GB2250740A

GB2250740A - Improved glaze

Info

Publication number: GB2250740A
Application number: GB9126510A
Authority: GB
Inventors: Mark Edward Crooks; Anthony Charles Airey; Robert Dennis Brett
Original assignee: British Ceramic Research Ltd
Current assignee: British Ceramic Research Ltd
Priority date: 1990-12-13
Filing date: 1991-12-13
Publication date: 1992-06-17
Also published as: GB9126510D0; GB9027063D0

Abstract

An essentially lead and cadmium free glaze with a reduced rate of volatilization upon firing. The glaze preferably comprises loss than 5% B2O3 and may be essentially boron free. The glaze preferably comprises 50-70% SiO2 and desirably 60-66% SiO2.

Description

Improved Glaze This invention concerns improvements in or relating to glazes, and particularly but not exclusively glazes for application to ceramic articles and especially glazes for firing in open-flame conditions.

The term "open-flame conditions" when used in the specification is to be understood as referring to firing where the flame is in direct contact with the atmosphere around an article being fired.

In the ceramics industry energy considerations are dictating a move to rapid firing in open-flame conditions.

Open-flame firing has the following advantages over conventional firing: - Low capital outlay; - Lower running and maintenance costs; - Lower fuel costs.

Firing on a rapid schedule in open-flame conditions using unsupported single layer placing has the advantage of being amenable to automatic processing. Although many ceramic bodies can be fired under these conditions it has not been possible up to now to produce a tableware glaze with the required quality finish that will fire under these conditions.

The reason for this is that open-flame firing significantly changes the atmosphere in the kiln and the physics and chemistry of glost firing. The majority of glazes contain substantial quantities of boron oxide which can be steam distilled from the glaze by the moisture produced during open-flame firing. Furthermore, as a consequence of the turbulant atmosphere created by open-flame firing, volatile components of the glaze are continually swept away from the surface of the glaze resulting in depletion of these elements from the glaze surface. If higher temperatures are used to reduce the firing time this loss from the surface is accelerated. As a consequence glazes produced under these conditions are of poor surface quality and contain a large amount of bubbles.

A standard test for measuring ceramic glaze volatility can be carried out as follows. A rectangular piece of ceramic biscuit body with dimensions of approximately 9.5 x 3.5 x 1 cm is dried and weighed. The glaze to be tested is sprayed onto the largest face of the body to a thickness of approximately 0.25 mm. The glaze pieces are then dried and weighed.

The pieces are then fired in an electrically heated tube kiln and heated at 1000C per hour to a peak temperature of 11000C with a soak time of 24 hours. The kiln is then allowed to cool naturally i.e. at greater than 1000C per hour to 8000C and then subsequently to room temperature. The samples are then reweighed.

The batch compounding of the glaze is on the basis of a glaze frit suspended by china clay or organic suspending agent. Weight losses due to breakdown of clays or organic additives are determined by measuring the loss on ignition of the glaze to 10000C. This figure is subtracted from the weight loss figures to provide the percentage wt. loss due to volatilisation per surface area When used in this specification percentage propor t ions are expressed in oxide mol D.

According to the present invention there is provided a glaze with a reduced rate of volatilization upon firing.

The glaze is preferably usable on ceramic material and the rate of volatilization upon firing is desirably less fhan 5 weight loss per 33.25 cm2 when the above described test is applied.

The glaze is preferably substantially lead and cadmium free and desirably comprises 50 - 70% SiO2 and more desirably 60-66% SiO2.

The glaze also preferably comprises one or more of the following components in the following range of compositions.

Oxide Range(%) Al2O3 0-10 CaO 0-18 MgO 0-5 K20 0-5 Na20 0-5 8203 0-10 Li20 0-15 SrO 0-18 ZnO 0-15 P205 0-10 Lea 203 0-5 CeO 0-5 Nod 203 0-5 7r02 0-7.5 TiO2 0-7.5 Other rare earths 0-20 The glaze preferably comprises less than 5, more preferably less than 3, and desirably substantially no B203. When the glaze is substantially free of B203 it preferably comprises Lea2O3 The glaze preferably comprises less than 3, Ó MgO and desirably substantially no MgO.

The total proportion of the constituents SrO and ZnO is preferably at least 10%.

The total proportion of the constituents CaO and SrO is preferably at least 6o ó The total proportion of alkali (K,Na,Li) oxides is preferably less than 5% for low expansion glazes for use with vitrified and porous earthenware.

The glaze desirably comprises one or more of the following components in the following range of compositions.

Oxide Range (%) Al2O3 4-8 CaO 6-12 K20 1-4 Na2 0 0-3 Li20 2-8 SrO 7-11 ZnO 4-11 P205 0-0.5 Lea 203 0-2 CeO 0-2 Nod 203 0-1 ZrO2 0-3 TiO2 0-3 Other rare earths 0-1 Embodiments of the present invention will now be described by way of example only.

Twelve glazes A-L are prepared with 95% of the glaze as a frit, the remainder comprising china clay. The composition of these glazes is given below along with an indication of their thermal expansions.

Oxide/glaze A B C D E F SiO2 63.20 63.09 62.56 60.98 64.16 65.10 Al203 7.35 7.41 7.34 7.35 6.40 5.40 CaO 6.74 6.21 6.15 6.73 10.00 11.60 MgO 1.72 1.47 1.45 1.71 2.49 2.90 K2O 1.95 2.09 2.07 1.95 2.00 2.00 Na20 0.96 0.95 0.95 0.96 1.00 1.00 B2O3 - - - 2.27 - ZnO 10.07 - 4.00 10.01 10.00 10.00 Li20 8.01 8.13 8.06 8.04 3.95 2.00 SrO - 10.65 7.42 - - Thermal Exp. 0.32 0.41 0.38 0.37 0.31 0.28 (, Ó) in the range (20-500 C) G H I J K L SiO2 65.1 65.1 61.4 62.6 62.6 62.6 Al203 5.4 5.4 9.1 4.9 4.9 4.9 CaO 11.6 11.6 6.1 - 8.9 MgO 2.9 - 1.6 - - K20 1.0 1.0 1.8 3.7 3.7 3.7 Na20 0.4 0.4 1.0 8.2 10.2 7.2 B2O3 - 2.1 2.3 2.3 2.3 ZnO 10.0 12.9 9.0 7.4 7.4 7.4 Li20 3.6 3.6 7. 1 2.0 - 3.0 SrO - - - 8.9 - 8.9 ZrO2 - - 0.8 - - - Thermal Exp.LOW LOW HIGH HIGH HIGH HIGH Glazes A-F were applied to conventional vitreous ceramic tableware bodies and quality glaze finishes were obtained firing in open-flame gas fired roller-hearth kilns and schedules of 30 to 60 minutes to peak temperatures in the range 1120doc to 1160o. Good quality glaze finishes were obtained with glazes G-L when a rapid electrical firing programme was used.

The table below which includes glazes A-D illustrates the low volatility of the glazes according to the invention compared to conventional presently commercially available glazes X and Y when the above described test is applied.

Glaze , Ó Loss A 4.7 B 2.3 C 2.8 D 0.0 X 7.1 y 12.7 The reduced volatility of glazes according to the invention as illustrated above permits these glazes to be used in open-flame firing conditions thus providing for the advantages outlined in the introduction of this specification. However, these glazes could also be used in muffle and electric kilns with the advantage that vapour attack on the heating elements is eliminated or significantly reduced thereby providing a longer working life.

There are thus described a number of glazes with reduced rates of volatization upon firing all of which are essentially lead and cadmium free. It is normal to provide a significant proportion of boron in glazes as this element has a negative coefficient of expansion and also acts as a very good flux, thereby permitting firing at lower temperatures. All of the above described glazes have a greatly if not wholly reduced boron content, with a mixture of other components to provide the glaze characteristics usually provided by the boron. The glazes have a relatively high silica content but the mixture of other constituents ensure that exceedingly high firing temperatures are not required. These glazes have been found to have good detergent resistance.

The presence of magnesium has been found to promote crystallisation on subsequent decoration firing, thereby giving a halo and providing a deterioration in the glaze transparency. It has therefore been found preferable to omit magnesium from the glaze, which has also been found to improve detergent resistance.

If the glaze is essentially boron free it is advantageous to include some lanthamum which tends to clear bubbles during firing which is normally performed by the boron.

If a low expansion glaze is requierd e.g. for use on vitreous china, then the total proportion of alkali oxides should be less than 5 The glazes described are given for example only and other different compositions could be used. The glazes may be made up with a lower or higher percentage of china clay or another suspending agent such as bentonite or an organic compound. Alternatively the glazes may be fully fritted with the particulate material suspended by organic additions to the water phase. As a further alternative the glazes may be raw and comprise a range of minerals formulated together to give the required composition.

The frits may be prepared from a range of commercially available oxides, carbonates and hydrates. Minerals such as feldspar or spodumene etc. can also be used. The glazes may be applied by spraying or dipping.

Claims

Claims:

1. A glaze with a reduced rate of volatilization.

2. A glaze according to claim 1, which is usable on ceramic material and with which the rate of volatilization upon firing is less than 5% weight loss 2 per 33.25cm when the above described standard test is applied.

3. A glaze according to claims 1 or 2, which is substantially lead and cadmium free.

4. A glaze according to any of the preceding claims, which comprises 50-70 Ó SiO2.

5. A glaze according to claim 4, which comprises 60-66R ó SiO2.

6. A glaze according to claims 1 to 5, which comprises less than 5% B203.

7. A glaze according to claim 6, which comprises less than 3% B203

8. A glaze according to claim 7, which comprises substantially no B20 .

9. A glaze according to claim 8, which comprises La2O3.

10. A glaze according to any of the preceding claims, which comprises one or more of the following components in the following range of compositions.

Oxide Range(%) Al 2O3 0-10 CaO 0-18 MgO 0-5 K20 0-5 Na20 0-5 B2O3 0-10 Li20 0-15 SrO 0-18 ZnO 0-15 P205 0-10 Lea 203 0-5 CeO 0-5 Nod 203 0-5 ZrO2 0-7.5 TiO2 0-7.5 Other rare earths 0-20

11. A glaze according to claim 10, which comprises one or more of the following components in the following range of compositions.

Oxide Range(%) Al 203 4-8 CaO 6-

12 K20 1-4 Na20 0-3 Li20 2-8 SrO 7-11 ZnO 7-11 P205 0-0.5 Lea 203 0-2 CeO 0-2 Nd203 0-1 ZrO2 0-3 Ti0-2 Other rare earths 0-1 12. A glaze according to claims 10 or 11, which comprises less than 3% MgO.

13. A glaze according to claim 12, which comprises substantially no MgO.

14. A glaze according to any of claims 10 to 13, in which the total proportion of the constituents SrO and ZnO is at least 10.

15. A glaze according to claim 14, in which the total proportion of the constituents CaO and SrO is at least 6%.

16. A glaze according to claim 15, in which the total proportion of alkali oxides is less than 5.

17. A glaze substantially as hereinbefore described.

18. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.