron. Iron compounds are found in some soils, so in these copper iron sulfides can form underground. Dauhree has studied Roman bronze medals and coins, and he identified chalcopyrite CuFeSz, hornite CusFeS4,and tetrahedrite (CuFe)lzSh S,3 (18).
-
278 / Journal of Chemical Education
The presence of sulfur dioxide in air can result in the conversion of copper oxides to a mixed salt, CuSO&u(OH)z, which is gradually changed to brochantite (CuSOc3Cu(OH)z). The reactions may he represented by the equations:
-+ -
Cu + Hz0 + % 0 2 CU(OH)Z Cu(0H)z+ S O F CUSOI+ %OH)or
+
4 CuO SOP+ 3H20 % 0 2 CuSO* 3Cu (OHh I t is interesting to note that the Statue of Liberty in New York, although surrounded by sea so that there is a bigh chloride concentration in the air, is covered with mainly hrochantite with basic copper chloride as only a minor constituent (19).
Long contact with sodium chloride in saline soils or desert sands can give basic cupric chloride, atacamite (CuC ~ Z ~ C U ( O(15). H ) ~This ) reaction may be represented by the equation:
-
Cu + NOH)- + 2C1- CuC12 3Cu(OH)z+ 8eAtacamite is dark ereen and non-nrotective so hronzes with " it are usually fissured and distorted. Another aggressive chloride is cunrous chloride. nantokite CuCl which is unstable. wax and is the cause of bronze has the appearance of disease (15). Otto has analyea hy X-rays many samples d t h e patina from curwer allov d~iectsthat have heen expmed for hundreds 'He found that the major producG were azurite, maof lachite, atacamite, and para-atacamite. The composition of the patina was not related to that of the alloy substrate (20). The results of atmospheric corrosion on copper roofing have been studied by Vernon and Whitby (21).They found that the sulfate ion nredominated in the corrosion nrocesses and that the carbonate ion was present in low concentrations. In areas with low artificial pollution and chloride ion may predominate over the sulfate ion, hut this predominance was reversed in areas where urhan and marine conditions coincided. Thus in urhan environments an analysis of the coating gave 10-41% sulfate, 1.1-7.1% carbonate, 0-5.0s chloride and 0.3-2.6% sulfur. I t is interesting to compare the corrosion products on the south side and the north side of the same church roof which was about 300 years old. The north side had a black, appearance and consisted of 11.8%copper sulfate, 4.7% copper carbonate, 22.6% lead sulfate, 1.5% lead sulfide with the remainder mainly copper oxidelhydroxide; the south side was green and the corresponding figures were 25.6%,1.4%,5.3% and a trace. This data confirnis the view of Otto that the composition of the patina is mainly related to the type of atmosphere, prevailing wind etc. Vernon et a1 (21) showed that the composition of the surface product changed with time according to the stages: (1) initially a brown film (2) after several months, crystala of copper sulfate formed (3) the surface then became black (4) after about 4.5years basic sulfate and a little sulfide and carbonate were present ( 5 ) then a green patina formed. This scquence dcprnds upon local variatiuns in the environment hut is probably generally applicahle. The corrosion of buried cupper ohjecti is rather more complicated than that in air. The tyDeoisoil. thesaltj nrrsent,. the -pH and conduc.. tivity are all important as well as local variations in the compaction of the earth. Some soils, such as dry sand, give very little attack whereas water-logged acidic clays, peats, ortidal marshes can he verv corrosive. There is no direct relationship hetween the rate ofcorrosion ot'copper and any single feature of the soil composition according to Markovic et alr22). Corrosion of Bronze
The benefits bestowed on copper by alloying it with tin unfortunately have to he offset against the reduction in the
corrosion resistance. Tin is a more reactive metal than copper as shown by the relative potentials in the galvanic series, the value for copper heing -0.09 v and for tin -0.15 v. Tin compounds are thermodynamically more stable than the corresponding copper compounds, the values of the enthalpies heing cuprous oxide -40 K, cupric oxide -37 K and stannic oxide -139 K callmol, and for cupric hydroxide -107 K and stannic hvdroxide -207 K callmol. Thus duriue corrosion the tendency"is for the tin-rich areas to be attackedbreferentially. Scholes and Jacob have shown that during atmosoheric corrosion the rate of penetration of copper was less than that of alloyed copper, (23). The corrosion of bronze is similar to that of copper and many of rhe copper cvmpoundi already discussed are produrrd. Thr main alloying elrmrnti of tin and lead do produce modifications and generally bronzes corrode faster than copper objects (23,24). When tin corrodes it generally forms stannic oxide Sn02,which is similar to cassiterite, although other comnonnds have been fonnd. inclndine stannous oxide. romarchite SnO, and the hydrated form, hidroromarchite; 5 Sn0.2 Hz0 (15). Tin oxide has a whitish appearance but is often colored brown or black by iron salts or green or hlnishgreen by copper salts. As the volume of the oxide (16) is similar to that of the metal (only 33% greater) the oxide that forms is hard and compact. Because the stannic oxide is formed in situ it is present within the patina and does not affect the original surface shape. It has been observed in disconnected seams throughout the malachite on corroded bronzes (25). When formed on the surface of hronze this oxide withstands oxidation and the resistance increases as the tin content is raised to about 40%. I t is this oxide which forms the thin hard reflective surface found on manv ancient mirrors. Thus the early Chinese made mirrors with hronze containing up to about 50% tin. (5) Lead is often present in hronzes to improve the fluidity of the molten alloy or to modify the patina produced and Roman bronzes usually contained 10-27% lead (10). It is only partially soluble in bronze and forms roughly spherical inclusions. When i t corrodes in moisture-containing carbon dioxide, lead carhonate, cerussite. PhCOn is formed. Chinese knife coins made in Chou times had lead contents of 17 to 55% and these hronzes were used because of their low cost and good fluidity (26). Cerussite is often found within the matrix of the basic comer carhonate. malachite. If chlorides are nresent the lead compound cotunnite PhClz may he produced.'Under corrosive conditions in which brochantite is formed the lead mav combine with copper to form caledonite, corresponding to thk double basic sulfate of these metals ( 5 ) . Gettens examined ancient bronze vessels about 2500 years old made of 73%copper, 21% tin, and 4% lead. He fonnd that the cross-sections consisted of three zones (25): (1) an uncorroded metal core, (2) an intermediate, partially mineralized zone, and (3) an onter, completelymineralized zone. The inner corrosion product was mainly nantokite with a middle zone of cuprite. The tin corrosion product, cassiterite, is present throughout the cuprite. Bronzes may have a low tin content and consist of a single phase, solid solution called a. In these alloys the tin is mainly dissolved in the comer matrix but often seereeation occurs during casting so tl;& the tin concentration becomes greater at the arain boundaries. The initial reaction occurs a t the surfaceand the corrosion products eventually form a patina. In general, corrosion is concentrated along grain boundaries which are more reactive and anodic compared with the grains. The segregation of tin to the grain boundaries enhances this localized attack. Severe attack in these areas, combined with general corrosion proceeding into the grains, can eventually lead to the complete disintegration and collapse of the article. Within the grains consisting of the solid solution the tin may he selectively attacked, a process best known in brasses, and called dezincification.
Bronzes which contain more than about 10 wt%tin consist of two phases known as cu (copper rich) and 6 (tin rich). The 6 phase consists of CuaSn with about 31.8% tin. Corrosion (27) usually commences a t this phase on the outer surface and occurs intergranularly, so that eventual disintegration may occur. Corrosion of the a nhase mav also occur. These nrocesses would he expected from thermodynamic considerations because the more reactive phase is preferentially attacked. Under certain circumstances, however, the reverse may occur and the a phase corrodes as in high alloyed bell bronzes. Careful analysis of various alloys (27) has shown that the concentration of zinc oresent in the hronzes was imoortant. This 6 phase was pref&entially corroded when the ailoy contained 1to 9% zinc, hut when it had onlv traces of zinc it was less reactive and the a solid solution was attacked. Under immersed conditions it is possible that both phases are corroded together within a particular alloy. In some of the alloys, the copper that is dissolved together with the tin is then precipitated on the onter corroded regions. Although on bronze the normal patina of cuprite and malachite is stable and nrotective. there is often a laver of cunrous chloride below it on.the metai surface. In dry conditions this chemical is stable, but if moisture can penetrate through the outer layers the cuprous chloride becomes unstable, absorbs water, and is converted to basic cupric chloride which can produce powdery light green spots on the surface. This condition is known as bronze disease. The reactions mav he reoresented by the equations:
--
+
2CuClt Hz0 CUZO 2HC1 2HC1 t 2Cu t %01 2CuCI + Hz0 Thus the overall reaction is the conversion of copper in the alloy to copper oxide via copper chloride:
-
2Cu t 'Iz02 Cup0 The process is antocatalytic because the acid is liberated during the reactions and is free to attack more metal. Acetic acid from wood shavines has also been fonnd to cause attack of many bronzes a t the Fitzwilliam Museum in Cambridge. The acid combined with the cuprous chloride to give soluble copper acetate which was then converted to basic carhonate or sulfate and the acid was liberated and caused further attack. There are several techniques which can he used to reduce or eliminate bronze disease, and these are discussed later. Protection
The existing patina on many structures is decorative as well as protective. The surface layers formed may well have given protection to the underlying metal over hundreds or thousands of years. Kalish recommends that new monuments are not covered with wax, lacquer, oil, or lubricant but that corrosion should he allowed to start. He considers that the oatina that forms is stable and protective (28). Rieder has studied the corrosion oroducts that have formed on bronzes (29) and found that, as well as the usual basic salts, compounds such as gypsum had been formed by reactions between dust and atmospheric contaminants. I t isinteresting to note that he found several strata within the surface layers, including one containing soot which was characteristic of the winter season. As a protection against further attack he recommended a coating of oil and wax over the natural patina. Regular washing with water containing a detergent was fonnd to he beneficial. Bronze disease has been investigated since 1894 (25,3&33) and may be reduced or eliminated bv various techniques in(11 Keeping the hronze article in a dry atmosphere with a relative humidity ofless than 35%to prevent hydrolysis (34,35). (2) Removine " all the cuorous chloride hv a suitable immersion
treatment such as with sodium sesquicarbonate (35,361.
(3) Making the article cathodic in distilled water and removing the chloride ions from active areas (37). Volume 57, Number 4, April 1980 / 279
(4) Treating the active areas with a silver salt (such as silver oxide), which can seal the active area with a coating of silver chloride
(hornsilver) (35,37,38). (5) The unstable cuprous chloride can he reduced by applying zinc filings and strong sulfuric aeid to the local areas of disease (39): (6) The use of zinc nib of wire in hydrochloric aeid to give a local current at disease spots, then repeatingwith phosphoric acid and then sodium carbonate solution (40). (7) The use of a corrosion inhibitor,benzotriazole, to combine with the euprous chloride to form astable copper benzotriazole complex (34.41). More details of these techniques to prevent bronze disease can he from the~ aouro~riate articles. ..ohtained .. . - ~ ~~ . ~~
~
..
In general the protective nature of the patina on copper and bronze can he reinforced hv additional surface treatments. T h e application of special iarnishes to protect statues has been successful in the National Archaeological Museum in Athens (42) and other museums (43).A treatment involving immersion or spraying with a hot solution of benzotriazole followed by drying and coating with a varnish (44)or resin (45) has been found to he particularly beneficial. D e t a i l s u i u ~ h t~r rrh n i q u ~ sused rtr preserve c o r d e d a r t i f a c t s cnn be d ~ t a i n ~frmn d numeruu; houks \ 16 -5Q1and nrticler. Literature Cited (I1 Gowhnd, W. J..lnsl. MeLob 7.23 (19121. (2) Mareehsl, T. R.. Metour-Corrosion Ind., (3971 377 (1956). (3) Fink,C. G.,sndPolushkin. E. P.. Metols Tech. 3, (2)693(19361. (4) Varaufskis. G. J., Hirt. Met.. 8. (2) 95 (1974). (5) Collins, W. F., J . Insl.M~lob,45.23 (1931). (61 Boni, B.. Fonderio Ital.. 22.361 (1973). (7) "ChillLCast Tin Bronzes. (Editors Hanson, D..PeU-Wdple. W.T.).Arnold.landon,
280 / Journal of Chemical Education
(13) Young, W. J., "Art and Technolom: A Sympasium on Claraical Bronzes'' (Editors: Dmringer J..etal.).M.I.T. Prelis,Cambridge. Mass., 1070,p.35. (14) Char1es.J. A.. Arehasomefry. 16,105 (1973). (15) Gettens. R. J., '"Art and Technology: A Sympaium on Claaaicd Bronzes" (Editor: Doeringer S. et el). M.I.T. P ~ e s Cambridge, s Mass.. 1970. (16) Pilling, N. B.,andBedworthR.E., J. lnsf Motob, 29,529 (1923). (17) Collinga,W. F., J.lnsl. Melola, 44.389(1930). (181 Daubree,A:, C.R. Acod. Sci, Paris, 93.572 (1881). (19) &born. D. H.. Moterials in DaOn Engineering, 1963, p. 8(1. (201 Otto. H.. Fmibergor Forschungsh, 837.66 (1959). (21) Vernon, W. H. L a n d Whitby L.J . Insf Metals, 42.181 (1929): 44.389 (19301. Vernon, W. H. J., J . Insf. Mefols, 49,153 (1932). (22) Markovic, T., Sevdic. M., and Rubinic, L., Werksf u Korroaion, 11,37 (1960). (23) scholes,~.~ . , a n d ~ a c oW. b , R.,~ l n s r~. e t o l s98,272(1970). , (24) Raivef, N. S.. Trons.lndionlnst. Met., 29,254,305 (19761. (25) Gpttena,R. J., J. CHEM. EDUC..28.67 (19511. (26) Cheng,C.F.,andSchwitter, C. M.,Amer. J . Archosology, 61.351 (1957). (27) Wemer,D.,Plakf. Mdallogr.. 4.3i1967). (23) Kalish, M. K., Prirodn, l L 6 1 (1974). (29) Rieder,J.. Werkd. u Korrosion. 23.1097 (19721. (30) Berthelof,M.,C.R.Acad.Sei. Paris. 118.768i1S941. (31) Csley, E. R.. Proc. Phil. Soe., 84.689 (1941). (32) 0rgan.R. M.,Sfud. Coueruotion, 8.1 (1963). (33) Smith,C.S.,Archoenmelry, 16.114 (1976). (34) Madsen, H. B.,SLud. Conseruation. 12, I63 (1967). (35) Organ. R. M., Museum J., 51.2 (1961). (36) Oddy, W. A. and Hughes. M. J., Stud. Conseruolion, 15,183 (1970). (37) Gettens, R. J.. Mausrion, 35-36.119 (19361. (38) Fsltermrier, K. J., Propprioior, 16,33 (1970). (39) Plenderlcith, H. J.,'lThe Conaervstionof Antiqueaand WorLaofAn,"OxfordPr~. 1962.p. 193. (40)Evans. U. R.. Chsmistniondlndusfry (London) 710 119511. (41) Msdsen. H. B., Stud. Co~emotion.16.120 (19711. (42) Varoufakia. G. and Stathia, E.C.,MelollurRico, 83.141 (1971). (43) Muehlethder. B., Nolurwlss8lsnsch~/k~n,60,382 (1973). (44) Marabelli, M. aod Guidohaldi,F., Quad. Ric. Sci.. 81.95 (1972). (45) Mambelli, M., Tarrnatecnica, 2'7,302 (19731. (4s)'Cormionand Metal Anifaetr. ADUogue Batmen Conservator8 and Arehaeologhtr and CormsionSciontists," (Editors Br0wn.B. F..et d l . Naf.Bur. StandardaPublication No. 479, W~shingonOC,U.S.A., 1977. (47)"Tho Con~ewatiiiof Antlquifiessnd Works of Art," Plenderleifh. H. J.. and Werner. A. E. A , Oxford Uniu. Presa, London, 1971. (48) "Recent Advances in Conservation," (Editor Thomaon. G.). Butteworths. London, 1963. (49) "Consarvetion in Field Archaeology," (Editor: Dowman, E. A,), Methuen, London. 1970. (50) "Art and Technology," (Editor Doorlnger, S. etal), M.I.T. P-, Cambridge, M a s , U.S.A.and London, 1071.
