1

On the Microstructure of the Reflecting Surface of Aranmula

Speculum Metal Mirror

E.A Nazimudeen 1, a)

, T.E Girish 2, b)

1, 2 Dept. of Physics, University College, Thiruvananthapuram, Kerala, India – 695034,

a) nazimudeenea@gmail.com

b) Corresponding author: tegirish5@yahoo.co.in

Abstract

The invention of speculum metal must have been an achievement of considerable importance in the bronze-age

world. In modern scientific world, it has some brilliant engineering applications apart from its

archaeometallurgical importance. We report our experimental investigations on detailed chemical composition

and microstructure of the reflecting surface of cast and thin film form of speculum metal mirror for the first time

using an integrated approach combining EDS, XRD and AFM analyses techniques. The reflecting surface of

cast and thin film form of speculum mirror is found to be semi- amorphous in nature with nanoscale particles as

evident from XRD and AFM analyses. We could also infer the intermetallic phase Cu6.25Sn5 at the reflecting

surface of cast and Cu5.6Sn at the reflecting surface of thin film form of mirror respectively. The optical

reflectance of the cast Aranmula speculum mirror material is found to increase by about 10 % on an average in

the visible region of the spectrum when it is prepared in thin film form in glass substrate by thermal evaporation

method. Combined use of XRD and optical reflectance studies suggested an inverse relationship between

crystallinity and optical reflectance of the mirror material.

Keywords: Speculum metal, Aranmula mirror, reflectivity, microstructure, thin films.

1. INTRODUCTION

Speculum metal is a peculiar type of high tin bronze alloy that is used as a vital component in reflecting

telescopes and other optical precession instruments as mirrors, gratings etc. from the time of Isaac Newton in

17th

century and continuing up to early decades of 20th

century [1]. The idea of telescope design involving

mirrors was brought forward as early as during Galileo’s lifetime. But more advanced systems were suggested

by Gregory and Cassegrain in 1661 [2]. From Newton to Lord Rosse, over a period approaching two hundred

years (1670- 1850), the earliest telescope mirrors were made of speculum metal. Subsequent inventions of

practical methods of silvering glass diverted attention from metallic specula for many decades. However, the

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rapid development of space technology and its applications, new environments and improvements in

manufacturing technology have forced a relatively recent return to more extensive use of metal mirrors [3].

The archaeometallurgical studies on speculum metal

mirrors have always been of great interest to archaeologists and other scientists. In the past, the name speculum

applied exclusively to mirrors made from the cast copper- tin alloy, but within recent years a technique has been

perfected enabling these alloys to be electrodeposited on steel and other basic materials [4]. Speculum metal

now find excellent modern engineering applications such as internal combustion engine [5], nickel- cadmium

batteries, alternative to nickel undercoating for gold or chromium electroplating [6], solar water heater [7],

storage battery for vehicles [8], slit material in transmission electron microscopy [9], anode material for lithium-

ion batteries, etc. [10].

Speculum metal has long been known for its excellent

resistance to tarnishing and for its pleasing white color [11]. Since classical times bronze mirrors have been

made from an alloy of copper and tin, to which various minor additions might be made in an attempt to increase

whiteness, ductility or freedom from pores [1]. A great deal of investigations on the nature and properties of the

copper – tin alloys which constitute speculum metal has been carried out by metallurgists during the last few

decades. Speculum metal mirror in cast and thin film form is not reported in modern studies after the work of

Tolansky in 1937 [4]. Also the detailed microstructure of reflecting surface of speculum metal mirror is not

reported previously, which is the main focus of the present work. We have carried out experimental

investigations using EDS, AFM, XRD and UV- Visible Spectroscopy analysis techniques on Aranmula metal

mirror made in Kerala.

2. MATERIALS AND METHODS

Aranmula mirror is a kind of good quality speculum mirror, cast and polished using traditional techniques from

Kerala in southern India. There exist literary articles that describe its metallurgical and technical aspects of

casting and polishing methods [12, 13, 14]. The alloy of copper and tin is melted in a jug shaped crucible- cum-

mold of clay using a furnace fired with a coconut shell- charcoal to get the cast mirror blank. A small quantity of

the melt is removed from the crucible and left to solidify on the ground. The casting disk is then broken and

remelted to cast it as the mirror with the required thickness. A closed crucible process was used to prevent

oxidation losses when the metal is cast and also to minimize the formation of an oxide skin that would be

detrimental to the polishing of the mirror. For this, the mold is formed with two burnt clay discs, each of which

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has one polished surface separated by small spacers. The mold is covered with clay on all sides except the ‘in-

gate’, which act as the connector to the crucible. The mouth of crucible is then covered with a fine piece of linen

cloth soaked in a mixture of alluvial silt with cow- dung past and then sealed with more wet clay. The finished

crucible- cum- mold is heated in an open pit furnace for melting. The cast disc blank, which is retrieved by

breaking the mold, is mounted in heated resin on to a wooden handle. It is then polished and lapped for about 2-

3 hours for a day over four or five days over hessian and velvet cloth placed on a flat wooden board to get a

mirror finish. When a satisfactory finish is achieved, the disc is heated just enough to separate it from the

wooden handle, and the mirror is mounted on a brass frame. A typical Aranmula mirror brought from a

commercial outlet is shown in figure 1.

Figure 1. Typical Aranmula metal mirror from a commercial outlet

Aranmula mirror mounted on a brass handle was purchased from a reliable Govt. agency during the year 2006.

The mirror portion with an aperture of 0.05m, separated from the brass base is sectioned into number of

fragments using diamond cutter and some fragments are grinded to powder form, which are used for

experimental investigations. We could successfully make thin films of the cast mirror material for the first time

using thermal evaporation method in Thin Film Lab, Department of Physics, CUSAT, Cochin. The film was

deposited on a glass substrate at room temperature using a molybdenum boat by applying a current of about 120

A with a coating pressure of about 2 x 10−5 mbar in a coating time of around 5 minute. Thin film form of cast

Aranmula mirror is shown in figure 2.

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Figure 2. Thin film form of cast Aranmula mirror

The identification of intermetallic compounds on the basis of composition was carried out by Bruker’s XFlash-

6110 Energy Dispersive Spectrometer at an energy level of 15 keV. The surface morphologies of cast and thin

film coated mirror samples are monitored by Atomic Force Microscopy (Digital Instruments Nano Scope E,

Si3N4 100) instrument in contact mode at a scanning rate of 2 µm/s with bias voltage 0.5 V. The roughness of

the mirror surface and mean particle size is estimated using Bruker’s Nanoscope Analysis Software and WSxM

5.0 Develop 8.0 Analysis Software. X-Ray diffraction (XRD) patterns were recorded directly on the mirror

sample by using XPERT PRO -83005153 diffractometer with copper anode and irradiated with monochromatic

𝐴1Kα X-rays at wave length of 1.54 Å.

Thermal properties of Aranmula mirror in the form of powder is

studied up to 1000℃ in Nitrogen atmosphere at a rate of 20℃ /min by means of Perkin Elmer, Diamond

TG/DTA instrument with TG sensitivity 0.2 mg and DTA sensitivity 0.06 mV. The optical reflectance of the

cast and thin film coated mirror samples are measured in a region of wave length ranging from 200 nm to 900

nm using JASCO - 550V UV- Visible Double Beam Spectrophotometer.

3. RESULTS

3.1. Identification of Chemical Composition by EDS

The normalized chemical composition at the reflecting surface of cast, thin film and powder form of Aranmula

mirror samples at different locations are shown in Table 1.

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Table 1. The normalized weight of chemical composition of the cast, thin film and powder form of

Aranmula metal mirror

Mirror sample

Position

Normalized weight of chemical compositions ( Wt.% )

Cu

Sn

Zn

As

Ag

Au

Al

Pb

Fe

O

P

Ni

S

Cast mirror

1.

68.87

30.16

0.76

0.08

0.06

…..

0.05

…..

0.02

2.

69.23

29.70

0.83

0.09

…..

0.08

…..

0.07

…..

3.

68.64

30.06

0.85

0.19

0.14

…..

0.03

0.09

…..

4.

69.03

29.96

0.71

0.18

…..

0.07

…..

0.05

…..

5.

69.15

29.85

0.69

0.17

…..

0.05

0.03

…..

0.06

Average

68.98

29.95

0.77

0.14

0.1

0.07

0.04

0.07

0.04

Powder form of

cast mirror

1.

63.87

29.75

0.22

0.06

0.85

0.12

1.89

…..

…..

3.11

0.03

0.01

0.07

2.

62.78

29.98

0.24

…..

1.03

…..

2.18

…..

0.01

3.51

0.16

0.10

…..

3.

64.13

29.24

0.26

0.04

0.78

0.28

1.68

…..

0.01

3.52

…..

0.06

…..

4.

62.43

30.09

0.19

…..

1.07

0.07

1.89

0.11

0.06

3.85

0.14

0.08

0.03

5.

65.21

28.44

0.25

…..

0.80

0.25

1.39

0.11

0.09

3.27

…..

0.19

…..

Average

63.68

29.5

0.23

0.05

0.91

0.18

1.81

0.11

0.04

3.45

0.11

0.09

0.05

Thin film

coated mirror

1.

62.22

31.21

0.15

…..

1.00

0.07

…..

0.10

4.95

0.20

…..

0.12

2.

61.21

31.97

0.02

0.09

1.19

0.42

…..

0.14

4.53

0.27

0.02

0.14

3.

62.11

31.84

…..

0.23

1.02

0.30

…..

0.09

4.07

0.19

0.03

0.11

4.

62.01

31.69

0.04

0.14

1.05

…..

…..

0.08

4.51

0.22

0.15

0.12

5.

62.00

31.67

0.09

0.08

1.03

0.29

…..

0.14

4.40

0.20

0.03

0.07

Average

61.9

31.68

0.08

0.14

1.06

0.27

0.11

4.49

0.22

0.06

0.11

EDS measurements indicate that the alloying system in Aranmula mirror is based on two major elements

including copper (62 ± 1.56 % - 69 ± 1.31 %) and Tin (30 ± 0.81 % - 32 ± 0.69 %). Observed other significant

minor constituents that are included in the measurements of the reflecting surface of cast mirror are Zinc (Zn),

Arsenic (As) and Lead (Pb),which constitute only about 1% of the total weight percentage of the chemical

composition. The mirror surface was also found to contain traces of Iron (Fe), Phosphorous (P), Sulphur (S) and

Nickel (Ni), whose individual content is less than 0.1 Wt. %.

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The significant minor constituents such as Zinc (Zn), Silver (Ag), Gold (Au), Lead (Pb), Aluminium (Al) and

phosphorous (P) that are observed in the measurement of the powder form of cast mirror constitute about 1.9 %

of the total weight percentage of elemental composition. It also contains traces of Arsenic (S), Iron (Fe), Sulphur

(S) and Nickel (Ni). But in the EDS measurements of thin film mirror, the minor constituents such as Arsenic

(As), Silver (Ag), Gold (Au), Iron (Fe), Phosphorous (P) and Sulphur (S) constitute about 2.1% of the total

weight percentage of chemical composition. It was also found to contain trace elements such as Zinc (Zn) and

Nickel (Ni). The minor constituent Lead (Pb) is not observed in thin film coated mirror sample, but which is

included in both the reflecting surface of cast and powder form of the mirror samples. The inclusion of oxygen

affects significantly the weight percentage of copper and tin. The normalized chemical compositions of the

reflecting surface of a new cast Aranmula mirror sample with and without oxygen are shown in table 2.

Table 2. The normalized weight of chemical compositions at the reflecting surface of a new cast Aranmula

metal mirror with and without oxygen

Observation

Positon

.

Normalized weight of elemental chemical compositions ( Wt.% )

Cu

Sn

Ag

Al

p

As

Fe

Zn

Ni

S

Pb

Au

O

Without oxygen

1.

66.48

30.32

1.42

0.97

0.32

…..

0.18

0.06

…..

0.11

0.05

0.09

2.

65.96

30.54

1.53

1.02

0.32

0.27

0.20

…..

0.05

0.11

…..

…..

3.

65.35

30.93

1.18

1.06

0.39

0.20

0.25

0.15

0.12

0.11

…..

…..

4.

65.78

30.53

1.55

1.17

0.40

…..

0.22

0.19

…..

0.11

…..

0.01

5.

65.58

30.77

1.50

1.05

0.36

0.05

0.20

0.1

0.09

0.06

…..

…..

Average

65.83

30.62

1.44

1.05

0.36

0.19

0.21

0.14

0.09

0.10

0.05

0.05

With oxygen

1.

63.3

28.75

1.29

0.91

0.28

0.24

0.18

0.17

0.09

0.05

…..

…..

4.74

2.

63.57

28.57

1.34

1.01

0.3

0.05

0.21

0.21

…..

0.09

…..

…..

4.65

3.

63.0

28.83

1.23

0.91

0.29

0.2

0.24

0.18

0.11

0.09

…..

…..

4.92

4.

63.67

28.52

1.32

0.87

0.24

0.27

0.19

…..

0.05

0.09

…..

…..

4.78

5.

64.2

28.32

1.20

0.82

0.24

…..

0.17

0.08

…..

0.1

0.05

0.08

4.74

Average

63.5

28.6

1.28

0.90

0.27

0.19

0.2

0.16

0.08

0.08

0.05

0.08

4.77

3.2. Morphological Studies Using Atomic Force Microscopy

The three dimensional AFM morphologies of cast and thin film coated mirror samples at 2 µm are shown in

figure 3 (a) and 3 (b). The three dimensional morphology of the reflecting surface of cast mirror sample at 2 µm

shows a streak or texture pattern with porous structure. It also consists of many fine grains and each grain

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consists of elongated rulings with crystals arranged in a uniform pattern with RMS roughness of about 3.38 nm.

The average particle size estimated is about 69.68 nm. The morphology of thin film coated mirror sample at 2

µm consists of dense particles, which also shows a texture pattern with porous structure. The estimated value of

average size of the particle and RMS roughness of thin film coated mirror are about 29.21 nm and 1.61 nm

respectively. Thus AFM morphology of the reflecting surface of cast and thin film form of mirror samples show

a texture pattern with porous structure with a small value of roughness, which is the direct evidence for high

value of reflectance compared with other ordinary mirrors.

Figure 3 (a). 3D AFM morphology of the reflecting surface of cast mirror sample at 2 µm

Figure 4 (b). 3D AFM morphology of the reflecting surface of thin film form of cast mirror at 2 µm

3.3. Structural Evaluation by means of X-Ray Diffraction

The XRD spectra of the reflecting surface of cast, thin film and powdered form of Aranmula mirror samples are

shown in figure 4 (a), (b) and (c).

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Figure 4 (a). XRD spectrum of the reflecting surface of cast Aranmula mirror

Figure 4 (b). XRD spectrum at the reflecting surface of thin film form of cast mirror

Figure 4 (c). XRD spectrum of the powder form of cast Aranmula mirror

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The XRD spectra of the reflecting surface of cast and thin film form of Aranmula mirror samples suggest an

amorphous nature with some intense multiple peaks of the intermetallic phases of Cu- Sn alloy system. The two

intense peaks at the positions 43.09° and 60.52° in the reflecting surface of cast mirror sample confirmed that

both copper and Cu6.25Sn5 phases present in the as synthesized material. The peak at the position 42.38° in the

reflecting surface of thin film form of mirror indicated the presence of intermetallic phase Cu5.6Sn of the bronze

alloy system. Both spectra of the mirror sample also contain a line broadening, which may be due to lattice

parameter changes associated with relative diffusional growth of adjacent intermetallic layers. The estimated

average particle size is found to be about 50.55 ± 0.46 nm in the former case and that in the latter case is about

18.85 ± 0.52 nm.

The XRD spectrum of the powdered form of mirror sample

is almost crystalline in nature and it also consists some intense peaks, which confirmed the presence of

Cu10Sn3, Cu6.25Sn5, Cu41Sn11, Cu81Sn22 phases of the Cu- Sn alloy system. The average particle size is found

to be about 74 ± 3.75 nm. The XRD data at the reflecting surface of cast, thin film coated and powder form of

Aranmula speculum metal mirror samples are shown in table 3.

The XRD data obtained from mirror samples were

compared with the data in the JCPDS reference file, but it does not give a pattern specifically for the delta phase

suggested in early literature [12, 13]. However we could find a distinct transition from crystal to amorphous

phase as the material is polished in cast mirror or made in the form of thin film. The nature of decreasing

crystallinity for different cases is shown in figure 5.

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TABLE 3. XRD data at the reflecting surface of cast, thin film and powder form of Aranmula metal mirror

Mirror

sample

Position

(2θ)

d spacing

(nm)

Intensity

( % )

Particle size

. (nm)

Phase

h k l

values

JCPDS

Reference

Cast mirror

29.1465

3.06393

10.81

45.791

𝐂𝐮𝟔.𝟐𝟓𝐒𝐧𝟓

1 0 1

47-1574

43.0922

2.10200

100

31.143

Cu

1 1 1

00-004-0836

60.5247

1.52849

67.66

84.503

𝐂𝐮𝟔.𝟐𝟓𝐒𝐧𝟓

1 0 3

47- 1574

72.6019

1.290352

12.5

40.7274

𝐂𝐮𝟔.𝟐𝟓𝐒𝐧𝟓

2 1 1

47- 1574

Thin film

42.387

2.13073

100

18.648

𝐂𝐮𝟓.𝟔𝐒𝐧

1 1 1

00-031-0487

Powder form

of cast mirror

24.368

3.64980

1.8

70.686

𝐂𝐮𝟔𝐒𝐧𝟓

1 1 2

01- 072- 8761

25.834

3.44586

5.9

74.110

Not identified

29.880

2.98787

7.1

78.322

𝐂𝐮𝟒𝟎.𝟓𝐒𝐧𝟏𝟏

6 0 0

00- 031-0485

34.643

2.58724

4.2

74.984

𝐂𝐮𝟒𝟏𝐒𝐧𝟏𝟏

4 4 4

65- 7047

35.655

2.51604

0.7

77.283

𝐂𝐮𝟖𝟏𝐒𝐧𝟏𝟐

5 5 1

03- 065- 1778

37.503

2.39624

5.2

71.112

𝐂𝐮𝟏𝟎𝐒𝐧𝟑

1 2 0

01- 071- 0339

42.736

2.11412

100

68.258

𝐂𝐮𝟏𝟎𝐒𝐧𝟑

3 0 0

00- 026- 0564

47.492

1.91290

5.0

72.947

𝐂𝐮𝟒𝟏𝐒𝐧𝟏𝟏

6 6 4

01- 031- 0485

49.708

1.83269

2.1

68.413

𝐂𝐮𝟒𝟎.𝟓𝐒𝐧𝟏𝟏

8 4 4

00- 071- 0339

50.538

1.80452

1.3

75.746

𝐂𝐮𝟒𝟏𝐒𝐧𝟏𝟏

9 3 3

01- 071- 0094

51.925

1.75956

3.3

76.849

𝐂𝐮𝟏𝟎𝐒𝐧𝟑

3 1 0

01- 071- 0339

62.634

1.49510

19.2

82.044

𝐂𝐮𝟔.𝟐𝟓𝐒𝐧𝟓

2 0 2

00- 047- 1575

63.917

1.45530

2.2

80.735

Not identified

68.324

1.37178

2.1

78.071

𝐂𝐮𝟖𝟏𝐒𝐧𝟐𝟐

10 6 6

03- 065- 1778

71.175

1.32366

1.4

65.575

𝐂𝐮𝟒𝟏𝐒𝐧𝟏𝟏

12 6 2

65- 7047

72.985

1.29523

3.0

73.212

𝐂𝐮𝟔𝐒𝐧𝟓

0 2 7

01- 072- 8761

74.494

1.26834

2.4

69.942

CuSn

0 0 4

03- 065- 3434

78.196

1.22144

12.5

73.658

𝐂𝐮𝟖𝟏𝐒𝐧𝟐𝟐

12 6 6

03- 065- 1778

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Figure 5. Nature of decreasing crystallinity in different cases

The combined use of AFM and XRD analyses of the reflecting surface of cast and thin film form of mirror

reveal the presence of a transparent, thin, semi- amorphous layer on the mirror surface, which may be due to

clever polishing of mirror material during its construction. It also revealed that the reflecting surface of

Aranmula mirror contains nano-particles that are considered to be semi-crystalline in nature. These results are

broadly in agreement with the report of early literature [14, 15, 16].

3.4. Thermal Characterization by means of TG/DTA

TG/DTA curve for mirror sample up to 1000℃ in Nitrogen atmosphere is shown in Figure 6.

Figure 6. TGA and DTA curves of cast Aranmula mirror sample

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The material is found to be thermally stable up to a temperature of 600℃ and a notable mass change is

observed afterwards. To confirm this feature we have carried out DTA analysis and a clear discontinuity is

observed at a temperature of 736℃ , which is very close to the melting point of the mirror sample reported in

early literature [17].

3.5. Optical Reflectance Studies by UV- Visible Spectroscopy

The reflectance spectra for cast and thin film form of the mirror samples are shown in Figure 7. The optical

reflectance values for thin film form of the cast mirror material are shown as dotted line and its higher optical

reflectance is clearly seen in figure. Optical spectra of cast and thin film form of Aranmula mirror showed more

uniform reflectance across the entire visible region. Both spectra gave a maximum value of reflectance in the IR

region and absorption was observed in the lower wave length region, which results in a significant loss of

reflection. The average optical reflectance of the cast Aranmula mirror sample in the visible region was found to

increase from 61.35 % to 71.02 % when it was prepared in the form of thin film. The reflectance spectrum of

Aranmula mirror obtained matched with that of speculum metal mirrors reported in literature [18]. Tolansky

obtained thin film form of speculum metal mirror for the first time, which also had enhanced optical reflectance

[4].

Figure 7. Optical reflectance spectra of cast and thin film form of Aranmula mirror samples

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4. DISCUSSION

Speculum metal is commonly used for making traditional mirrors and art objects in medieval periods. Minor

constituents are added to speculum metal to modify its color, mechanical, physical and optical properties since

historical times. Lead is added to Chinese mirrors belonging to the periods of Han dynasty and later through

Tang and Sung Dynasties [19]. Many of the mirrors used in Roman times were made using high tin leaded

bronze, with tin content of 20- 24 % and lead variable typically 5- 12 % [20]. The minor constituent arsenic is a

very common element in ancient copper- tin alloys, either as an accidental or an intentional addition [21]. It

plays a crucial role in early reflecting telescopes of Newton, James Nasmyth and William Lassell [1, 22]. In

medieval period, bronze mirrors with surface amalgamated with mercury are believed to have been used [23,

24]. The EDS measurements performed on our experimental specimen of Aranmula mirror indicated that it was

essentially a speculum mirror containing about 62- 69% copper and 30 – 32% tin. The minor constituents

constitute only about 1- 2% which included As, Zn, Ag, Pb, P, S, Al, and Fe.

Srinivasan et al. [12] reported that Aranmula mirror

consists of Cu 64.73%, Sn 32.47% and the rest 2.8% and Pillai et al. [13] pointed out that it consists of Cu

70.4%, Sn 29.4%, Zn 0.06%, Fe 0.034% and Ni 0.052%. Also Sekhar et al. [14] reported that the ancient mirror

alloy consists of Cu 32.1%, Zn 0.082%, Ni 0.068%, Al 0.02% and rest of copper. Our studies provide for the

first time the evidence for the presence of minor constituents such as As, Ag, and Pb in this speculum mirror

making. The presence of minor constituents may alter the mechanical, physical and optical properties of

speculum mirrors. The EDS measurements of mirror samples at different locations reveal a clear indication that

the compositions are not homogeneous everywhere and also the inclusion of oxygen and number of elements

added in the analysis affect significantly the weight percentage of copper and tin.

The present work reports the detailed microstructure of

speculum mirror for the first time using XRD and AFM analysis techniques. The information about some

physical properties such as particle size, crystalline phases and level of crystallinity are obtained through

microstructural investigations of mirror material. The XRD spectrum of mirror material in powder form is found

to be mostly crystalline in nature and its crystallinity formation is about 87%. However the cast mirror surface is

found to be 26% crystalline and 74% amorphous. The thin film surface is also 14% crystalline and 86%

amorphous.

14

The XRD pattern of the powder form of cast mirror mainly consists of intermetallic phases Cu6.25Sn5,

Cu41Sn11, Cu10Sn3 and Cu81Sn22 of Cu- Sn alloy system, in which particle size varied between 66- 82 nm.

The reflecting surface of cast mirror consists of an intermetallic phaseCu6.25Sn5, in which the average particle

size is found to be about 74 nm. The thin film surface also contains only one intermetallic phase Cu5.6Sn whose

particle size is found to be about 18 nm. The variation in particle size and intermetallic phases may affect the

homogeneity of the mirror surface. In this sense thin film speculum is most homogeneous at the micro level.

The XRD data obtained from our studies does not give a

specific pattern for delta phase suggested in early literature. Srinivasan et al. pointed out that the presence of this

delta phase is optimized by a clever casting and polishing process [12]. Meek suggested that delta phase is not

formed on normal tinned surfaces, but it can occur on heat- treated tinned surfaces. However it does normally

occur in the body of a cast or worked bronze as the eutectoid and has a characteristic microstructure, which is

only derived from cooling the bronze through the 520℃ isothermal temperature. He also suggested that delta

phase has the cubic γ- brass structure with lower symmetry [25].

The three dimensional AFM morphologies at the

reflecting surface of cast and thin film form of mirror sample at 2 µm shows a streak or texture pattern with

porous structure with small value of roughness. The combined use of XRD and AFM analyses at the reflecting

surface of cast and thin film form of mirror suggest that on the mirror surface there exist a transparent, thin,

semi- amorphous layer, which may be due to clever polishing of mirror materials as found from metallurgical

studies. The estimated average size of the particles from XRD and AFM indicated that they are nano structured.

The thermal stability of the mirror sample obtained from TGA analysis clearly rules out earlier speculations of

herbs and the presence of other organic materials in the traditional Aranmula mirror making. A clear

discontinuity is observed at a temperature of about 736℃ , which is very close to the melting point of the

Aranmula mirror reported in early literature by Nagee et al. [18].

The average optical reflectance of Aranmula speculum

mirror in the visible region during the time of purchasing is found to be 66%. The historic speculum mirrors

made during 18th

century to early 20th

century had an optical reflectance varying between 60- 64%. In this case

the weight percentage of copper varied between 66- 68 % and tin varied between 29 – 33 % [1]. From these data

we can infer that Aranmula mirror is a special kind of cast speculum mirror with highest optical reflectance.

Tolonsky found that for a bronze surface with 55 % copper and 45% tin possessed an average visible optical

15

reflectance of 67% in electroplated form and 71% in thin film form using vacuum deposition. Recent

investigations on the reflecting surface of cast mirror suggest an average optical reflectance of 62% in the

visible region and 72% when it is prepared in the form of thin film. Thus the average optical reflectance of cast

mirror in the visible region is found to increase by about 10%when it is prepared in the form of thin film. The

decreasing nature of recent optical reflectance from the time of purchasing suggests that interaction with

atmosphere causes oxidation in the reflecting surface of speculum mirror. It also suggested that optical

reflectance of cast speculum increased when it polishes and further increased when the mirror material is made

in the form of thin film. From XRD and reflectance studies we could infer an inverse relationship between

crystallinity and optical reflectance as the cast mirror material is transferred to thin film.

5. CONCLUSIONS

The microstructure and chemical composition at the reflecting surface of cast and thin film form of speculum

metal mirror is studied in detail using XRD, AFM and EDS analyses techniques. The reflecting surface of

speculum metal mirror is found to be semi- amorphous in nature with particle size varying between 18 – 74 nm,

which reveal that the constituent particles in the reflecting surface of cast and thin film form of mirror exist in

nanoscale, which are semi- crystalline in nature. The intermetallic phases Cu6.25Sn5, Cu5.6Sn of Cu- Sn alloy

system are found from these studies. The thermal stability of Aranmula speculum mirror sample up to 600℃ as

evident from TGA analysis, clearly rules out earlier speculations of herbs and the presence of other organic

materials in the traditional Aranmula mirror making. The average optical reflectance of Aranmula speculum

mirror specimen under investigation is found to be relatively higher than similar cast speculum mirrors reported

till dates. Its optical reflectance is likely to depend on the magnitude of crystallinity and homogeneity and also

the presence of minor constituents such as As, Ag, Al and Au. The optical reflectance of the cast Aranmula

speculum mirror material is found to increase by about 10 % on an average in the visible region of the spectrum

when it is prepared in thin film form in glass substrate by thermal evaporation method.

16

ACKNOWLEDGEMENTS

We are extremely grateful to Dr. S. Narayana Kalkura, director, Crystal Growth Centre, Anna University,

Chennai, Dr. K. MuraleedharaVarier, Emeritus scientist, Dept. of Physics, University College, Trivandrum, for

their constructive comments and valuable suggestions, Dr. B. Pradeep, Mr. Anuroop, Dept. of Physics, CUSAT,

Cochin, Dr. V.P.Mahadevan Pillai and his staff, Dept. of Optoelectronics, University of Kerala, Kariavattom,

The director, NCESS, Trivandrum, The staff of XRD unit, Dept. of Physics, University of Kerala, Kariavattom,

The Director, STIC, Cochin University, Kochi for providing useful reference materials and laboratory facilities

to conduct experimental analysis, Dr. Abdul Kalam, Dept. of Physics, Iqbal College, Peringammala, Mr.

Sumesh Gopinath and all teaching faculties, Dept. of Physics, University College, Trivandrum for their

continuing support and contributions during the entire work.

REFERENCES

[1] A.A. Mills, R. Hall, The Production of a Plane Surface: As Illustrated by Specula from Some Early

Newtonian Telescopes, Notes Rec. Roy. Soc. London, 37(1983), P.147- 166.

[2] Mikhail M. Miroshnikov, Sergei V. Ljubarsky, Yurii D. Khimich, Mirrors for Optical Telescopes, Opt.

Eng, 31(1992), P.701- 710.

[3] J.R. William P Barness, Basic Properties of Metal Optics, Opt. Eng, 16(1977), P.320- 323.

[4] D. Tolansky, W.K. Donaldson, The Reflectivity of Speculum Metal, J. Sci. Instrum, 24(1947), P.248 –

249.

[5] J.B. Hicks, Euclid, Ohio, Internal Combustion Engine, Serial No. 569717, Appl. Mar 6, (1956), P.1- 6.

[6] Akira Ito, Kuaiki Murase, Takashi Ichii, Hiroyuki Sukimura, Cu- Sn Alloy Metallization of Polymer

through Reduction – Diffusion Method Using Ionic Liquid Bath at Medium- Low Temperature, ECS

transactions, 16(2009), No. 49, P.677- 679.

[7] Fred M Huntoon, Solar Water Heater, Serial No. 895761, Appl. Aug 11, (1908), P.1- 2.

[8] Akira Sakamoto, Storage Battery for Vehicles, Serial No.895964, Appl. July 22, (1975), P.1- 4.

17

[9] A.V. Crewe, M. Isaacson, D. Johnson, A High Resolution Electron Microscopy for Use in

Transmission Scanning Electron Microscopy, Rev. of Sci. Instrum, 42(1971), No. 4, P.411- 420.

[10] Jianguo Ren, Xiangming He, Liwang, Weihua Pu, Changyin Jiang, Chunrong Was, Nanometer

Copper- Tin Alloy Anode Materials for Lithium- Iron Batteries, Electrochimica Acta, 52(2007),

P.2447- 2452.

[11] Charls L Faust, Wilbur G Hespenneid, Electrodeposition of Bright Cu- Sn Alloy, Serial No. 2658032,

Appl. June 11, (1949), P.1- 16.

[12] S. Srinivasan, I. Glover, Skilled Mirror Craft of Intermetallic Delta High-Tin Bronze (Cu31Sn8, 32.6 %

tin) from Aranmula, Kerala, Current Sci,93(2007), P.35-40.

[13] S.G.K. Pillai, R.M. Pillai, A.D. Damodaran, T.R. Ramachandran, The Thermomechanical Processing

of High- Tin Bronze- An Old Practice in a South Indian Village, JOM, (1994), P.59 –62.

[14] J.A Sekhar, A.S Mantri, S. Yamjala, S. Saha, R. Balamuralikrishnan, P. Rama Rao, Ancient Metal

Mirror Alloy Revisited: Quasicrystalline Nanoparticles Observed, JOM, 67(2015), P.2976- 2983.

[15] G.M. Ingo, P. Plescia, E. Angelini, C. Riccucci, T. De Caro, Bronze Roman Mirrors: The Secret of

Brightness, Appl.Phys, 83(2006), P. 611- 615.

[16] La Niece Susan, Craddock Paul, Metal Plating and Patination: Cultural, Technical and Historical

Development, Edited by Susan La Niece, Paul Craddock, Butterworth Heinemann Ltd., Jordhan Hill,

Oxford, 1993, P.247- 275.

[17] T. Nagee, S. Srinivasan, S. Ranganathan, R.M. Pillai, Y. Shimizu, Thermographical Analysis of

Continuing Tradition of Mirror Casting in Kerala, ISIJ International, 54(2014), P.1172 – 1176.

[18] T.E. Girish, Speculum Metal Mirror as a Solar Energy Concentrator, [in] Proceedings of internatinal

conference on Solar Cells, CUSAT, 2008, P.199- 200.

[19] A. Gutkind Bulling, Isabella Drew, The Dating of Chinese Bronze Mirrors, Archives of Asian Art,

25(1971/1972), P.36- 57.

18

[20] David A. Scott, Metallography and Microstructure of Ancient and Historic Metals, Edited by Irina

Averkieff, Getty Trust Publications, Tien Wah Press Ltd., Singapore, 1991, P.19.

[21] O. Oudbashi, S.M. Emami, M. Malekzadeh, A. Hassanpor, P. Davami, Archaeometallurgical Studies

on the Bronze Vessels from Sangtarashan, Luristan, W- Iran, Iranica Antiqua, 48(2013), P.147- 174.

[22] A.A. Mills, D.J. Turvey, Newton’s Telescope: An Examination of the Reflecting Telescope Attributed

to Sir Isaac Newton in the Possession of the Royal Society, Notes. Rec. R. Soc. Lond., 33(1974),

P.133-135.

[23] William F Collins, The mirror- Black and Quicksilver Patinas of Certain Chinese Bronzes. Royal

Anthropological Institute of Great Britain and Ireland, 64(1934), P.69- 79.

[24] Edison Pettit, The Reflector, Astronomical Society of the Pacific, (1956), No. 331, P.249- 256.

[25] N.D. Meek, Tin-Rich Surfaces on Bronze – Some Experimental and Archaeological Considerations,

Archaeometry, 28(1986), P.133- 162.