Gold

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Contributors: Ainslie Harrison, Kim Cullen Cobb, and Harriet "Rae" Beaubien
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Copyright: 2011. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works. The Objects Group Wiki pages are published for the members of the Objects Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.



THIS ENTRY IS A DRAFT



Begin with a brief summary or introduction (1-3 sentences) to your material of object type (most likely the title of your page, for example glass, ivory, basketry ). Don't forget to add images as appropriate!


Materials and technology

This section should be used to provide a basic synopsis or refresher of the topic being discussed. Many materials and object types already have excellent references in the form of journal articles, books, websites, Wikipedia entries, and more. Rather than repeat all of that information here, this is a good place to reference significant publications or resources in summary and refer readers directly to the original sources.


===History ===
If appropriate: brief summary or introduction to historical context, art historic background, function, use, etc.

Materials

Raw materials; composition; definitions; parts; types; substrate(s); surface decoration; finish; chemical, optical, physical, or thermal properties; etc. *Remember:

Sub-subheadings use this formatting

Type your text here.

Technology

Gold can be made into three dimensional objects (e.g. jewelry, coins, inlay, votive figures, armor, etc.) or it can be applied to surfaces of other materials as gilding. Due to the ductility of gold, it can be hammered to a very fine thickness. Once it reaches the thinness of a fine foil, it is called gold leaf. Gold leaf applied to surfaces with an adhesive material (e.g. oil or hide glue) followed by burnishing is called gilding. Gilding can be found on paintings, architectural elements, furniture, and sculptures. It can also be used on glass to create mosaic tesserae. A thin gold surface layer can also be added to metal objects through electrochemical plating, chemical plating, mercury gilding, and fusion gilding (Scott 2000). A gold surface can also be achieved on gold alloy objects through a surface enrichment process such as depletion gilding or mise en couleur, which is essentially the removal of copper at the surface through pickling with acids followed by burnishing of the porous gold surface. Objects made of gold or gold alloy can be formed by hammering or casting. The same casting techniques used with other metals can also be used to cast gold (see section on Metals Technology). Different alloys of gold have different characteristics when casting. For example, alloys higher in copper have a lower melting temperature and become harder and less ductile upon casting. Varying the alloy composition and therefore the melting point can also be used to “cast on” additional sections or components to a gold alloy piece. This technique can be used to create bimetallic objects. Selective surface enrichment, on the other hand, can be used to create the appearance of a bimetallic object at the surface. Gold objects can also be formed by hammering from gold ingots, bars, blanks, or nuggets. The higher the gold content, the more malleable the alloy, which in turns allows a thinner sheet to be produced. Periodic annealing is necessary to relieve stresses in the metal, otherwise cracks and breaks can occur. Gold sheet that is thicker than leaf can also be used to cover other materials as sheathing, plating, or overlay, either by burnishing over the substrate to cause the gold to conform to that shape, crimping, or riveting.

Joining Techniques

The procedures for joining gold and gold alloys include hot working processes such as fusing, soldering/brazing, the formation of eutectic systems, and sintering (Harrison et al. 2012). Cold working methods include crimping, burnishing, and riveting.

Fusing is a metal bonding process using heat to unite two homogeneous metals without the addition of other materials. Adjacent surfaces are heated until they become fluid and diffuse into each other. This process takes place at the liquidus or melting temperature of the metal. Heat is rapidly transmitted through gold, silver and copper, owing to their superior thermal conducting properties, so the possibility of melting the entire metal piece during the fusing process is greatly increased (Brepohl 2003). Although lack of variation in the metallic bond may make detection difficult, tell-tale signs of a roughened surface or wavy edges can be reasonable visual indicators of a fused seam.

Soldering (alternatively called hard soldering or brazing when higher temperatures are required) is a method of joining metals by using heat to flow an alloy of a lower melting temperature metal between the two surfaces of a metal with a higher melting temperature. For gold and gold alloys, the solder is frequently formulated of gold, copper and silver in a ratio that reduces the melting temperature below that of the original alloy sufficient to flow the solder without melting the object. The solder required to achieve this disparity in melting temperatures may not be visible under magnification. However the compositional variation between the solder and original alloy may be measured by a variety of analytical techniques including SEM-EDX (see Scott and Doehne 1990, Duval and Eluere 1986, and Demortier 1988).

Another method of joining gold elements is the use of a eutectic system to form a bond. Eutectics are also referred to as proto-brazing ([[#ref10|Griffin 1986), reaction-soldering (Echt and Thiele 1995), or – when attaching small granules of metal – granulation (Wolters 1981). Gold alloys with a low percentage of copper will form a eutectic when copper salts in an organic adhesive are applied between the surfaces to be joined, followed by heating. Carbonization of the adhesive reduces the salt to metallic copper, which forms an alloy with the original metal. The localised reduction in melting temperature results in bonding at the points of contact. The slight change in composition of the eutectic system has been identified in several studies using both surface analysis with SEM-EDX and analysis of cross-sections (Eluere 1989, Duval et al. 1989).

A bond can be formed using powdered metal of the same composition as the bulk metal, which is packed into the seam with an organic adhesive. This process is called sintering. When the metal powder is heated to 65-80% of its solidus temperature the grains coalesce, closing the seam. Sintering may be distinguished by the granular appearance at the surface of the join; however, metallography highlighting the dendritic structure of the metal in the seam is a more reliable indicator (Echt and Thiele 1995).

Deterioration

The deterioration of gold is highly dependent on what it is alloyed with and in what quantities. Relatively pure gold will only corrode in aqua regia and selenic acid. Under alkaline conditions, gold can react with sodium or potassium cyanide to form soluble complexes. Gold is also soluble in mercury, forming an amalgam at room temperature, which is useful for gilding or alloying. Under normal conditions, however, gold is incredibly stable and is more often susceptible to damage from mechanical pressures (scratching, distortion, etc.) rather than corrosion and other chemical processes. If it is in association with other materials, however, gold may become encrusted with corrosion products or accretions from other metals (e.g. gold inlaid in iron).

If alloyed with less than 55% gold in the alloy (Selwyn 2004), then a number of different deterioration processes can occur. According to Scott (Scott 1983), there are at least four different pathways of degradation for archaeological gold-copper-silver alloy artifacts (an alloy called Tumbaga in Central and South America), which include tarnishing, dissolution of anodic constituents, stress corrosion cracking, and order-disorder transformations. Depending on the artifact structure, forming techniques, and burial or storage conditions, any combination of these may be present.

Almost all gold-silver alloys are susceptible to tarnishing if exposed to atmospheric pollutants. The development of a grey-brown film and whisker-like crystal growths is typically due to production of silver sulfide when exposed to sulfur rich pollutants like hydrogen sulfide (Scott 1983). Studies have found that a reddish tarnish layer may be attributed to silver gold sulfide (Frantz and Schorsch 1990). Tarnishing of silver in gold-silver alloys can occur even when very low percentages of silver are present, and up to 95% gold content may be required to inhibit tarnishing in the presence of sulfides (Scott 1983).

In the burial environment, gold-copper-silver alloy artifacts are especially prone to deterioration by dissolution of anodic constituents due to the types of metals present in the alloy microstructure (Scott 1983). Gold, copper, and silver are relatively distant from each other in the electromotive series, and therefore have very different electrochemical potentials. The presence of metals with such different electrochemical potentials adjacent to each other can lead to problems from preferential anodic corrosion. Due to its lower electrochemical potential, copper serves as the anode in the corrosion process, dissolving as electrons are lost, releasing metal ions.

Stress corrosion is another major factor in the deterioration of Gold-Copper-Silver artifacts. Due to the internal stresses that develop in ternary alloys, cracking and embrittlement along the grain boundaries are common problems found in archaeological gold alloy artifacts. As discussed earlier, anodic dissolution can cause corrosion between grains in the metal microstructure. If allowed to continue, mineralization between grains can result in structural weakness along these boundaries. Stresses induced during the forming process, however, may also contribute to those caused by anodic dissolution. Burnishing and cold working gold-copper-silver objects can introduce stress into the structure of an object even before it is used (Scott 1983).

Yet another source of potential deterioration is order-disorder transformation, which can include ordering, age hardening, and spinodal decomposition. Ordering occurs when ordered crystal phases such as AuCu crystallize in the alloy. While this most often occurs during the forming process, it is also possible for ordered phases to develop over long periods of time (Scott 1983). Ordered phases are less ductile than the normal crystal structure and so are generally avoided by quenching the metal during the initial working. If ordering does occur, however, the loss of ductility and increased hardness can cause enriched gold areas to become brittle and fracture easily (Scott 1983).

Age-hardening and spinodal decomposition are also changes in the metal microstructure that can occur during forming or over long periods of time. Age-hardening is a diffusion process in which small particles of an impurity phase precipitate out in the solid metal. While age-hardening, also called precipitation hardening, can be desirable to harden metals during forming, over time it can lead to embrittlement and damage (Scott 1983). Spinodal decomposition instead refers to the separation of the alloy constituents into distinct regions with different concentrations. This material segregation leads to higher hardness values and the differences in composition can also lead to enhanced anodic dissolution (Scott 1983).

Conservation and care

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

  • To learn how you can care for your personal heritage, please visit AIC's Resource Center.


The conservation and care of gold is highly dependent on whether the object is made of high purity gold or whether gold is present in lower quantities in the alloy.

Documentation

Gold is very high density (high atomic #) so it can be difficult to x-ray if the object is very deep/thick. Often the best way to learn about structure and fabrication is through metallography; however, this is destructive testing as a small sample must be removed from the objects, mounted in epoxy, and polished. In addition to compositional analysis (XRF, PIXE, LA-ICPMS, etc.), noting the color of the gold can be useful in understanding the type of alloy.

Preventive conservation

Gold can be very soft and easily scratched or distorted, so proper handling and storage are very important. Refer to [[Gold can be very soft and easily scratched or distorted, so proper handling and storage are very important. Refer to Preventive Care section of main AIC wiki.

Interventive treatments

Interventive treatments depend on the type of condition issues present. Generally high purity gold requires little treatment unless it is crushed or distorted or else other materials in association with it have deposited on the surface. Alloys, however, can have a wide range of condition issues requiring different types of treatment.

For gilded materials, refer to the relevant specialty group (WAG / ASG / PSG) Wiki sections.

Cleaning

High purity gold can usually be cleaned with solvents such as a non-ionic detergent in distilled water, ethanol, or acetone (Hockey 2001).

Reduction of corrosion products on gold

Archaeological gold objects encrusted with corrosion products from associated metals can be carefully cleaned under magnification using chemical and/or mechanical methods. Due to the softness of the metal, mechanical cleaning should be done extremely carefully and always with a microscope (Cronyn 1990). While this will help to remove dirt and encrustations, corrosion product can be removed using a variety of chemical methods. Complexing agents such as ammonium citrate or the di-sodium salt of EDTA can be used to remove copper corrosion on the object surface; however, in some cases these reagents may be too strong and could potentially damage the copper below the surface (Scott 1983). Dilute formic acid acts more slowly and can be used safely to reduce copper corrosion products so long as care is taken not to let it seep under the surface of the object (Scott 1983). The method of removing tarnish from gold alloy artifacts depends upon the condition of the artifact and extent of tarnish. For light tarnishing, the common practice of using a mild abrasive such as calcium carbonate, followed by degreasing and lacquering, may be appropriate. If the artifact is brittle, however, a complexing reagent such as ammonium thiosulphate can be applied in a dilute solution using swabs (Scott 1983). Acidified thiourea has also commonly been used to remove tarnish from gold and silver surfaces (Wharton 1989). Recent research, however, has shown that it is difficult to impossible to remove all of the residues from thiourea from silver and a number of permanent changes to silver surfaces can occur, including change in colour, etching, and surface enrichment (Contreras-Vargas et al. 2013). O’Connor et al. (2015), however, found that thiourea was the least damaging method for removing tarnish from a gilded silver waterpipe as even the gentlest mechanical cleaning removed some of the gold from the surface. If the object is severely degraded, chemical methods should not be used for tarnish removal as pieces of the artifact can detach during the cleaning process.

Stabilization

For archaeological gold, consolidation of either the base alloy or the gilded surfaces may be necessary depending on the condition of the object. If the entire object is severely degraded or mineralized, immersion in a consolidant such as Paraloid B-72 may be advisable. If the equipment is available and the artifact is strong enough to withstand the additional pressure, vacuum impregnation can be performed to ensure maximum penetration. If only the surface is brittle and flaking, an adhesive such as Paraloid B-72 can be applied in solution and allowed to wick under the detaching pieces, holding them to the substrate upon drying.

Structural treatments

If the object is fragmentary and requires adhesion of pieces, special care should be taken to ensure that the individual fragments are stronger than the adhesive used. If this is not the case, or cannot be ascertained, the fragments to be joined can first be strengthened by consolidation in a suitable resin (Scott 1983). A resin adhesive can then be used to join the pieces.

Conservators at the British Museum also consider creases, tears, and exposed broken edges to be threatening to the structural integrity of delicate gold objects and will therefore attempt to repair these problems to ensure lasting stability (Hockey 2001). In these case the defects are gently manipulated into the proper alignment, adhered using an appropriate adhesive, and supported with Nylon gossamer or an appropriate tissue backing material (Hockey 2001). Under no circumstances, however, should soldering be used to re-attach gold or gold alloy fragments. Lead-tin solder can dissolve gold readily, causing holes to develop (Selwyn 2004). In addition, intermetallic compounds of gold with tin and lead can form, resulting in a rough and brittle solder, subject to cracking (Selwyn 2004). For heavily distorted gold sheet or sheathing, some re-shaping may be possible if there isn’t significant work hardening and the gold content is high enough. In this case, a conservator with expertise in metal forming can gently re-shape the metal using a variety of rounded tools. Heating to anneal the metal should not be attempted as it can cause irreversible changes to the microstructure of the metal.


====Aesthetic reintegration ====
Loss compensation, fills, casting, molding, re-touching, finishing, etc.

Surface treatments

Since archaeological gold objects alloyed with significant quantities of copper are also susceptible to active copper corrosion, much in the same way as bronze artifacts, a corrosion inhibitor can be used to halt further deterioration. High relative humidity (RH), acidity, and contaminants such as chlorides can accelerate degradation and lead to potentially devastating outbreaks of active corrosion. If these factors can be completely avoided through preventive conservation methods, such as storage in inert materials at low RH, the use of corrosion inhibitors can be avoided. If ideal environmental conditions cannot be guaranteed, however, gold-copper alloy artifacts can be treated with the corrosion inhibitor benzotriazole (BTA), either in a 3% solution with ethanol or as a component in a consolidant. The benzotriazole molecules form complexes with Cu(I) atoms, which in turn form a Cu-BTA polymer film over the copper surfaces of the artifact (Sease 1978). The polymeric film acts as a cathodic and anodic inhibitor and protects the metal surface from water and contaminants (Scott 2002). After treatment with BTA, a protective lacquer should be applied in order to help protect the Cu-BTA film on the artifact and to prevent contamination from handling or penetration of moisture.

Gold alloy and gold plated objects with significant copper and/or silver content that have not been treated with BTA may also be lacquered to protect against contamination and moisture. Coating can be carried out following standard procedures for lacquering metals.


====Other treatments====
If appropriate: This section heading should be used if the treatment being discussed does not fit into any of the other heading categories.

References

Brepohl, E. 2003. The Theory and Practice of Goldsmithing, Brynmorgen Press, Portland.

Contreras-Vargas, J, J L Ruvalcaba-Sil, and F J Rodríguez-Gómez. Effects of the Cleaning of Silver with Acidified Thiourea Solutions. ICOM Metals 2013. Pg.223

Cronyn, J. M. 1990. The Elements of Archaeological Conservation. London: Routledge.

Demortier, G., ‘The soldering of gold in antiquity’, in Materials Research Society Symposium Proceedings No. 123, Symposium held April 6-8,1988 in Reno, Nevada, eds. E. Sayre, P., Vandiver, J., Druzik and C. Stevenson, Materials Research Society, Pittsburgh. 193-198.

Duval, A.R. and Eluere, C. 1986. “Solder characterization on ancient gold artifacts with the electron microprobe”, Scanning electron microscopy 4, 1331-1335.

Duval, A.R., Eluere, C., Hurtel, L.P. and Menu, M. 1989. ‘The use of scanning electron microscopy in the study of gold granulation’, in Archaeometry: Proceedings of the 25th International Symposium. 325-333.

Echt, R. and Thiele, W.R. 1995. ‘Sintering, welding, brazing and soldering as bonding techniques in Etruscan and Celtic goldsmithing’, in Prehistoric Gold in Europe: Mines, Metallurgy and Manufacture, eds. G. Morteani and J.P. Northover, Kluwer Academic Publishers, Boston. 435-450.

Eluere, C. 1989. ‘A ‘gold connection’ between the Etruscans and early Celts?’, Gold Bulletin 22 (2) 48-55.

Frantz, James H., and Deborah Schorsch. 1990. ‘Egyptian Red Gold.’ Archeomaterials 4, p133–52.

Griffin, J.G. 1986. ‘An investigation in joining metals’, in Metalurgia de America Precolombina, ed. C. Plazas, Banco de la Republica, Bogota. 369-376.

Harrison, A.C., Cullen Cobb, K., Beaubien, H.F., Jett, P., and J. Mayo. 2012. A Study of Pre-Columbian Gold Beads from Panama. In Historical Technology, Materials and Conservation: SEM and Microanalysis, eds. Nigel Meeks, Caroline Cartwright, Andrew Meek, Aude Mongiatti. Archetype: London.

Hockey, M. 2001. Approaches to the Conservation of Gold at the British Museum. In Metal 2001: Proceedings of the ICOM Committee for Conservation Metals Working Group: ICOM, 263-269.

O’Connor, A., J Lauffenburger, M Craft, and G Gates. 2015. “Silver or Gold? Surprising Challenges in Cleaning a 19th Century Persian Water Pipe.” Talk given at 43th Annual AIC Meeting in Miami, FL. http://www.conservators-converse.org/2015/06/43rd-annual-meeting-objects-specialty-group-session-may-16-2015-silver-or-gold-surprising-challenges-in-cleaning-a-19th-century-persian-water-pipe-by-ariel-oconnor-with-julie-lauffenburge/

Scott, D. A. 1983. The Deterioration of Gold Alloys and Some Aspects of their Conservation. Studies in Conservation 28 (4):194-203

———. 2000. A Review of Gilding Techniques in Ancient South America. In Gilded Metals: History, Technology and Conservation. London: Archetype.

———. 2002. Copper and Bronze in Art: Corrosion, Colorants, Conservation. Los Angeles: Getty Conservation Institute.

Scott, D.A. and Doehne, E. 1990. “Soldering with gold alloys in ancient South America: examination of two small gold studs from Ecuador”, Archaeometry 32 (2)183-190.

Sease, C. 1978. Benzotriazole: A Review for Conservators. Studies in Conservation 23 (2):76-85.

Selwyn, L. 2004. Metals and Corrosion: A Handbook for the Conservation Professional. Ottawa: Canadian Conservation Institute.

Wharton, Glenn. 1989. “The Cleaning and Lacquering of Museum Silver.” In WAAC Newsletter, Volume 11, Number 1, Jan. 1989, pp.4-5.

Wolters, J. 1981. ‘The ancient craft of granulation: a re-assessment of established concepts’, Gold Bulletin 14 (3) 119-129.

Further reading

Eilertsen, K. 1981. A Mounting Technique for Fragile Metal Objects. Studies in Conservation 26 (2):77-79.

Untch, K. 1990. Two Gold Alloy Birds from Central America. In Papers presented by Trainees at the Fifteenth Annual Art Conservation Training Programs Conference, April 28-29, 1989.

West, R. C. 1994. Aboriginal Metallurgy and Metalworking in Spanish America: An Overview. In Quest of Mineral Wealth: Aboriginal and Colonial Mining and Metallurgy in Spanish America, edited by A. K. Craig and R. C. West. Louisiana: Geoscience Publications




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