US20160009591A1

US20160009591A1 - Glass-ceramic of lithium aluminosilicate type containing a solid solution of b-spodumene

Info

Publication number: US20160009591A1
Application number: US14/770,233
Authority: US
Inventors: Kamila Plevacova; Cécile Jousseaume; Pablo Vilato; Emmanuel Lecomte
Original assignee: Eurokera SNC
Current assignee: Eurokera SNC
Priority date: 2013-02-28
Filing date: 2014-02-28
Publication date: 2016-01-14
Also published as: US20170129799A1; JP2016509987A; US9593040B2; DE202014010349U1; US20140238971A1; US10160685B2; JP6166389B2; FR3002532A1; CN105209400A; FR3002532B1; TR201811741T4; US20190077694A1; EP2961704A1; ES2685663T3; CN105209400B; EP2961704B1; WO2014132005A1

Abstract

A glass-ceramic of the lithium aluminosilicate type includes a solid solution of β-spodumene and exhibits, for a thickness of 4 mm; a light transmittance within a range extending from 0.3 to 2%, an optical transmittance for a wavelength of 625 nm of greater than 3.0%, an optical transmittance for a wavelength of 950 nm within a range extending from 50 to 75%, an optical transmittance for a wavelength of 1600 nm of at least 50%, and L*, a*, b* colorimetric coordinates in diffuse reflection for an illuminant D65 and a reference observer at 2° such that 15.0≦L*≦40.0, −3.0≦a*≦3.0 and −10.0≦b*≦3.0. The chemical composition of the glass-ceramic includes the following constituents, varying within the limits by weight defined: SnO₂0.2-0.6%, V₂O₅0.015-0.070%, Cr₂O₃0.01-0.04% or Bi₂O₃0.05-3.0%, Fe₂O₃0.05-<0.15%, AS₂O₃+Sb₂O₃ <0.1%.

Description

The present invention relates to the field of glass-ceramics. It relates more particularly to a glass-ceramic exhibiting a specific appearance in reflection and also a controlled transmittance curve, and also to articles made of such a glass-ceramic, in particular cooktops, and to the precursor glasses of such glass-ceramics.
In an application as cooktop, in particular for heating devices of the radiant type, it is necessary for the cooktop to meet certain requirements with regard to its optical properties, both in the visible region and in the infrared region. It is in particular important for the heating elements to be able to be concealed when not operating but for them to be clearly visible when they are operating. The energy efficiency of the devices is also important, in order to reduce as much as possible the time for cooking the foodstuffs. The esthetic appearance of the cooktop is also a parameter to be taken into account, in particular for the cooktop to be fully integrated in kitchens. The commonest glass-ceramic cooktops on the market, for example described in the patent U.S. Pat. No. 5,070,045, are glass-ceramics of the lithium aluminosilicate type comprising a crystal phase essentially composed of a solid solution of p-quartz; they are transparent but have a very low light transmittance and exhibit a black appearance which fits into the majority of kitchens. It can be advantageous, in order to further improve this incorporation, to have available cooktops exhibiting other appearances. In particular, cooktops having a grey color would be particularly nice.
The use of transparent glass-ceramics comprising a solid solution of β-quartz as main crystal phase has proved to be unsuited to the achievement of such optical appearances as, because of their very low light transmittance (set by the application), they always appear black, whatever their true color in transmittance.
In order to obtain such colors, one possibility consists in coating the surface of a clear glass-ceramic with a colored coating, for example made of a heat-resistant resin or of an enamel. However, few resins are compatible with the high temperatures undergone by the cooktop in the case of radiant heating. In addition, the deposition of resin or enamel involves a costly additional stage in the process for the manufacture of the cooktops. There thus exists a need to have available cooktops which are suitable for radiant heating and which exhibit the desired optical appearance without having to add a resin or an enamel over the whole surface of the cooktop.
To this end, the subject matter of the invention is first of all a glass-ceramic of the lithium aluminosilicate type comprising a solid solution of β-spodumene and exhibiting, for a thickness of 4 mm;

- a light transmittance within a range extending from 0.3 to 2%, in particular from 0.6 to 1.7%,
- an optical transmittance for a wavelength of 625 nm of greater than 2.0%, in particular of greater than 3.0%
- an optical transmittance for a wavelength of 950 nm within a range extending from 50 to 75%,
- an optical transmittance for a wavelength of 1600 nm of at least 50%, and
- L*, a*, b* colorimetric coordinates in diffuse reflection for an illuminant D65 and a reference observer at 2° such that: 15.0≦L*≦40.0, −3.0≦a*≦3.0 and −10.0≦b*≦3.0.

Said glass-ceramic is in addition such that its chemical composition comprises the following constituents, varying within the limits by weight defined below:


	SnO₂	0.2-0.6%, in particular 0.25-0.5%
	V₂O₅	0.015-0.070%, in particular 0.015-0.050%
	Cr₂O₃	0.01-0.04% or Bi₂O₃0.05-3.0%
	Fe₂O₃	0.05-<0.15%
	As₂O₃+ Sb₂O₃	<0.1%, in particular <0.05%.

In the present text, all the contents are expressed as percentages by weight.
The content of V₂O₅is preferably at most 0.060%. The solid solution of β-spodumene preferably represents at least 20% by weight, in particular 30% by weight or 40% by weight, of the total crystalline fraction. Preferably, the glass-ceramic according to the invention comprises a solid solution of β-spodumene as main crystal phase. In some cases, the solid solution of β-spodumene can represent more than 50% by weight, in particular 60% by weight and even 70% by weight or 80% by weight of the total crystalline fraction. The glass-ceramic according to the invention can even sometimes comprise a solid solution of β-spodumene as sole crystal phase. In addition to the solid solution of β-spodumene, the glass-ceramic can comprise a solid solution of β-quartz. The crystalline fraction can thus advantageously comprise a mixture of solid solution of β-spodumene and of solid solution of β-quartz, in proportions by weight of at least 20:80, in particular 40:60, indeed even 50:50 and even 60:40 or 70:30. In some cases, this proportion can even be at least 80:20, 90:10 or 95:5. The amounts of a given crystal phase can be determined in x-ray diffraction by the Rietveld method. The crystal fraction normally represents at least 60% by weight, in particular 70% by weight and even 75% by weight of the glass-ceramic.
The inventors have developed this combination of optical and chemical characteristics, which makes it possible to achieve unique properties, both in terms of esthetic appearance (color which is more or less light grey and possibly tending toward brown) and of functional characteristics. The various advantages of the invention will become apparent as the present description goes by.
The light transmittance (within the meaning of the standard EN 410) for a thickness of 4 mm is within a range extending from 0.3 to 2%, in particular from 0.6 to 1.7% and even from 1.0 to 1.7%, more particularly from 1.1 to 1.6%. For low light transmittances, the heating elements (in particular of the radiant type) are not visible when they are in use, which presents safety problems. On the other hand, when the light transmittance is too high, the heating elements are visible even when not in use, which presents an esthetic problem.
The optical transmittance for a wavelength of 625 nm is greater than 2.0%, in particular greater than 3.0% or 4.0%, preferably greater than 4.5% or even 5.0%. It is normally at most 50%. In that way, red indicators, commonly used for cooktops, are fully visible through the glass-ceramic.
The optical transmittance for a wavelength of 950 nm is within a range extending from 50 to 75%, in particular from 55 to 70%, which makes possible use of conventional electronic control keys, emitting and receiving at this wavelength.
The optical transmittance of a wavelength of 1600 nm is at least 50%, in particular within a range extending from 55% to 80% and even from 60% to 75%. This transmittance affects the thermal performances of the cooktop, which are too low if the transmittance is itself too low and, when the transmittance is too high, can bring about excessive and dangerous heating.
The color in reflection obtained is such that the L*, a*, b* colorimetric coordinates in diffuse reflection for an illuminant D65 and a reference observer at 2° comply with the following inequalities: 15.0≦L*≦40.0, −3.0≦a*≦3.0 and −10.0≦b*≦3.0.
The colorimetric coordinates preferably observe at least one of the following inequalities, indeed even two or three of these inequalities:

- 20.0≦L*≦30.0,
- −1.5≦a*≦1.5,
- −5.0≦b*≦1.0.

The colorimetric coordinates are calculated from a diffuse reflection spectrum, obtained using a spectrophotometer equipped with an integrating sphere, under perpendicular incidence and after subtraction of the specular reflection and of a baseline obtained by the same measurement carried out on Spectralon®.
The inventors have been able to demonstrate, after lengthy studies, that the desired transmittance curve and the desired color in single reflection could, surprisingly, be achieved by virtue of the presence of a solid solution of β-spodumene in combination with a specific choice of colorants and refining agents, in very precise contents. The high values of L* testify to the importance of the diffuse reflection, due to the presence of crystals of β-spodumene.
The composition includes SnO₂as refining agent. The refining becomes easier to carry out and becomes more effective in proportion as the amount of SnO₂present increases. However, it is advisable to minimize, indeed even to avoid, any phenomenon of devitrification and to control the influence of said SnO₂on the optical transmittance and the optical reflection. This is because the tin oxide is capable of reducing the vanadium and the iron present, during the ceramization. An SnO₂content of 0.2 to 0.6% by weight is required, in particular of 0.25 to 0.5%, indeed even of at most 0.4%.
The glass-ceramics according to the invention include neither As₂O₃nor Sb₂O₃or include only traces of at least one of these toxic compounds, SnO₂being present instead of these conventional refining agents. If traces of at least one of these compounds are present, it is as contaminating product; it is a priori due to the presence, in the charge of vitrifiable starting materials, of recycled materials of cullet type (resulting from previous glass-ceramics refined with these compounds). In any case, only traces of these toxic compounds are capable of being present: As₂O₃+Sb₂O₃<1000 ppm, indeed even 500 ppm.
V₂O₅is a main colorant of the glass-ceramics according to the invention. This is because V₂O₅, in the presence of SnO₂, significantly darkens the glass during its ceramization (see above). V₂O₅is responsible for the absorption, mainly below 700 nm, and it is possible, in its presence, to retain a sufficiently high transmittance at 950 nm and in the infrared. An amount of V₂O₅within a range extending from 0.015 to 0.070%, in particular from 0.015 to 0.060%, more particularly from 0.015 to 0.050%, indeed even from 0.015 to 0.035%, has proved to be adequate.
The invention employs the combination of V₂O₅with at least one other main colorant chosen from Cr₂O₃, Bi₂O₃and their mixture.
According to one embodiment, the glass-ceramic comprises V₂O₅and Cr₂O₃(in the abovementioned contents and those explained below) but not Bi₂O₃. Thus, the content by weight of Cr₂O₃is within a range extending from 0.01 to 0.04%, in particular from 0.015 to 0.035%, indeed even from 0.02% to 0.03%, and the Bi₂O₃content is zero.
According to another embodiment, the glass-ceramic comprises V₂O₅and Bi₂O₃(in the abovementioned contents and those explained below) but not Cr₂O₃. Thus, the content by weight of Bi₂O₃is within a range extending from 0.05 to 3.0%, in particular from 0.1 to 2.0%, indeed even from 0.2 or 0.3 to 1.0%, and the Cr₂O₃content is zero.
According to another embodiment, the glass-ceramic comprises V₂O₅, Bi₂O₃and Cr₂O₃(in the abovementioned contents and those explained below). In this case, the content by weight of Cr₂O₃is preferably within a range extending from 0.01 to 0.03%, in particular from 0.015% to 0.025%, and the content by weight of Bi₂O₃is preferably within a range extending from 0.1 to 1%, in particular from 0.2 to 0.5%.
When it is present, Cr₂O₃is advantageously included in a content by weight within a range extending from 0.015 to 0.035%, in particular from 0.02 to 0.03%. When it is present, Bi₂O₃is advantageously included in a content by weight Within a range extending from 0.1 to 2.0%, in particular from 0.2 to 1.5%, indeed even from 0.2 to 1.0%.
The glass-ceramics according to the invention generally have an optical transmittance for a wavelength of 450 nm of less than 0.1%.
The iron oxide results in an absorption mainly in the infrared and its content must be at least 500 ppm, advantageously at least 700 ppm, in order to obtain the required transmittance. If its content reaches or exceeds 1500 ppm, the absorption in the infrared is too high in the glass-ceramic but also in the starting glass, which makes it more difficult to melt and to refine. Advantageously, the content by weight of iron oxide is between 700 and 1200 ppm (0.07 to 0.12%).
It is not excluded from the scope of the invention for the composition of the glass-ceramics to. comprise, in a more or less significant amount, in addition to V₂O₅, Fe₂O₃, Bi₂O₃and Cr₂O₃, at least one other colorant, such as CoO, CuO, MnO₂, NiO or CeO₂. The manganese oxide MnO₂can be used to provide a brownish tint. However, the presence of said at least one other colorant must not significantly influence the targeted optical transmittance curve and the targeted appearance in reflection. It is advisable in particular to watch out for possible interactions which are capable, even with low contents of colorants, of significantly modifying said optical transmittance curve or said appearance in reflection, and the like.
Thus, CoO can a priori be present only in a very low amount in so far as this element strongly absorbs in the infrared and not insignificantly at 625 nm and confers a blue coloring in reflection. Preferably, the chemical composition of the glass-ceramic comprises less than 200 ppm, advantageously 100 ppm, indeed even 50 ppm or 30 ppm of cobalt oxide.
Likewise, the NiO content is preferably at most 500 ppm, in particular 200 ppm, indeed even zero, with. the exception of inevitable traces. The CeO₂content is preferably at most 0.5%, in particular 0.1%, indeed even zero, except for inevitable impurities. The MnO₂content is preferably at most 0.5%, in particular 0.1%, indeed even zero, except for inevitable impurities. The CuO content is preferably at. most 500 ppm, in particular 200 ppm, indeed even zero, except for inevitable impurities.
Preferably, the composition of the glass-ceramics according to the invention does not comprise refining aids, such as F and Br. With the exception of inevitable traces, it does not comprise F and Br. This is particularly advantageous in the light of the price and/or toxicity of these compounds. Within the compositions of the invention, the presence of refining aid(s) is a priori superfluous in so far as SnO₂, present in the amounts indicated.
The base composition of the glass-ceramics of the invention can vary to a large extent. Preferably, the chemical composition of the glass-ceramic comprises the following constituents, varying within the limits by weight defined below:


	SiO₂	60-72%
	Al₂O₃	18-23%
	Li₂O	2.5-4.5%
	MgO	0-3%
	ZnO	1-3%
	TiO₂	1.5-4%
	ZrO₂	0-2.5%
	BaO	0-5%
	SrO	0-5%
	With BaO + SrO	0-5%
	CaO	0-2%
	Na₂O	0-1.5%
	K₂O	0-1.5%
	P₂O₅	0-5%
	B₂O₃	0-2%

These compositions lend themselves perfectly to the melting of glass precursor and then to the ceramization, and in particular to the development of solid solution of β-spodumene.
Preferably, the content by weight of MgO is at most 2%, in particular 1%. The content by weight of CaO is advantageously at most 1%. The sum of contents by weight of Na₂O and K₂O is preferably at most 1%, in particular 0.5%. The content by weight of BaO is preferably at Most 3%, in particular 2% and even 1%. These different preferred ranges, alone or in combination, make it possible to reduce the thermal expansion coefficient of the glass-ceramic.
Preferably, the sum of the contents of SiO₂, Al₂O₃, Li₂O, MgO, ZnO, TiO₂, ZrO₂, BaO, SrO, CaO, Na₂O, Li₂O, K₂O, P₂O₅, B₂O₃, SnO₂, V₂O₅, Fe₂O₃, Cr₂O₃and Bi₂O₃is at least 98%, in particular 99%.
Another subject matter of the invention is an article comprising a glass-ceramic according to the invention. Said article is advantageously completely composed of a glass-ceramic according to the invention. Said article is in particular a cooktop, a cooking utensil or a microwave oven floor. The cooktop is advantageously incorporated in a cooking device, in particular of the radiant type, comprising at least one element which heats by radiation. The cooktop can be coated with an enamel decor.
In particular when the cooktop has elements which heat by radiation (of the “radiant” type), the linear thermal expansion coefficient between 20 and 700° C. of the glass-ceramic is advantageously at most 10.10⁻⁷/K.
A further subject matter of the invention is a lithium aluminosilicate glass which is a precursor of the glass-ceramics of the invention as described above. Said glass exhibits the bulk composition of said glass-ceramics, as explained above. It may incidentally be noted that said precursor glasses advantageously exhibit an optical transmittance, for any wavelength between. 1000 and 2500 nm, of greater than 60%, for a thickness of 3 mm. The melting and refining thereof are then facilitated.
Finally, a subject matter of the invention is a process for the preparation of a glass-ceramic according to the invention, comprising the heat treatment of a charge of vitrifiable starting materials under conditions which successively provide for the melting, refining and then ceramization, wherein said charge exhibits a composition which makes it possible to obtain a glass-ceramic according to the invention.
The ceramization is carried out at temperatures which make possible the development of a solid solution of β-spodumene. The appropriate ceramization temperature, which can vary as a function of the glass matrix, can be chosen after differential thermal analysis, which makes possible the determination of the crystallization temperature of the β-quartz phase and then, at a higher temperature, of the temperature at which the transformation (irreversible) of the β-quartz into β-spodumene takes place. The ceramization temperature is preferably at least 950° C., in particular 970° C. and even 1000° C. or 1020° C.
The examples which follow illustrate the invention in a nonlimiting manner.
Different glasses, the chemical compositions (contents by weight of oxides) of which are given in tables 1 and 2 below, were melted in a known way.

TABLE 1

C1	1	2	3

SiO₂	65.53	65.45	65.02	65.20
Al₂O₃	20.48	20.65	20.48	20.48
Li₂O	3.75	3.75	3.75	3.75
Na₂O	0.56	0.60	0.56	0.56
K₂O	0.23	0.22	0.23	0.23
MgO	0.24	0.37	0.24	0.24
ZnO	1.51	1.52	1.51	1.51
BaO	2.55	2.46	2.55	2.55
CaO	0.36	0.45	0.36	0.36
TiO₂	3.02	2.96	3.02	3.02
ZrO₂	1.35	1.30	1.35	1.35
SnO₂	0.29	0.31	0.29	0.29
V₂O₅	0.0171	0.0355	0.0200	0.0200
Fe₂O₃	0.0804	0.1220	0.1200	0.1200
Cr₂O₃		0.0215		0.0200
Bi₂O₃			0.50	0.30
CoO	0.0289	0.0015

TABLE 2

4	5	6	7	8

SiO₂	65.59	65.57	65.55	65.49	64.91
Al₂O₃	20.48	20.69	20.68	20.48	20.73
Li₂O	3.75	3.76	3.76	3.75	3.75
Na₂O	0.28	0.3	0.6	0.28	0.6
K₂O	0.12	0.12	0.22	0.12	0.22
MgO	0.24	0.37	0	0.37	0.37
ZnO	1.51	1.52	1.52	1.51	1.51
BaO	2.55	2.46	2.46	2.55	2.45
CaO	0.36	0.45	0.45	0.36	0.44
TiO₂	3.02	2.97	2.97	3.02	2.96
ZrO₂	1.35	1.3	1.3	1.35	1.3
SnO₂	0.29	0.31	0.31	0.29	0.31
V₂O₅	0.0200	0.0355	0.0355	0.0200	0.0550
MnO₂				0.4
Fe₂O₃	0.1200	0.1220	0.1220	0.1200	0.1248
Cr₂O₃	0.0200	0.0215	0.0215	0.0200	0.0233
Bi₂O₃	0.30
CoO		0.0015	0.0015
As₂O₃					0.083

Composition C1 (comparative) comprises neither Bi₂O₃nor Cr₂O₃. The other compositions are in accordance with the invention.
The glass plates were subsequently ceramized according to different cycles, characterized by different ceramization temperatures and stationary-state times at this temperature. The ceramization cycle employs rapid heating up to 650° C., then a rise up to 820° C. at a rate of 5° C./min and, finally a rise up to the ceramization temperature at a rate of 15° C./min, followed by maintenance at this temperature.
The results obtained are summarized in tables 3 and 4 below, the following being shown:

- the type of glass used for the ceramization,
- the ceramization temperature, denoted T and expressed in ° C.,
- the stationary-state time at the ceramization temperature, denoted t and expressed in minutes,
- the thickness of the plate, denoted th and expressed in mm,
- the transmittance properties at the true thickness, calculated from a transmittance spectrum recorded by spectrophotometry: the light transmittance within the meaning of the standard EN 410, denoted LT, and the transmittances for a wavelength of 625 nm (denoted T625), of 950 nm (denoted T950) and of 1600 nm (denoted T1600),
- the L*a*b* colorimetric coordinates in diffuse reflection, calculated from a diffuse reflection spectrum, obtained using a spectrophotometer equipped with an integrating sphere, under perpendicular incidence and after subtraction of the specular reflection and of a base line obtained by the same measurement carried out on Spectralon®,
- the type of crystals: Q denotes β-quartz as main crystal phase and S denotes the significant presence of β-spodumene as crystal phase.

TABLE 3

A	B	C	D	E	F

Glass	1	1	1	C1	2	3
T (° C.)	930	1020	1020	1020	1020	1020
t (min)	4	10	12	10	10	10
th (mm)	4.08	3.90	3.90	4.21	4.24	3.67
LT (%)	1.1	0.85	0.62	0.86	0.40	1.90
T625 (%)	3.6	3.3	3.8	34.6	22.1	37.8
T950 (%)	61.1	60.9	59.7	66.3	51.5	65.9
T1600 (%)	65.4	64.9	64.9	62.7	66.3	70.1
L*	1.7	22.6	23.2	20.9	38.4	25.3
a*	0.1	−0.5	−0.5	7.7	1.8	0.8
b*	−0.6	−2.5	−2.6	−19.4	−7.8	−6.8
Crystals	Q	S	S	S	S	S

TABLE 4

G	H	I	J	K

Glass	4	5	6	7	8
T(° C.)	1020	1020	1020	1020	1020
t(min)	10	10	10	10	10
th(mm)	4.14	3.89	3.82	4.00	4.00
LT (%)	1.12	0.67	1.04	0.45	1.04
T625(%)	29.6	2.4	3.7	2.1	3.95
T950(%)	59.7	60.1	64.3	52.0	65.0
T1600(%)	67.3	65.9	68.9	50.9	65.0
L*	29.9	17.0	20.1	16.5	21.6
a*	−1.5	1.4	1.0	0.5	−0.4
b*	−2.6	−6.6	−7.9	−4.3	−1.3
Crystals	S	S	S	S	S

Comparative example A, while it has advantageous transmittance characteristics in accordance with the requirements, exhibits an appearance in reflection which is not that desired as it appears black. It is crystallized essentially in the β-quartz form, as a result of a ceramization temperature which is too low to make possible the growth of crystals of β-spodumene.
When the same glass is ceramized at a higher temperature, so as to bring about the growth of the crystals of β-spodumene type, which is the case of the examples according to the invention B and C, the transmittance properties are substantially retained but the appearance in diffuse reflection is completely modified, as is testified by the higher L* value. The samples exhibit the desired grey color.
In the case of comparative example D, the glass-ceramic having the comparative composition C1, even ceramized so as to bring about the growth of the crystals of β-spodumene type, on the other hand exhibits a blue color in reflection, characterized in particular by a very negative b* value and an excessively high a* value.
In comparison with examples B and C, example E has both a lower light transmittance and a lighter grey color in reflection. Examples F to K are other examples according to the invention.

Claims

1. A glass-ceramic of the lithium aluminosilicate type comprising a solid solution of β-spodumene and exhibiting, for a thickness of 4 mm;

a light transmittance within a range extending from 0.3 to 2%,

an optical transmittance for a wavelength of 625 nm of greater than 2.0%,

an optical transmittance for a wavelength of 950 nm within a range extending from 50 to 75%,

an optical transmittance for a wavelength of 1600 nm of at least 50%, and

L*, a*, b* colorimetric coordinates in diffuse reflection for an illuminant D65 and a reference observer at 2° such that 15.0≦L*≦40.0, −3.0≦a*≦3.0 and −10.0≦b*≦3.0,

said glass-ceramic being such that its chemical composition comprises the following constituents, varying within the limits by weight defined below:

SnO₂0.2-0.6%,

V₂O₅0.015-0.070%,

Cr₂O₃0.01-0.04% or Bi₂O₃0.05-3.0%

Fe₂O₃0.05-<0.15%

As₂O₃+Sb₂O₃<0.1%.

2. The glass-ceramic according to claim 1, wherein the content of V₂O₅is at most 0.060%.

3. The glass-ceramic according to claim 1, comprising a solid solution of β-spodumene as main crystal phase.

4. The glass-ceramic according to claim 1, preceding claims, wherein the content by weight of SnO₂is at most 0.4%.

5. The glass-ceramic according to claim 1, wherein the content by weight of Fe₂O₃is at least 0.07% and at most 0.12%.

6. The glass-ceramic according to claim 1, wherein the content by weight of Bi₂O₃is at least 0.1% and at most 1.5%.

7. The glass-ceramic according to claim 1, wherein the content by weight of Cr₂O₃is at least 0.02% and at most 0.035%.

8. The glass-ceramic according to claim 1, the chemical composition of which comprises less than 200 ppm of cobalt oxide.

9. The glass-ceramic according to claim 1, the chemical composition of which comprises the following constituents, varying within the limits by weight defined below:

SiO₂60-72%

Al₂O₃18-23%

Li₂O 2.5-4.5%

MgO 0-3%

ZnO 1-3%

TiO₂1.5-4%

ZrO₂0-2.5%

BaO 0-5%

SrO 0-5%

With BaO+SrO 0-5%

CaO 0-2%

Na₂O 0-1.5%

K₂O 0-1.5%

P₂O₅0-5%

B₂O₃0-2%

10. The glass-ceramic according to claim 9, wherein the sum of the contents of SiO₂, Al₂O₃, Li₂O, MgO, ZnO, TiO₂, ZrO₂, BaO, SrO, CaO, Na₂O, Li₂O, K₂O, P₂O₅, B₂O₃, SnO₂, V₂O₅, Fe₂O₃, Cr₂O₃and Bi₂O₃is at least 98%.

11. An article comprising a glass-ceramic as claimed in claim 1.

12. A lithium aluminosilicate glass, precursor of a glass-ceramic, a chemical composition of which corresponds to that of a glass-ceramic as claimed in claim 1.

13. A process for the preparation of a glass-ceramic, comprising carrying out a heat treatment of a charge of vitrifiable starting materials under conditions which successively provide for the melting, refining and then ceramization, wherein said charge exhibits a composition which makes it possible to obtain the glass-ceramic as claimed in claim 1.

14. The glass-ceramic according to claim 1, wherein

the light transmittance is within a range extending from 0.6 to 1.7%,

the optical transmittance for a wavelength of 625 nm is greater than 3.0%,

SnO₂0.25-0.5%

V₂O₅0.015-0.050%

As₂O₃+Sb₂O₃<0.05%.

15. The glass-ceramic according to claim 8, wherein the chemical composition comprises less than 100 ppm of cobalt oxide.

16. The glass-ceramic according to claim 10, wherein the sum of the contents of SiO₂, Al₂O₃, Li₂O, MgO, ZnO, TiO₂, ZrO₂, BaO, SrO, CaO, Na₂O, Li₂O, K₂O, P₂O₅, B₂O₃, SnO₂, V₂O₅, Fe₂O₃, Cr₂O₃and Bi₂O₃is at least 99%.

17. The article according to claim 11, wherein the article is a cooktop.