Touching Precious MetalsWalo WlchliSchweizerische Edelmetallkontrolle, Basel, Switzerland

The technique of touching as a means of assaying gold and other

precious metal alloys has been extensively used by goldsmiths for severalmillenia. In this article the historical origins of touching are discussedand the process, and its tools and applications today are reviewed.

Assay and hallmarking offices have the difficult taskof assaying, accurately and preferably non-destructively, valuable jewellery articles which areoften in finished form, polished, set with stones orpearls and which may even have undergone lometreatment to improve the appearance or technical pro-perties of their surfaces. Two methods are currentlyavailable for such assaying. The first involves thesampling by scraping for subsequent fire assay andthe second is the touchstone test. Both methods haveadvantages and disadvantages, but a detailed discus-sion of their comparative merits falls outside thescope of this article.

Swiss assayers have a long tradition of successfullytouching finished jewellery articles and have acquiredby experience the ability to use the method at an ex-ceptional level of accuracy. Thus, differences inprecious metal contents as small as 10 to 20 parts perthousand may be established with confidence by thetouchstone test. In testing against hallmarkingstandards, where negative deviations only are of con-cern, a better accuracy is normally required.Therefore, if the result of touching leaves the assayerin any doubt, he resorts to fire assay. It is not unusualfor the result of the Jatter, very accurate method to bewithin 5 parts per thousand of that of the preliminarytouchstone test!

HistoryTouching is among the oldest methods of assaying

for noble metals. It was first referred to by the Greekpoet and moralist Theognis of Megara (6th centuryB.C.) who made repeated symbolic mention of the`gold testing stone' in his verse. A manuscript of theArthagástra, the 5th or 4th century B.C. treatisesattributed to Kautilya, describes the use oftouchstones in ancient India. Pliny the Elder (23 to79 A.D.) recorded in his 'Natural History' that, usinga touchstone to test ore,

`Persons of experience. ..can tell in a moment theproportion of gold there is in it, how much silver, orhow much copper; and this to a scruple, their accuracybeing so marvellous that they are never mistaken.' (BookXXXIII, chapter 43).

The Leyden Papyrus X, from Thebes, which wasprobably written early in the 4th century A.D.,indicates that touchstones were employed by theancient Egyptians.

From the 12th century onwards, goldsmiths' andsilversmiths' guilds became established throughoutEurope. The charters and rules of these associationsmake extensive mention of touching in connectionwith the control of official standards for gold- andsilverware.

The introduction of printing, towards the end ofthe 15th century, led to the publication of the firstassay manuals which detailed the procedures oftouching, and gave recipes of acid mixtures anddescriptions of the touch-needles. However, it wasonly after the discovery of mineral acids by thealchemists that significant progress was made intouching and assaying. The use of nitric acid (firstknown as aqua fortis) and of aqua regia began in thefirst half of the 17th century. Some 150 years later,proper test acid solutions were formulated and thefirst reference to them was made by the Frenchchemist Vauquelin in 1799.

Scope of the MethodTouching is part of the art and science of assaying

noble metals. Because of its simplicity, the methodhas found widespread application for testing noblemetal articles of all types. It may be used with itemsof any shape, be they raw materials, semi-finishedproducts or delicate pieces of jewellery. Touchingdoes not cause significant damage, it requires aminimum of material and equipment, and may beperformed very rapidly. In most cases, the methodnot only identifies qualitatively the constituents of aprecious metal alloy, but also allows their quantitativedetermination. Touching is of particular interestwhen very little material is available for sampling andanalysis — a typical test requires only about 0.5 mg ofalloy. Moreover, it is well suited to the testing of inex-pensive articles, when the cost of fire assay cannot bejustified, or of very valuable pieces, for whichsampling by scraping is unacceptable.

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A gold alloy brooch and the accessories thatare required to test it by touching

In summary, touching is used by theassayer to: -(1) Identify precious metals in alloys(2) Identify many base metals in alloys(3) Assay finished articles(4) Test samples before using finer analy-

tical methods(5) Obtain some indication of the thickness of

precious metal coatings. (This is done by produc-ing from the sample a wavy linear rubbing of suf-ficient length to ensure wear through the coating.Then, the base metal portion of this trace isdissolved with an appropriate acid solution. Thelength of the remaining precious metal trace canfinally be interpreted by the assayer. The methodobviously requires much experience.)

The Procedure

Touching is essentially a comparative methodwhich consists of the following operations:

First, the object made of the alloy to be tested isrubbed under firm and even pressure onto a hard,acid-resistant, lightly oiled touchstone until adjacentrubbings cover completely and uniformly an area 20to 30 mm in length and 3 to 5 mm in width. Then,next to this, a similar film of metal is rubbed onto thestone from a needle made of a standard alloy of knownfineness and matching colour. It is important thatboth the test and reference rubbings should be ofsimilar Bizes and intensities. Finally, the rubbings arewetted with an appropriate test acid solution. Theacid preferentially attacks the base metals in the alloy,as well as silver. When sufficient time has elapsed forthe action of the acid on the rubbings, these are wipedwith filter paper. It is then possible to make a visualestimate of the fineness of the sample. Usually, thedegree of attack, and therefore the fineness, arerevealed by the coloration of the area of the rubbingwhich has been in contact with the acid: the lighterthis is, the higher the fineness, and vice-versa. This isbecause, as a rule, alloys of low fineness are morereadily and more completely dissolved than thosewhich are rich in precious metals. Similar behaviour

and appearance of the test and reference rubbings areindicative of identical fineness.

Accuracy

Generally speaking, approximate results only canbe expected from touching. Under optimum condi-tions, an experienced assayer can detect differences infineness of 10 to 20 parts per thousand. Among thevariables which affect accuracy are the quality of thestone, the type and range of needles at the disposal ofthe assayer, the strength of the acids he uses, his ex-perience and the ambient lighting conditions.However, the most important factors are the proper-ties and composition of the alloy tested. Very softmaterials, such as fine gold or alloys containing morethan 920 parts per thousand of gold, often do notleave rubbings on the stone, but 'smear'. Very hardalloys, such as some white golds, on the other hand,scratch the stone and are not amenable to touching.

The Tools and Accessories

It is apparent from the above that a touchstone,needles and acids are all the equipment that isrequired for touching.

TouchstonesIn contrast to earlier times, when only black and

dark grey stones were used, present practice employsboth black and red touchstones.

The red stones, which are particularly well suitedfor touching gold, are called radiolarites, as they con-sist of radiolaria. These marine protozoa havesiliceous skeletons which accumulate as radiolariansludge which is a red, clayish, deep-sea sediment.Eventually time and pressure turn this sludge into thesedimentary rock radiolarite.

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The black stones consist of a siliceous slate which isa hard, dense and brittle mineral consistingpredominantly of quartz and chalcedony. Its origin issimilar to that of radiolarite, but in this instance thesedimentary sludge has been submitted to muchhigher pressure and possibly exposed to heat.Siliceous slate with a beautiful black colour impartedby carbonaceous constituents is called lydite.

A good quality touchstone is of uniform deep blackor brownish red colour. It is free from veins or spots,fine-grained, acid-resistant and appropriately hard.The surface has to be ground to a matte finish andshould not be polished, otherwise the rubbings wouldnot adhere to the stone. Before use, the stone is lightlyimpregnated with a pure oil, as oil-free stones acceptthe metal rubbings only with difficulty and inpatches. Touchstones are easily cleaned with a fineabrasive and.-can be used almost indefinitely if wellcared for.

Touch-NeedlesThe alloys of known compositions, which are

required by the assayer for reference, are commer-cially available as touch-needles or stars.

A needle consists of a brass or German silver shankto one end of which a piece of standard precious metalalloy has been soldered. The gold and silver contentsof the standard alloy, and usually the details of otherconstituents in it, are inscribed on the shank.

Five- or six-pointed stars are made of base metalwith lens-shaped pieces of standard precious metalalloys soldered to the tips of the branches.

The higher the accuracy demanded from touching,the larger the number of touch-needles which must beavailable to the assayer. When touching gold alloys, itis essential to have needles not only of variousfinenesses, but also of various colours for eachstandard caratage. Thus, wide assortments of needlesof identical fineness and covering a range of coloursare found in all assay offices.

AcidsThe acid solutions used for touching gold, which

should under no circumstance attack the rubbings tooviolently, consist either of diluted aqua regia or ofnitric acid with copper chloride (Table I). These solu-tions are kept in bottles with stoppers suitably shapedto transfer the required amount of acid onto the rub-bings. Assayers also use acid-resistant plastic dropp-ing bottles.

Touching Gold

Touching may be used either to determine whetheran alloy contains any gold at all or to determine thefineness of an alloy known to contain gold.

Establishing the Presente of GoldAfter attack by diluted aqua regia or nitric acid, the

rubbings left by an alloy of unknown compositionappear as reddish brown or brownish black residues ifgold is present in small quantities. These residues aredissolved in stronger aqua regia which is thenabsorbed onto filter paper. A drop of tin chloridesolution is applied to the wet spot on the paper andthe presente of gold is revealed by the development ofa light to dark purple halo (Purple of Cassius).

Using a red, or especially a white, touchstone(Arkansas grinding stone, for example), this test ispositive for gold concentrations in the alloy down toabout 50 parts per thousand.

Coloured Gold AlloysIn order to determine the fineness of a coloured

gold alloy of unknown caratage, a rubbing is testedwith concentrated nitric acid, while a second one istested with the acid solution for 18 carat alloys. If thefirst rubbing is attacked, then it may be concludedthat the gold content of the alloy is below 500 partsper thousand. The situation is similar with the secondrubbing, where attack indicates a gold content lessthan 750 to 800 parts per thousand.

Table 1

Acid Solutions Used for Touching Gold Alloys

Alloy fineness, Solution composition,parts per thousand ml

800 to 1000 45 HNO3 + 3 CuCIZ* + 2 H20

650 to 800 (a) 40 HNO3 tont. + 1 HCI tont. + 15 H 20

(b) 41 HNO3 + 2 CuCI; - + 7 H 20 + 0.1 HCI

500 to 650 (a) 30 HNO3 tont. + 0.5 HCI tont. + 70 H20

(b) 30 HNO3 + 1 CuCl2' + 15 H 20

375 to 500 5 HNO3 + 15 CuCl2 * + 20 H 20

D = 1.284From a solution of 6 to 7 g CuCIZ in 20 ml H20

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Testing of three side by side rubbings of gold alloys (585 parts per thousand at the bottom and in the tentre, and 750 partsper thousand at the top)Left: The rubbings as made from gold alloysCentre: Attack by an acid solutionRight: After wiping the acid, the different behaviour of alloys of various finenesses is clearly visible

After this first approximate indication of thecaratage of the alloy has been obtained, a moreaccurate determination of the fineness is made bycomparing the behaviour of rubbings of the samplewith that of rubbings of appropriate needles whichshould match closely the alloy to be identified notonly in fineness, but also in colour. The process ispursued to the point where rubbings of the pieceunder test and of a particular needle behave in anidentical manner when submitted to attack by thesame acid solution. Apart from the fineness, themethod also provider information on the approximatesilver and copper contents of ternary gold-silver-copper alloys.

When touching for gold, it is important to bear thefollowing in mind, in order to ensure maximumaccuracy:(1) If the action of the acid is excessively prolonged,

the residues of the rubbings darken as a result ofoxidation and the visual perception of small dif-ferences in fineness becomes difficult or evenimpossible

(2) It is often advantageous to place the stone underrunning water for the observation of the results ofthe attack

(3) Silver — present in most gold alloys — produces amilky precipitate of silver chloride, which turnsblueish and is detrimental to accurate observa-tions. As silver chloride is soluble in ammonia, itis recommended that the rubbings are treatedwith a solution of the Jatter after a first attack bythe acid solution. Acid is then applied again for ashort time before wiping with filter paper. Thisprocedure assists in distinguishing small dif-ferences in fineness

(4) Upon removal of the acid, the residues of the rub-bing tend to tarnish rapidly with time. Thisundesirable effect is particularly noticeable withcopper-rich and zinc-containing gold alloys

(5) Attack on red gold rubbings is characterized bythe evolution of gas. The attack is not completeand the excess acid should not be removed untilafter this evolution of gas has ceased

(6) Dilute gold alloys (those with a gold contentbelow 400 parts per thousand) are advantageouslytested on a red or white touchstone such as anArkansas grinding stone, for instance. The testacid for these alloys is usually a dilute solution ofnitric acid. A brown residue remains afterremoval of the acid, which is darker for alloys ofvery low gold contents. Finenesses down to about100 parts of gold per thousand can be determinedin this way.

White Gold AlloysAs a rule, it is not feasible to touch white gold alloys

with the accuracy that is possible for their colouredcounterparts. The main reason for this is that allwhite golds of fineness between 333 and 916 parts perthousand are of rather similar greyish white colour.Thus, it is not possible to derive any preliminaryassessment of the gold content or the composition ofthese alloys from their colour, and the assayer has torely entirely on his observations of the behaviour ofthe rubbings during attack by the acids in order to ob-tain such information. However, this behaviour is notdependent solely on the gold content, but is stronglyinfluenced by the nature and relative amounts ofother constituents in the alloy. This factor makes theselection of the appropriate test-needles very difficult

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and these play a much lesser role in touching whitegolds than yellow golds. Therefore, the resultsarrived at by touching white alloys may differsignificantly from actual finenesses.

Nitric acid — concentrated or diluted — is normallyused on rubbings of white golds of fineness belowabout 650 parts per thousand. The residues have areddish colour after attack, which becomes lighterwith increasing gold content and vice-versa. Thisfeature allows the detection of large differences infineness, especially if a slowly acting acid solution isused for the test.

Rubbings of white golds containing platinum groupmetals are attacked with far greater difficulty thanthose of alloys bleached with nickel or nickel andzinc. The development of an easily recognizablebrown colouration of the acid drop during attackreveals the presence of palladium which is a consti-tuent of many white golds.

Influence of Other Alloy Constituents

The effect of silver content on touching gold hasbeen mentioned earlier in this article.

A clear indication that an unknown alloy containsadditions to gold other than silver and copper is theimpossibility of matching its colour with that of anyneedle in a comprehensive assortment of gold-silver-copper standards. If this is the case, the result oftouching should be regarded as unreliable.

The presence of zinc and/or cadmium acceleratesthe rate of attack of rubbings by test acids, therebyresulting in different behaviour for the sample undertest and a gold-silver-copper alloy needle of identicalfineness. Consequently, the net result of touchingalloys containing zinc and/or cadmium is usually anunderestimation of their gold content. Platinum, onthe other hand, has the opposite effect, even if presentonly in small quantities. Another effect of the addi-tion of platinum to coloured golds is to impart tothem a paler colour than that of platinum-free alloys.

Touching SilverThe presence of silver in an article of unknown

composition is established by attacking a rubbingwith dilute nitric acid, adding a drop_ of sodiumchloride solution or a small crystal of sodium chlorideand checking for the precipitation of silver chloridewhich dissolves in ammonia solution. Alternatively,the rubbing may be wetted with a solution ofpotassium dichromate and sulphuric acid, in whichcase silver forms its red-brown dichromate if present.

Usually, the fineness of silver alloys is determinedsimply by comparison of the colours of test rubbingswith those of rubbings from needles of known com-position. Test acids are employed only in specialcircumstances.

Touching is simplest on binary silver-copper alloys,owing to their wide range of colours — from red for afineness of about 500 parts per thousand, pink around600, yellow between 700 and 800 and yellowish whiteto bright white for silver-rich alloys.

There is always an element of doubt concerning theresults of touching silver, because this metal is seldomalloyed with copper alone. Additional metals, thepresence of which cannot be deduced from theappearance of the sample, can have a significant effecton the colour of the rubbings. Thus, platinum-,palladium-, zinc- or cadmium-containing silver alloysproduce rubbings of lighter colour than referencesilver-copper binary alloys of identical fineness,which leads to an overestimation of the silver contentof the former.

Touching PlatinumThe presence of platinum is established by attack-

ing or dissolving a rubbing with aqua regia whilegently heating the touchstone, absorbing the aquaregia onto filter paper and adding a drop of zincchloride solution to the damp spot on the paper. Thedevelopment of a yellow to orange colour indicatesplatinum.

The determination to a reasonable accuracy of thefineness of platinum alloys by touching is restricted tothose containing at least 900 parts per thousand of themetal. Alloys of lower fineness, which contain otherprecious metals and appreciable quantities of basemetals, cannot be assayed for platinum by touching.

Platinum-rich alloys are tested on special, acid-resistant stones. After preparation of the rubbings,the stone is immersed in a beaker with hot aqua regiato which potassium nitrate has been added. As soon asthe attack is seen to begin, the stone is removed fromthe acid and placed under running water forobservation.

There is no touchstone test for the other platinumgroup metals, including palladium. However, thelatter can be detected qualitatively with a tin chloridesolution, by the development on the touchstone of abrownish red colour which rapidly changes to dullgreenish blue.

Concluding RemarksThis article has attempted to give the reader an in-

sight into an important method, little known outsidethe jewellery industry, of analyzing for gold, silverand platinum (and sometimes other constituents) inalloys. The technique of touching has acceptableaccuracy in most cases. Moreover, it is rapid and hasthe advantage of requiring only a very small amountof test alloy. It will undoubtedly retain its value as amethod for the testing of precious metals and, inparticular, gold alloys.

158 Gold Bull., 1981, 14, (4)