Glaze Color

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    LIQUID PH SE SEP R TION N GL ZESOR SINGLE FIRED POROUS W LL TILE

    by J . Apart siv , M V Nufiez , Moreno?', M J . Orts ' ?' Esmalglass, s.a. Villarreal In stitu to de Tecnologia Ceramica . Universitat Jaume Castellon.Asociacion de Investi gacion de la s Industrias Ceramicas. (AICE). Castellon (Espana ).

    ABSTRACT

    Th e appeara nce of colou r differences in fast single-fired porous wall tile with transparent orpractically t ra ns parent glaze is one of t he d efects considerably impairing the quality of th e fin ishedproduct.Th e sepa ra ti on of immiscible liquid phas es during cooling of th e glaze in th e ki ln was shown tobe the physico-chemical trans formation responsible for changes in colour. Th e colour of these glazeswas shown to be very sensitive to small chan ges in th e composition an d limited alterations in th ecooling cycle of the ware in the kiln . A method is proposed, based on th e measuremen ts of th echromatic coordinates of the glaze obtaine d by subjecting it to differen t th erm al treatmen t, fordet ermining th e sens it ivity of th e glaze to changes in colour by alte rations in th e cooling cycle.1. INTRODUCTIONThe ap pearance of colour differences in fast single-fired porous wall tile with virtually tran spar en tglaze is one of th e de fects conside ra bly impai ring th e quality of th e finish ed product.Th e colour of these gl azes ha s bee n shown industr ially to be extre mely sensitive to small changesin th e compositi on and in th e cooling cycle. Th e la t te r seem s to indi cate th at a series of physicochemica l transformations (immiscible liquid-phas e separat ion, crystallization of new ph ases) takes

    pl ace du rin g cooling o he ware in th e kiln, which appear macroscopically as a change in colou r ofth e glaze.Th is k in d of gl aze is characte rized by bearing a high pe rcentage of alkaline earth oxides and zincoxide as well as a cons idera ble am ount of boron oxide. These materials ma y th erefore be expectedto show a great te nd ency to separate into immiscible liquids or even to devitrify crystalline phases,

    as may be observed from the cor respond in g immiscibil ty diagrams (1) (2) (Figure 1 . Th er e is in fac ta broad immiscibil ity domain for th ese systems where many of th e compositions used as glazes forsingle-fired tiles a re to be found.

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    coO OCoO

    5 i02

    Zn O

    ZnO 5 0 2 o

    2. AIMSFigure 1. Immiscibility diagrams of someternary systems.

    In this study the causes of changes in colour of th e glaze are analyzed with a view to acquiringenough information to formula te new compositions which avoid or minimize this defect.A series of experiments was therefore programmed to achieve the following aims: To determine the nature of th e physico -chemical tran sformations responsible for the changein colour of the glaze on cooling in order to elimin ate or minimize this defect.ii To determine the temperature interval in wh ich these changes in colour take place at th e

    greatestrate in order to choosea suitable cooling cycleand establish the influence of this firingstage on this phenomenon.

    3. MATERIALS AND EXPERIMENTAL PROCEDURE3.1 MaterialsIn order to carry out this study a group offr its used in obta ining transparent glazes over porousbodies by fast single fir ing were utilized. Their chemical compositions are detailed in Figure 2.Alth ough differences in composition among the differ en t frits are to be observed they all bear a grea t

    amount of eaO and ZnOas well as BaO in some instances.The sealing t ransformation and softening temper at ure values of th ese frits are detailed inFigures 3 4 and 5. All present the typical values for frits used in single-fired porous wall t ilemanufacture

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    fo7 .... - - - - - - - - - - - - - - - - - -

    4321o

    1 .3

    CaO MgO Na2 K aO ZnO igure Interval of compositi s studied

    3.2 Experimental3.2.1 Sample preparationGlaze suspensio ns were prepar ed bygrinding each fr it in a laboratoryball mill with 5kaolin and0.3C.M.C.Operating conditionswere as follows:Su spensiondensity 1.65 g/cm,and2grind oversizefraction at 40 urn. Th e glaze suspension wa s sprayed onto a single-fired porous redware body which

    had previously been fired aU1 OOC,whilethe amount ofglaze applied, 0.7 g/crn, washeld as constantas possible.3.2.2 Thermal treatment3.2.2.1 Non -isothermal experiments .A series of expe riments was ru n with ea ch of th e fr its, keeping th e firing cycle cons ta nt and

    modifying the kindof cooling in order to compare the changes in colouring th e glazes undergo duringth e cooling stage of the firi ng operation and to determ ine the temperature interval in which thisphenomenon takes place at the grea test rat e.Th e th ermal cycles used are shown in Figure 6. As maybe observed in this figure , each test specimen was fired up to 11OOC and cooled in th e kiln accordingto a slow (A) or a fast (B)cycle un til reaching a cer ta in temperature.The test specimen was the n takenout of the kiln an d left to cool in ambient conditions. The arrows in the figure indicate thetemperat ure at which removal from th e kiln took place.Th e thickness of th e glaze layer and its chromatic coordinates (Section 3.2.3) were determ ined inthe cold test pieces. Th e test pieces with a markedly different thickness in their glaze layer from th epreset val ue were rejected.

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    25

    20

    15

    10

    5 o -86 87 88 89 9 9 92 930

    Figure 3 Sealing temperatures of th e frits used

    25

    2

    15

    10

    5

    6 62 63 64 65 66 67igure Transfonnation temperatures of the fri ts used

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    30 ,----- - - - - - - - - - - - - - - - - - - - - -

    25

    20

    1

    5

    0 74 76 78 8 820 840Figure Softe ning temperatures of the ftits used.

    400

    21000 3456 7 8800 8

    H OC A600

    zoo

    10 30 50 70t minFigure 6Non-isothermal thermal treatment used. The arrows indicate temperature and time at whicht t piec were removed from the kil n.

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    3.2.2.2 Isoth ermal experim en tsA series ofi sothennal experiments was conducted to detennin e th e influence of temperature onth e rate at which changes in colour take place in the glaze.The thermal t reatment carried out is schematized in Figure 7. It involved subjecting the fired testpieces taken from the kiln at llOOC to differing th ermal treatment.

    0 C 1

    T

    t mlnutos

    Temp e ra tura delt ratamientoi sote r mo

    Figure Isothermal treatment method determine the rate o ch nge in colour of the glazes

    3.2.3 Determination of the change in colour in th e glazes.The changes in colou r in each glaze , which take place on cooling of th e ware, were quantified as

    the difference between th e values of th e chromatic coordinates of th e glaze applied on th e test pieceremoved from the kiln at llOOC and th e values corresponding to those involved in th e remainingthermal treatmen t. As pointed out above Section 3.2.1 , glaze was applied on bodies previously firedat 11 C so that any possible change in colour of the body would not affect th e results.3.2.3.1 Determination of th e chromat ic coordinates of the glazed piece.These were detenn inedby mean s ofa t rist imulus colorimeter, model LFM-3, supplied by the finnDr. Lange. The CIE 1976 CIELAB mea suring system was used. This is based on colour descriptionby using th r ee rectangular coordinates L a and b , The relation between th ese parameters andcolour are schematized in Figure 8. In order to reduce experimental error, five measuremen ts were

    run on each test specimen and th e mean value was computed.

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    ..L- bla n o

    bamar illo

    -averde

    a zul - neg ro

    - a{rcic}

    Figure 8. CIEL system ofchromatic coordinates. (green;light green; light yellow; yellowred; dirty red;blue black; blue)

    3.2.3 .2 Calculation of colour differences.The standard ASTM D-2244-85 method 0 was used.According to th i s method, colour difference (6C) b etween a certain colour C ) described by its

    chromatic coordinates (L, a and b ) and the reference colour (C with coordinates (L: , a : andb: ) is given by the equation:6C = [(6L* + (6a + (6b*) ]

    where:6 L* = L* - Lo*6b * = b* - boo

    A glazed piece fired at 11OO C and removed from th e kiln at th is temperature was taken asreference colour (Co ). Th e values for 6L*, 6a* and 6b* the re fore indicate th e difference between th evalues of th e chromatic components white, red and blue ofa glazed piece and th ose of the referencepiece.4. RESULTS AND DISCUSSION

    4.1 Ch a nges colo ur undergone by the glazes according to the cooling cycle they we res ubjec ted to .In Figures 9 and 10 the bounding colour difference va lues found for a glaze fir ed at 11OOC and

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    r emoved from th e kiln at th is temperature and th e findings ob ta ined on let ting it cool in side the k il nto ambi en t temperature are shown for the glazes st ud ie d , according to each of th e two abovem en tion ed cycl es Figur e 6).60 r - - - - - - - - - - - - - - - - - - - ----,40

    20

    o

    - 20

    + 6 L b la nco )

    - 40 -- - ---- - ---- JFigure 9. Colour difference and chromatic coordinates and between th e glaze fired

    and removed from the kiln at 0C virtually transparent and the glaz es after cooling by a slo w cycletype A). Cooling time from 1100 to 750C was 25 min .

    30 - - - - - - - - - - - - - - - - - - - - ---

    20

    10

    10

    -20

    + 6 L (blanco)

    t r nsp r nt

    +68. rojo)

    - 68. verde)+6b (amar illo)

    - 6b azul)-30 L

    Figure 10. Colour difference and chromatic coordinates and between the glaze firedand removed from the kil n at 1l00C virtually transparen t) and th e gla zes after cooling by a fas t cycletype B). Cooling time from 1100 to 750C was 3.5 min .

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    Th e following may be concluded on analysis of these figures:Th e magnitude of the change in colour th e glaze undergoes on cooling decreases considerablywhen th e cooling ra te of th e ware increases.All the studied glazesshowed changes in colour to a greateror lesser extent on cooling. However,some performed better and even managed not to show any appreciable changes in colour whencooled at high rates.In each case th echangein colour was due toan increase in theblue -t.b*)andwhite t.L*)chromaticcomponent.

    The change in colour one of th e studied frits underwent when this glaze was cooled according tocycle A, is plotted in Figure 11.6 - - - - - - - - - - - - - -

    TlPO DE ENFRIAM IEN TO A

    4

    20

    o

    /BL ANCO

    il X 0

    VERDE

    2 il l \6 -+ 6 b

    AZUL 6 C *40 1 1 10 5 0 10 0 0 95 90 0 8 50 800 75

    TEMPERATURA Gigure 11 ariation of th e differences between the values of the chromatic components of th e gl ze and

    those corresponding to the reference piece with cooling timeIn this glaze the change in colour is observed to take place in a temperature interval between 1000and 900 C.Analogousfindings wereobta inedfor th e other glazes studied. although the temperatureinterval in which the change in colour takes place at th e greatest ra te and th e magnitude of thetransformation are differen t.The above seems to indicate th at the change in colour of th e glaze on cooling is due to immiscibleliquid-phase separation and/or devitrification of crystalline phases . In fact, the maximum rate of

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    change in colour takes place at a t emperature interval in which the glaze is fluid enough to allow theabove-mentioned transformations to take place. On the contrary,at low temperatures viscosityoftheglaz e is so great th at such transformations do not take place at an appreciable rate. At hightemperatures , close to peak temperature , the thermodynamic tendency to nucleate is so slight thatalthough viscosity of the glaze is also low, no nuclei can form. 3) 4)4.2 Kinetics of changes in colour of the glazeA ser ie s of isothermal experiments was conducted with one of the frits which showed a grea ttendency to ch a nge in colour on cooling, according to th e proc edure described in Section 3.2.2.2.In Figure 12 th e differences are pl ot ted between the values of the chromatic coordinates of th eglazed pieces treated at different temperatures and dwell times and those ofthe reference piece takenfrom th e kiln at llOOC), in order to determine the temperature interval in which changes in colourtake place at the greatest rate.

    40r-s min

    30 0 1 5 min t 1 0 min t 1 5 min20L , L10

    ,

    - - - - 1 : ~ : : : : 1 r - _ _ _ 7 r -

    1

    20

    3

    11000000000- 40 ----...l700

    Figure 2 volution of the chromatic components of the glaze with isoth al temperature treatmentfor different dwell ti mes.

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    In Figure 13, the evolutio n of the chromatic coord inate s blue and whi te are plot ted wit h dwellt imes for the temperatures at which changes in colour develop at the grea test rate , in order todetermine th e effect of te mperatu re on t he r ate of change in colour of th e glaze.40 r

    AZULo

    - 10

    - 3 0

    T 9 C BL ANCO30 T 9 5 C

    T1 00C

    ti L 20

    10

    f , b 20

    2 050- 40 l l o

    TIEP MIN.igure 3Variati on of th e chromatic coordinates of th e gl ze with dwell times at different temper tures

    On analyzing these plots , the following conclusions may be drawn:i) The temperature interv al in wh ich colour changes take pla ce in the glaze at a n appreciabl e

    rate lies between 800 and 1050C Figu r e 12).i i) Th e te mperatur e at whi ch th e colour diffe rence of the pie ce is t he greatest for both blue and

    white), in cr eases as dwell ti me decreases Figu re 12).iii) The white chromatic component increases as dwell ti me of th e glaze increases for a llthe temperatures tested Figure 13). On th e contrary, the blue ch romatic component r each esits maximum value after a certain ti me and then d rops . In fac t for this glaze, at temperaturesclose to 900C, maximum blueness is reach ed after a te n-minute dwell t im e. Bluen essdecrea ses progressively and whiteness in crea se s on inc remen ting dwell time at thiste mperature Figure 13).

    iv) On comparing the evolutio n of t he chromatic components with dw ell ti me at a certaint emp erature Figu re 13), t h e change in colors of th e gla zes is observed to follow the sequence :

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    Tran sparent - blue - bluish white - white, in each case.Th e fact that in both isothermal as well as non-isoth ermal treatment Section 4.1, blu enessappears as th e fir st observed change in colour of the glaze, to the n tum white, indicates that thephysico-chemical tran sformation responsible for this phenomenon is immiscible liquid-phasesepa ra tion. In fact, blueness is due to ma in ly the shortest wavelength radiation of th e visible

    spectru m being absorbed by th e glaze. This absorption is characteristicfor glazes bearing nucleatedphase sepa ration in which th e size of the drops dispersed in the matrix is limi ted. On increasing thedurati on of th ermal treatmen t, drop size grows and/or th ey coalesce, which entails absorption ofradi ation with a longer wavelength and the glaze therefore appears whi te and opaque 6.

    4.3 Nature of the physico chemical transformations responsible for changes in colourof the glaze on coolingIn th e foregoing section, immiscible liquid-phase separation was suggested as being th e likelycause of changes in colour of the glaz e on cooling. In order t o confirm this assumption someisothe rmally tre ated test pieces were chosen for observation by scanning electron microscopy SEM .Transparent glazes were shown to conta in nucleated separation of immiscible liquid phases witha very small drop size 2Ilm Figure 14.Furth ermore, bluish tran sparent glazes Figure 15 and 16 were also observed to bea r nucl eatedphase separation. However , drop size was greater 2Ilm so that on mainly ab sorbingshorter visiblewavelength radiation th ey turned bluish.

    Figure 4Microstructure of a transparent glaze

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    Figure 5 Microstructure of a bluish gl ze

    Figure 16. Microstruc ture of a bluish glaze.Whi te opaque gl zes were similarly observed to be r drops of imm iscible liquid pha s ewhich hadcoalesced Figu res 17 nd 18. h e in crease in size of the dispersed ph se involves ove r ll bso rption

    in the visible ligh t spectrum thus giving th e gl ze a white ppe r nce.

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    Figure 17 Microstructure of a bluish white glaze

    Figure 18 Microstructure o white glazeIt m y be concluded from th e foregoing that th e colour of th e glaze will dep end upon th e num erof dispersed drops nd th eir size. Th erefore, s nucleation of sm ll drops nd their growth and/orcoalescence t ke place t gret r te and at prtially overlapping temper ture in terv ls in this glaze,holding the colour of th e glaze or per iodically obt ining transprent slightly bluish glazes would beextremely complicted Figu re 19.

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    Intens idadde lco lo r

    I AZ ULII

    BL ANCO

    TEMPERATURA C

    Figure 19. Nucleation I) and growth I l) curves.

    4.4 Isochromatic curves.Th ese curves were experimen tally obtained by mean s of isothermal techniques Section 3.2.2.2)and plot the pair s of values , temperature an d dwell t ime at tha t temperature, where the glaze reachesthe same blue ness Figu re 20) or whit eness ind ex values Figure 21). Th e following inform ation may

    easily be acquired by using th is k ind of plot : i) The sensitivity of a glaze to change in colour on cooling. In fact, the shorter the t ime in whi cha certa in change of colour takes place th e more difficult it becomes for the gla ze to keep it sorigina l colour on cooling.i i) Th e te mpera ture at which the change in colour o he glaze develops a t th e grea test rate.That

    is, the temperature at which the glaze undergoes a certain change in colour with the shor testdwe ll tim e.iii Establishing the temperature and time interva ls in which the re is no change in glaze colour.

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    \ \I/

    I 5 10 t .I n.

    lot. Zot 3D l

    Z

    / /\. b ) for a glaze with littletendency to liquid-phase separation.1000

    v900

    TC C I

    800

    70010

    L ..S to 6L 1 l 6L . 15 - l ZO :

    100t mi

    1000

    Figure 23. Curves of th e same wh iteness index values 6 L*) obtained from isother-mal treatmentIt has bee n possible to obtain frits for single-fired porous tilewith little te ndency to change in colouron cooling, on the ba si s ofthe influence exerted by the fri t composition on liquid-pha se separation aswell as by u sing th e te chniques described above.Th e isochromat ic diagrams corresponding to one of th es e Frits are plotted in Figures 22 and 23.It has likewi se been observed that slight variations in frit composition consid erably infl uence th e

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    tendency for changes in colour to develop on cooling of the glaze. This all entails th e need to disposeof a very strict monitoring system both for th e raw materials as well as th e frit elabora tion processto obtain good tr ansparent glazes.5. ON LUSIONSThe following conclusions may be drawn from the findings obtai ned in this study: Immi scible liquid-phase separation was shown to be th e physico-chemical transformation

    responsible for changes in colour of the glaze on cooling in th e kiln, for th is kind of glaze .However, the magnitude of this phenomenon largely depends on frit composition and thecooling ra te used on firing.(ii) On studying the evolution of th e chromatic coordinates of th e glazes subjected to differenttherma l treatment, change in colour was in each case shown to follow th e sequence:transparent -blue - bluish white - white. The study of th e microstructures of these glazesshowed these changes in colour tobe associated with growth and/or coalescence ofimmisciblephases separa ted by nucl eation.(iii) The isochroma tic diagrams described above, which provide data concerning the sensitivity

    ofthe glaze to changesin colour andthe temperature at which the transformation takes placeat the greates t rate allow frit compositions and the most suitable cooling cycles to be chosento avoid these chan ges in colour of th e glaze. is possible to obtain good tr ansparent glazesfor single-fired porous wall tiles by mean s of the se diagrams, with the usual cooling cycles.

    6. REFEREN ES(1) Rinc6n, J . Se paraci6n de fases en vidrios , Ed. S.E.C.V. Madrid (1982)(2) Vogel, W. Chemistry of Glass . Ed. Am. Cer. Soc. U.S.A. (1985)(3) ASTM Standards on Color and Appearance Measurement Ed. ASTM Ph iladelphia(1987)(4) G. GEIRNAERT, Verres et Refracta ires, 21-30, 25(1) (1971)(5) E. PLUMAT, Silicates Industriels; 5-13,1 (1967)(6) L. PROD HOMME, Verres Refra ct ., 604-613,22(6) (1968)

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