872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 15

On the Kautilyas characterization tests for the purity of silver

and its experimental replication

RK Dube

Department of Materials and Metallurgical Engineering Indian Institute of Technology Kanpur 208016 (UP) India

A R T I C L E D A T A A B S T R A C T

Article history

Received 26 February 2007

Received in revised form

12 November 2007

Accepted 28 January 2008

Development of characterization tests for ascertaining the purity of precious metals was an

important task and skill in antiquity The statement of Kau

tilya (Fourth century BC) on the

characteristic features of pure silver is presented and discussed According to Kau

tilya if the

cast silver pieces or ingots are of white colour like that of coagulated milk or curd and have

projections similar to a cocks comb on its surface it should be considered as pure It has been

shown that the projectionsformedon the surface ofcastpuresilverare due to theevolution of

dissolved oxygen during the solidification of the air-melted silver The results of macro- and

microscopic examinations of a cast pure silver surface are also presented showing the

observation made by Kau

tilya

copy 2008 Elsevier Inc All rights reserved

Keywords

Kau

tilya

Arthaśā stra

Silver purity

Spitting

Sprouting

Oxygen in silver

1 Introduction

One of the important uses of silver in early times was as asource of concentrated wealth and a medium of exchange intrade and commerce Cast silverin the form of pieces andmini-ingots were used in business transactions The value was fixedby weight However the value of the silver pieces depended onits purity One of the important tasks before ancient assayerswas to develop a method for testing the purity of silver

Kau

tilya (Fourth century BC) was the key advisor to theIndian king Candragupta Maurya He was a statesman and

philosopher of high repute and was also known by the namesof Cā nakya and Visnugupta He wrote the famous treatise

called Arthaśā stra [1] which contains discussions on polityeconomics law and justice state administration and war inits wider sense He has also included discussions on miner-alogy metallurgy gemology and town planning Kau

tilya has

described the tests used for ascertaining the purity of silver inhis discussion related to metallurgy This reference has notattracted the attention of the metallurgical community and isbeing discussed here for the first time

The paper reports and discusses the characterizationmethods used for testing the purity of silver as stated byKau

tilya Results of the experimental study of the method arepresented and explained Microstructures showing the crestsproduced on silver as a test of purity are also presented

2 The Statement of Kautilya on Testing thePurity of Silver and its Experimental Replication

Kau

tilya discussed the mineralogy and metallurgy of silver inPrakarana 29 Chapter13 entitled lsquoThe Superintendent of Goldin the Workshoprsquo in his Arthaśā stra He described the

M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

Tel +91 512 2597769E-mail address rkdiitkacin

1044-5803$ ndash see front matter copy 2008 Elsevier Inc All rights reserveddoi101016jmatchar200801019

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 25

characteristics of pure silver which should be used for testing its purity The relevant stanza in question is as follows

lsquoudgatcū likamacchabullm bhr ā ji

s

nu dadhivar

nabullm ca ś uddham|rsquo

[English translation That (silver pieces or ingots) in whichthere areprojectionslike that of a cocks comb whichis clear andshinning and of the colour of coagulated milk or curd is pure]

Kau

tilya stated two qualitative tests for ensuring the purityof silver The first test is based on the colour of silver Puresilver should have white colour similar to that of coagulatedmilk or curd This test would be valid for both cast and

wrought silver If its colour is splendid white it would give ashinning appearance The second test is that the surface of thecast pure silver ingots or pieces should exhibit an appearanceof projections similar to a cocks comb In other words the topsurface of the pure silver ingot has a rising appearance atcertain places In fact this is a reference to spitting and

sprouting behaviour of silver This testis with reference to castsilver as shown later

The second test of Kau

tilya is scientifically more interest-ing and will be discussed in detail An experimental investi-gation was conducted to study the spitting and sprouting behaviour of pure silver Approximately 125 g of silver of 9999 purity was melted in a refractory crucible used by agoldsmith in a gas fired furnace The silver was kept in amolten state for about 30 min Subsequently the cruciblecontaining the molten silver was taken out of the furnace and

the silver was allowed to solidify in the crucible in open air Ahemispherical mini-ingot having a top surface diameter of approximately 45 mm and a central height of approximately15 mm was obtained The top surface of the cast silver ingotwas found to be rough Fig 1 shows a macro-photograph of thefront view of the top surface of the silver mini-ingot It can be

Fig 1 ndashMacro-photograph of the top surface of the cast silver

ingot showing crests Arrow shows large crests

Fig 2ndashSEM micrograph of the top surface of the cast silver

ingot showing several crests

Fig 3ndashSEM micrograph of the top surface of the cast silver

ingot showing the area near the crest at higher

magnification

Fig 4 ndashSEM micrograph of the top surface of the cast silver

ingot showing small sized projections

278 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 35

seen that several crests of different heights are present Thebiggestcrest is approximately25 mm in heightThe densityof such projections over the surface was not uniform At someplaces it was high while at others it was very lowFig 2 showsan SEM micrograph of the top surface of the cast silver ingotshowing both large sized and very small sized crests Thesmall sized projections gave a rough appearance to the topsurface Fig 3 shows an SEM micrograph of the surface of alarge crest at a higher magnification It is interesting to note

that there is a swirling type of action around theprojection AnSEM photograph of the cast silver surface showing severalsmall sized crests or combs is shown in Fig 4 These

projections of smaller heights produced roughness on thesurface

An interesting observation was also the formation of alarge pore and several small sized pores on the top surfaceof the mini-ingot (Figs 5 and 6) The large pore is approxi-mately 4 mm in diameter and has a depth extending up to

approximately 50 of the thickness of the ingot There was anindication of the swirling type action around the pore Thesmall pores were of the order of a few microns in size

3 Discussion

The colour of silver depends on its purity Pure silver has asplendid white colour In early times silver obtained after extraction from the ores was further purified by the addition of lead and the process was called lsquoCupellationrsquo If silver is notpurified properly during cupellation it would contain someresidual lead The colour of such silver would have a black tintCopper has been a common alloying element in silver for increasing the hardness Such recycled impure silver can alsobe purified by the cupellation process If the copper present inthe silver is not removed completely the colour of the resulting silver will not havea splendid white colour The deterioration inthe white colour of silver depends upon the amount and natureof the impurities present in it Thus the statement of Kaut

ilya

on the colour of pure silver is understandable This test wouldbe valid for both cast and wrought silver

The spitting behaviour of silver for testing the purity of silver as described by Kautilya can be explained as follows

Oxygen dissolves readily in molten silver In 1909 Sivertsand Hagenacker [2] measured the amount of oxygen dissolvedin molten silver under an oxygen pressure of 760 mm Hg atdifferent temperatures and the values are shown inTable 1 Itcan be seen that thesolubility of oxygen in solid silver at 1 atmoxygen pressure and just below the melting point is

approximately 5 cm3 per 100 g of silver The solubility of oxygen in molten silver just above the melting point under thesame condition is approximately 213 cm3 In other words itcan be stated that molten silver dissolves approximately 20times its own volume of oxygen near the melting point at1 atm pressure of oxygen Just below the melting point thesolid silver can dissolve oxygen only up to half its own volumeunder similar conditions The large difference in solubility of oxygen in the liquid and solid state causes the evolution of oxygen during solidification of molten silver

Siverts and Hagenacker [2] also studied the effect of oxygenpressure on its dissolution in molten silver The amount of oxygen dissolved in molten silver at 1075 degC and differentoxygen pressures is shown in Table 2 It is interesting to notethat the [amount of dissolved oxygen(oxygen pressure)12]ratio is approximately constant over the oxygen pressure

Fig 5 ndashSEM micrograph of the top surface of the cast silver

ingot showing a large pore

Fig 6 ndashSEM micrograph of the top surface of the cast silver

ingot showing micro-pores

Table 1 ndash Solubility values of oxygen in silver at different temperatures at 760 mm Hg pressure

TemperaturedegC

Solubility of oxygen per 100 g

of silver at760 mm Hg

pressure cm3

Volume of oxygen

dissolved per unit volume of

silver

wtoxygen

923 54 054 ndash

973 2135 2038 03051024 2056 1953 02951075 1939 1842 0277

1125 1849 1756 0264

279M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 45

range 39ndash1203 mm Hg This law was later designated asSiverts law

Silver melted in air is therefore a silver ndashoxygen alloy Animportant effect of the dissolution of oxygen in molten silver is to depress its freezing point and also to cause freezing tooccur over a temperature range Furthermore the depressionof the freezing point is also influenced by the pressure of

oxygen in the atmosphere above the molten silver Allen [3]conducted experiments from which he determined liquidusand solidus temperatures of molten silver alloys at differentoxygen partial pressures Table 3 shows the value of thesolidus and liquidus temperatures of the silver ndashoxygen alloyunder different oxygen pressures

In common practice silver is melted in air From the abovediscussion one can find out the amount of oxygen dissolved inmolten silver from air Assuming that air contains 21 vol of oxygen it can be deduced from Table 2 that the concentrationof oxygen is about 10 cm3 per 1087 g of molten silver at1075 degC Assuming the densityof molten silver is 932 g per cm3

[4] it can be said that the dissolution of oxygen is about 9

volumes per unit volume of molten silver Further whenmolten silver is kept in air for a long time to reach a conditionof equilibrium solidification starts at about 951 degC and thesilver completely solidifies at about 932 degC (Table 3)

As discussed earlier silver melted in air is actually a silver ndashoxygen alloy During its solidification crystals of silvercomparatively free from oxygen separate out first while theoxygen content in the residual liquid increases This processcontinues as the temperature decreases during solidificationuntil the composition of liquid is such that the pressure of oxygen that would be in equilibrium with it is not less than theexternal pressure Bubbles of oxygen are then given offresulting in ldquospitting rdquo at the free surface

As a result liquid silver from the interior is ejected on thesurface of the ingot and a shape similar to a cocks comb isformed on solidification There is possibly another way thatsmall sized combs or protrusions can form If the pressure of

oxygen generated in silver just below a solidified skin on thefree surface is sufficient to deform the metal surfaceprotrusions or bubbles would form and an appearance of arising surface will result (Fig 4) A higher ejection pressuregives rise to formation of holes on the surface (Fig 5 and 6)The size of the pore will depend on the pressure

If small amountsof copper or other base metals are presentin molten silver the dissolved oxygen will combine with these

metals to form respective metal oxides In such a case oxygenwill not evolve during the solidification of silver and the topsurface of the ingot would be smooth and no spitting andsprouting would occur In ancient times silver was refined bythe process of cupellation which used lead Improper refining would result in silver containing small amounts of lead Insuch a situation the phenomenon of spitting would not be

observed Also if the refining of copper-bearing silver scrapwas not carried out to the fullest extent the recycled silver would contain some copper Such silver would also not exhibitthe spitting phenomenon

4 Summary and Concluding Remarks

The value of precious metals has been greatly dependent onits purity since early times Different types of tests weredeveloped in different times for testing the purity of preciousmetals Kautilya recognized the specific features present onthe surface of cast pure silver which was formed during the

solidification of air-melted pure silver Oxygen from air dissolves in molten silver and it evolves during solidificationleading to the formation of crests or combs on its top surfaceHe used this behaviour of silver to formulate a test for its

purity According to him the surface of the cast pure silver should exhibit a feature similar to a cocks comb The abovestatement of Kautilya has not been discussed in literature Ascientific explanation for the phenomenon is presented for the first time in this paper An experimental replication of thecasting of a pure silver ingot shows the formation of suchprojections of different heights

The surface also shows pores of sizes ranging from big tosmall This phenomenon is due to the unique property of silver of dissolving about 9 volumes of oxygen per unit volumeof silver when melted in air When such a silver ndashoxygen alloyis solidified the solidification occurs over a temperatureinterval Most of the oxygen evolves vigorously just beforethe solidification is complete giving rise to the phenomena of spitting and sprouting

Kautilya is the first classicalwriterto consider the presenceof spitting and sprouting as an index of the purity of castsilver If impurity metals such as copper and lead are presentin the silver theoxygen will combine with theseelements andform respective oxides The surface of such impure cast silver will not exhibit a feature similar to a cocks comb Kautilyaalso recognized the effect of impurities on the colour and theshining appearance of silver The residual copper and lead

Table 3 ndash Liquidus and solidus temperatures of silver ndashoxygen alloys

Oxygencontent in theatmospherevol

Liquidustemperature

degC

Solidustemperature

degC

Depressionof liquidus

temperaturedegC

0 961 ndash ndash

78 958 929 3206 951 932 10640 942 931 19974 938 930 23

Table 2 ndash Volumes of oxygen absorbed per 1087 g of silver at 1075 degC and different pressures

Pressure of oxygen(P) mmHg

Volume of oxygen absorbed per 1087 g of silver at NTP (m) cm3

mffiffi

Pp

ratio

1203 2691 078760 2101 076488 1702 077346 1453 078209 1175 081150 1009 082128 887 078

39 475 076

280 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 55

give a blackish tint to the colour of cast or wrought silver andas a result it will not have a splendid white colour

R E F E R E N C E S

[1] Kautilya Arthaśā stra Gairola V editor VaranasiChowkhamba Vidya Bhawan 1962

[2] Siverts A Hagenacker Joh Uumlber die loumlslichkeit von wasserstoff und sauerstoff in festem und geschmolzenem silber Z PhysChem 190968115ndash28

[3] Allen NP The effect of pressure on the liberation of gases frommetals (with special reference to silver and oxygen) J Inst Met193249317ndash46

[4] Lide DReditor CRCHandbook of Chemistryand Physics vol 4Boca Raton Taylor and Francis 2005 p 131 86th edition

281M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 35

seen that several crests of different heights are present Thebiggestcrest is approximately25 mm in heightThe densityof such projections over the surface was not uniform At someplaces it was high while at others it was very lowFig 2 showsan SEM micrograph of the top surface of the cast silver ingotshowing both large sized and very small sized crests Thesmall sized projections gave a rough appearance to the topsurface Fig 3 shows an SEM micrograph of the surface of alarge crest at a higher magnification It is interesting to note

that there is a swirling type of action around theprojection AnSEM photograph of the cast silver surface showing severalsmall sized crests or combs is shown in Fig 4 These

projections of smaller heights produced roughness on thesurface

An interesting observation was also the formation of alarge pore and several small sized pores on the top surfaceof the mini-ingot (Figs 5 and 6) The large pore is approxi-mately 4 mm in diameter and has a depth extending up to

approximately 50 of the thickness of the ingot There was anindication of the swirling type action around the pore Thesmall pores were of the order of a few microns in size

3 Discussion

The colour of silver depends on its purity Pure silver has asplendid white colour In early times silver obtained after extraction from the ores was further purified by the addition of lead and the process was called lsquoCupellationrsquo If silver is notpurified properly during cupellation it would contain someresidual lead The colour of such silver would have a black tintCopper has been a common alloying element in silver for increasing the hardness Such recycled impure silver can alsobe purified by the cupellation process If the copper present inthe silver is not removed completely the colour of the resulting silver will not havea splendid white colour The deterioration inthe white colour of silver depends upon the amount and natureof the impurities present in it Thus the statement of Kaut

ilya

on the colour of pure silver is understandable This test wouldbe valid for both cast and wrought silver

The spitting behaviour of silver for testing the purity of silver as described by Kautilya can be explained as follows

Oxygen dissolves readily in molten silver In 1909 Sivertsand Hagenacker [2] measured the amount of oxygen dissolvedin molten silver under an oxygen pressure of 760 mm Hg atdifferent temperatures and the values are shown inTable 1 Itcan be seen that thesolubility of oxygen in solid silver at 1 atmoxygen pressure and just below the melting point is

approximately 5 cm3 per 100 g of silver The solubility of oxygen in molten silver just above the melting point under thesame condition is approximately 213 cm3 In other words itcan be stated that molten silver dissolves approximately 20times its own volume of oxygen near the melting point at1 atm pressure of oxygen Just below the melting point thesolid silver can dissolve oxygen only up to half its own volumeunder similar conditions The large difference in solubility of oxygen in the liquid and solid state causes the evolution of oxygen during solidification of molten silver

Siverts and Hagenacker [2] also studied the effect of oxygenpressure on its dissolution in molten silver The amount of oxygen dissolved in molten silver at 1075 degC and differentoxygen pressures is shown in Table 2 It is interesting to notethat the [amount of dissolved oxygen(oxygen pressure)12]ratio is approximately constant over the oxygen pressure

Fig 5 ndashSEM micrograph of the top surface of the cast silver

ingot showing a large pore

Fig 6 ndashSEM micrograph of the top surface of the cast silver

ingot showing micro-pores

Table 1 ndash Solubility values of oxygen in silver at different temperatures at 760 mm Hg pressure

TemperaturedegC

Solubility of oxygen per 100 g

of silver at760 mm Hg

pressure cm3

Volume of oxygen

dissolved per unit volume of

silver

wtoxygen

923 54 054 ndash

973 2135 2038 03051024 2056 1953 02951075 1939 1842 0277

1125 1849 1756 0264

279M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 45

range 39ndash1203 mm Hg This law was later designated asSiverts law

Silver melted in air is therefore a silver ndashoxygen alloy Animportant effect of the dissolution of oxygen in molten silver is to depress its freezing point and also to cause freezing tooccur over a temperature range Furthermore the depressionof the freezing point is also influenced by the pressure of

oxygen in the atmosphere above the molten silver Allen [3]conducted experiments from which he determined liquidusand solidus temperatures of molten silver alloys at differentoxygen partial pressures Table 3 shows the value of thesolidus and liquidus temperatures of the silver ndashoxygen alloyunder different oxygen pressures

In common practice silver is melted in air From the abovediscussion one can find out the amount of oxygen dissolved inmolten silver from air Assuming that air contains 21 vol of oxygen it can be deduced from Table 2 that the concentrationof oxygen is about 10 cm3 per 1087 g of molten silver at1075 degC Assuming the densityof molten silver is 932 g per cm3

[4] it can be said that the dissolution of oxygen is about 9

volumes per unit volume of molten silver Further whenmolten silver is kept in air for a long time to reach a conditionof equilibrium solidification starts at about 951 degC and thesilver completely solidifies at about 932 degC (Table 3)

As discussed earlier silver melted in air is actually a silver ndashoxygen alloy During its solidification crystals of silvercomparatively free from oxygen separate out first while theoxygen content in the residual liquid increases This processcontinues as the temperature decreases during solidificationuntil the composition of liquid is such that the pressure of oxygen that would be in equilibrium with it is not less than theexternal pressure Bubbles of oxygen are then given offresulting in ldquospitting rdquo at the free surface

As a result liquid silver from the interior is ejected on thesurface of the ingot and a shape similar to a cocks comb isformed on solidification There is possibly another way thatsmall sized combs or protrusions can form If the pressure of

oxygen generated in silver just below a solidified skin on thefree surface is sufficient to deform the metal surfaceprotrusions or bubbles would form and an appearance of arising surface will result (Fig 4) A higher ejection pressuregives rise to formation of holes on the surface (Fig 5 and 6)The size of the pore will depend on the pressure

If small amountsof copper or other base metals are presentin molten silver the dissolved oxygen will combine with these

metals to form respective metal oxides In such a case oxygenwill not evolve during the solidification of silver and the topsurface of the ingot would be smooth and no spitting andsprouting would occur In ancient times silver was refined bythe process of cupellation which used lead Improper refining would result in silver containing small amounts of lead Insuch a situation the phenomenon of spitting would not be

observed Also if the refining of copper-bearing silver scrapwas not carried out to the fullest extent the recycled silver would contain some copper Such silver would also not exhibitthe spitting phenomenon

4 Summary and Concluding Remarks

The value of precious metals has been greatly dependent onits purity since early times Different types of tests weredeveloped in different times for testing the purity of preciousmetals Kautilya recognized the specific features present onthe surface of cast pure silver which was formed during the

solidification of air-melted pure silver Oxygen from air dissolves in molten silver and it evolves during solidificationleading to the formation of crests or combs on its top surfaceHe used this behaviour of silver to formulate a test for its

purity According to him the surface of the cast pure silver should exhibit a feature similar to a cocks comb The abovestatement of Kautilya has not been discussed in literature Ascientific explanation for the phenomenon is presented for the first time in this paper An experimental replication of thecasting of a pure silver ingot shows the formation of suchprojections of different heights

The surface also shows pores of sizes ranging from big tosmall This phenomenon is due to the unique property of silver of dissolving about 9 volumes of oxygen per unit volumeof silver when melted in air When such a silver ndashoxygen alloyis solidified the solidification occurs over a temperatureinterval Most of the oxygen evolves vigorously just beforethe solidification is complete giving rise to the phenomena of spitting and sprouting

Kautilya is the first classicalwriterto consider the presenceof spitting and sprouting as an index of the purity of castsilver If impurity metals such as copper and lead are presentin the silver theoxygen will combine with theseelements andform respective oxides The surface of such impure cast silver will not exhibit a feature similar to a cocks comb Kautilyaalso recognized the effect of impurities on the colour and theshining appearance of silver The residual copper and lead

Table 3 ndash Liquidus and solidus temperatures of silver ndashoxygen alloys

Oxygencontent in theatmospherevol

Liquidustemperature

degC

Solidustemperature

degC

Depressionof liquidus

temperaturedegC

0 961 ndash ndash

78 958 929 3206 951 932 10640 942 931 19974 938 930 23

Table 2 ndash Volumes of oxygen absorbed per 1087 g of silver at 1075 degC and different pressures

Pressure of oxygen(P) mmHg

Volume of oxygen absorbed per 1087 g of silver at NTP (m) cm3

mffiffi

Pp

ratio

1203 2691 078760 2101 076488 1702 077346 1453 078209 1175 081150 1009 082128 887 078

39 475 076

280 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 55

give a blackish tint to the colour of cast or wrought silver andas a result it will not have a splendid white colour

R E F E R E N C E S

[1] Kautilya Arthaśā stra Gairola V editor VaranasiChowkhamba Vidya Bhawan 1962

[2] Siverts A Hagenacker Joh Uumlber die loumlslichkeit von wasserstoff und sauerstoff in festem und geschmolzenem silber Z PhysChem 190968115ndash28

[3] Allen NP The effect of pressure on the liberation of gases frommetals (with special reference to silver and oxygen) J Inst Met193249317ndash46

[4] Lide DReditor CRCHandbook of Chemistryand Physics vol 4Boca Raton Taylor and Francis 2005 p 131 86th edition

281M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 45

range 39ndash1203 mm Hg This law was later designated asSiverts law

Silver melted in air is therefore a silver ndashoxygen alloy Animportant effect of the dissolution of oxygen in molten silver is to depress its freezing point and also to cause freezing tooccur over a temperature range Furthermore the depressionof the freezing point is also influenced by the pressure of

oxygen in the atmosphere above the molten silver Allen [3]conducted experiments from which he determined liquidusand solidus temperatures of molten silver alloys at differentoxygen partial pressures Table 3 shows the value of thesolidus and liquidus temperatures of the silver ndashoxygen alloyunder different oxygen pressures

In common practice silver is melted in air From the abovediscussion one can find out the amount of oxygen dissolved inmolten silver from air Assuming that air contains 21 vol of oxygen it can be deduced from Table 2 that the concentrationof oxygen is about 10 cm3 per 1087 g of molten silver at1075 degC Assuming the densityof molten silver is 932 g per cm3

[4] it can be said that the dissolution of oxygen is about 9

volumes per unit volume of molten silver Further whenmolten silver is kept in air for a long time to reach a conditionof equilibrium solidification starts at about 951 degC and thesilver completely solidifies at about 932 degC (Table 3)

As discussed earlier silver melted in air is actually a silver ndashoxygen alloy During its solidification crystals of silvercomparatively free from oxygen separate out first while theoxygen content in the residual liquid increases This processcontinues as the temperature decreases during solidificationuntil the composition of liquid is such that the pressure of oxygen that would be in equilibrium with it is not less than theexternal pressure Bubbles of oxygen are then given offresulting in ldquospitting rdquo at the free surface

As a result liquid silver from the interior is ejected on thesurface of the ingot and a shape similar to a cocks comb isformed on solidification There is possibly another way thatsmall sized combs or protrusions can form If the pressure of

oxygen generated in silver just below a solidified skin on thefree surface is sufficient to deform the metal surfaceprotrusions or bubbles would form and an appearance of arising surface will result (Fig 4) A higher ejection pressuregives rise to formation of holes on the surface (Fig 5 and 6)The size of the pore will depend on the pressure

If small amountsof copper or other base metals are presentin molten silver the dissolved oxygen will combine with these

metals to form respective metal oxides In such a case oxygenwill not evolve during the solidification of silver and the topsurface of the ingot would be smooth and no spitting andsprouting would occur In ancient times silver was refined bythe process of cupellation which used lead Improper refining would result in silver containing small amounts of lead Insuch a situation the phenomenon of spitting would not be

observed Also if the refining of copper-bearing silver scrapwas not carried out to the fullest extent the recycled silver would contain some copper Such silver would also not exhibitthe spitting phenomenon

4 Summary and Concluding Remarks

The value of precious metals has been greatly dependent onits purity since early times Different types of tests weredeveloped in different times for testing the purity of preciousmetals Kautilya recognized the specific features present onthe surface of cast pure silver which was formed during the

solidification of air-melted pure silver Oxygen from air dissolves in molten silver and it evolves during solidificationleading to the formation of crests or combs on its top surfaceHe used this behaviour of silver to formulate a test for its

purity According to him the surface of the cast pure silver should exhibit a feature similar to a cocks comb The abovestatement of Kautilya has not been discussed in literature Ascientific explanation for the phenomenon is presented for the first time in this paper An experimental replication of thecasting of a pure silver ingot shows the formation of suchprojections of different heights

The surface also shows pores of sizes ranging from big tosmall This phenomenon is due to the unique property of silver of dissolving about 9 volumes of oxygen per unit volumeof silver when melted in air When such a silver ndashoxygen alloyis solidified the solidification occurs over a temperatureinterval Most of the oxygen evolves vigorously just beforethe solidification is complete giving rise to the phenomena of spitting and sprouting

Kautilya is the first classicalwriterto consider the presenceof spitting and sprouting as an index of the purity of castsilver If impurity metals such as copper and lead are presentin the silver theoxygen will combine with theseelements andform respective oxides The surface of such impure cast silver will not exhibit a feature similar to a cocks comb Kautilyaalso recognized the effect of impurities on the colour and theshining appearance of silver The residual copper and lead

Table 3 ndash Liquidus and solidus temperatures of silver ndashoxygen alloys

Oxygencontent in theatmospherevol

Liquidustemperature

degC

Solidustemperature

degC

Depressionof liquidus

temperaturedegC

0 961 ndash ndash

78 958 929 3206 951 932 10640 942 931 19974 938 930 23

Table 2 ndash Volumes of oxygen absorbed per 1087 g of silver at 1075 degC and different pressures

Pressure of oxygen(P) mmHg

Volume of oxygen absorbed per 1087 g of silver at NTP (m) cm3

mffiffi

Pp

ratio

1203 2691 078760 2101 076488 1702 077346 1453 078209 1175 081150 1009 082128 887 078

39 475 076

280 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 55

give a blackish tint to the colour of cast or wrought silver andas a result it will not have a splendid white colour

R E F E R E N C E S

[1] Kautilya Arthaśā stra Gairola V editor VaranasiChowkhamba Vidya Bhawan 1962

[2] Siverts A Hagenacker Joh Uumlber die loumlslichkeit von wasserstoff und sauerstoff in festem und geschmolzenem silber Z PhysChem 190968115ndash28

[3] Allen NP The effect of pressure on the liberation of gases frommetals (with special reference to silver and oxygen) J Inst Met193249317ndash46

[4] Lide DReditor CRCHandbook of Chemistryand Physics vol 4Boca Raton Taylor and Francis 2005 p 131 86th edition

281M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1

872019 Materials Character is at Ion

httpslidepdfcomreaderfullmaterials-character-is-at-ion 55

give a blackish tint to the colour of cast or wrought silver andas a result it will not have a splendid white colour

R E F E R E N C E S

[1] Kautilya Arthaśā stra Gairola V editor VaranasiChowkhamba Vidya Bhawan 1962

[2] Siverts A Hagenacker Joh Uumlber die loumlslichkeit von wasserstoff und sauerstoff in festem und geschmolzenem silber Z PhysChem 190968115ndash28

[3] Allen NP The effect of pressure on the liberation of gases frommetals (with special reference to silver and oxygen) J Inst Met193249317ndash46

[4] Lide DReditor CRCHandbook of Chemistryand Physics vol 4Boca Raton Taylor and Francis 2005 p 131 86th edition

281M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 2 7 7 ndash 2 8 1