INFLUENCE OF PORCELAIN PRODUCTION CONDITIONS ON ITS

WHITENESS

V. M. Loginov, A. S. Vlasov, and A. M. Kurbanov

UDC 666.5

Whiteness plays an important role in porcelain articles used for decorative and domestic purposes.

At the present time studies are being made of the influence on the whiteness of porcelain of the dispersion of the nonplastic materials in the bodies (quartz sand, quartz, and pegmatite); the quality of the original raw materials Creplacing quartz sand and KF-3 kaolin with in- creased concentrations of coloring oxides and kaolin KF-2 with a lower content of coloring oxides in the same percentage ratios); and the firing temperature for the biscuit firing.

The raw materials consist of kaolin KF-3, Chasov Yar clay, Chupinsk pegmatite, Glukhovets quartz sand, and also porcelain pitchers (scrap). The temperature of the first firing is 850~ and that of the second firing (glost) 1350~ The residue on screen No. 0063 (I0,000 aper- tures/cm 2) for the nonplastics in the prepared body was 1.2%. The whiteness was 60-61%.

On the basis of light wave theory in strongly turbid bodies, especially in the multi- phase and polydispersed media, such as porcelains, a hypothesis was developed about the in- fluence of interfaces between the glass and of the crystalline phase on the scattering of the light in porcelain~ In this case, the porcelain contains two phases: the quartz and the mul- lite. If the quartz is present in the form of rather coarse crytals, which influence the whiteness of the porcelain to a slight degree, then the fine crystals of mullite, whose di- mensions are commensurate with the wavelength of the light, exert a marked effect on the white- ness. It was decided to test the stated hypothesis by regulating the amount and sizes of the mullite crystals with the dispersion of the quartz sand, which influences the mullitization processes and the crystallization of mullite from the melto

To obtain the necessary dispersion for the sand, it was necessary to determine experi- mentally the milling time of the starting material. With the aim of saving time and energy, on grinding the other nonplastic, the pegmatite, we carried out combined milling of the ma- terial with quartz sand in a ballmill.

In order to study the influence of the dispersion of the nonplastics on the porcelain's whiteness we took control points with respect to the residues of these materials -- 1.2% (re- sidue in the production bodies)9 2o09 2.5, 3.09 3.5% (residues in the test bodies) on the control screen No~ 0063. Using existing production technology we prepared the required bodies. From each batch we then preparedthe test specimens. The temperature of the first firing was 850~ and the second 1350~

The highest whiteness was exhibited by specimens made of bodies with residues of 2.5-3.0% nonplastics on screen No. 0063 (Table i). The particle sizes of quartz in these specimens of porcelain (Nos. 3 and 4) are within the region of 15 ~m.

This method facilitates an increase in the whiteness of 2.5-3~0%, and cuts the milling time by a factor of 1.5, which in turn makes possible a reduction in energy outlay.

It was noted earlier that the maximum scattering of light in a heterogeneous medium is noted for particle sizes in the matrix located within the limits 0.4-0.8 ~n, i.e., when the sizes of the particles coincide with the wavelengths of visible light. In porcelain these dimensions may exist only for crystals of mullite which are formed during firing. With in- crease in the temperature, the sizes are increased. It is very important to carry out the firing so that during the maturing of the porcelain in its structure there is a predominance of mullite crystals measuring about 0.5 ~m; then the light scattering will be maximum and the reflection will be at its highest.

"Gzhel'" Industrial Association. D. Io Mendeleev Institute of Chemical Technology, Mos- cow. Translated from Steklo i Keramika, No. I, p. 6-7, January, 1989.

10 0361-7610/89/0102-0010512o50 �9 1989 Plenum Publishing Corporation

TABLE i

Properties Specimens from body

1 2 I 3 4

esidue pl tic, 00,3, o, non- ' :,o' 2.3 2.5 Whiteness~ % 61,O 63,5 63,8 63,3

I \ {2z ) ~ ~

23 ,, ~ o

22 \ } " 161 15 Zl >; =~ 750 8,90 850

. 7 5 0 8 5 O 35O Temperature, ~ F i rs t f i r i ng temperature

Fig. 1 Fig. 2

Fig. i. Relationship between the content of quartz in the specimen and the first firing temperature.

Fig. 2. Influence of the first firing temperature on the amount of mullite (i) and quartz (2) in the specimens after the second firing.

The formation of mullite, its amount, and the sizes of the crystals will depend on the firing schedule. Therefore, in this work we paid attention to the first firing when the commencement of the formation :fo the crystals occurs. The first firing of the specimens pre- pared from production bodies was done at 750, 800, 850, 900, 950, and 1000~ The upper temperature limit was restricted by the loss of the porcelain's facility to take decoration of a good standard by painting.

An increase in the temperature of the first firing from 750 to 1000~ hardly affected the porosity of the specimens, which changed from 35.2 to 29.5Z. Petrographic and x-ray analyses showed that an increase in the first firing temperature in the above limits does have an effect on the content of quartz in the specimens (Fig. I).

The principal processes during which the porcelain's structure is created (that is, the scattering medium) occurs in the second firing. This firing was done at a single maximum temperature of 1350~ after which the specimens were again subjected to petrographic and x-ray study. The data obtained indicate that with an increase in the first firing tempera- ture the specimens undergo marked changes in the phase composition, and also the microstruc- ture~

The temperature of the first firing markedly affects the amounts of mullite in the porce- lain, and the cross sectional dimensions of its crystals (Figs. 2 and 3), and also the reduc- tion in the content of crystalline quartz. The maximum whiteness was noted in the specimens in which the average sizes of the mullite crystals over the c axis was about 2 ~m, i.e., they were located outside the limits of the stated wavelengths of light.

It was found that the most influence (together with the sizes of the particles of mullite) on the whiteness of the porcelain during its production from one and the same material, is exerted by the amount of scattering phase, namely the mulliteo Therefore, to obtain the maxi- mum amount of mullite changes were made in the schedules for the two firings. As a result the whiteness of the porcelain was successfully increased by 2.5-3.0Z.

As is known, KF-3 kaolin and quartz sand contain a larger amount of coloring oxides than KF-2 kaolin, and pure quartz. Therefore, in the porcelain body at the Gzhel' Production Asso- ciation the KF-3 and quartz sand were replaced by KF-2 and pure quartz, which should contribute to an improvement in the whiteness of the porcelain.

ll

TABLE 2

Porcelain body

Whiteness o f speci- mens, ~ at ~irlng temDerature~ ~

850 ~ C 900 ~ C 950 ~

Production with residue of non- plastics on No.

~ 006~, %: 1.27 -- 2 . 7 ~ : ,

and quartz with residues of non- plastics on mesh No. ~0063, % : -

- 1.2 ........

~7-:~ .~

61,0 63,7

6z.s 65,2

62,2 64, I 64,9 66,8

~3.7 65.16 66.~+.L 6a..3

2,O

| ~,5 N

ta

,..)

| 0,5

~ a 750

65

1 ~ / ' ' ' 7 ~ 6[+

/ g

// 'I" Go ~

58 850 950 Temperature, ~

Fig. 3. Influence of the temperature of the first firing on the average size of the mullite crystals (i) and the whiteness of the porcelain (2).

Using the existing production methods we prepared two porcelain bodies. In the first we inciuded the KF-3 kaolin and the quartz sand; and in the second kaolin KF-2 and pure quartz. From each batch we prepared specimens whose first firing temperature was 850~ and for the second 1350~ It was found that replacing the KF-3 kaolin and the quartz sand with KF-2 and pure quartz increases the porcelain's whiteness by 1.5-2.0Z~

We also made bodies for comparative analysis of the combined influence of the three fac- tors -- the quartz particle sizes, the firing temperature, and the use of purer raw materials. The first firing of the specimens of all batches was done at 850, 900, and 950~ and the sec- ond at 1350~

Table 2 shows that the combined influence of these factors on the whiteness facilitates an increase in the whiteness of 7.0-7.5Z, compared with the production bodies.

Thus, to boost the whiteness of porcelain (bearing in mind the preservation of the tech- nological parameters) milling the nonplastics of the body should be done to a residue of 2.7- 2.8Z on mesh No. 0063. The optimum temperature of the first firing is 950~

12