THERMOSTABLE LOW-ALKALI

FAIENCE MAJOLICA

Yu. G. Shteinberg

S T R O N T I U M G L A Z E FOR'

UDC 666.295 :~66 ;642.3

The State Scientific Research Institute of the Ceramics Industry (GIKI) has developed a thermostable lead-f~ee strontium-magnesium low-alkali glaze for faience majolica fired in an electric roll-table furnace to I020-I040~ In August 1969 this glaze (No. 313) was introduced at the M.,I. Kalinin Konakovskii Faience Plant in collaboration with the plant workers (chief engineer M. I. Fadeev, chief technologist A. A. Khodina, chief of the central laboratory L. M. Soshnikova, research engineer N. Z. Barabanova, and others).

Glaze No. 313 contains (in wt. %): 29-31 feldspar, 16-18 quartz sand (GOST 7031-54), 6-7 Prosyanov- skii kaolin (GOST 6138-61), 12-14 strontium carbonate (VTU 3071-57), 24-26 boric acid (GOST 9656-61), 2.7-3.2 lithium carbonate (MPTU 2277-49), 2.5-3 magnesium oxide, 0.8-1.2 sodium sulfate. In excess of 100% for bluing the frit we added 0.045 parts by weight of glaze DKZ No. 3101.

The feldspar was added, as usual, with Chupinskii pegmatite with appropriate recalculation of the quantity of quartz sand to allow for the content of quartz in the pegmatite.

In place of lithium carbonate it is possible to use a cheaper product - spodumene concentrate with a Li20 content not less than 4.5%, and in place of magnesium oxide a cheaper natural magnesium carbonate - the magnesite of the SatkInskii deposit, which is used at the M. I. Kalinin Plant. The chemical composi- tion of frit No. 313 in this case should be left unchanged, namely (in wt. %): 50-52 SiO 2, 13-13.5 A1203, 16- 17 B203, 8.5-10 SrO, 2.5-2.8 MgO, 0.5 CaO, 2.3-2.7 K20, 2.8-3.1 Na20, 1.0-1.2 Li20, 0.5 Fe203, and 0.5

TiO 2 .

The mill composit ion of the glaze contains 93% fr i t and 7% Veselovsk clay grade DVO. In developing the glaze we used as the base the previously synthesized glaze No. 1 for coating radio porcela in par t s [1] with ~20_400oc = 40 �9 10 -7 1 /~ Glaze No. 313 has the same coefficient of expansion. Such a low coefficient of expansion of glaze in compar ison with that of faience (about 80 �9 10 -7) was necessa ry in order to obtain high thermostabi l i ty of the coating at the p resc r ibed low tempera ture of the second firing in the rol l - table

furnace.

During the investigation the composition of glaze No. 1 was changed in accordance with the r equ i re - ments of majol ica production at the M. I. Kalinin Plant , in par t icu lar zinc oxide was eliminated owing to its unfavorable effect on the color of cer tain underglaze decorat ions and toxicity of zinc in the composition

of glaze.

As we see f rom the chemical composition of f r i t No. 313, its low coefficient of expansion is due to the small content of alkali oxides, which are introduced only with pegmati te , and the high content of boron oxide and alumina. The increase of concentrat ion of these oxides at the expense of SiO 2 and, in addition, the introduction into the R20 group of lithium oxide, in the p resence of which the t e rnary eutectic of alkali oxides (silicates) forms, cause the low viscosi ty of the melt of f r i t No. 313. In connection with this, after f ir ing in the ro l l - tab le furnace we obtained a normal lus ter and spread of the glaze coating, and also sp read- ing of runs , despite glazing by the high-speed method (without cleaning the runs).

The small content of alkali oxides revealed a new feature of this glaze which had a great effect on r e - ducing runs after glazing. It consists In stable coagulation of the suspension under the effect of a small , one- t ime addition of l ime milk, in the p resence of which the normal degree of "suspension" lasted the entire

State Scientific Resea rch Institute of the Ceramics Industry. Transla ted from Steldo i Keramika,

No. 5, pp. 37-38, May, 1971.

O 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 ~/est 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher, d copy of this article is available from the publisher for $15.00.

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working shift (8 h). This made it possible to deliver to the job si tes glaze with a p resc r ibed addition of milk of lime of cer ta in concentrat ion which provided a constant and sufficiently high modulus of elastici ty of the suspension, equal to 0.1 (the pyknometr ic density is 1.33-1.35 and the ae romet r i c density is 1.44-1.46). In this case the runoff of the glaze suspension on the ver t ical wall of an ar t ic le and consequently the formation of runs decreased owing to la rge shear ing res i s tance .

If glazes with a normal content of alkali oxides a re used (total R20 of about 10-12%) it is not possible to obtain such a high modulus of elast ici ty even with a large addition of l ime and, fu r thermore , this modulus drops to zero after 15-20 rain. For coagulation of such a glaze suspension it was often necessa ry to add milk of lime , whichledto crumbling of the fr iable layer of glaze powder from the ar t ic le owing to excessive coagulation of the suspension, and then to the formation of runs as a consequence of its sagging. This was due to the smal le r chemical stability of such fr i ts and consequently leaching of pept izators , alkali oxides, f rom them into the aqueous medium.

On faience majol ica subjected to a f i rs t f i r ing up to 1230-1250 ~ glaze No. 313 after the second fir ing in the ro l l - tab le furnace to 1020-1040 ~ has a high thermostabfl i ty (7-10 heating - cooling cycles after Har- court), which exceeds the established norms considerably (4 h e a t i n g - c o o l i n g cycles). In connection with this, in the case of an insufficient t empera tu re of the f i rs t firing (1200 ~ and lower), which, as is known, leads to a dec rea se of a of faience, c raze appears even on such thermostable glaze.

In connection with this we established a more r igorous control over observance of the final t empera - ture of the f i rs t f ir ing, which is facili tated by the small t empera ture difference (one Seger cone) in a tunnel furnace.

Glaze No. 313 was p repared in the following way. All mate r ia l s of the batch after measur ing were mixed in a mechanical mixer and del ivered pneumatical ly to the melt ing furnace. The fri t was melted in a rotat ing furnace lined with shaped f i reclay br icks grade B at a t empera tu re of 1300~ for 5.5 h. The p r e - pared mel t was granulated on water . The glaze of the mill composit ion indicated above was ground in a ball mill with a weight rat io mater ia l :bal ls :water = 1 : 1.3 : 0.8 to a res idue of 0.03-0.08% on a 10,O00 hole / cm 2 sc reen .

The p repared glaze suspension was strained through a 4900 h o l e s / c m 2 vibrating sc reen and the pykno- met r i c density of the suspension was brought to 1.34-1.36 by dilution with water and the a reomet r i c density to 1.44-1.45 by the addition of l ime milk.

The ar t ic les were glazed by the dipping method. Glaze No. 313 upon mixing with all DKZ glazes fired in the ro l l - tab le furnace produces the same color hues as with the previously used majol ica glaze except for a slightly lower intensity of pink colors , which did not, however, spoil the appearance of the ar t ic les , and also yellow lead color DKZ No. 450. This color was replaced by l ead- f ree z i r con ium-vanad ium yellow pigment DKZ No. 55.

During the second fir ing in the ro l l - tab le furnace the glazed ar t ic les a re placed in a single row on iron plates lined with f i reclay slabs and move on the ro l ls together with the conveyer.

Some of the a r t ic les which were coated on the outside with low-melt ing, high-lead DKZ glazes are placed on t r ihedra l runners to avoid runoff of these low-viscous melts onto the f i reclay slabs.

At p resen t the fir ing t ime of the a r t ic les in the ro l l e r - t ab le furnace is 6 h 50 rain. The firing t ime can be reduced in compar i son with the original (8 h 15 rain) by lengthening the firing zone at the expense of the warming zone. There is no t empera tu re difference in the furnace. The daily output of the furnace is J_2,000 majol ica ar t ic les .

According to p re l imina ry calculations, the annual economic effect from the introduction of the new thermostable glaze is about 230,000 rubles.

1.

LITERATURE CITED

Yu. G. Shteinberg, Strontium Glazes [in Russian], Stroiizdat (1967).

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