Optical Properties of Tin Opacified Glazes

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Archaeometry 42, 2 (20 00), 325-340 . Printed in Great Britain

OPTI CAL PROPERTI ES O F T I N - O P A C IF I E D G L A Z E S *

M . V E N D R E L L . l J . M O L E R A ' and M. S. T I T E 2

'Department of Crisrallography and Mineralogy. University of Barcelona, U Marti' i FranquPs. s/n, 08028 Barcelona,

Spain

Research Labo ratory fo r Archaeology and the History ofArt, Oxford University, 6 Keble Road, Oxford, OX1 3QJ, UK

Tin glazes contain SnO2 particle s, with a diameter similar to the wavelength of visible light,

which are responsible fo r glaze opacification. In this pa pe r a theoretical model to explain the

optical behaviour of these glazes is developed. This model is tested by measurement of the

refectance and transmittance spectra for diyerent optical paths in Spanish Islamic and

mudejar glazed ceramics and it is shown to be valid by total refectance non-destructive

measurements on a wide range of other Islamic and related glazed ceramics.

KEYWORDS: SPAIN, MURCIA, PATERNA, ISLAMIC, MEDIEVAL, MUDEJAR,

SPECTROPHOTOMETRY, CERAMICS, GLAZE, LEAD, TIN, O PACITY, TECHNOLOGY

I N TR OD U C TI ON

One of the finest medieval ceramic products was the w hite glazed ware decorated with green,

black, blue and lustre. The decoration was usually applied to a white opacified glaze, the quality

of the ceramic being related to the 'whiteness' of this glaze. This tradition began in the early

Islamic period and has continued through to the present day with local imitations of this Islamic

tradition being made, both as deco rative ceramics and as souvenirs for tourists.The opacification is produced by the distribution of small particles of tin oxide within a

colourless glaze, resulting in the reflection and the scattering of the light. The extent of the

opacification achieved is determined by (i) the number of particles (which has the effect of

increasing the path length for the light); (ii) the s ize of the particles; (iii) the presence of clean

faces to the crystalline particles; and (iv) the difference of refractive index betw een crystals and

glass matrix. Accordingly, a significant proportion of the incident light is reflected within the

glaze. Thus, the best quality glazes are achieved when the colour of the ceramic body (w hich is

normally reddish or creamy) cannot be seen through the glaze and the appearance of the piece is

white. To conceal the body colour and ensure whiteness, the am ount of opacifier an do r the

thickness of the glaze must be increased. In both cases the econom ic cost also increases.

In ancient ceramics, the concentration of tin ox ide varied from 3-4% (Iraq in the eighth

century AD) up to 20-25% (Italy in the thirteenth century AD ) (Mason and Tite 1997), but the

opacification is normally achieved with concentrations between 5 and lo%, as in most of the

Islamic and mudejur workshops in Spain (Molera 1996). Several classic studies of opaque glazes

were m ade in the first half of this century, relating the concentration, distribution and crystal size

of the opacifier to its efficiency. Von Weiman (1926) and Beds er ul. (1951) studied the

opacification with titanium oxide and suggested that a good opacification is produced w hen the

sizes of the crystals fall between 200 and 400nm. Stutz and Pfund (1927) gave an empirical

explanation for the opacification on the basis of the reflected and scattered light, and the

*Received 4 March 1999, accepted 25 January 2000.

325



326 M . Vendrell, J. Molera and M . S. Tite

relationship between the crystal size and the wavelength of the light. Molera er al. (1999) have

demonstrated that tin oxide reacts during the glaze formation and recrystallizes during heating,

forming SnOz particles of a few hundred nanometers in size.

The primary aim of this paper is to assess the usefulness of total reflectance measurem ents for

quantifying the opacity and whiteness of tin-opacified glazes and, thus, for prov iding a tool forcomparing and contrasting the optical properties of d ifferent glazes. In order to assist in the

interpretation of the total reflectance data, a model for the total reflectance was developed in

terms of reflection from the glaze surface, the internal reflection andor scattering from the

opacifier and the reflection at the glaze-bo dy interface. The validity of this m odel was tested by

means of detailed reflectance and transmission measurements on six Islamic and rnudejur glazed

sherds from Spain, together with non-destructive total reflectance measurements only on a

diverse range of Islamic and Italian glazed sherds spanning the period AD 700-1600. For both

groups of sherds, the tin oxide and lead oxide concentrations of the glazes and the tin oxide

particle sizes were determined by examination in an analytical scanning electron microscope

(SEM). These compositional data were also used to assess the potential of total reflectancespectra to provide , non-destructively, a measure of the tin and lead oxide contents of a glaze .

HISTORICAL BACKGROUND

The origin of tin-opacified glazes has been traditionally related to attempts, around the ninth

century AD, to imitate Chinese porcelain. However, recent studies seem to indicate an earlier

origin. Kleinmann (1986) and Mason and T ite (1997) point out that opacified glazes were being

produced in Iraq during the Achaemenian period (c . 550-330 BC) and continued into the

subsequent Parthian (c. 250 BC to AD 230) and Sassanian (c. AD 230-640) periods. The glazes

of these periods were opacified through the presence of crysta lline phases, such as wollastonite,

diopside, quartz and feldspar, and a ir bubbles without any contribution from tin ox ide (H edges

and Moorey 1975; Hedges 1976; Kleinmann 1986; Mason and T ite 1997).

According to both C aiger-Smith (1973) and Mason and T ite (1997). the first use of tin oxide as

an opacifier was in Iraq during the first half of the e ighth century AD. However, in this case, the

tin oxide was not distributed through the glaze but was applied as a slip between the ceramic

body and the glaze , which also con tained crystals of calcium silicates. The beginning of the true

tin-opacified glazes, with tin ox ide scattered through alkaline glazes instead of being applied as a

slip between body and glaze, was in the second half of the eighth century AD.

Around 975 AD the potters from Basra (and probably from other places) moved to Fustat and

started to produce tin-opacified glazes and lustre decorated ceram ics. Mason and Tite (1997)

distinguish four different phases each with d ifferent compositions of both the g lazes and theceramic bodies. During the second half of the eleventh century AD, the technique was

transferred to Syria and Iran. In this same period, some potters left Fustat to establish workshops

in the Iberian peninsula in the caliphate of Medinat al-Zahra. Here, the ceramic bodies were

made from an illitic clay from the Guadalquivir valley and the glazes contained around 15% tin

oxide and between 20 and 40% lead oxide (Escudero-Aranda 1990).

Tin-opacified lead glazes continued to be used in Islamic Spain with different decorative

styles and, later, after the Christian invasion, survived as a mudejar production technique.

Several excavated workshops from these periods have allowed these ceramics to be stud ied bothfrom the typological point of view (Navarro Palazdn 1990; Rosselld Bordoy 1995; Am igues and

Mesquida 1993; Gisbert 1990) and from the technological point of view (Molera 1996; Molera



Optical properties of tin-opacijied g lazes 327

Figure 1

that occur.

et al. 1997; Perez-Arantegui and Castillo 1997 and forthcoming). All the Spanish glazes are

lead-rich and contain from 5 to 10% tin oxide as small crystals of less than 1 pm diameter

(Molera 1996).The technique of tin-opacified glazes subsequently diffused northwards reaching

Barcelona (fourteenth century AD), Marseille (thirteenth century AD) and Italy (thirteenth-

fifteenth centuries AD), among other places in Europe.

Although stonepaste bodies were sometimes used in Islamic Egypt and the Near East (Mason

and Tite 1994), in Spain and the rest of Europe tin-opacified glazes were normally used with

calcareous clay bodies which fired to a buff or cream colour. On the archaeological evidence,

production involved a double firing with the glaze being applied to a previously fired biscuit

body.

Schematic diagram showing the path of a ray of light passing through a glaze, together with the rejlections

OPTICAL MODEL FOR TIN-OPACIFIED GLAZED POTTERY

Tin-opacified glazes consist of a layer of glass, typically 100-500 pm thick, with small crystals

of tin oxide (cassiterite) scattered through its thickness. The outer surface of the glaze is a flat

surface (at least at the millimetre scale) and, therefore, results in specular reflection. In contrast,

the contact between the glaze and the body paste is a rough surface and this, therefore, acts as a

diffuser of light. Figure 1 shows the path of a ray of light passing from the air through such a

glaze together with the several reflections occurring at different points in the glaze.First, there is the reflection at the outer surface of the glaze. This reflection is specular and, to a

first approximation, can be evaluated for normally incident light by using the Fresnel formula for

dielectric substances (Galopin and Henry 1972):

R , = I , / I ~= ( n l - n212/(nl+ n2)2

where n l is the refractive index of the glass and n2 that of the air (n2= I for our calculations).

Assuming n1 = 1.6, the reflection is around 5%.

Next, the light travelling on through the glaze interacts with thousands of discontinuities

associated with the faces of the minute crystals of the tin oxide opacifier phase. At each one of

these faces, the light is reflected, the amount again being evaluated using the Fresnel equation. In



328 M. Vendrell, J. Molera and M. S. Tire

this case, nl is the refractive index of the glass matrix while n2 is that of the cassiterite. Assuming

n2= 2.00 (average of the refractive index of the tin oxide), each reflection is about 1.2%. The

spectral response of the n ormal reflection is controlled by the d ispersion of the refractive index

of the glass in the v isible range (Cauchy equation). Since the d ispersion for the glass is low, the

reflection in the visible spectrum can be considered as colourless.If the tin oxide crystals are small enough (i.e., diam eter less than the w avelength of the light),

a different phenomenon also takes place; that is, the scattering of the light by each of the

particles. In this case the intensity of scattered light is controlled by the Rayleigh equation

(Jenkins and White 1987):

where I , is the scattered intensity, I , the inciden t intensity,k a co nstant depending on the p article

size and the m edium, and X he wavelength of the light. According to this formula, the scattered

intensity is higher for the blue end of the spectrum than for the red end which is, for exam ple, the

reason that one sees a blue sky and a red sunset or sunrise. Thus, if scattering is the mostimportant phenomenon in the opacification of the glaze , the g laze will be sligh tly blue, as noted

by Friedberg et al . (1948).

The absorption by the glass matrix of the glaze is not usually considered since the glass ac ts as

a dielectric and the absorption coefficient is near to zero. H owever, for lead-rich glasses, there is

an important absorption in the near ultraviolet (wav elength less than 380 nm ) (Stroud and Lell

1971) which can be detected in the blue end of the v isible spectrum. This absorption is the reason

for tin-opacified lead glazes appearing white, even when the crystal size of the tin oxide is small

enough to produce scattering with a higher intensity in the blue .

Finally, the light reaching the interface between the glaze and body paste is reflected at the

surface of the paste and then trave ls back again through the thickness of the glaze to the outer

surface. As the surface between the glaze and paste is rough, this reflection is diffuse and,

therefore, can not be evaluated by the F resnel formula. Instead, its intensity depends on the state

of the su rface and the refractive indices of both media. As the paste is normally a fired clay

containing several per cent of iron oxide, the colour of this diffuse reflection is reddish or creamy

rather than colourless; that is, the reflected intensity is higher for the red than for the blue or the

green parts of the visible spectrum.

Total reflectance relationship

Assuming that a ray of unit intensity falls normally on the surface of a tin-opacified glaze (F ig. l ) ,

the specular reflection at the surface, calculated using the Fresnel equa tion, is Re and, therefore,

the intensity of the light transmitted to just under the g laze su rface is (1 -Re).As the light passesthrough the glaze, its intensity is reduced as a re sult of internal reflection and sca ttering by the tin

oxide crystals, the transmitted light being calculated from the g eneral expression:

(1 )

where Ti is the internal transmittance of the glaze, (Y is the absorption index, as determ ined by the

extent of reflection and scattering from the tin oxide, and t is the thickness of the glaze

transversed. Therefore, the intensity of the light reaching the glaze-paste interface is:

T .--

where t is now the actual thickness of the glaze.



Optical properties of tin-opac$ed glazes

The intensity of the light reflected from the glaze-paste interface is then given by:

329

(1 - R,)e-*' Rp

where Rp is the diffuse reflectance from the rough surface of the body paste. Because this

reflection is diffuse, the light returning to the surface of the glaze is in the form of a cone andonly those rays forming an angle with the normal less than the c ritical angle ( i , ) will exit from

the glaze back into the air. In order to allow for this effect, the thickness of the glaze for the

return light path is taken to be f' which correspon ds to an incident angle of iJ2 and which is

given by the expression:

t' = tlcos ( i J 2 ) ( 2 )

Therefo re, the intensity of the light originating from the reflection at the glaze-paste interface

that finally emerges from the glaze surface is given by:

e RP1 - R ) 2 , - * ( t f r ' )

Thu s, the total light reflectance observed at the su rface of a tin-opacified glaze (RT)will be the

sum of the origin al reflection at the surface of the glaze (Re) , he internal reflection and scattering

by the opacifier (R; ) ,and the light finally em erging from the glaze after reflection at the g laze-

body interface; that is, RT is given by:

The parameter R; depend s on both the thickness of the glaze and the internal characteristics of

the glaze. Therefore, a new param eter, called the 'characteristic reflectance' (R,) , is introduced

which is dep endent only on the concentration and particle size of the tin oxide crystals, their

distribution being assum ed to be uniform.

If we consider a ray of light reach ing an infinitesimally thin layer ( d f ) t a depth o f t from the

front surface of the glaze, this ray will have been reduced in intensity in accordance with

equation (1) . The internal reflection and scattering from this layer will then be given by the

characteristic reflectance ( R J . Since this reflection and scattering will be diffuse, its return light

path back to the fro nt surface of the glaze will be t', as given by equation (2).Therefo re, the total

internal reflectance (R; )will be the sum of the reflections from each infinitesimal layer making

up the thickness of the glaze and is given by the expression:

(4)

Provided that th e blue scattered light is compensated by the blue absorption of the lead glaze , the

reflectance coefficient (R,,) s the only parameter that introduces colour into the reflected light. Itis, therefore, the magnitude of the term within the square brack ets in equation (3) that determines

the observed colour of the g laze. As a result, either by increasing the absorption index (a) y the

addition of a higher concentra tion of tin ox ide, or by inc reas ing the thickness of the glaze ( t ) , he

intensity of any colour originating from the body paste will be reduced and a w hite glaze will be

achieved.

R .- Y R , e- * f + dt- 0

EX P ER I M EN TA L P R OC ED U R ES

Reflectance and transmittance measurements were made on two Islamic pottery sherds from a

ninth-century w orkshop in Murcia and for four mudejar pottery sherds, one of the thirteenth



330 M. Vendrell, J. Molera and M. S. Tite

fSample

Figure 2 Schematic diagram showing the ex perimental arrangement or measuring the refiectance and transmittance.

century and three of the fourteen th century, all of them from the w orkshops of Paterna. Forall

these measurements, an integrating sphere of ba rium su lphate was used to co llect the scattered

light emitted from the samples. The spectropho tometerwas a double beam VIS-UV, model UV-

2101PC, and the spectra were acquired from 380 to 800nm at 1 nm interva ls with a band width

of 2 nm. In all cases, the measured area was a 4mm diameter circle.

The total reflectance of the ceramic (i.e., tin glaze plus body paste) (RT) was measured by

placing a surface, which was as flat as possible, at the ex it window of the integrating sphere. To

measure the diffuse reflectance of the body paste ( R J , a ground surface of a body after removal

of the glaze was placed a t the measurement slit. In this case, a drop of imm ersion oil and a cover

glass were attached to the surface to be measured in order to sim ulate the effect of the glaze on

the paste. One should note that the saturation of the colou r observed when the paste is wetted is

higher than when it is dry because of a reduction in the light reflected by quartz and feldspargrains, which are almost white. Correction for the reflection from the front of the cover glass is

not necessary because the incidence is norm al and, therefore, because of the geom etry of the

integrating sphere, this reflection is not collected by the photomultiplier (Fig. 2).

In order to determine the reflectance and transm ittance of the g laze itself, a flat surface of the

glaze was glued with resin to a 1 mm thick glass slide and the body was then ground away to

obtain a slice of tin-opacified glaze with two parallel faces. In order to avoid diffuse reflection at

the upper surface, it was coated with a d rop of imm ersion oil (DIN 58884 nD= 1.515) and a

cover glass (Fig. 3). The reflectance from this mounted glaze slice (Roil)was measured by

placing it at the exit slit of the integrating sphere, and its transmittance (Toil) y placing it at the

entrance window (Fig. 2).These raw data (Roiland Toil) eed to be corrected for the reflections from the front and rear

glasses between which the glaze slices were mounted. The calculations have been made by

assuming normal incidence, which is considered to be a good approximation because the variation

of the am plitude with the angle of incidence is not very importan t for these d ielectric materials

(Galopin and Henry 1972). In these calculations, the reflection at the oil-glaze interface has not

been considered because, according to the Fresnel formula, it is lower than 0.1. Also, there is no

reflection at the o il-glass interface as both med ia have the sam e refractive index.

Correcting for the front and rear reflections at the glass-air interfaces, the measured

transmittance (Toi l ) an be expressed as:

Toil = (1-Ra)* i



Optical properties of tin-opaciJied glazes 331

Figure 3 Schematic diagra m showing the method of mounting fo r measurement of the reject anc e an d transmittance of

the glaze itself; together with the rejections that occur when a ray of light passes through the mounted glaze.

where R, is the reflectance of the glass which can be calculated from its refractive index. The

transmittance of the glaze itself (Ti) can thus be determined. Then, using equation (l), the

absorption index (a) an be calculated.

Similarly, correcting for the first order reflections at the glass-air interfaces, and for second

order reflections at the front glass, the measured reflectance from the mounted glaze (Roil) an be

expressed as:

Roil = R, + Ri + [Ti2R, (1 - R,)2]

The total internal reflectance ( R i )of the glaze can thus be determined. Then, using equation (4),

the characteristic reflectance (R,) can be calculated.

In addition to the detailed spectral measurements undertaken on the tin-opacified glazed

pottery from Spain, non-destructive spectral data were obtained for 20 Islamic pottery glazes

from Iraq (eighth-tenth centuries AD), Egypt (tenth-eleventh centuries AD) and Turkey (Iznik

ware, sixteenth century AD), and for Italian maiolica (fourteenth-fifteenth centuries AD). In thiscase, the measurements of the total reflectance (RT) were made using a ‘portable’ compact

spectrophotometerMinolta CM-503, equipped with an integrating sphere, three basic sources of

illumination and a multichannel analyser. The equipment determined the total reflectance from

400 to 700nm at 20nm intervals.

For both groups of pottery, the colour coordinates (x and y) for the CIE 1931 diagram,

together with the colour attributes, luminosity (Y), excitation purity be) nd dominant

wavelength ( AD ) , were calculated from the spectral curves for the total reflectance. Data on

the compositions and thicknesses of the glazes and the tin oxide particle sizes were

obtained by examination of polished sections through the glazes and bodies using an analytical

SEM.



332 M. Vendrell, J. Molera and M. S. Tire

0.50-

0.40-

0.30-b

0.20-

0.10-

9

O . 7I T 821

"13Figure 4 Spectral curvesfor Spanish ceramicsfor (a) total rejectance of tin-opac ijed glaze plus body paste (RT) nd

(b) d if is e rejlectance of body paste (R,,).Dashed lines are usedfor the Islamic sherds and solid linesfor the mudejar

sherds.

RESULTS AND DISCUSSION

The spectral curves for total reflectance (i.e., tin-opacified glaze p lus body paste) (RT) for the

Spanish ceramics are presented in Figure 4 (a) and the corresponding curves for the diffuse

reflectance of the body pastes (R,) are presented in Figure 4 (b), the latter exh ibiting the stepped

shape characteristic of calcareou s clays. The spectral curves for the glaze transmittance (T i ) , he

absorption index (a),he total intern al reflectance (Ri)nd the characteristic reflectance (R,) fo r

the six Spanish ceram ics are presented in Figure 5 , the ord inates for a and R, being arbitrary.

Starting with the curves for the diffuse reflectance of the pa ste (R,), the total reflectance (RT)

can be calculated for different thicknesses of glaze using equations (3) and (4) in combination

with the experimental values for R, and a. he resulting curves are presented in Figure 6. In all

cases, the agreement between the experimental and calculated total reflectance is reasonably

good, the difference at the red end of the spectral curve for the sherd T903 being due to the

presence of a red spot on the surface of this glaze. The inflections in the 450-550nm range

observed in both the experim ental and corre sponding calculated total reflectance curves for

sherds T903 and TVOl are the result of the colour of the paste being 'seen' through the glaze.

Otherwise, the curves show that the thicknesses of the tin-opacified glazes are sufficient to

conceal the colour of the u nderlying body paste.

The extent to which tin-opacified glazes concea l the colour of the underlying body paste canalso be represented on a CIE 1931 diagram. In Figure 7, the colour coordinates ( x and y ) for the

calculated total reflectances for sherds T921 and TVOl are plotted on a CIE 1931 diagram,

together with those for the corresponding experimental reflectances. From this figure, it can beseen that the colou r coordinates for the calculated total reflectances follow, w ith increasing glaze

thickness, a line from the point assoc iated with the reflectance from the body paste (R,) to a point

corresponding to so-called infinite thickness. In the case of sherd T921, the colour coordinate

point for the experimental total reflectance coincides with the calculated infinite thickness point

(Fig. 7 (left)), indicating that the glaze is sufficiently thick to conceal the colour of the

underlying body. In contrast, for sherd TVOl, whose glaze is insufficiently thick to conceal

completely the colour of the underlying body paste, the colour coordinate point for the



Optical properties of tin-opacifed glazes 333

Co'601

Ri

d

s .01-

0.01-

T 602

0.00Io o 5 0 0 8 0 0 7 0 0 8 0 0 40 0 500 800 7 w 800

wvaenpm(W -Figure 5 Spectral curves fo r Spanish ceramics fo r (a) glaze transmittance (Ti), (b) absorption index (a),c ) rota1

internal refrectance (Ri) nd (d) characteristic refrectance (K).Dashed lines are used fo r the Islamic sherds and solid

lines for the mudejar sherds.

experimental total reflectance lies between the calculated infinite thickness point and the point

associated with the reflectance from the body paste (Fig. 7 (right)).

Next, one needs to consider the relationship between the observed values for the parameters,

Ti,, i nd R,, and the compositions of the glazes (Table 1) .The characteristic reflectance (&),

which is independent of glaze thickness, exhibits the expected correlation with the tin oxidecontent of the glaze, the R, values being higher for sherds T921, SN63 and T903 (10-13% Sn02)

than those for sherds SN66, TVOl and T902 (5-7% SnOz) (Fig. 5 (d)). Similarly, there is some

correlation between the absorption index (a), hich is again independent of glaze thickness, and

the tin oxide content of the glaze, the a values being higher for sherds T921 and SN63 (10- 11%

SnOZ), but not T903 (13% Sn02), than for sherds SN66, TVOl and T902 (5-7% Sn02)

(Fig. 5 (b)).

The negative slopes of the total internal and characteristic reflectance curves (R i and R,

respectively) in the 600-800 nm range (i.e., an increase towards blue end of spectrum) (Fig. 5 (c)

and (d)), together with the corresponding positive slopes of the transmittance curves (Ti)Fig. 5

(a)), are an indication that scattering from the tin oxide crystals predominates over reflection.



334

0.50-0.40-

0.30-R

0.20-

0.10-

M. Vendrell, J. Molera and M. S. Tite

0.80-

0.60-

R 0.40-

0.20-

0.60-SN63

R

R

0 . 0 0 1 ,

4 0 0 5 Q 1 e W 7 w e o o 4 0 0 5 0 0 w o 7 0 0 0 0- m -igure 6 Comparison of the experimental and calculaied speciral curves o r ihe rejec ianc es o r Spanish ceramics. The

lower solid line represents ihe experimental d i f is e reflectance of the body paste (Rp) and ihe upper solid line ihe

experimenial ioial rejeciance (RT). he calculaied values of the iota1 reje cianc e fo r differeni glaze ihicknesses are

ploiied as doiied lines, wiih a dashed line o r the calculaied ioial reflectance corresponding to ihe aciual glaze thickness.



Optical p roperties of tin-opacijied glazes 335

0.44 1 0.441

Y 0 . 4 0 1

T V - 011-921

0.361 & + +

0.321 , , I , , , , 0 , 3 2 4 , , I , , ,

0.32 0.36 0.40 0.44 0.32 0.36 0.40 0.44X X

Figure 7 CIE 1931 diagrams showing the colour coordinates (x and y) or ihe rejlectances for Spanish ceramics, (left)

sherd lY21 and (right) sherd TVOI. The colour coordinaies fo r the calculated total rejleciances a re represented by

crossFs (+), wiih open squares and (p) being the points corresponding to the experimental total rejlectance and the d if is e

rejleciance from the body pasie, respectively.

Therefore, the diameter of a high proportion of the tin oxide crystals must be less than the

wavelength of visible light. However, there is no obvious difference between the curves for the

Islamic sherds (SN63, SN66) and those for the mudejur sherds (T902, T903, T921, TVO1). This

is in spite of the fact that there is a uniform distribution of very small tin oxide crystals (around

200 nm) in the Islamic glazes and a more heterogeneous distribution of larger tin oxide crystals

(in the range 350- 1200 nm) in the mudejar glazes. In contrast, the experimental total reflectance

( R T )curves for the Islamic sherds are the only ones with negative slopes (Fig. 4 a)). However,rather than relating to tin oxide particle size, this more probably reflects the contribution to the

total reflectance spectra from the underlying body colour since the calculated values all tend

towards a negative slope for infinite glaze thickness (Fig. 6) .

Finally, the curves of Ti, and Ri show an important absorption in the blue and near UV

region of the spectrum (Fig. 5 (a)-(c)) that is characteristic of lead-rich glazes. The curves for

R , (Fig. 5 (d)) are not valid in this spectral region because the values calculated for this

parameter are very sensitive to small changes in the experimental data. This absorption in

the blue region helps to compensate for any increased scattering of light in the blue region

by the tin oxide crystals and, as a result, the glaze appears white rather than either creamy or

bluish.

Total reflectance measurements

The spectral curves for total reflectance (RT) or the Islamic pottery from Iraq, Egypt and Turkey

(Iznik) and the Italian maiolica, all measured with the portable Minolta spectrophotometer, are

presented in Figure 8. The colour coordinatesfo r theCIE 1931diagram and the colour attributes

(i.e., luminosity, dominant wavelength and excitation purity), calculated from the measured total

reflectance data, are given in Table 1, together with the corresponding values calculated from the

total reflectance curves (Fig.4 a)) for the Spanish sherds, measured using the double beam VIS-

UV spectrophotometer. Also included in the table are the glaze thicknesses and tin oxide and

lead oxide concentrations.



336 M . Vendrell. J. Molera and M . S. Tite

Table 1 Optical characteristics,' glaze thicknesses and tin oxide and lead oxide concentrations o r Islamic and related

ceramics

Origin Samples X Y Y p e AD 7%ickness(pm) SnOZ8 P b 0 8

Murcia SN63

SN66

Patema T921

T902

T903

Tvo1

Egypt P172*

P56

P181*

P144-INT

P49

P8740-EXT

P8755-INT

P8755-EXT

P8739-INT

P8739-EXT

P8759

P37

P69-INT

P64-INT

P64-EXT

Iraq P8740-INT

Italian 3834Maiolica F11

F1

Iznik IZNIK*

0.3613

0.3544

0.3442

0.3427

0.3588

0.3649

0.3255

0.3389

0.3369

0.3414

0.3195

0.3409

0.3446

0.3405

0.3444

0.3433

0.3438

0.3464

0.3403

0.3371

0.3469

0.3680

0.34560.3386

0.3346

0.3369

0.3681 43

0.3641 65

0.3493 69

0.3483 42

0.3593 53

0.3517 48

0.3495 45

0.3546 51

0.3605 43

0.3587 49

0.3375 70

0.3592 45

0.3602 34

0.3563 44

0.3589 44

0.3618 41

0.3583 56

0.3589 39

0.3588 52

0.3557 65

0.3613 48

0.3759 37

0.3538 470.3476 46

0.3532 56

0.3519 57

27

24

18

18

25

24

13

18

19

20

8

20

21

19

20

21

20

21

19

18

22

30

1914

16

16

578

577

578

576

579

584

568

574

57 1

573

567

573

574

574

575

573

575

575

573

572

575

578

577579

572

574

150

200

150

150

90

70

200

300

500

200

200

400

400

600

600

800

400

400

400

lo00

350

350

300300

150

100

10

5

11

5

13

7

5.4

10.7

5.7

9.2

15.5

3.3

2.9

7.4

5. 8

3.2

5.5

8.0

6.5

7.6

8.4

3.7

7.57.0

9.0

3.5

55

51

46

41

31

42

36

34

34

25

28

1.5

0.6

6.4

4.4

1.1

1.1

7.8

5.9

7.6

8.2

4.2

1616

22

29

' Colour coordinates (x and y) , luminosity (0. xcitation purity @ and dominant wavelength (AD) for total reflectance from glaze

* Stonepaste body.

surface.

For pottery from a specific region, there is some correlation between increasing tin oxide

content of the glaze and increasing total reflectance (asmeasured at 700 nm). This Correlation is

particularly pronounced in the case of the pottery from Egypt (Fig. 8 (b)), with the totalreflectance at 700nm increasing from sherds P172 and P181 (5-6% S n0 2) through sherds P56and P144 (9-11% Sn 02 ) to sherd P49 (15.5% Sn02). Again, there is some correlation for the

Spanish mudejur pottery (Fig. 4 a)) with the total reflectance at 700 nm increasing from sherd

T902 (5% Sn0 2) o sherds T903 and T921(11- 13% Sn 02) .Although the co rrelation is less good

for the pottery from Iraq (Fig. 8 (a)), the glazes with tin oxide con tents less than 4% Sn 02 sti ll

tend to have lower total reflectances at 700nm than those containing more than 5%S n02 .Part of

the reason for this poorer correlation could be that the Iraqi glazes were opacified by both tin

oxide and other crystalline phases (e.g., wollastonite, diopside) rather than tin oxide by itself

(Mason and Tite 1997). In the case of the Italian maiolica glazes (Fig. 8 (c)), a slight increase in

the total reflectance at 700 nm is observed with increasing tin oxide content from 7-9%. Further,



Optical properties of tin-opac$ed glazes 337

7 a "1

400 500 800 700

m len p th (nm)

Figure 8

plus Italian maiolica.

Spectral curves for total rejlectances (RT)or Islamic ceramics from (a) Iraq, (b) Egypt and ( c ) Turkey (Iznik)

it is interesting to note that one of the highest total reflectance values is obtained for the Isnik

pottery (Fig. 8 (c)) which has a stonepaste body with a very low iron oxide content, but only a

very low concentration of tin oxide particles in the glaze, the majority of the tin oxide (3.5%

Sn02) being in solution (Tite et al. 1998).

The correlation between total reflectance and tin oxide content is re-enforced when glaze

luminosity (Y) which is also a measure of reflectivity, being equal to 100 for a perfectly

reflecting surface, is plotted against tin oxide content (Fig. 9 (a)). Again, when the data forparticular pottery types such as Egyptian Islamic, Spanish mudejar and Italian maiolica are

considered, the plots approximate to linear relationships. As in the case of the total reflectance at

700 nm data, the spread in the luminosity data is due to the influence of other factors, such as the

thickness of the glaze, the crystal size and distribution of the tin oxide, and the presence of other

crystalline phases.

When the excitation purity be), hich varies from zero for pure white to 100 for a pure

spectral colour, is plotted against tin oxide content (Fig. 9 (b)), the expected inverse correlation

between excitation purity and tin oxide content is not observed. Only sherd P49 from Egypt with

the lowest observed excitation purity (8) and highest tin oxide content (15.5%) and sherd P64-

EX T from Iraq with the highest excitation purity (30)and a low tin oxide content (3.7%) exhibit



338 M. Vendrell, J. Molera andM. S. Tire

12 -

?m 8 -

1

4 -

E "1 E112-

53" 6 -

1

4-

T

M

E

T

SNE

0 7

30 40 50 0 70 5 1 0 1 5 2 0 2 5 3 0

L u n M L y M Po

Figure 9 Plors of glaze luminosity (Y) left) and excitation purity (p,) (right) versus tin oxide content for Islamic

poner yfrom Spain (SN), Iraq OR), Egypt ( E ) and Turkey (Imik) (IZ );f orSpanish mudejarpo ttery (T); ndfo r Italian

maiolica (M).

the expected correlation. For excitation purity, in addition to the factors influencing the spread in

total reflectance at 700nm and luminosity values, the spread could be further increased

depending on the extent to which the preferential scattering by the tin oxide in the blue

region of the spectrum is compensated for by the absorption in the blue by the lead oxide in the

glaze. Again, there is not obvious correlation between dominant wavelength (AD) , which spans

only a relatively small range from 567 nm to 584 nm (correspond ing to the green-yellow region

of the visible spectrum), and the tin oxide contents for these glazes.

The negative slopes of the total reflectance curves in the 600-7OOnm range (i.e., increase

towards the blue end of the spectrum ) for som e six sherds (SN63, SN66, P172, P18 1,87 39 and

F1: Figs. 4 (a) and 8) indicate that scattering from the tin oxide crystals predominates over

reflection, the diameter of a high proportion of the tin oxide particles being less than the

wavelength of visible light. However, the absence of a negative slope for the majority of

the sherds does not necessarily indicate that the reflection predom inates since, as suggested for the

Spanishmudejar sherds, there could be a con tribution to the total reflectance from the underlying

body co lour. Certainly, there a re no obvious differences in the tin oxide particle sizes between

glazes that exhibit and those that do not exhibit negative slopes in the 600-700 nm range.

Finally, as shown in Figure 10, the expected increase in the absorption in the blue and nearW

region, as measured by the decrease in total reflectance between 420nm and 400nm,with

increasing lead oxide content of the glaze is generally observed. Thus, the Iraq glazes with leadoxide con tents of less than 10%exhibit the low est absorp tions (i.e., 1-4% decrease in 420-

400 nm range). The majority of the Egyptian, Iznik and Italian maiolica glazes with lead oxide

contents in the range 15-40% exhibit interm ediate absorptions (i.e., 3-6% decrease) and the

majority of the Spanish sherds with lead ox ide contents in the range 30-55% exhibit the highest

absorptions (5-10% decrease).

CO NCL US I O NS

The agreement observed between the experimen tal values for the total reflectance (RT) f the

Spanish ceramics and the values calculated from the glaze reflectance and transmittance data



Optical properties of tin-opaciJiedglazes 339

"1 SN

0 2 4 6 8 1 0

,m T (4m-40onm)

Figure 10 Plot of decrease in total reflectance between 420nm and 400nm versus lead oxide content for Islamic

pottery from Spain (SN), Iraq (IR), gypt (E) and Turkey (Iznik) (1 Z) ;fo rSpanish mudejarpottery ( T ) ;and for Italian

maiolica (M).

provides overall confirmation of the validity of the model proposed for the optical properties of

tin-opacified glazed pottery. Further, the detailed reflectance and transmittance data for the

Spanish ceramics together with the total reflectance data only obtained for a much wider range of

tin-opacified glazed ceramics provide confirmation of the expected correlations between the

optical properties of the glazes and the glaze compositions and tin oxide particle sizes. First,

the effectiveness of glazes in concealing the colour of the underlying body paste increases

with both the tin oxide content and the thickness of the glaze. Second, the absorption of glazes

in the blue and near UV region of the spectrum, that serves to compensate for increased

scattering in the blue region, thus enhancing the whiteness of the glaze, tends to increase

with increasing lead oxide content of the glaze. Finally, the observation that the glaze

reflectance increases from the red towards the blue end of the spectrum, with a corresponding

decrease in glaze transmittance, indicates that scattering of light predominates over

reflection for the full range of tin oxide particle sizes present in the different glazes studied

for this paper.

In summary, the total reflectance spectrum for a glaze, together with the luminosity

calculated from this spectrum, provides a valid and useful measure of the opacity of a glaze

in terms of its ability to conceal the underlying body colour. Further, this total reflectance

spectrum, together with excitation purity calculated from it, provides a measure of thewhiteness of the glaze. If data are also available for the composition and tin oxide particle

sizes for the glazes, then the total reflectance spectra provide parameters for assessing and

comparing the effectiveness of these different glaze types in achieving opacity and

whiteness. Conversely, if no compositional data for the glaze are available, then the total

reflectance spectra, obtained non-destructively with a portable spectrophotometer, can be used

to provide an indication of the tin oxide and lead oxide contents of the glaze. However, the

total reflectance also depends on the glaze thickness, the tin oxide particle size, the presence

of other crystalline phases in the glaze and the body colour. Therefore, it will rarely be

possible to provide other than very approximate values for the tin and lead oxide contents by

this method.



340 M. Vendrell, J. Molera and M. . Tite

A C K N O W L E D G E M E N T S

We thank Dr Trinitat Pradell (Universitat Polytecnica de C atalunya) for helpful discus sions and Serv eis Cientifico-

Tecnics (Universitat de Barcelona) for technical assistance. We are indebted to DrMesquida (Museu de Ceramica de

Paterna), Dr N av m o Palaz6n (Cen tro de Arqueologia de M urcia) and Professor Jam es Allan (Ashmolean Museum,

Oxford) for providing the ceramic sam ples. We are also indebted to an anonymous referee for encourag ingus to reframethe questions posed in this paper and thus, we believe, enha nce its con tribution to the study of a ncien t opaq ue glazes.

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Optical Properties of Tin Opacified Glazes