A Study of Ca-Mg Silicate Crystalline Glazes

——An Analysis on Forms of Crystals*

LIU Pei-de* *

, YU Ping-li and WU Ji-huai

I nstitute of M aterial P hy sical Chemistry , H uaqiao Univ ersity , Quanz hou 362011, P . R . China

Receiv ed Dec. 5, 2003

* Suppor ted by the Natura l Science Foundation of Fujian Province( No . D0210012) .

* * To w hom cor respondence can been addressed.

　In the study on Ca-Mg silicate cry stalline glazes, we found some disequilibra ted cry st allization phenomena,

such as non-cry st allog r aphic small ang le forking and spher oidal g r ow th, pa rasitism and w edg ing-form of cr ys-

tals, dendr itic gr ow th, secondar y nucleat ion, etc. Those phenomena possibly r esulted fr om two facto rs:

( 1) par tial t emperature gr adient , w hich is caused by heat asymmetr y in t he electr ical resistance furnace,

w hen cry stals cr ystalize fr om silicate melt; ( 2) constitutional super coo ling near the sur face of cr ystals. T he

disparity o f disequilibrat ed cr ystallizat ion phenomena in different main cr ystalline phases causes var ious mo r-

pholog ical featur es o f the cry st al ag gr egat es. A t the same time, disequilibrated cr ysta llization causes g reat

str ess ret ained in the cr ysta ls, which result s in cr acks in g lazes w hen the tem perature dr ops. Acco rding to

t he r esults, the aut ho r s analyzed those phenomena and displayed cor relative figures and data .

Keywords　Cry st alline g laze, Co stitutional super cooling , Heat dynamical condition, Disequilibr ated cry st al-

lization, Mo rpholog ical feature o f cry stal ag g regates

Article ID　1005-9040( 2004) -02-200-05

Introduction

Tr aditional ceramic glaze is basically is silicate

glass. The crystall izat ion state of cry stalline in

glass direct ly af fects the appearance and the perfor-

mance o f the glaze. Crystal lization has a bad im-

pact on t ransparent glazes, and abundant micro-

cry stals faciliates opaque glazes. When a few cry s-

tals become phaner ocry stalline, they are probably

have values for appreciat ion. Cry stalline glazes are

glazes of o rnamental por celain decorated by cry s-

tals. Based on good glaze f it , the morpholog ical

featur es o f single crystals or cry stal ag gregates de-

term ine the values o f cry stalline glazes.

The thickness of cr ystalline g lazes ranges from

1. 5 to 2 mm, which is thicker than that of common

glazes. T he crystalline grow th is r estr icted by

space. U sual ly, crystals gr ow in three dimensions

at the init ial stag e, and the gr ow th quickly changes

into one dimensional or tw o dimensional growth

along the cry stalline direct ions, and finally the

cr ystals become f ibrous aggr egates o r tabular ag-

gregates.

While the temper ature of the silicate melt

dr ops about 100 ℃ fr om the f iring region to the

gr ow th region, it is in the condit ions of heat dise-

quilibrium and const itut ional super coo ling . Even if

it lasts fo r a certain per iod at the temperature of

gr ow th region, the temper ature field is not com-

pletely symmetrical in the electr ical resistance fur-

nace. The undulat ion of composit ion and heat dy-

nam ic condit ion af fects the gr ow ing crystals, and

leads to disequilibrated cry stallizat ion forms.

M oreover, g row ing under disequilibr ated condi-

t ions, the crystals retain much st ress, w hich dam-

ages the glaze fit and porcelain′s value. The forms

of cr ystals and aggr egates direct ly influence the

value for appreciat ion. T herefore, studying silicate

melt crystallization is impo rtant for obtaining good

cr ystalline g lazes. According ly, the autho rs w ill

gener ally analy ze some forms of silicate crystals

and the possible inducements o f them.

Mineral Combination in GlazesThe components of silicate melts are complex .

T he autho rs only sem iquant itat ively studied the re-

lat ionship betw een the melt composit ions and the

miner al combinat ions. T he results are display ed in

T able 1. When the rat io of m ( CaO ) /m ( MgO) is

higher than 1. 5, the main cr ystalline phases are

made up of w ol lastonite and cyclow ollastonite; and

when the rat io is clo se to 1, the main cry stalline

phases are clinopy roxene and melilite. In the same

sample, ther e are several mineral combinat ions:

CHEM. RES. CHINESE U . 2004, 20( 2) , 200—204

wollastonite + cyclow o llastonite, w ollastonite +

clinopyroxene, clinopyr oxene + melilite, w o llas-

tonite+ clinopy roxene+ melilite, cy clow ol lastonite

+ clinopy roxene.

Table 1　The relationship between the ratio of m(CaO) /

m(MgO) and mineral combinations in crystal-

line glazes system*

m (C aO) / m( MgO)m( SiO 2) / m( Al 2O 3)

13 15 17 19 20

2. 467 A A A/ B A/ B A/ B

1. 836 A B/ C B B

1. 229 D C/ B B

0. 733 C/ D C C

　　* A. Cyclow ollastonite; B. w ollastonite; C. clinopyroxene;

D. melilit e.

Analysis on Forms of Crystals and Aggre-

gates1　Basic Growing Forms of Crystals

In crystalline g lazes, basic forms of crystals

are determ ined by crystal st ructures and glazes′

space. In the cyclow ollastonite crystal st ructure,

the arrangement of [ SiO 4 ]4-

tet rahadra is an

[ Si3O 9 ] ring . T he cry stal shape of cy clow o llas-

tonite is usually equig ranular. Because of the dise-

quilibrated cr ystallizat ion caused by melt composi-

tions and heat dynamic conditions in supercooling

glaze melt , cyclow ollastonite gr ow s in 2-dimension

and becomes flaky cry stals. Hexagons are the cen-

ters o f cry stals of cyclow ollastonite, w hich become

hexagonal skeleton crystals. As the skeleton cry s-

tals develop, the hexagonal o r irregular flaky cry s-

tals superpo se and become ret iculated agg regates

( F ig . 1) .

Fig. 1　Six-sided crystal center and net-shaped crystal

aggregate of cyclowollastonite ( d = 1. 5 mm±

RLM ) .

RLM: Ref rected ligh t m icros cope. d : long axis of vi-

su al area.

T he crystal st ructure of w ollastonite is a chain

st ructure fo rmed by bi-tet rahedra [ Si2O 7 ] and te-

tr ahadra [ SiO 4 ] , w hich ar e par al lel to b-ax is and

appear al ternatively . And this st ructure is fav or-

able to the forming of tabular or fibrous w o llas-

tonite par allel to b-axis. The cry stal behavior is ob-

vious in crystall ine glazes and the cr ystals ag gre-

gates are radiat ing ( Fig . 2) .

Fig. 2　Wollastonite [ crystal aggregate of

several rounds, d= 5. 4 mm( - ) ] .

( - ) : Polarizing microscope with s ingle polar.

The crystal st ructure of diopside makes its

cr ystal co lumnar and the columnar-gr anular ag gre-

gates are common in nature[ 1]. The diopside cry s-

tals appear in a columnar form in cry stalline glazes;

the aggregates o f diopside are radiat ing w ith vari-

ous appearances( F ig. 3 and Fig . 4) .

Fig. 3　Pyroxene [ radial crystal aggregate,

d= 5. 4 mm( + ) ] .

( + ) : Polariz ing m icros cope w ith cross polars .

Fig. 4　Pyroxene [ radial crystal aggregate,

d= 5. 4 mm( + ) ] .

Melilite belongs to a tet rag onal system and the

tabular cr ystals have square tr ansects. Small-sized

melilite in the crystall ine glazes is g ranular w hich is

close to a square or polyg on with salient angles of

90°. Under a micr oscope, the crystals of melilite

are actually singular or penet rat ing dimorphous

( Fig . 5) . Big ones are limited by the space. T he

t ransects of the singular cry stals are square, and

the edges of the crystals of penet rat ing tw ins ex-

po se on the surface of glazes which makes the cry s-

tals r adiating.

No 201. 2 L IU Pei-De et al. 　　

Fig. 5　Melilite[ insert twin, square section of

crystal, d= 5. 4 mm( - ) ]

2　Unequi librated Crystall ization Phenomena

2. 1　 N oncry stallographic Smal l A ngle Forking

and Spheroid al Grow ths

In the init ial gr ow ing stag e, both w ol lastonite

and clinopy roxene show obvious sphero idal

gr ow th. A low rate of molecule movement is fav or-

able to the spheroidal g row th of cry stals[ 2, 3] .

Therefo re, silicate melt′s high viscosity and a large

amount of impurities are the important outer causes

to the spher oidal grow th.

The spherical crystals are formed when the

so lid-liquid interface is disturbed and the cry stalline

ang le deviates slight ly. Because of small angle

fo rking , tw o ends o f the crystals cont inually g row

in mult iple direct ions, first they become dendrit ic

or radiat ing crystals, then spherical cr ystals. In

cr ystalline g lazes, spher ical cry stals are limited by

thin melt , and f inally they form radiat ing planar

agg regates.

The ang les of small angle forking ar e indefinite

and have no connect ion with cr ystal lat t ice. All the

spherical crystals, w hich ar e fo rmed by the small

ang le forking o f the singular crystals, ar e made up

of fibrous crystals radiated f rom the same center [ 4] .

In crystall ine glazes, when tw o or more cry stal

centers ar e nex t to each other, the growth of fi-

br ous cry stals of the spherical crystals of w o llas-

tonite may be limited by the space; and the unlim-

ited part cont inuously gr ow s into a part of the

sphere ( Fig . 2) . T he indefiniteness o f the branches

of small ang le fo rking makes the f ibr ous radiat ing

cr ystals of w o llastonite bend w ith various radiat ing

fo rms. Non-cry stallog raphic small ang le for king al-

so appears in clinopyroxene aggr egates w ith various

radiat ing and dendritic cr ystals. Compared w ith

that of w ollastonite, the ability of non-crystallo-

graphic small angle for king of clinopy roxene is

much w eaker, w hich makes the clinopyr oxene ag-

gregates and w o llastonite agg regates dif fer from

each other.

2. 2　Outl ine of Dend ritic Grow th

In crystall ine glazes, the cry stallizat ion o f cy-

clowo llastonite most ly begins w ith dendrit ic

gr ow th and with hexagonal skeleton cry stals as

centers( F ig . 1) .

Dendrit ic g row th o f clinopyr oxene is obvious

and occurs in every stage of crystallizat ion. In the

beg inning of the gr ow th, the branches are formed

w ith the radiat ing crystals of spherical cry stals as

t runks( Fig . 6) .

Fig. 6　Dendrite of pyroxene[ d= 2. 16 mm( + ) ] .

During the grow th, because the dissipat ion of

the latent heat at the tips of the cry stals is faster,

and the deg ree o f supercooling is high, w hich is

beneficial for the g row th of the f langes and the for-

mat ion of slender tr unks of the branches. The in-

stability of the interface of the supercoo ling liquid

around the t runks causes the fo rmat ion of branch

cr ystals. Because of the cont inuous crystallizat ion,

the inter-spaces o f branches ar e f illed w ith or cov-

er ed w ith newly-fo rmed cr ystals, which makes the

formed br anch cry stals unobservable. A sudden

dr op in temperature breaks the crystall ization

rapidly, and the g row ing form of the branch cry s-

tals r emains( Fig . 7) .

Fig. 7　Pyroxene( dendritic growth at the edge of

crystal aggregate, SEM ) .

In the same radiat ing aggr egate o f py roxene,

the slender t runks of the branch crystals can be

st raight o r cur ved( F ig. 8) , which means that there

is dif ference in dynamic conditions and composi-

t ions among different regions in the melt . When

cr ystals are g row ing, either the compositional dif-

ference in the melt beside the t runks or disturbance

202　　 CHEM. RES. CHINESE U . Vol. 20

to the stabil ity of sol id-liquid w ill cause the growth

of the cry stal to deviate slight ly and the t runks to

bend. T he occurrence of the py roxene br anch cry s-

tals and the bend o f the t runk show the instability

of the solid-liquid interface. A ll the phenomena of

dendrit ic gr ow th show that during the process of

dendrit ic grow th at a constant temperatur e, there

are some instable facto rs in the melt such as the

composit ion of the mel t and heat-dynam ic condi-

tions.

Fig. 8　Pyroxene [ radial crystal aggregate and curved

radiated tuf ts, d= 1. 5 mm(RLM ) ] .

2. 3　Parasit ism and W edging-f orm of Cry stal s

The occurr ence of parasit ism and w edging-

fo rm of cry stals is obv ious in melilite. The singular

melilite has a square crystallized center , it s out line

is similar to crystal agg regates ( F ig. 9) . The four

ang les of the singular melil ite are w edged. T he

w edging-form of cry stals is the result of the faster

Fig. 9　Melilite ( square center of crystal,

wedge-shaped crystal at corner, SEM ) .

gr ow th at the t ips of the crystals. The fast dissipa-

tion on the t ips of the crystals and a high degree of

surper cool ing around the cr ystals lead to the fast

gr ow th. T he depo sit ion o f the so lute in the melt

cr eates a large amount of parasit ical crystals in the

mother-cry stals. T he direct ion of parasit ical cry s-

tals is parallel to the mother -crystals and the t ips

are higher than the other part . The parasit ical

cr ystals and mo ther-cry stals show ex tinct ion prop-

er ty under a cro ssing-polarizing microscope, w hich

belong to a parallel or ientat ion. Parasit ical crystals

have the same gr ow ing state as the mother -cry s-

tals. The parasit ical crystals in diagonal direct ion

are w edged while the par asitical crystals par allel to

the sides of the mother-crystals ar e clo se to a

square.

Parasit ical cr ystals and wedg ing-fo rm ar e like-

ly to occur in regions w ith higher st ress and de-

fects, w hich indicates the ex istance of st ress in the

pr ocess of cry stallizat ion, and st ress is a v ital fac-

tor causing cracks in glazes.

2. 4　S econdary N ucleation

The nucleat ion over ex ist ing cr ystals is called

secondary nucleat ion[ 5] . Secondary nucleat ion oc-

curs around existing aggregates, and enriches cry s-

tal pat ter ns of crystall ine glazes. Pyro xene aggre-

gates have various secondary nucleat ion forms.

Secondary nucleat ion o f pyro xene in cry stalline

glazes forms branches ( F ig . 10) and various f label-

late aggr egates( Fig . 11) . T he shapes of ag gregates

bring the magnificent f ringes of cr ystalline glazes.

Fig. 10　Pyroxene crystal aggregate [ dendrite caused by

secondary nucleation, d= 2. 16 mm( + ) ] .

Fig. 11　Pyroxene crystal aggregate [ f an-shaped

edge caused by secondary nucleation,

d= 2. 16 mm( + ) ] .

Secondary nucleat ion usually occurs w hen the

temperatur e of cr ystal grow th fal ls[ 5] . For the pur-

po se of chang ing the forms of crystal agg regates,

w hich makes crystalline glazes mor e valuable for

appreciat ion, w e can change the constant tempera-

ture of cry stal grow th.

DiscussionFr om the analyses above, the autho rs found

that cr ystalline phases w ith dif ferent m ineral com-

binations have various cry stalline phenomena

caused by the disequilibrated condit ion. Small an-

gle forking and spheroidal grow th phenomena occur

most obviously in w ollastonite, secondly obviously

No 203. 2 L IU Pei-De et al. 　　

in pyr oxene. Dendrit ic g row th mainly occurs in

some minerals, such as py roxene and cyclow o llas-

tonite. M elilite is characterized by parasit ism and

w edge format ion of crystals. Secondary nucleat ion

is obvious in pyro xene aggr egates.

The inducements of those disequilibrated cry s-

tall ized forms above are heat dynam ic condit ions

and the hetero geneity of the melt components,

such as local temperatur e gradient and components

fluctuat ion.

Crystallizat ion changes the melt composit ion

near the so lid-liquid interface, w hich leads to melt

composit ion changing w ith distance aw ay from the

interface, and even causes the composit ional dif fer-

ence betw een the tw o sides of the interface. T he

composit ional difference of melt po ssibly results

fr om heat dynam ic factor s, such as: ( 1) melt

movement caused by slight temperature f luctuat ion

in the isothermal process; ( 2) compositional f luc-

tuat ion of the melt surface caused by the heat con-

vect ion in elect rical resistance furnace. Heat dy-

nam ic unstable factors possibly result f rom :

( 1) the temperatur e asymmetry in the elect rical re-

sistance furnace; ( 2 ) the different temperature

fields in the dif ferent r eg ions of solid-l iquid inter-

face, w hich is caused by the latent heat produced

by cry stallizat ion. T he br anches and sphero idal

cr ystals caused by small ang le for king are produced

when the stability of the so lid-liquid interface is de-

st roy ed.

In addition, the patterns of growing crystals

are lim ited by space in glazes. In crystal growth

pr ocess, cer am ic g laze is a shallow layer of a sili-

cate melt . T herefo re, three-dimensional g row th of

the cry stals is lim ited. One dimensional or tw o di-

mensional cr ystal g row th commonly occur s, and fi-

br ous or tabular-co lumner crystals come out fre-

quent ly. Cry stal agg regates and interface inter sect

to fo rm or namental “crystal flow ers”. Take w ol-

lastonite for instance, the surface o f g lazes and

but t-glaze inter face limit the spheroidal cry stal for-

mat ion, and crystal agg regates appear as round ra-

diat ing cr ystals made up of fibrous cr ystals on the

surface o f the glazes. While mel il ite crystals pre-

sent interpenet rated sect ions of singular or mult i-

cr ystals. Pyro xene can form branch crystals or

dendrit ic crystals at any t ime. T he dendr it ic cry s-

tals or br anches formed later fill the interspaces of

former crystals and can no t grow up for the limited

spaces( F ig . 12) . T he existence of radiating crystals

and small dendritic cr ystals, as w el l as the sector

edge formed in the secondary nucleat ion, make the

macro-features o f the radiat ing pyroxene aggr egate

change and therefore, reinfo rce the outer value for

appreciat ion.

Fig. 12　Pyroxene [ small radiated tufts in

aperture between large-sized

radiated tufts, d= 5. 4 mm( - ) ] .

Disequilibr ated crystallizat ion keeps a high

st ress in glazes. T he st ress does harm to porcelain

because the st ress can low er capability of combina-

t ion fo r porcelain, w hich makes the crystals crack,

the glazes cr ack or f lake, and so on.

Disequilibr ated crystallization is unavoidable in

cr ystalline glazes. How to control it properly and

to make full use of cr ystal ag gregates′appearance

formed by unstable factor s, and to low er the harm-

ful inf luence on the g laze quality are quest ions to be

studied.

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204　　 CHEM. RES. CHINESE U . Vol. 20