GB2237016A - Glass-ceramic material and production thereof - Google Patents

Abstract

Glass-ceramic bodies are derived from a vitrifiable mixture containing slate, a nucleating agent and, if necessary to lower the melting point of the mixture to 1400 to 1450 DEG C, a flux. A homogeneous melt of the mixture is cast into a mould and allowed to cool to form a vitrified body which is subsequently devitrified by heat treatment.

Description

GLASS-CERIIC t1ATERIAL AND. PRODUCTION TEERSOF The present invention relates to glass-ceramic materials and provides a method of manufacturing glassceramic material from slate.

Glass ceramics are devitrified (i.e. crystallized) forms of glass which are hard and resistent to corrosion and wear. These properties make glass ceramic bodies particularly suitable for use such as linings and pipes for the transportation and handling of coal, ore and similar abrasive materials. They are also used as, for example, flooring and wall tiles and cladding in the building industry and as corrosion resistant linings in the chemical industry.

Glass ceramics have been known for about 30 years (US-A-2920971). They are manufactured by forming a homogeneous melt of a vitrifiable (i.e. glass-forming) mixture which usually contains a nucleating agent and, if necessary to lower the melting point of the mixture, a flux. The melt is cast into a mould and allowed to cool to form a vitrified body, which is then devitrified to the glass-ceramic state by heat treatment to nucleate and then crystallize the cast mass.

Usually, the glass-ceramic is formed from basalt, which is a basic igneous rock consisting of the minerals feldspar, pyroxene and magnetite. However, more acidic rocks and metallurgical slags have been used instead of basalt. In particular, copper refining and blast furnace slags have been used. Typical compositions of the rocks and slags useâ are set forth below in Table I: It has now been found that good quality glass ceramics can be prepared from slate. Slate is a metamorphic sedimentary rock of clay or silt grade which cleaves into thin layers. It is different mineralogically from basalt and the acidic rocks and metallurgical slags previously used for the manufacture of glass ceramics. It is also chemically different, especially in that it has a significantly lower CaO content than basalt.

In its broadest aspect, the present invention provides the use of slate in the manufacture of a glass-ceramic body. Nore specifically, the present invention provides a method of making a glass-ceramic body which comprises the steps of: (a) forming a homogeneous melt of a vitrifiable mixture containing a nucleating agent and, if necessary to lower the melting point of the mixture, a flux; (b) casting the homogeneous melt into a mould; (c) allowing the cast melt to cool to form a vitrified body; (d) heat treating the vitrified body to devitrify it to form a glass-ceramic body; (e) cooling the glass-ceramic body, wherein the major component of the vitrifiable mixture is slate and the mixture melts at 1,400 to 1,450 C.

It is preferred that the slate is of the kind found in XJales, especially North Wales and contains, in percentage by weight: SiO2 45 - 65% A1203 11 - 25% Fe2O3 3 - 15% MgO 2 - 7% K2O 1 - 6% Na2O 1 - 4% CaO 1 - 5% Ti02 0.5 - 2% A particularly suitable slate is Penrhyn or similar slates containing, in approximate percentages by weight: Si02 - 55% A12O3 - 21% Fe2O3 - 10% MgO - 8% K20 - 3% Na2O - 2.58 CaC - 2% TiO2 - 0.5% Usually, the slate will be crushed to form a powder which is then mixed with the nucleating agent(s). Any of the nucleating agents conventionally used in the manufacture of glass ceramics can be employed, for example, Cr203, Fe3O4' TiO2 or ZrO2.Presently, Cr2O3, IiO2 and mixtures thereof are preferred.

The use of TiO2 is particularly preferred when the homogeneous melt is formed under reducing or weakly oxidizing conditions. Usually, TiO2 will be present in the vitrifiable mixture in an amount of 0.2-7%, preferably 4 to 6%, by weight of the vitrified mixture.

then Cr2O3 is present as the sole nucleating agent or with TiC2 as joint nucleating agents, the Cr2C3 or Cr203/Ti02 usually will be present in an amount of 0.30 to 3%%, preferably 0.35 to 0.75%, by weight of the vitrified mixture.

Some nucleating agents, for example TiC2 are also effective fluxing agents and, depending upon the type of slate used, the mixture of slate and nucleating agent may melt within the required temperature range of 1400 to 1450"C. However, it usually will be necessary to add a flux in order to cause the mixture to melt within the said range. Any flux conventionally used in the manufacture of glass ceramics can be employed but it is presently preferred that CaO, Na2O or a mixture thereof is employed as the flux. Conveniently from the conservation point of view, the flux can be used or waste soda-lime glass. The use of such glass will also increase the SiC2 content of the vitrifiable mixture, which increase is desirable for certain glassceramic uses.

;;hen CaO is used, will usually be present in an amount of 10 to 30%, preferably 15 to 20%, by weight of the vitrified mixture. Na2O can be used in equivalent amounts.

It also is preferred that the vitrifiable mixture contains added SiC2 additional to that present in the slate. Suitably, the amount of added SiG2 increases the SiC2 content to 45 to 60%, especially to about 50%, by weight of the vitrified mixture. The addition of SiC2 increases the chemical resistence of the resultant glass-ceramic but increases the viscosity of the homogeneous melt.

In a preferred embodiment, CaO and SiC2 are present in the vitrifiable mixture as Portland cement.

The cement contains less chemically combined gases for example water and carbon dioxide than the raw materials, especially raw limestone, and hence frothing is at least reduced, if not eliminated, during formation of the homogeneous melt. As a result, the time required to make the melt homogeneous is reduced.

In addition, little or no heat is absorbed due to dissociation during melting, which occurs when limestone is used as a flux.

Suitably, the components of the vitrifiable mixture are thoroughly mixed and the mixture fused in an electric resistance furnace, preferably at a temperature between 1430 and 1460"C. Fusion can take place under varying degrees of oxidizing and reducing conditions. As is known in the art, the properties of a glass ceramic are dependent inter alia upon the conditions under which the vitrifiable mixture is fused. The oxidation condition usually will be determined by the crucibles used to contain the vitrifiable mixture during fusion.Oxidizing conditions suitably are provided by using alumina crucibles with the addition of a small quantity of ammonium nitrate and reducing conditions can be provided by using Plumbago clay-graphite crucibles of the "Salamander" type. ('Salamander" is a Trade Mark of Morganite Crucible Limited).

After fusion, the mixture is maintained in its molten state for a sufficient time to attain the required degree of homogenisation. Usually, approximately 3 hours are required. The homogeneous melt is then cast into, for example, metal or sand moulds. The solidified body from a metal mould should be immediately placed in an annealing furnace at, for example, 650"C, in order to avoid cracking. For this reason, it is preferred to use pre-heated sand moulds unless the body is required to have a smooth surface.

Advantageously, the said moulds are made from silica sand bonded with sodium silicate and are pre-heated to 300"C.

The vitrified body is heat treated to the devitrify it to form the glass-ceramic body. Usually, the heat treatment is carried out in two stages. The first stage is nucleation in which the vitrified body is heated to a nucleation temperature and maintained at that temperature for a sufficient time to allow the required degree of nucleation to take place. Suitably, the nucleation temperature is between 650 and 700"C and that temperature is maintained for up to 16 hours.

After nucleation, the temperature is increased to a crystallization temperature and that temperature maintained for sufficient time to cause the required degree of crystallization. Suitably, the crystallization temperature is between 900 and 1000 C and is maintained for a period of about 5 hours.

61hen the homogeneous melt is cast into sand moulds, it is preferred that the vitrified body is nucleated and crystallized whilst still in the moulds.

This procedure can prevent the vitrified body from cooling below about 600"C before nucleation and provides support for the body during crystallization.

without such support, the rate of heating may need careful control to avoid distortion of the body.

The invention is illustrated in the following non-limiting examples.

EXAMPLES Crushed Penrhyn slate was mixed with the components specified in Table II below to form a vitrifiable mixture.

The slate had the following approximately composition, in percentage by weight: Si02 - 55.2% Al2O3 - 20.9% Feo2O3 - 9.8% MgO - 7.9% K2C - 3.1% Na2C - 2.4% CaO - 2.05% TiO2 - 0.3% The resultant composition of the vitrifiable mixture for each of the examples is set forth in Table IV below. In the case of Example 19, the composition is only approximate because the composition of the window glass was not accurately known.

Each of the compositions was heated in an electric resistance furnace at a temperature between 1430 and 14600C for approximately 3 hours. In Examples 1, 7 and 9, the mixture was fused under oxidizing conditions by using alumina crucibles with up to 5% ammonium nitrate added. In the case of the remaining examples, the mixture was fused in "Salamander"-type Plumbago claygraphite crucibles.

The homogeneous melt was cast into tiles in metal or sand moulds. Metal moulds were used in Examples 1, 2, 3, 4, 13, 14, 15, 17 and 19. In the remaining examples, moulds made from silica sand bonded with sodium silicate were used. XIhen using metal moulds, the solidified tile was immediately placed in a furnace at 650"C for annealing. Flhen sand moulds were used, they were pre-heated to 3000C prior to casting.

The solidified tiles were nucleated by heating to between 650 and 700"C for up to 16 hours and the temperature then increased to 900 to 1000"C for 5 hours for crystallization. The actual temperatures and times for each of the Examples are set forth in Table V.

The metal mould cast tiles were slowly heated from the nucleation temperature to the crystallization temperature in order to prevent distortion. In the case of the sand cast tiles, they were placed in the nucleation furnace still in their moulds and accordingly were more rapidly heated to the crystallization temperature.

In the case of Lxample 11, the nucleating step was omitted and the sand mould was heated to 9000C and then slowly cooled. No crystallization occurred but re-heating to 1000 C producted a crystalline glass ceramic material.

The crystalline material was cooled and the following physical tests carried out on the product of Examples 4 to 10 and compared with a commercially available cast basalt tile manufactured in Czechoslovakia (see Table IV).

Hardness Vickers Diamond Penetration Hardness using a microhardness microscope and scratch hardness according to the tioh hardness scale.

Transverse Strength Determination of the modulus of rupture by 3 point loading at a loading rate of 600 lbf/min (2669 N/min).

Abradability According to the Morgan-Marshall abrasion test (British Standard 1902 : Section 4.6 : 1985) Density Determined on transverse strenth speciments.

X-Ray Defraction To determine degree and type of crystallization.

Optical tAicroscopy To determine crystal morphology and size.

Initial work on the slate-based ceramic-glass without the addition of nucleating agents (Examples 1 and 2) other than the inherent iron content of the slate, showed that the crystallization of these glassceramics was very dependent upon the melting conditions. When low casting temperatures (1375 C) were used, no crystallization was apparent when tiles were heat treated at 6500C to nucleate and 900"C to crystallize regardless of whether fusion was under oxidizing or reducing conditions. At a higher casting temperature (1480"C), the oxidized glass was found to have crystallized to form a glass-ceramic but the reduced glass showed no apparent crystallization.

Having regard to the aforementioned variability in crystallization, TiO2 was added to the vitrifiable mixture in an amount of 39/1009 slate (Example 3).

This improved the formation of crystals in the glasses melted under reducing conditions but only after a long heat treatment at 9000 C. The amount of TiC2 was therefore increased to 5g/100g slate (Examples 4 and 5). Good crystallization was obtained after heat treatment with glasses obtained under oxidizing or reducing conditions.

A aood glass-ceramic also was produced when the amount of TiO2 was increased to 89/1009 slate and Cr2G3 added as an additional nucleating agent in an amount of lg/1009 slate (Example 6).

The series of glass-ceramics of Examples 1 to 6 were made with a 15g CaO addition to 1009 slate.

Satisfactory glass-ceramics also were obtained when the amount of CaO was increased 25% and TiC2 (6%) and Cr2G3 (0.5%) used as nucleating agents (Examples 7 and 8). A similar product was obtained omitting the TiG2 content and using only Cr203 (C.5%) as the nucleating agent (Examples 9 and 10).

In order to investigate whether crystallization will occur without the need to hold the cast glass at 650 to 700"C to nucleate, a glass was cast into a sand mould and held at 900 C before slowly cooling (Example 11). No crystallization occurred but upon re-heating to 100"C, a glass ceramic was obtained.

Examples 12 and 13 were conducted in order to investigate the minimum additions necessary to producing glass-ceramics from slate. Two glasses were produced with a 15% CaO addition to the slate together with 0.5% Cr203. On heating, the viscous glasses crystallised to glass ceramics.

A further series of more acid glasses with a higher SiC2 content was investigated by decreasing the CaC addition and/or increasing the SiC2 addition (Examples 14, 15 and 16). These glasses were more viscous at the melting temperatures and, therefore, did not flow easily into the mould. However, after casting and the heat treatment, glass ceramics again were obtained.

Examples 17 and 18 illustrate that increasing the SiG2 content and decreasing the CaC content also resulted in viscous glasses which were effectively crystallised by heat treatment.

The glass made by adding 33% window glass to slate with 0.58 Cur203 had a lower CaO content than most of the other examples but a higher Na2C content. The glass and slate fused to give a high viscosity glass which was difficult to cast but heat treatment after casting produced a glass-ceramic product.

As can be seen from Table IV, only one of the glass ceramics (Example 4) which had been cast into a metal mould had a strength (modulus of rupture) and wear resistence (abradability index) comparable to the commercial cast basalt tile. Both of these tests are very dependent upon the surface condition of the test pieces. Sand casting produces a rougher surface texture and this is probably a reason for the lower values, especially those of wear resistence. Another factor which may have contributed to the lower values is that the degree of crystallization in the slatebased glass-ceramics may be less than that in the commercial cast basalt tile. The degree of crystallization is dependent upon the nucleation and crystallization temperatures.No detailed investigation of the optimum temperatures for these treatments has been conducted but control of these temperatures will increase the degree of crystallization and hence improve the wear and strength properties.

The colour of the slate-based ceramic glasses varied between light grey/blue, green and black depending upon composition and heat treatment. These colour variations indicate differences in crystal phases and morphology. For examples the colours of the glasses of Examples 1, 3, 7, 9, 10 and 12 were as follows: Example Colour 1 Grey-green 3 Grey-black 7 Black 9 Blue-grey 10 Grey 12 Green-grey Petrographic and X-ray defraction analysis of the commercial cast basalt tile and samples of the slatebased glass ceramics indicate that the crystalline phase present was predominantly pyroxene.The crystalline form varied; in the basalt-based material the matrix had a spherulitic texture, being composed entirely of centres of crystallization with radial crystal growths of 20-30 ym diameter. Within the matrix were pyroxene (50-200/um) with occasional plagiocase feldspar (100-2509sm). In contrast, the slate-based materials were characterized by an amorphous-cryptocrystalline matrix with very fine inclusions of dendritic or feathery crystals of pyroxene. There was a higher proportion of feldspar phases present compared to the basalt-based material.

The crystalline phases were generally much finer in the slate-based glass ceramics than in the cast basalt material. Generally, the dark grey-black slate glass-ceramics showed a more spherulitic crystal morphology with the green-blue being of a dendritic form.

X-Ray defraction semi-quantatitive analysis indicated that a larger proportion of amorphous glass phase (30%) was present in the sample of slate-based glass-ceramic as compared to the cast basalt (20%).

It will be appreciated that the invention is not restricted to the particular compositions and procedures described in the examples and that numerous modifications and variations may be made to the compositions and procedures without deviating from the invention as defined in the following claims: TABLE I Source France Germany U.S.S.R.Czechoslavakia Poland Germany Rock/Slag Basalt Slag Basalt Diabaso Basalt Basalt Slag SiO2 42.80 44.70 49 47 45.6 44.97 47.25 Al2O3 11.02 11.92 20 14 11.7 14.34 16.88 CaO 11.12 10.45 13 12 10.1 10.38 18.35 MgO 10.04 8.47 10 9 10.1 11.11 7.53 Fe2O3 6.26 ) ) 3 5.6 4.31 0.11 ) 16.56 ) 9 FeO 7.04 ) ) 12 7.1 7.14 3.72 Na2O 2.57 ) ) ) 2.02 2.97 1.60 ) 5.06 ) 2.5 ) 4 K2O 1.79 ) ) ) 1.17 1.70 3.14 TiO2 3.63 1.00 1.10 1.43 0.80 P2O5 0.68 1.36 0.33 Cr2O3 - TABLE II Added Components in Percentages by weight (based on weight of slate) Example CaO SiO2 TiO2 Cr2O3 Glass 1 & 2 15% 10% - - 3 15% 10% 3% - - 4 & 5 15% 10% 5% - - 6 15% 10% 8% 1% 7 & 8 25% 20% 6% 0.5% 9, 10 & 11 25% 10% - 0.5% - 12 & 13 15% - - 0.5% 14 - 10% 5% - 15 & 16 15% 25% - 0.5% 17 & 18 8% 25% 8% 0.5% 19 - - - 0.5% 30% * * Soda-lime window glass.

TABLE II Composition in percentages by weight Example SiO2 Al2O3 CaO MgO Fe2O3 Na2O K2O TiO2 Cr2O3 penryn slate 55.2% 20.9% 2.05% 7.9% 9.8% 2.4% 3.1% 0.3% Examples 1 & 2 55.2% 16.7% 14.2% 6.3% 8.0% 1.9% 2.5% 0,2% " 3 50.9% 16.3% 13.3 6.2% 7.7% 1.9% 2.4% 2.6% " 4 & 5 50.2% 16.1% 13.1% 6.1% 7.5% 1.9% 2.4% 3.9% " 6 48.7% 15.6% 12.7% 5.9% 7.3% 1.8% 2.3% 6.2% 0.75% " 7 & 8 45.6% 14.6% 18.9% 5.5% 6.8% 1.7% 2.2% 4.4% 0.34% " 9, 10 & 11 47.4% 15.3% 19.7% 5.8% 7.2% 1.8% 2.3% 0.2% 0.36% " 12 & 13 47.8% 18.1% 15.2% 6.8% 8.5% 2.1% 2.7% 0.3% 0.43% " 14 56.5% 18.5% 1.8% 7.0% 8.7% 2.1% 2.8% 4.1% " 15 & 16 57.3% 14.9% 12.2% 5.6% 7.0% 1.7% 2.2% 0.2% 0.43% " 17 & 18 52.2% 14.7% 7.1% 5.5% 6.9% 1.7% 2.2% 5.8% 0.44% " 19 58.1% 15.5% 4.1% 6.4% 7.3% 6.0% 2.4% 0.2% 0.49% TABLE IV Hardness Example Test Piece Bulk Density Abradability Index Modulus of Rupture Vickers Moh (g/cm3) (N/mm2) (200g) Basalt 1/1 2.94 32 48.6 1/2 - - 45.4 2/1 2.94 29 53.7 2/2 - - 51.9 MEAN 2.94 30 49.5 832 6-7 4 1 2.71 38 50.1 2 2.77 30 62.5 3 2.74 34 62.8 MEAN 2.74 34 58.5 803 6-7 5 1 2.59 68 26.5 2 2.65 108 28.0 3 2.59 109 23.4 4 27.9 MEAN 2.61 95 26.8 777 6-7 6 1 2.68 68.9 39.2 2 2.66 77.8 40.6 3 2.67 54.4 23.5 4 10.1 5 27.6 MEAN 2.67 67.0 28.2 774 6-7 7 1 2.84 77.7 46.5 2 2.83 88.2 33.1 3 2.83 99.1 65.9 4 41.0 5 38.9 MEAN 2.83 88.3 45.5 782 6-7 TABLE IV -/continued...

Hardness Example Test Piece Bulk Density Abradability Index Modulus of @upture Vickers Moh (g/cm3) (N/mm2) (200g) 8 1 2.76 90.5 35.0 2 2.73 65.1 39.8 3 29.9 4 36.6 MEAN 2.75 77.8 37.8 822 6-7 9 1 2.78 83.4 55.1 2 2.77 98.8 45.1 3 2.80 83.1 43.4 MEAN 2.78 88.7 47.9 778 6-7 10 1 2.74 79.9 7.4 2 2.82 70.9 MEAN 2.78 75.4 7.4 789 6-7 (Please provide corresponding data for any other tiles tested) TABLE V EXAMPLE NUCLEATION CRYSTALLISATION Time (hrs) Temp ( C) Time (hrs) Temp ( C) 1 2 650 2 900 2 2 650 16 900 3 2 650 12 900 4 5 670 2 980 5 3 680 4 950 6 3 670 10 920 7 5 670 5 900 8 5 680 5 950 9 5 680 7 930 10 4 660 8 900 11 4 650 5 1000 12 3 670 5 920 13 4 650 11 940 14 2 650 9 980 15 3 660 12 900 16 5 650 6 900 17 3 680 5 970 18 5 650 12 920 19 2 670 10 940

Claims (31)

1. A method of making a glass-ceramic body which comprises the steps of: (a) forming a homogeneous melt of a vitrifiable mixture containing a nucleating agent and, if necessary to lower the melting point of the mixture, a flux; (b) casting the homogeneous melt into a mould; (c) allowing the cast melt to cool to form a vitrified body; (d) heat treating the vitrified body to devitrify it to form a glass-ceramic body; (e) cooling the glass-ceramic body, wherein the major component of the vitrifiable mixture is slate and the mixture melts at 1400 to 1450 C.

2. A method as claimed in Claim 1, wherein the mixture is heated at 1430 to 1460"C to form the homogeneous melt.

3. A method as claimed in Claim 1 or Claim 2, wherein the slate contains, in percentage by weight: Si02 45 - 65% A12 3 11 - 25% Fe2O3 3 - 15% MgO 2 - 7% K2O 1 - 6% Na2O 1 - 5% CaO 1 - 5% TiO2 0.5 - 2%

4. A method as claimed in Claim 3 wherein the slate contains, in approximate percentages by weight: Si02 - 55% Al2O3 - 21% Feo2C3 - 10% DigC - 8% K2O - 3% Na2O - 2.5% CaG - 2% TiO2 - 0.5%

5. A method as claimed in Claim 4, wherein the slate is Penrhyn slate.

6. A method as claimed in any one of the precedings claims, wherein the nucleating agent is Tio2 and/or Cr203.

7. A method as claimed in Claim 6, wherein the vitrifiable mixture contains Tio2 in an amount of 0.2 to 7% by weight of the vitrified mixture.

8. A method as claimed in Claim 7, wherein the amount of TiC2 is 4 to 6% by weight of the vitrified mixture.

9. A method as claimed in any one of Claims 6 to 8, wherein the vitrifiable mixture contains Cr203 in an amount of 0.35 - 0.75% by weight of the vitrified mixture.

10. A method as claimed in Claim 9, wherein the amount of Cr203 is 0.5 to 1% by weight of the vitrified mixture.

11. A method as claimed in any one of the preceding claims, wherein the vitrifiable mixture contains a flux selected from CaO, Na2C and mixtures thereof.

12. A method as claimed in Claim 11, wherein the vitrifiable mixture contains CaO in an amount of 10 - 20% by weight of the vitrified mixture.

13. A method as claimed in Claim 12, wherein the amount of CaG is 15 to 20% by weight of the vitrified mixture.

14. A method as claimed in Claim 11, wherein the vitrifable mixture contains Na2C in an amount of 5-108 by weight of the vitrified mixture.

15. A method as claimed in Claim 14, wherein the Na2C is added as soda-lime glass.

16. A method as claimed in any one of the preceding claims wherein the vitrifiable mixture contains SiC2 additional to that present in the slate.

17. A method as claimed in Claim 16, wherein the amount of added SiC2 increases the SiC2 content to 45 to 60% by weight of the vitrified mixture.

18. A method as claimed in Claim 17, wherein the amound of added SiC2 content is 50% by weight of the vitrified mixture.

19. A method as claimed in any one of Claims 16 to 18, wherein SiC2 and CaO are present in the vitrifiable mixture as Portland cement.

20. A method as claimed in any one of the preceding claims, wherein the formation of the homogeneous melt takes place under oxidizing conditions.

21. A method as claimed in any one of Claims 1 to 19, wherein the formation of the homogeneous melt takes place under reducing conditions.

22. A method as claimed in any one of the precedings claims, wherein the melt is cast into metal moulds and the resultant solidified body immediately placed in an annealing furnace.

23. A method as claimed in any one of Claims 1 to 21, wherein the melt is cast into pre-heated sand moulds.

24. A method as claimed in Claim 23, wherein the filled moulds are immediately placed in a nucleating furnace.

25. A method as claimed in any one of the preceding claims, wherein the heat treatment of the vitrified body is carried out by heat a nucleating temperature and, after nucleation, the temperature is increased to a crystallization temperature.

26. A method as claimed in Claim 25, wherein the nucleating temperature is 650 to 700"C and the crystallization temperature is 900 to 1000"C.

27. A method as claimed in Claim 1 and substantially as hereinbefore described in any one of the Examples.

28. A glass-ceramic body whenever prepared by a method as claimed in any one of the preceding claims.

29. The use of slate in the manufacture of a glass-ceramic body.

30. A use as claimed in Claim 29 wherein the slate is as defined in any one of Claims 3 to 5.

31. A slate-based glass-ceramic body.