ArcAaeometry 26, 2 (1984), 139-154. Printed in Great Britain

A TECHNOLOGICAL STUDY OF CHINESE PORCELAIN OF THE YUAN DYNASTY

M. S . T I T E , I . C . F R E E S T O N E a n d M . B I M S O N

British Museum Research Laboratory. London WClB 3DG, U. K .

I N T R O D U C T I O N

Yingqing (qingbai) porcelain, which is the technological ancestor of virtually all porcelain made in China and Japan, was produced at Jingdezhen from at least the eleventh century A.D. and remained the primary product of the kilns in the Jingdezhen area until the fourteenth cen- tury A.D. It is characterised by a translucent glaze with a bluish tinge and most of the pieces produced are very thin and of small size. In the fourteenth century A.D. during the Late Yuan dynasty, a number of important technical developments occurred in the Jingdezhen area. First, a thick porcelain with a white opaque glaze, known by the name of shufu, was produced. Soon after porcelain items of substantial size and weight which were decorated with underglaze painting in blue and red were made. With these new developments, the production of yingqing porcelain ceased.

The raw materials used in the production of yingqing, shufu and underglaze blue porcelains have been the subject of several previous investigations. Sundius and Steger (1963) observed that the bodies of Southern Chinese ceramics, including yingqing ware, were characterised by a ‘scoriaceous’ quartz, the grains of which are ‘rich in rounded hollows filled with a low-refractory glass, probably a melted feldspar’. On the basis of this, they suggested that the porcelain bodies were made from a mixture of kaolin and a pulverised fine-grained quartz-feldspar rock. In contrast, Wood (1978a) suggested that, a t Jingdezhen during the Sung dynasty, yingqing porcelain bodies were made from a kaolinised porcelain-stone consisting of quartz, mica, albite (sodium feldspar) and kaolinite. This hypothesis was based on mineralogical and analytical data for a porcelain stone containing kaolinite which was in use at Jingdezhen in the nineteenth century (Vogt 1900) and those for a similar kaolinised porcelain-stone (Nankang stone) which was examined in the context of the technical aspects of the present-day porcelain production at Jingdezhen (Efremov 1956, Nikulina and Taraeva 1959). Wood (1984a) subsequently pre- sented further data t o support this hypothesis. In this later paper, he also demonstrated that the published physical properties (i.e. plasticity, shrinkage and refractoriness) of the Nankang stone (Efreniov 1956, Nikulina and Taraeva 1959) were such that it would have been suitable for making porcelain bodies without the addition of further kaolin. However, it is clear from the account of porcelain production a t Jingdezhen given in Pere d’Entrecolles’ letter of 1712 (Tichane 1983) that, a t least by the early eighteenth century, a kaolinised porcelain-stone was no longer being used. Pere d’Entrecolles reported that in his day kaolin was a necessary in- gredient and that the bodies were made from equal parts of kaolin and porcelain-stone (petuntse) for the best porcelain, from four parts of kaolin and six parts of porcelain-stone for medium quality porcelain, and from one part of kaolin and three parts of porcelain-stone for the poorest grade of porcelain. Similar mixtures of porcelain-stone and kaolin are also used at Jingdezhen in the present-day production of porcelain. Addis (1981) has suggested that kaolin

139

140 M. S. Tite. I. C. Freestone arid M. Bimson

was first added t o porcelain-stone t o make porcelain bodies at Jingdezhen during the Late Yuan dynasty and that this addition of kaolin was a crucial factor in the development of shufu and underglaze blue porcelain.

Wood (1978b) has reviewed the compositions and properties of the full range of lime-alkali glazes used on Chinese stoneware and porcelain. I n particular, he has re-emphasised the hy- pothesis first made by Nichols (1917) that Chinese porcelain glazes were produced by the addition of lime to the raw material used t o make the porcelain body. On the basis of Pere d'Entrccolles' account of porcelain production at Jingdezhen in the eighteenth century, it seems probable that the lime was added to the glazing mixture in the form of 'glaze ash' pre- pared by burning limestone with bracken. Wood (1984b) has further discussed this hypothesis in the context of yingqing, shufu and underglaze blue porcelain produced a t Jingdezhen. In this same paper, he presented data on the maturing temperature and general appearance of a series of glazes prepared from synthetic yingqing body material to which was added increasing amounts of slaked lime.

The aim of the present study was to contribute further to the identification of the raw materials used for the bodies and glazes of yingqing and underglaze blue porcelain of the Yuan dynasty. I n particular. evidence was sought for the possible change from the use of kaolinised porcelain-stone to that of a mixture of kaolin and porcelain-stone; in addition, the technological implications of such a change in the raw materials used were considered. Three sherds of Sung/Yuan dynasty yingqing porcelain and three sherds of Yuan dynasty underglaze blue porcelain were examined using scanning electron microscopy (SEM). optical microscopy, electron microprobe analysis and X-ray diffraction. Also examined were samples of porcelain- stone (Sanbaopeng), glaze-stone (Yaoli) and kaolin (Gaoling) collected from the Jingdezhen area (Addis 1981) and currently used in the production of porcelain.

E X P E R I M E N T A L I'ROC L D U R E S

Polished thin sections were prepared from slices cut from the porcelain-stone, the glaze-stone and the six sherds. The slices were cut perpendicular to the surface of the sherds so that sec- tions through the glaze and body were obtained.

The microstructure of the samples were investigated using both a petrological microscope and a scanning electron microscope (SEM).

The chemical compositions (table 1) were determined with a Cambridge Geoscan micro- probe tit ted with a Link Systems energy dispersive spectrometer. The operating conditions were 15 kV accelerating potential, 5 nA specimen current and 70" take-off angle. For analysis of the porcelain-stone, glaze-stone and the porcelain bodies, the microprobe beam was defocused to 300-400pni and a t least 3 individual analyses were averaged. For analysis of the porcelain glazes two approaches were used. For two samples ( 15883, 15886), a profile across the glaze perpendicular to the surface and consisting of between six and ten 3 p m spots a t 20pm intervals was averaged. For the remaining samples, the beam was defocused t o 70pni and analyses of several areas from the centre of the glaze taken at points along the length of the sample were averaged. In addition, some individual mineral phases were analysed (table 2). Relative standard errors were similar for the body and glaze analyses and were typically: S i 0 2 - 1%; AI2O3, K 2 0 1-2%; FeO, CaO, NazO - 5%; MgO, P,Os 10-20%. The mineral phases present in the porcelain stone, glaze-stone. kaolin and the porcelain bodies were identified by X-ray diffraction using DebyeScherrer powder cameras.

Tab

le 1

B

ulk

elem

enta

l and

min

eral

ogic

al c

ompo

sitio

ns

Oxi

de c

once

ntra

tions

(%w

t)=

M

iner

al c

once

ntra

tions

(%

wt)

3

Sam

ple

No.

D

escr

iptio

n of

sam

ple’

S

iO,

Al,

O,

h-eO

M

gO

CaO

N

a,O

K

,O

P,O

, To

tal’

A

M

K Q

-

Porc

elai

n-st

one

(San

baop

eng)

77

.8

14.5

0.

4 0.

1 0.

2 4.

6 2.

1 0.

3 97

.6

39.2

17

.4

-

Gla

ze-s

tone

(Y

aoli

) 77

.4

15.3

0.

4 0.

1 0.

2 3.

2 3.

1 0.

3 97

.0

27.7

25

.2

-

Porc

elai

n-st

one

(Vog

t, 1

900)

76

.1

18.0

1.

0 0.

2 0.

2 0.

4 4.

1 -

95.8

2.

6 27

.6

-

Porc

elai

n-st

one

(Nan

kang

) 77

.5

16.6

0.

6 0.

4 0.

8 0.

9 3.

2 -

99.9

1.

3 26

.6

1588

3 Y

ingq

ing

(193

8,4-

12,

21)

78.7

15

.7

0.8

0.1

0.4

1.1

2.9

0.3

93.1

9.

2 22

.7

1588

4 Y

ingq

ing(

1938

,4-1

2, 2

3)

78.1

15

.9

0.5

0.2

1.0

0.8

3.3

0.2

92.2

6.

6 26

.9

1588

5 Y

ingq

ing

(194

0,4-

13,

28)

79.2

16

.1

0.5

0.3

0.7

0.5

2.4

0.3

88.8

4.

5 13

.9

1588

6 U

nder

glaz

e bl

ue (1

921,

3-1

) 74

.3

19.7

0.

9 -

0.2

2.3

2.3

0.3

94.8

19

.7

19.1

15

887

Und

ergl

aze

blue

(19

26,6

-18,

35)

71

.9

20.6

1.

4 0.

2 0.

1 1.

6 4.

0 0.

2 95

.1

14.0

33

.0

1588

8 U

nder

glaz

e bl

ue (1

926,

6-1

8, 3

6)

71.1

21

.3

1.4

0.1

0.1

1.9

3.9

0.2

96.2

16

.2

32.3

1588

3 Y

ingq

ing

65.8

14

.5

0.9

0.5

14.6

1.

4 2.

1 0.

2 96

.9

14.0

20

.7

1588

4 Y

ingq

ing

66.8

14

.9

0.5

0.4

13.9

0.

8 2.

5 0.

2 98

.6

7.7

23.4

15

885

Y in

gqin

g 64

.6

13.7

0.

7 0.

6 18

.5

0.5

1.1

0.3

98.5

5.

4 11

.3

1588

6 U

nder

glaz

e bl

ue

68.7

14

.9

0.9

0.3

8.7

3.7

2.6

0.2

98.0

35

.2

23.1

15

887

Und

ergl

aze

blue

66

.0

16.0

1.

8 0.

5 9.

3 2.

2 4.

1 0.

1 98

.4

20.5

37

.5

1588

8 U

nder

glaz

e bl

ue

66.4

15

.7

1.1

0.4

10.1

2.

4 3.

7 0.

2 97

.7

22.7

34

.1

Porc

elai

n bo

dies

Porc

elai

n gl

azes

0.1

43.3

-

47.1

7.

0 62

.8

10.4

55

.7

9.9

58.2

9.

0 57

.5

17.9

63

.7

18.5

42

.7

11.0

42

.0

12.1

39

.4

13.6

51

.7

14.5

54

.4

25.2

58

.1

0.8

40.9

-

43.2

-

42.0

I B

ritis

h M

useu

m r

egis

trat

ion

num

bers

(D

ept.

of

Ori

enta

l Ant

iqui

ties

) ar

e gi

ven

in b

rack

ets.

Fo

r ea

se o

f co

mpa

riso

n, th

e an

alys

es h

ave

been

nor

mal

ised

to

100%

. The

ana

lysi

s to

tals

are

als

o gi

ven.

The

low

tota

ls a

re d

ue t

o th

e ir

regu

lari

ty a

nd

For

the

porc

elai

n gl

azes

, th

e li

me

cont

ent

has

been

sub

trac

ted

and

the

perc

enta

ge w

eigh

ts o

f al

bite

(A

), m

usco

vite

(M

), ka

olin

ite

(K)

and

quar

tz (

Q)

poro

sity

of

the

polis

hed

surf

aces

and

are

con

sequ

entl

y lo

wer

for

the

por

cela

in b

odie

s th

an f

or t

he g

laze

s.

have

bee

n no

rmal

ised

to

100%

.

b 9 3’ 3 m -+ x

m

142 M. S. Tite, I. C. Freestone and M. Birnson

N " 1 1 I I N " o o o o o o o o

A technological study of Chinese porcelain of the Yuan dynasty

R E S U L T S

143

1. Raw materials

Examination of polished thin sections of the porcelain-stone (Sanbaopeng) and glaze-stone (Yaoli) with the petrological microscope and SEM indicated that both consist of a fine aggregate of quartz, muscovite (potash mica) and albite (sodium feldspar). The principal difference between the porcelain-stone and glaze-stone is that the latter is finer textured (figures 1 and 2). The presence of quartz, muscovite and albite was confirmed by X-ray diffraction but no kaolinite was detected.

Microprobe analysis of the muscovite and albite phases (table 2) confirmed that the musco- vite contains only a small amount of Na20 and that the albite contains only a small amount of K 2 0 . Therefore, from the bulk analyses (table l), it was possible to estimate the percentage weight of the constituent minerals in the porcelain-stone and glaze-stone assuming that all the K 2 0 is associated with the muscovite ( K 2 0 - 3Al2O3 * 6Si02), that all the Na20 is associated with the albite (Na20 * A1203 6Si02), that any remaining A1203 is associated with kaolinite (A1203 * 2Si02) and that the remaining SiOz is present as quartz. The calculations confirmed that the amount of kaolinite present in these particular examples of porcelain-stone and glaze- stone was negligible (table 1). Analytical data (after Wood 1984a) for the two kaolinised porcelain-stones referred to in the Introduction are also included in table 1, together with the calculated percentages of the constituent minerals.

X-ray diffraction indicated that the kaolin sample (Gaoling) contained quartz, muscovite and albite in addition to kaolinite. However, the proportion of albite was significantly lower than that in either the porcelain-stone or the glaze-stone. It was not possible to obtain a repre- sentative sample of the kaolin for quantitative analysis by microprobe since the available sample was small and in the form of loose debris.

2. Porcelain bodies

The SEM examination of the porcelain sherds showed that the bodies consist of unreacted quartz set in a matrix made up of two poorly differentiated phases (figure 3). The first phase appears striated, has a high A1203 to Si02 ratio and probably represents areas of relict mica converting to mullite. The second phase appears to be homogeneous, has a low A1203 to Si02 ratio and represents areas of glass probably formed primarily from feldspar, quartz and kaolinite .

A number of phases, in addition to quartz, were identified in the bodies with the petro- logical microscope and the electron microprobe. A few well-defined pools of clear isotropic glass could be seen in most samples, although generally the glass is intimately mixed with the other phases. Some glass pools contained needles of a precipitated phase, probably mullite (figure 4). Occasional spongy, slightly birefringent areas gave a bulk analysis with an A 1 2 0 3 to Si02 ratio close to feldspar (table 2) and are interpreted as areas rich in original feldspar which have melted to quartz plus glass. Relict mica was relatively common, was fibrous in appearance (figure 5) and showed a low order birefringence with the straight extinction between crossed polars typical of sheet silicates. Microprobe analysis (table 2) indicated an A1203 to Si02 ratio similar to that of muscovite, but with low alkalis and an Na20 to K 2 0 ratio similar to that of the glass. The original mica is thus interpreted to have been converted to a mixture of mullite and glass. The presence of mullite in the bodies was confirmed by X-ray diffraction but no other crystalline phases, except quartz, were identified. The similar Na20 to K 2 0 ratios in relict mica, feldspar, glass and the bodies in bulk (tables 1 and 2), are interpreted to result from

144 M. S. Tite, I . C. Freestone and M. Binison

i.igurc 1 ,qrr.v). niuscoriri, (light gi-e?) and albite (dark grey).

SEM photomicrograph of’ porcelain-storre (Saribaoperig) showirig f ine aggregate of quart: (dark

1 . ipurt. Z rite (light Rrc,j,) and albite (dark grc.,)).

S l M photomicrograph of glaze-storiij f Saoli) shohing firie aggregate of quartz (dark grej’). tnusco-

A technological study of Chinese porcelain of the Yuan dynasty 145

Figure 3 SEM photomicrograph of the body o,f yingqing porcelain (I58831 showing upireacted quartz (dark grey) set in a matrix of two poorly differentiated phases (striated - high AI,O, , honiogeneous - low A1 ,O ,).

the relatively high mobility of the alkalis during firing, which allows the formation of a glass which permeates the other phases, and into which all the alkalis are segregated.

The bulk analyses (table 1) indicated that all the porcelain bodies have a high S i 0 2 content. Furthermore, SiOz, A1203 and the alkalis (KzO and NazO) make u p more than 98% of the bodies with the FeO, MgO, CaO and P,05 contents all being very low. To this extent, the body compositions are very similar to those of the porcelain-stones and glaze-stone. The principal composition differences between the two types of porcelain are that the yingqing bodies have higher S i 0 2 to A1203 ratios, and lower NazO concentrations than the underglaze blue bodies.

The various relict phases identified in the porcelain bodies and their similarity in composition to the porcelain-stones and glaze-stone provide strong evidence for assuming that the constituent minerals originally present in the body material were quartz, mica, albite and kaolinite. The percentage weights of these original constituent minerals in the porcelain bodies can therefore be estimated from the bulk analyses as in the case of the porcelain-stone and glaze-stone. The calculated percentage weights of the four minerals normalised to 100% are presented i n table 1 and the percentage weights of mica, albite and kaolinite, again normalised to 100%, are plotted on a ternary diagram in figure 7. From these data, it can be seen that the bodies of all three types of porcelain contain significant amounts of kaolinite. However, the yingqing bodies have significantly higher quartz contents and lower albite contents than the underglaze-blue bodies.

3 . Porcelain ghzes

The S E M and petrological examination of the porcelain glazes showed that they are generally homogeneous and clear. However, fine crystals of anorthite (CaO - A1203 2Si02) have developed

146 M. S. Titc, I . C. Freestone arid M. Bimson

1 igurc 4 precipitated in glass pool (field o f view = 1 2 5 p m across).

Optical photomicrograph of the bod!> of .vingqing porcelain (15885) showing mullite needles

IFrpirc 5 lath (,ii'eld of vie\<. = 12-ipni across).

Optical photomicrograph of the body o f underglare blue porcelain (15886) showing fibrous mica

at or close to the glaze-body boundary in most cases: this crystallisation is especially extensive 111 the underglaLe blue sherd 15888 (figure 6).

The bulk analyses (table 1) indicated that the glazes are all of the lime-alkali type with the lime (CaO) content increasing from 8-10% in the underglaze blue glaze to 13-19% in the

A technological study of Chinese porcelain of the Yuan dynasty 147

Figure 6 crystals of anorthite (dark grey) close to the glaze-body boundary (field of view = 615 pm across).

Optical photomicrograph of glaze and body of underglaze blue porcelain (15888) showing fine

yingqing glaze. As in the case of the corresponding bodies, the yingqing glazes have lower Na,O concentrations than the underglaze blue glazes. The variation in the CaO content was high along the length of the yingqing glaze sections (12.2-1 5.7% along 15884 and 10.2-1 7.7% along 15883), which suggests poor mixing of the raw materials used t o make the glaze for the yingqing ware as compared with the underglaze blue ware.

Apart from the lime content, the glaze compositions are similar in general terms to those of the porcelain-stones and glaze-stone and consistent with the views of Nichols (1917) and Wood (1984b) that the glazes were prepared by the addition of ‘glaze-ash’ (lime) t o the raw material for the bodies. Furthermore, analytical data for the ‘glaze-ash’ which most probably provided the lime content of the glazes indicates that its alkali content was negligible (Wood 1984b). The assumption that all the K 2 0 is associated with mica and all the Na,O with albite is therefore valid for the glazes and the percentage weights of the original constituent minerals (quartz, mica, albite and kaolinite) in the glazes can again be calculated from the bulk analyses. From the data presented in table 1 and figure 7, it can be seen that the yingqing glazes are similar mineralogically to the corresponding bodies. However, the underglaze blue glazes differ significantly from the corresponding bodies in that they contain negligible kaolinite and have higher albite contents.

DISCUSSION

1 . Raw materials In comparing the conipositions of the procelain bodies and glazes with those of the potential raw materials (i.e. porcelain-stone, glaze-stone and kaolin), the first point t o emphasise is that considerable variation in the mineral composition of the latter is t o be expected. This vari- ation occurs because the raw materials represent products of the alteration of pre-existing

148 M. S. Tite, I. C. Freestotie and M. Bimson Muscovite

/

80 60 40 20 Kaol in ite Albite

l.ipurt. I Ternarj, diagram showing percentage weights of mica. albite and kaolinite in bodies (closed symbols) and glares (open symbols) of yingqing (circles) and underglaze blue (squares) porcelain analwed in present study: corresponding body andglaze compositionsare linked b y tie-lines. Also p lo t t ed on the diagram are rhe mineral compositions of the porcelain-stone and glaze-stone analysed in the present stud)> (large stars1 and the kaolinised porcelain-stones previously anal.vsed (small stars).

quartz-feldspar rocks by high temperature saline hydrous fluids. The mineral assemblage pro- duced by these 'hydrothermal fluids' depends on the composition of the host rock as well as the temperature and composition of the fluids themselves. Typically (Grim 1968), sericitized rocks (i.e. mica-bearing porcelain-stone and glaze-stone) and kaolinised rocks are adjacent in a zonal body with a gradational contact (i.e. kaolinised porcelain-stone).

The analytical data for yingqing bodies are consistent with their having been made from a kaolinised porcelain-stone such as those referred to in the Introduction. Similarly, the glaze compositions are consistent with their having been prepared by the addition of lime t o the same kaolinised porcelain-stone as that used for the corresponding bodies. Thus, the present results entirely confirm the existing hypothesis regarding the raw materials used to make yingqing bodies (Wood 1978a and 1984a) and glazes (Wood 1978b and 1984b).

The analytical data for the underglaze blue bodies, taken in isolation, are consistent with their having been prepared from a kaolinised porcelain-stone, in the same way as the yingqing ware. However, when taken in conjunction with the data for the underglaze blue glazes, this seems less likely to be the case. I f lime is subtracted from the glaze compositions, the yingqing

A technological study of Chinese porcelain of the Yuan dynasty 149

glazes and bodies are essentially identical in composition whereas there are marked differences between the corresponding glaze and body compositions of the underglaze blue porcelain (table 1, figure 7). Unlike the bodies, the underglaze blue glazes show essentially no kaolinite component, and they are therefore likely to have been prepared by mixing ‘glaze ash’ with kaolinite-free porcelain-stone such as those analysed in the present study. In consequence, i t seems more probable that the underglaze blue bodies were similarly prepared by the addition of an impure kaolin to the kaolinite-free porcelain-stone used for the glaze. This is consistent with the hypothesis of Addis (1981) who suggested that the addition of kaolin was intro- duced at around the time of the introduction of underglaze blue ware.

The limited number of previously published analyses of yingqing, shufu and underglaze blue porcelain are in general agreement with data presented here and substantiate, in part, the proposed changes in the raw materials used. Pollard and Hall (1984) present body analyses for Sung dynasty yingqing porcelain and for Yuan dynasty underglaze blue porcelain, as well as for Yuan dynasty shufu porcelain which was introduced at about the same time as the underglaze blue porcelain (see Introduction). Guo Yenyi (1 982) presents both body and glaze analyses for Yuan dynasty yingqing, shufu and underglaze blue porcelain excavated at Dadu, the Yuan city at Peking. Chen Yao-cheng et al. (1978) present body and glaze analyses for Yuan dynasty underglaze blue porcelain. The percentage weights of muscovite, albite and kaolinite calcu- lated from these published analytical data and normalised to 100% are plotted on the ternary diagram in figure 8. The first point to note is that the lime contents of the glazes analysed previously (i.e. 12-17% for yingqing and 7-10% for underglaze blue) are similar to those for the corresponding glazes analysed in the present study. Second, the bodies of the Sung dynasty yingqing porcelain have similar mineral compositions to those of the yingqing porcelain analysed here and therefore could also have been produced using a kaolinised porcelain-stone. Third, both the bodies and glazes of the Yuan dynasty shufu and underglaze blue porcelain again have similar mineral compositions to those of underglaze blue porcelain analysed here, and therefore could similarly have been produced using a kaolinite-free porcelain-stone with kaolin being added in the production of the body. However, the mineral compositions of the Yuan dynasty yingqing porcelain analysed previously differ markedly from those of the yingqing porcelain analysed in the present study. In particular, both the bodies and glazes were high in albite and more importantly, the majority of the glazes contained negligible amounts of kaolinite. They therefore match in composition the shufu and underglaze blue porcelain and were thus again more probably produced using a kaolinite-free porcelain-stone with kaolin being added in the production of the body.

2 . Technological implications In considering the technological implications of the change, noted above, in the raw materials used to produce the different types of porcelain, the relevant parameters which can be inferred from the analytical data are plasticity, green-strength and refractoriness of the bodies and maturing temperatures of the glazes.

According to Wood (1978a), the mica in the porcelain-stone itself imparts a plasticity to the crushed stone. Therefore, the sum of the estimated mica and kaolinite contents of the porcelain bodies should be used to obtain an indication of the plasticity of the original raw materials. From the data presented in table 1, it can be seen that the total plastic materials in the underglaze blue bodies (37.6-44,4%) is consistently higher than in the yingqing bodies (3 1.8-35.9%). Since both plasticity and green-strength depend on the total effective surface

150 M. S. Tire, I. C. I-'reestone arid M. Birnson

Muscovite

I

A

Albite 80 60 40 2 0

Kaolinite

Figure 8 Ternar), diagram shorcing percentage weights of mica, albite and kaolinite in bodies (closed symbols) and glazes (open symbols) o f yingqing (circles). shufu (triangles) and underglaze blue (squares) porcelain previously anal.vsed: the Sung dynasty .vingqing porcelain is distinguished by a dash attached to the circle. Corresponding bodv and glaze compositions are linked bv tie-lines. Also plotted on the diagram are the mineral compositions o f the porcelain-stone and glaze-stone analysed in the present study (large stars), and the kaolinised porcelain-stones previously analysed (small stars).

area of the clay particles (Worrall 1964). the green-strength of the underglaze blue bodies would also have been higher. The higher plasticity of the underglaze blue body material would have made it easier to throw or mould the vessels and the higher green-strength would have been advantageous in the finishing processes of turning, decorating, glazing and setting in the kiln. These properties of the raw materials used would therefore have facilitated the production of the more substantial vessels which are typical of underglaze blue porcelain.

In order to assess the refractoriness of the porcelain bodies, their bulk compositions were plotted on the phase diagram K20-AI2O3 --SiOz (Schairer and Bowen 1947). For this purpose Na,O was considered equivalent t o K 2 0 on a molar basis and the CaO, FeO, MgO and P,05 contents were assumed negligible. From figure 9, it can be seen that the body compositions all fall within the niullite field of the phase diagram. It is generally agreed that the porcelains were fired in the 1200-1300°C range (Wood 1984a). Therefore, in order to compare their theoretical melting behaviour, the proportions of the phases (by weight) in each porcelain

A technological study of Chinese porcelain 0.f the Yuan dynasty 151

body at 1250°C were estimated from the phase diagram using standard geometric methods (Levin et al. 1956). These calculations indicated that at equilibrium at 125O"C, the under- glaze blue bodies would consist of - 85% glass with - 15% mullite and n o silica. The yingqing bodies, on the other hand, would consist of 50-70% glass, - 15% mullite and 15-35% silica. These figures strictly apply t o an equilibrium process and the phase assemblages of the ceramics are demonstrably not in internal equilibrium. Therefore, the percentages of glass in the bodies were also estimated from the ratio of the bulk KzO content in the body to the K 2 0 content in the glass itself, as analysed by the electron microprobe, the assumption being that all the KzO was in the glass phase. These latter calculations confirmed that the glass content of the underglaze blue bodies (76-84%) is higher than that of the yingqing bodies (58-73%). These data indicate that , principally as a result of their higher alkali contents, the underglaze blue bodies are less refractory than the yingqing bodies.

The maturing temperatures of the glazes are not purely a function of melting temperature.

V v 20 10

. .... \

K 2 0 L V

b mul l i te

Figure 9 Composih'ons (percentage weights) o f yingqing (circles) and underglaze blue (squares) bodies plotted on the high-silica part of the phase diagram K,O-AI ,O, - S O , (simplified after Schairer and Bowen 1947). Isotherms are shown every hundred degrees in the silica and mullite fields. The heavy dashed line is a projection between mullite and 1250°C on the tnullite-silica cotectic. Under equilibrium conditions, for compositions below this line all quartz will have melted b.v 1250°C; above the line quartz is present until higher temperatures.

157 M. S. Tire, I . C. Freestone and M. Binison

They are also dependent on a number of other parameters such as viscosity and surface tension, which determine if the glaze will run off the body, and the degree of reaction between the body and glaze. On the basis of the data obtained by Wood ( l 9 8 4 b ) for series of glazes prepared by adding increasing amounts of lime to a synthetic yingqing body, the maturing temperature for the glaze of the three yingqing sherds (13--19% CaO) is in the range 1230-1270°C and that for the three underglaze blue sherds ( 8 - 1 m CaO) is in the range 1240-1300°C. Furthermore, viscosity calculations using the method of Bottinga and Weill (1972) indicated that the viscosity of the yingqing glaze (7.6 x lo4 poise a t 1200°C) is substantially lower than that of the under- glaze blue glaze (3.2 x lo5 poise a t 1200°C and 8.7 x lo4 poise at 1300°C). This again implies ;L lower maturing temperature for the yingqing glaze in order to stop it running off the body.

There is an apparent inconsistency between the fact that the underglaze blue glaze has a higher maturing range than the yingqing glaze, whereas the underglaze blue body is less refrac- tory than the yingqing body. However, Wood’s data d o indicate an overlap in the maturing ranges of the glazes so that they could have been fired at the same temperature. In summary, therefore. i t seems unlikely that there was any very significant difference in the firing tenipera- tures employed in the production of the two types of porcelain under investigation. Certainly there is no evidence t o support the hypothesis (Wood 1984b) that the change in the raw materials used (i.e. from kaolinised porcelain-stone to a mixture of kaolinite-free porcelain- stone and kaolin) was prompted by the need for a higher firing body t o accommodate the lowel- l ime. higher maturing temperature underglaze blue glaze.

A further technological aspect requiring consideration is the fact that for each of the three types of porcelain, there is a specific glaze Composition. The shufu glazes are characterised by a low lime content (5-77c)(Guo Yenyi 1982) which results in an opaque glaze probably due t o the development of silica crystals. The underglaze blue glazes are characterised by an inter- mediate lime content (7-10%) with higher alkali content. According to Wood (1984b), these glazes are the best in the body clay-lime series with respect t o transparency and the absence of crazing. The yingqing glazes are characterised by a high lime content (12-19%) and are closer to the eutectic in the system CaO-Al2O3--SiO2. The spread of compositions within the yingqing group as well as the variation within a single glaze suggests that the composition and preparation of those glazes were not closely controlled. Presumably careful preparation was less impertant nearer the eutectic as variations in composition are less likely t o take the glaze into the silica field during firing and result in an opaque glaze, However, a disadvantage of glazes around the yingqing coniposition is a tendency t o craze (Wood 1984b). Opacity and crazing would have been highly undesirable on vessels decorated with underglaze painting. Thus the underglaze blue glaze compositions were carefully selected and controlled t o avoid precipitation of silica at low lime contents and crazing at high lime contents, so that the underglaze painting was best displayed.

C 0 N C L U S 1 0 N S

The results presented above significantly extend the available analytical data and indicate that there were two distinct traditions regarding the raw materials used in the production of yingqing, shufu and underglaze blue porcelain at Jingdezhen during the Yuan dynasty. The first involved the use of a kaolinised porcelain stone for making both the bodies and glazes with the addition of ‘glaze-ash’ (i.e. lime) in the latter case. The second most probably involved the use of a porcelain-stone containing a negligible amount of kaolinite. For making the body, kaolin was

A technological study of Chinese porcelain of the Yuan dynasty 153

added to the porcelain-stone and for making the glaze, ‘glaze-ash’ (i.e. lime) was added. On the basis of the present analytical data and those previously published, both traditions appear t o have been employed in the production of yingqing porcelain. However, it seems that the shufu and underglaze blue porcelain was always produced using the second tradition. The use of kaolinite-free porcelain-stone mixed with kaolin was therefore a later development but it is not yet certain a t what stage the changeover occurred.

The reasons for the change in the raw materials used t o make the porcelain are not clear-cut. it seems unlikely that there was any very significant change in the firing temperature employed in the production of the porcelain as a result of the change in the raw materials. However, the higher plasticity and green-strength of the bodies made from porcelain-stone mixed with kaolin would have been advantageous in the production of the more substantial vessels which are typical of underglaze blue porcelain. An alternative explanation is that by adding kaolin t o porcelain-stone, the control of the composition, and hence the physical properties, of the raw materials would have been easier than when using a kaolinised porcelain-stone. However, perhaps the most likely explanation for the change is that deposits of readily obtainable kaolin- ised porcelain-stone ran out and that alternative raw materials had to be sought. Since yingqing porcelain was also produced using porcelain-stone and kaolin mixtures, it may be that resource depletion provided the impetus for the change but that the associated technological advantages subsequently made it possible t o produce more readily more substantial vessels of underglaze blue porcelain.

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

We are indebted to the late Sir John Addis for providing the samples of porcelain-stone, glaze-stoi:e and kaolin, and for encouraging us to undertake the project. Mrs J. Rawson and Mr D. Gillman of the Department of Oriental Antiquities are thanked for providing the sherds examined and for helpful discussion during the course of the work. We are indebted to Dr A. C. Bishop, Keeper of the Department of Mineralogy. British Museum (Natural History) for allowing us to use the electron microprobe in his Department.

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154 M. S. Tite, I. C. Freestone and M. Birnson

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