The Electroforming of Gold and Its Alloys

7/27/2019 The Electroforming of Gold and Its Alloys

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THE ELECTROFORMING O F GOLDAND ITS ALLOYS

Anselm T. Kuhn * & Leslie V. Lewis *** Faculty o f Science & Technology, Harrow College, Harrow HA I 3TP, U.K.** B.J.S. Electroforming Ltd., London NW 6, U.K.

Electroforming is an area of gold technology which is of growing interest toworkers in fields as diverse as gold jewellery manufacturing and dentistry. Inthis short overall review of the subject, the authors focus on developme nts in bothapplications and techniques over the past two d ecades.

Electroforming can be described as a manufacturingprocess based on th e electrodeposition of a metal. U nlikeelectroplating, how ever, w here the deposited metal coversa substrate, in electroforming, at the end of the depositionprocess, that substrate (which is known as the mandrel),is removed, leaving the electrodeposit as an object in itsow n right. Th us electroforming, though it superficiallyresembles electroplating, is in fact a far more advancedtechnology. Not only is the p reparation of the mandrels acomplex procedure, but also many of the difficulties in-herent in electrodeposition, such as current distributionand the incidence of internal stress, become critically ob-vious in electroforming, w hereas in electroplating, thismight not have been the case. For every hundred com-panies in the electroplating industry, perhap s one mightinclude electroforming among their activities.In many w ays, gold and its alloys might be though t tobe ideal candidates for eleetroforming. The ability of thetechnology to p roduce sections far thinner than is possibleby casting, coupled w ith the near-p erfect reproduction ofsurface detail, are two m ajor advantages in manufactureof decorative gold objects. In fact, the electroforming ofgold and its alloys has, until the last five years or so, beenlittle more than a curiosity and only recently has th ere beensignificant comme rcial activity in this area. Th us thestandard reference w ork on electroforming by Spiro [1]published in 197 1 barely mentions the subject.Th e reason for this can be largely attributed to lack ofdevelopment of suitable electrolytes for electroforming ofgold. W hereas gold-plating has long been established, at-tempts to build up thicker gold deposits met w ith failure,

those deposits being prone to cracking. In the last fewyears, this situation has imp roved, and solutions capableof yielding thick deposits of gold and at th e same time dis-playing reasonable stability, have been developed. Thissuccess has prom pted further research into solutions forelectroforming of gold alloys w hich h as already been part-ly successful, but it is clear that further dev elopments w illbe announced before long.Solutions for Gold Electroforming

Most solutions for the electroforming of gold, as for itsplating, are based either on cyanides, sulphites, chloridesor a combination of these. C itrate solutions, though usedfor gold plating, app ear not to hav e found favour forelectroforming. Some suggested solution compositionsfor electroforming of gold are quoted by Reid and G oldie[2]. H ow ever, as far as the present authors are aware, theseare now of historical interest and are not used in practice.For obvious reasons, there is a degree of proprietarysecrecy cov ering those solutions wh ich are in commercialuse. In spite of this, some interesting information is avail-able.Rogers [3] h as described a bath containing 14.lg/1potassium gold cyanide, 18.3 1 gotassium cyanide,14.1 g/lpotassium carbonate and 11 .4 g/l boric acid. Op erated ata current density of 3.2 A /dm 2 at 65C with agitation, thedeposition rate is 100-125mlh. He points out that owingto excessive rate of w ater loss under these conditions, it ispreferable to work at 60C to give a deposition rate of75 1.lm/h. T his results in a virtual halving of cathode ef-Gold B ull., 1988, 21(1)7


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Fig. 2 8 carat gold jewellery electroformed with the Auruna process (courtesy Degussa)

Fig. 1 Hand bound copy of the American Treaty of Independence withseals, cornerpieces and skippet cover insert electroformed in 24 carat gold(courtesy B.J.S. Electroplating Co.)

ficiency in the latter case. Too high a current density givesa brow nish burnt effect.The coronet illustrated in Figure 5 is stated to hav e beenformed in a neutral (pH=6 .5, no free cyanide) gold solu-tion containing not less than 2 8g/l and up to 3 6g/l gold,and based on gold p otassium cyanide neutralised w ithphosphate.Sulphite gold electroforming solutions are based onsodium, sodium and ammonium or potassium ions, anduse arsenic as grain refiner. DOS 2 249 658 (1972)describes the electroforming of gold layers up to 600 mthick, using such solutions.

The Electroforming of Gold AlloysIn the main, there are two approaches to the electro-forming of gold alloys, based on a). simultaneous and b).sequential deposition. There are also rep orts of proceduresw hich do not fall into either category.

Simultaneous DepositionHere, the alloy is electroformed by the simultaneouselectrodeposition of the two, and in some cases three, me-tals in question. The main problem with this approach isthat found in most cases of electrodep osition of alloys,namely that th e comp osition of the deposited alloy is afunction not only of the solution composition, but also ofcurrent density and temperature. Ina manufacturing situa-tion, an error in one direction results in the (w asteful)deposition of a higher carat alloy than that specified, wh ilein the oth er direction lies the danger of producing sub-standard alloy. These problems are further multipliedw hen th ree-dimensional articles are to be formed, in thatthe throw ing pow er of the individual metals differs.The first commercialprocess claiming to over-come at least some of thesedifficulties w as launchedby O.M.Z In this, thedeposition p rocess is con-trolled by a computer,which regulates bathtemperature to better than1C as w ell as using inputfrom sensors to m aintainthe concentrations of eachof the m etal ions by actuat-ing dosing pumps. Thecomputer also logs totalcharge passed and takesinto account the surfacearea of the work being

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Fig.3 Computer-controlled gold alloy electroforming plant (courtesy Degussa)

formed. The 14 or 18 carat alloys w hich are so formed(copper and cadmium being the oth er components) requiresubsequent heat-treatment in an inert atmosphere, and th eoverall process in one calling for a major cap ital invest-ment. The composition of the electrolytes used in thisprocess is not stated, though S w iss Pat. CH 5 29 843 mayindicate th e underlying thinking.More recently, a German firm, Degussa, has announceda somew hat similar process. At present, this is capable ofproducing only 8 and 9 carat alloys with silver as a secondcomponent, though 14 and 1 8 carat solutions are said tobe under development. A significant advantage of theDegussa process appears to be the absence of any need forheat treatment. A dditionally, th e alloy comp osition ap -pears to be less sensitive to variations in depositionparameters. Degussa suggest that an over-caratage of 1point is sufficient to eliminate any local under-caratage,w hich w ould otherwise cause the item to fail a hallmark-ing test. An interesting point is the recommendation bythese manufacturers that, after forming, a thin flash of puregold be deposited. This suggestion is an indication thatvariations in colour can arise, presumably as a result ofimperfect current distribution.An altemative to the true simultaneous deposition of aternary alloy is described in Japanese Pat. 59/80 788 A2 .The electrolyte contains the three m etals gold, silver andcopper. H owev er, the rate of stirring is cyclically increasedand reduced. At h igh stir-ring rates, the deposit isabout 95% Au-Ag (ofwhich 80-90% is Au), whileat low stirring rates, thedeposit is about 95 % Au-Cu(of which 50-70% is Au).Each such layer is about0.1m thick and the processis comp leted until the re-quired thickness has beenreached. After this, the w orkis heat-treated.Moving somew hat awayfrom alloys to the subject ofco-deposits, Japanese Pat.JP 53/6935 discloses themanufacture of electricalcontacts made o f electro-formed Au-W. Tungstenparticles of ca 0.5mdiameter are coated withelectroless Pd and then dis-persed in the electroform-

ing bath. The resulting composite contained 13wt.% W .Sequential DepositionIn this approach, the alloy is formed by deposition ofeach of the m etals of the ally in turn. This cycle may takeplace once only or may b e repeated several times so thatthe dep osit consists of numerous discrete layers of the dif-ferent metals. Following this, the deposit is heat-treated sothat interdiffusion occurs, the result being a homogeneousmaterial.Technology

Gold itself may be electroformed in simple and conven-tional electroplating p lant. A n accurate am p-m inutemeter, high ly desirable for electroplating of gold, is ar-guably less imp ortant in electroforming wh ere directthickness measurement is easier. In US Pat. 4 28 8 2 98,Rogers describes a gold electroforming tank w hich,though p rimarily intended for forming dental crow ns upto 200 m thickness, could equally w ell be used for othersimilar small items. It consists of a tank divided into th reecompartments, one of w hich is the main electroformingsection, one being for pumping and filtration, the third forsolution storage. Facing the cathodic plane, on w hich areset a number of the trow n mandrels, is the anodic plane,into which are set a number of stainless steel baskets. Sheet

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Fig. 4 Selection of 24 carat gold electroforms including the scab bard-top of an Arabian da gger, a gold elephant,and the lid of a small box showing a coun try scene

gold is inserted into these baskets, to serve as an anode andprovision is made for sw itching in or out of circuit, someof these baskets, to create the appropriate anode-cathodesymmetry. Elsewhere (U S Pat . 3 997 6 37), the sameauthor indicates the use of pumped electrolyte beingdirected by a fine tube to the recesses of concave sh apesbeing electroformed.A lthough gold anodes have been used for electroform-ing of the pure metal, current practice is almost invariab-

ly based on th e use of p latinised titanium anodes. S tain-less steel bas also been used but is not recommended.Equipment for Electroforming of Gold Alloys

The tw o firms previously mentioned, each supply com-puter-controlled plants for the electroforming of gold al-loys.In one of these designs, the w ork is mounted on a rotat-ing `carousel' jig. There are tw o gold anodes, one insideand one outside thecarousel. Each piece ofwork (and there may beover 10 0 m ounted in thisjig) thus passes continuous-ly between the two anodes,so ensuring the best pos-sible metal distribution.The computer carries outthe functions previouslymentioned and controls thebath temperature, to aprecision of 0.1C . It is alsoconnected to an electronicbalance, the purpose ofwhich is explained below.As with the electro-deposition of almost all al-loys, the composition of thedeposit is a function of thecurrent density. This im-plies, in the production ofdefined carat alloys, that thesurface area of the workbeing formed must beknow n. Furthermore, espe-cially in the case of smallerpieces, the surface area willgrow as metal is depositedon the surface. The com-puter is interfaced with therectifier to allow a constantcurrent density to be main-tained. During the electro-forming, the work isweighed four times. Oneach of these occasions,great care must be taken tofirst wash and then dry thew ork before w eighing. Thefirst weighing is made priorto any electroforming, thesecond after about 25% of

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the required deposit has been laid down, the third aftersome 50%, and the final weighing is at the conclusion ofthe process. Th e computer calculates the w eight gain ineach case and compares it with the total charge passed, inampere-minutes. Because gold is so much denser than theother metals in the alloy, this calculation allows an es-timate of the alloy comp osition to be made and, dep end-ing on the result (too rich or too poor in gold), the com-puter automatically adjusts the current (and so the currentdensity) to the p lant. Other tech nical details are found in[8, 12]..Power Supplies

A lmost all reports of gold electroforming emp loy DCcurrent. In Japanese Pat. 58/1302 93 A 2, the dep osition isunder potentiostatic control w hich is stated to give goodresults, a 150 m gold-silver alloy deposit show ing littlevariation in composition. In US Pat. 4 343 684, Lechtzindescribes a long series of test experime nts, in some ofw hich p eriodic reverse current (60s forward, ca 4s reverse)w as used, but the concludes there w as not benefit andabandoned its use. In Sw iss Pat. CH 52 9843 too, currentreversal is used, the cathode:anode period being 5-10 to 1.It is suggested that the amount of ripple on the D.C . af-fects the composition of the deposit and that by varyingthis ripple between 0 .1 and 4%, a degree of comp osition-al control may be achieved. Th e early work of G ardam &Tidswell [4] extolling the virtues of current interruption inthe deposition of gold and its alloys, app ears to have beenlargely overlooked.Re cognising the difficulties in obtaining thick andstructurally- sound gold deposits, at least two workers haveadvocated the use of ultrasonics. Vrobel [5] found the per-missible deposition current density to increase to1.8A/dm2 , corresponding to a deposition rate of 54 m/h.There w as also an insrease in the microhardness of thedeposit to 22 0kg/mm . A few years later, the same ideawas expressed in US Pat. 3 427 231. While Vrobelreferred only to electroplating, the latter Pat.ent explicitlydescribes both plating and forming of gold. Th e authorsof the Pat.ent claim that, under the ultrasonic field effect,current densities may be increased to `10kA/m2 orindefinitely'. In fact, in their ex amples, a current densityof ca 0 .5kA/m 2 is used. They also claim that, underultrasonic agitation, the need for both filtration and addi-tion of organic agents is obviated.Physical Properties of Electroformed Goldand its Alloys

Very little has be en reported under th is heading. Vrij-hoef et al. [6] compare the properties of conventional gold

& m rt"

as used in dentistry w ith the electroformed metal. Table Iis adapted from th eir paper and suggests that significant(and potentially most useful) differences are found.An earlier paper on the ph ysical and mechanical proper-ties of electroformed Au-Cu alloys in the range 65-90w t.%, is by W iesner & Distler [7] w ho used both flatsheet and tubular samples for their measurements. Th estrongest alloy is found at around the 7 5w t.% composi-tion, but the effects of heat-treatment (which is consideredin some detail) can exert a considerable influence w ith 3hat 350 C , yielding substantially stronger specimens (UTS= 120kg/mm 2) than those treated for this time at 400 or450 C. Interesting structural information is also given bythese authors. Apart from these facts, Desthomas [8] givesthe follow ing information on gold electrodeposits: hard-

ness, 220 -300HV; density, 17; wh ile in US Pat. 3 427 231,Knoop hardness values of over 125 are mentioned. Thereare also sketches o f the metallurgical structures of goldformed at several current densities.

Fig.5 Coronet for the coronation of HRH the Prince of Wales (courtesy,Worshipful Company of Goldsmiths, London)

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Fig. 6 Gold electroformed matrix on tooth preparation (left), and (right)the same after ceramic veneering (courtesy Dr O.W. Rogers)

Applications of Electroforming to Gold andits AlloysTh e main ap plications of electroforming of gold andgold alloys can be grouped under th e following h eadings:Jew ellery and other Decorative, D ental, and Technicaland M iscellaneous

Jewellery and other Decorative ApplicationsBy far the most famous electroformed piece, is theCoronation Crown of H.R.H. the Prince of Wales, whichw as made in 24 carat gold by B.J.S Electroformers andEngelhard Ltd in 1967 from the design of Osman. Thetotal weight of this single piece before trimming off, w asin excess of 7kg, making it easily the largest goldelectroform on record. Th e magnificence of this item hastended to overshadow the thousands of far smaller itemsmade using the same technology. A number of patentsdescribe specialised app lications of electroforming. USPat. 4 464 2 31 relates to the electroforming of miniaturehollow gold sph eres. These are formed using spherical

plastic mandrels. After forming, the plastic is removed bymaking a tiny hole in the sp heres and heating to v olatiliseit. The exam ples in the patent are based on successivedeposition of copper, silver and gold which are then heatedto form an alloy of gold.UK Pat. App. GB 2 167 444, extols the merits ofelectroforming for gold jewellery manufacture, suggeststhat the m echanical properties of the metal are better thanthose of w rought gold, but offers no data. It does embodythe idea that gemstones, after embedding in soft gold, maybe more securely h eld by a subseq uent electroforming ofgold above the setting metal.A very old idea in electroforming is based on the use offlowers, leaves, insects or other naturally occurring ob-jects as mandrels. By encasing these in electroformedgold, the beauty of their sh ape is retained but in a per-manent and more durable form. In some cases, the organicmatter involved is 'locked-in' by the electroforming. InUK Pat. App . 2031 0 24, this is not the case and the patentdescribes how a flower w as electroformed in gold. Theflower is then autoclaved for 2 h (temp erature not stated)and after cooling, a jet of high-pressure water is forcedthrough the stem end to eject the disintegrated organicmatter.Y et another app lication of decorative ap plication ofgold electroforming is related to w atches. Three Japanesepatents indicate the possible scope h ere. For example, Jap.Kokai Pat. 59/8078 8 A 2 (to Seiko W atch Co.) covers theuse of electroforming for 'external parts of w atches' basedon gold alloys.Dental Applications of Electroformed Gold

The demands of restorative dentistry - the manufactureof jacket crow ns or inlays, each one custom-fitted and of

Fig. 7 Same as Figure 6, but also showing prepared tooth (left) (courtesyDr O.W. Rogers)

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Fig. 8 Electroformed gold crowns with porcelain veneer. Top photo: in-terior of crowns, with plaster models from which they were formed; top -right: crowns in place; lower right: electroformed gold inlay

a highly complex shape, with the additional requirementof thin sections, high mech anical strength and corrosionresistance and yet the low est possible cost, exp lains whythis is one of the oldest of the proposed ap plications ofelectroformed gold. A nd yet, in spite of some extremelyprom ising results, it must be adm itted that the de ntalprofession has been slow to adopt the idea, perhaps be-cause this is not a technology offered by the dentallaboratories to which dentists have access. The most en-thusiastic protagonist in all of this is Rogers [9], with atleast half a dozen publications and patents on the subject.More recently, Vrijhoef and co-w orkers in the Nether-lands have also advanced the subject [6] and it is clear fromthe w ork of both R ogers and of Vrijhoef et al. that theyhave been successful in taking their work from thelaboratory to the point w here it can and is being used byclinicians.Technical and Miscellaneous Applications ofGold Electroforming

The m ain emphasis here seems to be on app lications inthe field of electronics, instrumentation and communica-tions. Some of these are listed below:X-ray photomasks Jap 58/114427 A2Jap 58/200535 A2Infra-red grid filters Chanin [10]Micrometer Scale (50 m)Young & Ogbum [11]Bump circuitry US Pat 4 125 441The micrometer scale, w hich is manufactured by theUS Bureau of Standards, is formed by alternate depositionof layers of nickel and gold which, because their thick-nesses can be controlled and determined by th e chargepassed, may be used to calibrate electron microscopes.

ConclusionThe auth ors are in no doubt that after a very slow start,the electroforming of gold and its alloys is a technologycapable of considerable further developm ent and that as

this takes place, so will the scale of its use increase.References

1 P. Spiro, Electroforming, Robert Draper, Teddington, 197 12 F.H . Reid and W. G oldie, Gold Plating Technology,Electrochemical Publications, Ayr, 19743 O.W . Rogers, Austral. Dent. J., 1979, 24, 163-170 and1977, 22, 371-3724 G.E. Gardam and N.E. T idsw ell, Trans. Inst. M etal Fin.,1954, 31 , 418-4255 L . Vrobel, Trans. Inst. Metal Fin., 1966, 44, 161-1646 M .M.A . Vrijhoef, A.J. Sp anauf, H.H. Rengli, et al.,Tijdschr. Oppervlaktetech. M ater., 1985, 29 (4), 123-126, (Chem. Abs. 103:92818u)7 H .J. Wiesner and W .B. Distier, Plating, 1969, 56 , 799-8048 G. Desthomas, Tijdschr. Oppervlaktetech. M ater., 1984,28 , 92-95, 102;idem., Aurum, 1983, (14), 19-26 and 1983, (15), 17-219 O.W . Rogers, Austr. Dent. J., 1976, 21(6) , 479-487, 1980,25, 205-2 08 and 1979, 24, 163-17010 G. C hanin, A strophys. Space Sci. Libr., 30 (Infra-redDet'n); Proc. 5th ESLABIESRIN Symp., 1971, 219-224 (Chem. A bs. 83:18794k)11 J.P., Young, F. O gburn and D. Ballard, Plating, 1980,78(8), 27-2912 G. D esthomas, Galvanotechnik, 1986, 77 (6), 1342-45Gold Bult., 1988, 21(1) 23

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The Electroforming of Gold and Its Alloys