technology and colour development of hispano-moresque lead-glazed pottery

8/6/2019 Technology and Colour Development of Hispano-moresque Lead-glazed Pottery

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24 J . Molera, M. Vendrell-Saz,M . Garcia-Val& and T. Prude11

colours; copper-like, shining decoration was produced instead of the gold-like decoration of the

thirteenth century. Therefore, the change appea rs to be intentiona l and m ay reflect a deliberate

change in the technology of manufacture of these ceramics.

The green colour of the lead glazes is traditionally related to the presence of Cu2+ or Fe2+ inthe glaze, whereas the yellow to brown colours are related to Fe3+ (Weyl 1959,95 -7; Parmelee

1973, 71). As no copper has been detected in these glazes, the change of the colour must be

related to the degree of oxidation of the iron contained in the glaze (Bamford 1977; Wakam atsu

et al. 1987). The green colour is always related to dark and blackish pastes and, therefore, a

reductive process has normally been accepted as a part of the firing of these ceramics. However,

from the study of the workshop, there is no archaeological evidence that green lead-glazed

pottery was produced separately from the other ceramics which are oxidized, either in the

thirteenth or in the fourteenth centuries. Green-g lazed ceramics have grey to dark brown reduced

pastes, while yellow-, honey- and brown-glazed pots have cream and red oxidized pastes.

The a ims of this paper are: (i) the chem ical and physical (colour) characterization of the green,

yellow, honey and brown g lazes; (ii) to establish the cause of the colour; and (iii) to determ ine

the technology of production responsible for the different glaze colours. As the glazes are

transparent, in order to determine how the ceramic body influences the colour, both glazes and

pastes have been analysed independently.

E X P E R I M E N T A L

Chemical and mineralogical analyses of the ceramic body were performed by X-ray fluor-

escence (XR F) and X-ray diffraction (XRD), respectively, after removal of the glaze layer. Thestate of oxidation of the iron contained in the pastes was also determined by measuring the

Mossbauer spectra at room temperature. The study of the morpho logy and chemical composition

of the glazes was performed on the outer surface and on polished sections by scanning electron

microscopy (SEM ) provided with energy and wavelength dispersive X-ray spectrom eters (EDS

and WDS). The degree of iron reduction of the glazes cannot be studied by Mossbauer

spectroscopy due to the presence of lead.

To quantify the colour of the ceram ics, optical spectra of d iffuse reflectance were measured

between 360 and 800nm, at intervals of 0.5nm using a spectrophotometer fitted with an

integrating sphere. The measurem ents were performed on the surface of the g laze-over-paste and

on the paste alone after removing the glaze. The real colour of the glaze was determined bymeasuring the optical transm ittance from 400 to 700 nm in a microphotometer adapted to a Carl

Zeiss Universal microscope, on thin sections (c. 5 0p m ) of four selected samples (from green,

yellow, honey and brown pots). From the spectral distribution of reflectance and transmittance

the colour coordinates were calculated (Wyszecky and Stiles 1967, 238-321).

R E S U L T S

Pastes

The chemical analyses of the pastes are shown in Table 1 . No significant differences were

observed between the pastes corresponding to the yellow- and green-glazed pottery of the

thirteenth century. All of them are more calcareous (> 6.5% CaO) and poorer in iron (< 5%

F e2 03 ) han the fourteenth-century honey-glazed pastes, which are less calcareous (< 5%CaO)



Hispano-Moresque lead-glazed pottery 25

Table I Chemical compositions of paste fw t. %, averuge nnd. ir i parentheses. standard deviation ) fo r the samples of

thirteenth- and fourteenth-centup lead-glazed pottery. I . 2. 3 and 4 identif?.each group in Figure 3

Si02 A1203 CaO KzO F e 2 0 3 MgO T i 0 2

1 Green glaze 60.23 16.08 6.83 4.14 4.74 2.17 0.72

(lo)* (1 .55) (1.77 ) (1.86) (0.32) (0.66) (0 .24) (0.04)13th

century 2 Yellow glaze 60.64 16.02 6.74 4.19 4.74 2.16 0.70

(4) (1.66) (1.52) (0.57) (0.10) (0.41) (0.29) (0.02)

14th 3 Honey-coloured 58.96 18.78 4.72 4.54 5.72 2.30 0.78

century glaze ( I 8) ( 1 . 1 1 ) (0.43) (1.02) (0.23) (0.40) (0.16) (0.01)

13th-14th 4 Brown glaze 69.90 13.76 1.86 2.98 4.65 1.27 0.72

centuries (25) (3.00) (1.22) ( I OX ) (0.27) (0.55) (0.17) (0.06)

* Number of samples of each group analysed.

and richer in iron (> 5% Fe 20 3) . Th e cooking pots of both centuries show similar chemical

compositions indicating the persistence of the sam e paste (highly siliceo us and below 2% of

Ca O content). In a previous study (Molera et a l . 1996) he use of the same clay with additions of

quartz sand or calcite to m ake the cook ing pots and the honey-coloured lead-glazed pastes of the

fourteenth century was demonstrated. Therefore, two different clays were mined to obtain

the three pastes (the authors' unpub lished results): a clay which was used in the production of the

yellow- and green-glazed pastes of the thirteenth century; another clay w hich, with the addition

of siliceous temper, was used in the production of the cooking pots during both centuries and,

with the addition of calcite, was used in the production of the honey-glazed pottery.

The mineralogical analysis of the pastes (Table 2 ) reveals the presence of neoformed phases

such as gehlenite and pyroxene (except in the cooking pots which have a very low calcium

content and, thus, a very low development of gehlenite). Accordingly, the typical firing

temperature may be established between 850 "C and 900 "C .The different con tents of hematite

Table 2 Mineralogical composition of the pastes (rvt. %. avercige and. in parentheses, stundard de viatio n) fo r the

samples of thirteenth- and fourteenth -century lead-glaze d pottery . I , 2, 3 and 4 identib each group in Figure 3

Quartz Calcite Hemutite K-feldspa r Melilite C a. Al . Fe Ca-

gehl-nker pvroxens feldspar

- -Green glaze 35.4 1 . 1 2.6 1.3 I .2

(lo)* (9.1) (1.0) (1.7) ( 1.4) ( 1 .O)

century 2 Yellow glaze 37.8 I .6 2.8 2.8 1.2 0.4 1.4

(4) (9.8) (1.7) (0.6) (0 .4 ) ( 1 . 1 ) (0.1) (3.2)

14th 3 Honey-coloured 22.2 2.6 3.7 3.4 2.5 3.1 3.3

century glaze (18) (3.2) (2.6) (1.7) (1.8) (3.4) (5.7) (4.8)

13th-14th 4 Brown glaze 44.2 1 . 1 3.4 2.2 0.6 - 4.8

centuries (25) ( 9 3 ) (0.4) (1.7) (3.9) (0.6) (5.7)

13th

-

*Number of samples of each group analysed



26 J . Molera, M . Vendrell-Saz, M . Garcia-Vallks and T. Pradell

are related to the several paste colours; cooking pots and honey-glazed pastes (red) have 3 to 4%

of hematite and the yellow-glazed pastes (cream y) have less than 3%. Green-g lazed pastes of the

thirteenth century (grey colour) rarely contain hematite, which may be related to a reducing

process during the firing inferred from the dark colours of the body.Mossbauer spectra of different pastes were obtained as shown in Figure 1. The hyperfine

parameters of the Mossbauer spectra of representative pastes of each group and their association

with the several mineral phases developed during firing are shown in Table 3. The degree of

reduction is related to the ratio Fe'+/Fe3+ and the phases in which Fe2+ and Fe3+ are present

indicate the range of temperatures at which the ceramic was oxidized or reduced. We found that

green-glazed pastes of the thirteenth century present several degrees of reduction, although

yellow-glazed pots of the thirteenth century and all glazed fourteenth-century samples (honey-

glazed) and the cooking pots were made with oxidized pastes. Those pots of the thirteenth

century exhibiting a darkish green-glazed surface correspond to the most reduced pastes

produced at a temperature over 850 "C (temperature reached while reduction is performed),

as indicated by the absence of iron oxides, hematite, maghemite and m agnetite, and the presence

of well-deve loped hercynite and Fe2+ in a silicate phase (Pradell et al. 1995). The green-glazed

ceramics of the thirteenth century also present a high degree of reduction: iron oxides are related

to a maghem ite phase, with part of them (about 20% of the total iron) transformed to hercynite

and Fe2+ in calcium silicates. These results indicate similar temperatures of reduction in both

pastes but a possibly more reducing atmosphereAonger time in those pastes with a deep green

glaze.

The pastes corresponding to the yellow-glazed pottery of the thirteenth century show no

apparent reduction, and the firing temperature was about 900 "C, determined from the Fe3+hyperfine parameters -QS = 0.97(1)- (Maniatis et al . 1982 ; Janot and Delcroix 1974). The

fourteenth-century honey-glazed pastes and the brown-glazed cooking pots are also well

oxidized with a higher presence of hematite than the thirteenth-century yellow-glazed pastes

with between 60%and 70% of the total iron of the sample as hematite. A broad distribution of

iron oxide grain sizes indicates, most probably, a poorly oxidizing atmosphere during the firing.

The variable development of hematite in non-reducing conditions may be related to the

incorporation of Fe3+ in Ca-silicates developed du ring the firing from the reaction of C aO w ith

the clay at temperatures over 700°C. Calcium content is as important as iron con tent in the final

colour of the paste (M aniatis et al . 1983). Moreover, the illitic or kaolinitic character of the clay

also contributes to the final mineral composition and colour since kaolinite collapses at lowertemperatures than illite (Mackenzie et al. 1987). In the fourteenth-century samples, remains of

collapsed illite were observed in the XRD patterns, suggesting a clay richer in illite than in the

thirteenth century , in which clay the remains of illite are lower.

Glazes

All these glazes are transparent, their thickness is fairly broad ranging from 80p m to 140pm

depending on the pot and also on the position in the same pot. The glazes are heterogeneous,

showing several crystalline phases developed in the glaze-paste interface during the firing and

bubbles of gas escaping from the paste surrounded by some of these crystals (Molera et al.

1993).

The chemical analyses of the glazes are summarized in Table 4 where the mean and the

standard deviation are given for each glaze colour (green and yellow of the thirteenth century ,



Hispano-Moresque lead-glazed potter?, 27

A

B

+

-12 -9 -8 - 3 0 3 6 9 12

Velocity (mm/s)

Figure 1 Mossbauer spectra ofpaste s corresponding to each of the pottery groups studied. (A) a highly reduced paste

with a deep green-glazed apparent colour. (B ) a reduced paste o f a green-glazed pot. (C) a paste corresponding to a

yellow-glazed pot in which the development of the sextet corresponding to hematite may be seen.



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Hispano-Moresque lead-glazed pottery 29

honey of the fourteenth century and brown of the cooking pots). No great differences may be

observed among them, showing a composition in weight per cent of about 55% of PbO, 31%of

Si02,5% A1203,2% Fe203. % K 2 0 , below 1% of MgO and between I-3% CaO depending on

the type of paste.The presence of elements such as K, Al, Ca and Fe in the glaze may be partially attributed to

the composition of the raw materials used to make the glaze, but also to the diffusion of

components from the paste to the glaze during the firing. Some diffusion profiles of K and A1

have been shown in the glazes, but not for Fe or Ca, which show a fairly homogeneous

distribution. However, in laboratory reproductions of lead glazes using short firing times, Fe and

Ca also show diffusion profiles (the authors' publication in preparation).

Some diffuse reflectance spectra of green-. yellow-, honey- and brown-coloured glazed

ceramics are shown in Figure 2. From these data, the colour coordinates have been calculated

and represented in a CIE (1931) diagram (plain numerals in Fig. 3), where the different groups

may be identified by the dominant wavelength (AD), which is nearly the same for each one

(578 nm for the green glazes, 585 nm for the yellow ones and 588 nm for the honey, light brown

and brown glazes). The dotted lines in Figure 3 represent constant AD. Only AD (the hue of the

colour) is considered here because other parameters such as excitation purity and luminosity are

influenced by the geometry of the surface illuminated for the measurements. For a better

understanding of the colorimetric concepts an Appendix has been added.

However, since the glaze is transparent, the apparent colour observed (and measured) is the

combination of the real colour of the glaze and the colour of the paste. To improve our approach,

both sets of colours (those of the glazes and those of the pastes) were calculated. To this end,

transmittance spectra of glazes of the four different colours (Fig.4) were also measured on thinsections parallel to the surface prepared by removal of the paste, and the colour coordinates were

calculated (squares in Fig. 3 ) . The diffuse reflectance spectra of flat surfaces of the pastes

prepared by removal of the glaze were also measured (Fig. 5) and the colour coordinates were

calculated (circles in Fig. 3 ) .

The dominant wavelength of the brown, honey and yellow glazes is the same (579 nmj, which

is typically yellow, due to the presence of Fe" in the glaze (Stroud 1971).In contrast, the green

glaze has an A D corresponding to green (570 nm), probably associated with the incorporation of

Fez+ into the glaze (Bamford 1977).

The colour coordinates of the creamy pastes corresponding to yellow-glazed pottery are

grouped together and exhibit a nearly common AD (585 nm), which is slightly different to that ofthe honey- and brown-glazed pottery, which is reddish (AD = 590 nm). This finding agrees with

the hematite content of these pastes. The dispersion of the paste colours corresponding to the

green pots is due to the several degrees of reduction. which is responsible for the final colour of

the paste.

Therefore, the colour of the pots is a superimposition of a yellow glaze over a creamy or a red

paste which will give the final yellow, honey and brown colours observed. The green pots are the

result of superimposing a green glaze over a dark paste (grey or dark brown). In order to obtain a

better understanding, the optical behaviour of the system glass/paste has been calculated on the

basis of the model presented in Figure 6 by using the data of glass transmittance and the diffuse

reflection of the pastes. In this model only normal reflection has been calculated, the interface

air-glass has been considered a perfect reflecting surface and the contact glass-paste a diffusor.

Let the incident intensity be lo ,he air-glass reflection R"" (calculated from the refractive

index by Fresnel's equation), so that the transmission of the air-glass interface is ( I - Roir) , he



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40'IHispatio-Moresque lead-glazed po ttery 31

- 305

20

10

0 1I I I I I

400 500 600 700 800

I I I

wavelength (nm)

Figure 2 Drffuse rejlectcince ciirws of sotiie .selected glii:ed ceriir1tic.s.f he d iff er mt co1our.s studied.

internal glass transmittance T , the d iffu se reflection o f the paste R,, and the intensity reflected by

the system I . Following Figure 6, light suffers two reflections on the air-glass interface, one

diffuse reflection on the interface glass-paste and is transmitted inside the glass body (which is

considered here as an homo gene ous transparent m edium in which k = 0).Thu s, the I value after

the first reflection is:

1 = In *( 1 - Rf"'')* T R,, *( 1 - R";")

where Itlo= R

Only one reflection has been considered for calculating because the intensity of the second

reflection is under 0.001% of lo . By using this model, the dispersion of the calculatedreflectances show the s am e shape as that of the expe rimen tal curve s but the values are slightly

lower (about 5% ) in the whole spectrum. This fact may be explained because only the normal

reflectance has been considered and, addition ally, there are som e internal reflections on the

discontinuities of the gla ze such as crystallites and bubb les which, und er observation cond itions,

would increase the amount of reflected light.

D I S C U S S I O N

Although the ceramics under study present four different colours (green, yellow, honey and

brown), the glazes are only yellow or green. Among the chemical elements form ing the glaze,

iron is the only one capable of giving colour to the transparent lead glaze. Therefore, if iron is

diffused from the paste as Fe3+ the glaze becomes yellow , but, if Fe2+ is diffused , the co lour of

the glaze is green. Laboratory e xperim ents made by the autho rs confirmed that Fe , Al, K, and so



32

0.40-

0.44 -

0.40 -X

0.36-

J . Moleru, M . Vendrell-Suz, M . Gar&-Vullks and T. Prude11

1.0 ,0.0

0.0

0.1

0.6

0.5

0.4

0 .3

0.2

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x coordinate

0.32-

0.32 0.36 0.40 0.44 0.48

Y

Figure 3 CIE (1931 diagram showing the chromatici@points o the transmission ojf our glaz es (numerals in squares).

the diffuse reflectance nj he paste (numerals in ci rc les) and the dlffuffuse eflectance of glazed pottery (plain nurnerals).

The numerals indicate the type nf potteff: ( I ) green-glazed; ( 2 ) yellow-glazed: (3) honey-glazed; (4 ) brown-glazed

(c oo kin g p c ~ t ~ ) .he dotted lines radiating from point (C ) represenr constant dominant wavelength hl, (see text and

Appendix).



Hispano-Moresque lead-glazed potter?, 33

0'7010 60

0 50

0 40

0 30

0 0

400

1

500 600 700

wavelength (nm)

on, can diffuse from the paste to the glaze during heating (the authors' publication in

preparation).

The observed colour corresponds to that of the glaze supe rimpo sed on the colo ur of the paste.

As may be observed in Figure 3, the colour coordinates corresponding to the yellow pottery

(marked as 2 ) fall between those of the yellow glazes (ma rked as 2 in a squ are) and those of the

creamy pastes (ma rked with 2 in a circle). The colour coord inates of the honey and brown pots (3

and 4, respectively) are between the yellow glazes (marked as 2 in a square) and the

corresponding reddish and red pastes (3 and 4 in circles).

The observations for the four groups of pottery here presented may be su mm arized as follows.

( 1 ) green glaze on a grey paste (colou rless of low luminosity). T he appa rent colour is green. A sthe degree of reduction of the paste is not uniform different hues of green may be observed.

( 2 )apparently yellow-glazed pottery: a yellow glaze on a cream y paste (m ode rate Ca O content

and low development of hematite). The colour observed is yellow.

(3) apparently honey-glazed pottery: a yellow glaze on a reddish p aste (m odera te Ca O content

but well-developed hem atite). Th e colour observed is light brown (hone y).

(4)apparently brown glaze of cook ing pots: a yellow glaze on a red paste (very low Ca O content

and good development of hematite). The colour observed is brown.

Th e glazes and pastes for yellow a nd green pots have the sam e chemical com positions but the

paste is oxid ized or reduced, respectively. How ever. this can not be attributed to a different firing

process because yellow and green colo urs, in som e cases, are found togethe r in the sam e pot. But

the green colours are alw ays related to glazes applied to both sides, and yellow c olours to one

side only glazed. And even when both colours are found in different p arts of the sam e pot, the

paste below green glaze is grey to brown and the paste below yellow glaze is red. The reason



34 J . Moleru, M . Vendrell-Saz, M . Gurcia-Vallks and T. Pradell

6o1 2 yellow

1 green

40 0 500 60 0 700 800wavelength (nm)

Figure 5 Diffuse reflectance curves of selected pastes obtai ned on smoothed surfaces after rem ova l of the glaze

may be that a reducing atmosphere is created during firing by the gases produced inside the

ceramic body enclosed by glaze on two sides.

During firing, reducing and inert gases such as CO, water vapour, COz, an d so on are produced

by the combustion of the wood which con sum es an important part of the oxygen inside the kiln.

Thus, the pottery cannot be completely oxidized.

As the glaze m elts at about 700"C (Nordyke 1984), when calcite deco mp oses both sides of the

pot are sealed by the glaz e and a reducing a tmosphere is produced inside the paste by the trapped

C 0 2 .Under these conditions Fez+ diffuses from the paste to the glaze which becom es green. If

Figure 6 Schematic path fol lowed by the light in the systeni glaze-paste.



Hispunn-Morrsque leLid-gIu:ed p o t t e p 35

only one side is glazed. the gases produced by decomposition of the calcite may escap e through

the porosity of the ceramic body by the non-glazed side. The reason for the higher reduction of

the pottery of the thirteenth century glazed on both sides (i.e.. high er diffusion of Fe” to the

glaze) may be the calcite content of the clay body. The paste used in the fourteen th century w asCa -po ore r and probably the calcite w as of bigger grain size as indicated by the high develop men t

of gehlenite.

During the cooling ph ase, there is no burning, the partial pressure of oxygen is 2076, and the

iron of the paste is oxidized w hile the temperature is relatively high (ove r 50 0° C ). If both sides

are glazed the ceramic body is sealed before the oxidizing atmosphere reaches the paste and,

thus, i t remains reduced due to the protective action of the glaze coverin g both sides.

When only one side is glazed, the oxygen reaches the paste through the porosity of

the ceram ic body and the iron is oxidize d. Thu s, the paste bec ome s more or less red depending

on the iron available as oxides. If most of this iron has incorporated into the silicate’s

structure (like gehlenite or pyroxene), less iron remains to form hematite in the cooling

stage. The grain size of calcite is also an important factor, as in the honey pottery of the

fourteen th century which develop ed more hematite than the oxidized pottery of the thirteenth

century.

T o support the hyp othesis of a single firing there are several exa mple s of pots of the thirteenth

century partially glazed on one and both sides. The ir colour chang es from yellow to green, and

the colour of the paste under the glaze from cre am to grey, in those parts glazed on one or two

sides respectively.

All these facts support the hypothesis that the diffusion of iron from the pas te to the gla ze is in

the form of Fe’+ or Fe” wh ile the process that takes place in the paste is oxidiz ing or reducingand, therefore, the colour of the g lazes is yellow or brown. o r green, respectively.

C O N C L U S I O N S

From these results the following important points may be made.

The real colour of the glaze is produced by iron diffused from the paste.

The colour observed is that of the glaze by transmittance (which acts as an optical filter)

superimposed to that of the underlying paste by diffuse reflection.

Green-glazed pottery is always glazed on both surfaces and, as a consequence, the paste is

reduced and the iron is diffused as Fe”. giving the glaze green coloration.In those ce ram ics glazed on one surf ace the paste is oxidized and the glaze beco me s yellow by

the diffusion of Fe”.

The difference of the apparent colours in the yellow, honey and brown pottery is due to the

different development of hematite in the paste because of a difference in the Ca content of the

different groups and the mineralogical nature of the raw materials. Additional to the Ca content,

the mineralogical analyses of the paste of yellow pottery (high gehlenite content) seem to

indicate a bigger grain size of calcite, which after transformation into CaO reacts with the

silicate surrounding to fo rm gehle nite in this Ca-rich enviro nme nt.

The same paste was used to produ ce green and yellow pottery during the thirteenth century . A

change in the paste used in the fourteenth ce ntury produce s a reddish co lour (hon ey ) probably

because of the smaller grain size of the calcite grains. When the CaO content is under 2%

(as in the cooking pots) the paste becomes red and the observed colour through the glaze is

brown.



36 J . Molera, M . Vendrell-Suz, M . Gurcia-Vullks uncl T. Prude11

There are no differences between cooking pots produced in the thirteenth and the fourteenth

centuries, either in the paste or in the co lour.

The ch ange from green and yellow pottery (thirteenth century ) to honey (fourteenth century)

is parallel to the change from green- and black-decorated tin-glazed pottery of the thirteenth

century to the lustre pottery of the fourteenth century ( M . Mesquida pers. co mm .). Th us , if the

honey colour is a voluntary de cision of the potters to follow a fashion, they appear to have had

good knowledge of the behav iou r of the raw materials and, perhaps, u ndertook em pirical tests to

check how different clays change under firing. No changes have been detected in the cooking

pots in which the most important property is their thermal behaviour instead of their colou r. Th is

fact reflects a high development in the ceramic industry and in the empirical knowledge of the

ceramic process by the Hispano-Moresque potters.

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

The authors want to thank Dr Merci Mesquida. archaeologist of the Paterna Council. for her collaboration i n supplying

the materials for analysis and giving her time to discuss the results i n the historical framework.

R E F E R E N C E S

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spectroscopy. in Archrreologicnl cerurnics (eds. J. S . Olin and A. D. Franklin). 97- 108. Smithsonian Institution.

Washington, DC.

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37

A P P E N D I X

I n order to clarify some of the aspects presented i n this paper concerning the colour measurements. the authors prepared

this Appendix which should not he conhidered as a text on colorimetry hut only ;in aid to following the graphics and

calculations presented.Colour is a psychological perception caused by the stimulus produced by the visihle light in the eye, that is to say.

when a particular combination of w avelengths reaches the eye it is detected by the brain a s a particular colour. Although

each wavelength has its characteristic colour response. the range of pure colours given by each wavelength does not

represent the total range of available colours.

Each colour has three attributes. namely luminosity (th e quantity of ligh t). hue (whi ch allows us to differentiate

between red. green. blue. and so on) and excitation purity (t hc quantity of white that, mixed with a pure colour.

reproduces a particular colour). As the response oft he eye depends on the stimulus. it is obvious that the illuminant Hux

has to be considered when q uantitative nieasureineiits of the colour ;ire carried out. The Com mission Internationa le pour

I 'Eclairage (C IE )established some standard illurninants one of which is known as C (which corresponds to the north sky

light) and is one of the most widely used.

Leonard0 d a Vinci said that almost all the colours niay be made by mixtures of three 'well selected' colours. If we

draw a triangle whose corners are pure green, red and blue (Fig. 7), avoiding the mathematical development which is

beyond the scope oft hi s text. we may establish a colour C , as the w m of a certain amount of green ,yI. blue hl and red r l .

then:

Cl = r I R + ~ I G f h l t l

Thi s is the basis of the algebra of the colours which a llows us to establish that a mixture of tw o colours C, an d Czmay he

determined as follows:

2'Ci = 0'1+ r ? ) R+ (gl + gz)G+ ( b ,+ h:)B

However. the pure spectral colours (monochromatic light) cannot be obtained by mixing other spectral colours, so that

th e locus of all the spectral colours falls outside the triangle. Accordingly. there are some colours that cannot be obtained

by additions of green, blue and red lights previously chosen (those points lying between the triangle and the locus line).

Bu t al l the possible colours lie inside the lociis of the pure spectral colours and the purple line betw een blue an d red. I t is

for this reason that Leonardo's statement includes the term 'well selected'. By mixing three spectral colours limiting a

small triangle only a limited variety of colours could be obtained.

So , if we want to mix three 'colours' to obtain m y of the posbible colours. we must choose three points outside the

curve. These points will he 'non-existent' colours hut from the mathematical point of view this does not present a

3R

Figure 7



38 J . Moleru, M. Vendrell-Suz,M. Gurciu-Valle's and T. Prude11

problem. After experiments with hundreds of observers the CIE established the spectral distribution of three standard

disrribittiori cweJrcirrits ( x h . and : A ) and developed a representation system (CIE 193 I ) i n which all the coordinates are

positive (Fig. 8) .

Colour culculution

The radiant flux reflected or transmitted by an object has to be specitied by its spectral distribution RhLhAhor Th Lh A h .

where R , is the sp ectral reflectance. T i the spectral transmittance and LA the spectral distribution of the Rux illuminating

the object. The tristiniulus values X . Y an d Z of a reflecting object are given by (an d similarly for a transparent body ):

x = kCRhLhA-hAA; Y = kCRhLh>jhAh: an d z z CRALhzhAh

where .rh.F~ and z h are the distribution coefficients, and the normalizing factor k takes the value

k = lOO/CLhyhAh

so that. for a perfect reflecting or trans mitting objec t ( R hor T h equals one for all wavelength) Y = 100.Then. the value of

Y represents the Iumiriosiiv of the colour of this object.The chromaticity coordinates are calculated from the tristimulus X , Y an d Z

. r = X / ( X i Y + Z ) ; y = Y I ( X + Y + Z ) an d : = Z / ( X + Y + Z )

The colour coordinates of the illurninant may also be calculated from its spectral distribution and i t represents the

achromatic point ( C n our case).

Donzinanf wavelength and excitation purity

The dominant wavelength. AD . o fa particular colour is the wavelength of the spectrum wh ich, mixed with the achromatic

stimulus. matches the given colou r. So that, it represents the h e fthis colour. I n the CIE ( 193 ) diagram it is represented

1 o

0.9

0 .8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

4 52 0

j f i 55 0

9 00

1480L/ti0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

x coordinate

Figure 8

technology and colour development of hispano-moresque lead-glazed pottery

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