Crystallized glass and method for manufacturing the same

A crystallized glass and a manufacturing method therefor are disclosed. The crystallized glass comprises .beta.-spodumene solid solution as the predominant crystal phase, and is produced by forming a melted raw glass, cooling and thereafter heat-treating it. The raw glass has a composition comprising, expressed in terms of weight percent: 52-65% SiO.sub.2 ; 8-15% Al.sub.2 O.sub.3 ; 0-4% P.sub.2 O.sub.5, 3-8% TiO.sub.2, and 0-4% ZrO.sub.2, wherein the following inequality is satisfied, 0.1<(% of P.sub.2 O.sub.5 +% of ZrO.sub.2)/(% of TiO.sub.2)<0.8; 0-10% ZnO, 0-10% MgO, and 0-5% BaO, wherein the following inequality is satisfied, <(% of ZnO+% of MgO+% of BaO)<15; 3-12% Li.sub.2 O; 0-4% K.sub.2 O; and As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3, wherein the following inequality is satisfied, 0<% of As.sub.2 O.sub.3 +% of Sb.sub.2 O.sub.3 <2.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a crystallized glass and a method of 
manufacturing the same, for example, to a crystallized glass adapted for 
various kinds of materials which require a fine processing or a precision 
processing, e.g., material for electronic parts, material for a substrate 
for electronic parts, material for magnetic disk substrates, or the like. 
2. Description of Related Art 
A crystallized glass comprising .beta.-spodumene solid solution as the 
predominant crystal phase is used as a high-strength material because of 
its high mechanical strength and low expansion coefficient, and is also 
used for dishes which double as cooking utensils, a top plate for an 
electromagnetic cooking device and the like because the appearance thereof 
has a white colour and gives an impression of cleanliness and stains 
thereon easily come out. In the electronic industry, such a crystallized 
glass is used for a setter for burning for electronic parts and the like, 
an insulating protection tube for a heater, and the like. Recently, 
because a crystal having a length of about 1 .mu.m can be deposited, the 
crystallized glass is also used for material for electronic parts, and 
material for a substrate. 
Such a crystallized glass comprising .beta.-spodumene solid solution as the 
predominant crystal phase is generally obtained by melting a raw glass, 
forming, and cooling the material, and then carrying out a heat-treatment 
for nucleation and by carrying out a subsequent heat-treatment for 
crystallization. 
Japanese Patent Application Publication (Examined) No. Tokuko-Hei 1-57058 
discloses a crystallized glass, the strength of which is improved by 
properly controlling growth of both .beta.-spodumene crystal and 
cristobalite crystal to deposit during crystallizing a raw glass. Japanese 
Patent Application Publication (Unexamined) No. Tokukai-Sho 58-49633 
discloses a glass that contains fine crystal particles and has a greater 
mechanical strength and a lower coefficient of expansion, which is 
produced by combining F and As.sub.2 O.sub.3, which are necessary 
constituents for crystallization, with crystal nucleating agents. In 
addition, U.S. Pat. No. 4,192,665 teaches crystallized glass produced by 
crystallization of raw glass of composition: Li.sub.2 O--Al.sub.2 O.sub.3 
--SiO.sub.2 --MgO--ZnO--TiO.sub.2 at a crystallization temperature lower 
than 1000.degree. C. 
However, in those conventional crystallized glass of the above references, 
there have been some problems that because the raw glass is melted at a 
high temperature not less than about 1500.degree. C., clarification in 
melting tends to decrease. Also, both temperatures for nucleating and 
crystallization of the above products of crystallized glass are high, and 
in particular the crystallization temperatures are 900 to 1000.degree. C. 
so that coarse particles are likely to form in crystals to pose problems 
in quality. In addition, in crystallized glass which requires F as a 
necessary constituent, it has been disadvantageous that glass homogeneity 
is impaired because of evaporation of F upon melting. 
SUMMARY OF THE INVENTION 
The present invention was developed in view of the above-described 
problems. An object of the present invention is to provide an improved 
crystallized glass having a good heat resisting property and a mechanical 
strength in addition to an excellent workability, which enables lowering 
the temperatures for both melting and heat treatment for crystallization, 
and thereby enables obtaining enhanced clarification in melting of raw 
glass, improved homogeneity of glass, and fine-grained crystal. 
In order to achieve the above-mentioned objects, the present inventors have 
extensively studied to find that a crystallized glass containing 
fine-grained crystals can be obtained by utilizing TiO.sub.2, ZrO.sub.2 
and P.sub.2 O.sub.5 as nucleating agents and by controlling the 
composition ratio of TiO.sub.2, ZrO.sub.2 and P.sub.2 O.sub.5 when a raw 
glass of SiO.sub.2 --Al.sub.2 O.sub.3 --Li.sub.2 O system is used for 
manufacturing a crystallized glass. In addition, it has been found that 
the presence of Mgo, BaO and ZnO together with the above nucleating agents 
can significantly lower the melting temperature of raw glass to produce 
crystallized glass excellent in uniformity. 
The present invention has been accomplished on the basis of the above 
findings. In accordance with one aspect of the present invention, the 
crystallized glass comprises .beta.-spodumene solid solution as the 
predominant crystal phase, and is produced by forming a melted raw glass, 
cooling and thereafter heat-treating it, wherein the raw glass has a 
composition comprising, expressed in terms of weight percent: 52-65% 
SiO.sub.2 ; 8-15% Al.sub.2 O.sub.3 ; 0-4% P.sub.2 O.sub.5, 3-8% TiO.sub.2, 
and 0-4% ZrO.sub.2, wherein the following inequality is satisfied, 
0.1&lt;(% of P.sub.2 O.sub.5 +% of ZrO.sub.2)/(% of TiO.sub.2)&lt;0.8; 0-10% ZnO, 
0-10% MgO, and 0-5% BaO, wherein the following inequality is satisfied, 
3&lt;(% of ZnO+% of MgO+% of BaO)&lt;15; 3-12% Li.sub.2 O; 0-4% K.sub.2 O; and 
As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3, wherein the following inequality is 
satisfied, 
0&lt;% of As.sub.2 O.sub.3 +% of Sb.sub.2 O.sub.3 &lt;2. 
Accordingly, because the raw glass has the composition as described above, 
the melting temperature of raw glass can be not only made lower, but also 
the temperature to have crystal nucleus formed and crystallize is lowered. 
Thus, the clarification in melting of raw glass is improved, resulting 
crystallized glass has fine particles of crystals, and good homogeneity 
can be obtained. 
In accordance with another aspect of the present invention, the 
crystallized glass comprises .beta.-spodumene solid solution as the 
predominant crystal phase, and is produced by forming a melted raw glass, 
cooling and thereafter heat-treating it, wherein the raw glass has a 
composition comprising, expressed in terms of weight percent: 52-65% 
SiO.sub.2 ; 8-15% Al.sub.2 O.sub.3 ; 0-4% P.sub.2 O.sub.5, 3-8% TiO.sub.2, 
and 0-4% ZrO.sub.2, wherein the following inequality is satisfied, 
0.1&lt;(% of P.sub.2 O.sub.5 +% of ZrO.sub.2)/(% of TiO.sub.2)&lt;0.8; 2-8% ZnO; 
2-8% MgO; 0.5-4% BaO; 3-12% Li.sub.2 O; 0-4% K.sub.2 O; and As.sub.2 
O.sub.3 and Sb.sub.2 O.sub.3, wherein the following inequality is 
satisfied, 
0&lt;(% of As.sub.2 O.sub.3 +% of Sb.sub.2 O.sub.3)&lt;2. 
Accordingly, the melting temperature of raw glass can be not only made 
lower, but also the heat treatment temperature of crystal nucleating is 
lowered. Thus, the clarification in melting of raw glass is improved, 
resulting crystallized glass has fine particles of crystals, and hence 
good homogeneity can be obtained. 
In accordance with another aspect of the present invention, the method of 
manufacturing a crystallized glass comprising .beta.-spodumene solid 
solution as the predominant crystal phase, comprises the steps of: 
melting a raw glass, forming, and cooling the material, wherein the raw 
glass has a composition comprising, expressed in terms of weight percent: 
52-65% SiO.sub.2 ; 8-15% Al.sub.2 O.sub.3 ; 0-4% P.sub.2 O.sub.5, 3-8% 
TiO.sub.2, and 0-4% ZrO.sub.2, wherein the following inequality is 
satisfied, 
0.1&lt;(% of P.sub.2 O.sub.5 +% of ZrO.sub.2)/(% of TiO.sub.2)&lt;0.8; 0-10% ZnO, 
0-10% MgO, and 0-5% BaO, wherein the following inequality is satisfied, 
3&lt;% of ZnO+% of MgO+% of BaO&lt;15; 3-12% Li.sub.2 O; 0-4% K.sub.2 O; and 
As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3, wherein the following inequality is 
satisfied, 
0&lt;% of As.sub.2 O.sub.3 +% of Sb.sub.2 O.sub.3 &lt;2; 
carrying out a first heat-treatment to the cooled material at a first 
temperature at which nucleation develops in the material; and thereafter 
carrying out a second heat-treatment to the material at a second 
temperature which is higher than the first temperature and at which 
crystallization proceeds in the material. 
In accordance with another aspect of the present invention, the method of 
manufacturing a crystallized glass comprising .beta.-spodumene solid 
solution as the predominant crystal phase, comprises the steps of: 
melting a raw glass, forming, and cooling the material, wherein the raw 
glass has a composition comprising, expressed in terms of weight percent: 
52-65% SiO.sub.2 ; 8-15% Al.sub.2 O.sub.3 ; 0-4% P.sub.2 O.sub.5, 3-8% 
TiO.sub.2, and 0-4% ZrO.sub.2, wherein the following inequality is 
satisfied, 
0.1&lt;(% of P.sub.2 O.sub.5 +% of ZrO.sub.2)/(% of TiO.sub.2)&lt;0.8; 2-8% ZnO; 
2-8% MgO; 0.5-4% BaO; 3-12% Li.sub.2 O; 0-4% K.sub.2 O; and As.sub.2 
O.sub.3 and Sb.sub.2 O.sub.3, wherein the following inequality is 
satisfied, 
1&lt;% of As.sub.2 O.sub.3 +% of Sb.sub.2 O.sub.3 &lt;2; 
carrying out a first heat-treatment to the cooled material at a first 
temperature at which nucleation develops in the material; and thereafter 
carrying out a second heat-treatment to the material at a second 
temperature which is higher than the first temperature and at which 
crystallization proceeds in the material. 
Preferably, the first temperature is in the range of 500-650.degree. C. The 
second temperature is preferably in the range of 700-900.degree. C. 
According to the method of the present invention, the raw glass of each 
composition described above is after melted and cooled, first heat 
treatment is applied at a temperature permitting nucleating, for example, 
in a range of 500 to 650.degree. C., and subsequently second heat 
treatment is applied at a crystallization temperature higher than the 
first heat treatment temperature, for example, in a range of 700 to 
900.degree. C. This indicates that the raw glass can be melted at lower 
temperatures than that of conventional methods, and that nucleating and 
crystallization can occur at lower temperatures than conventional ones. 
Accordingly, the clarification in melting of the raw glass of the present 
invention is improved, and the crystallized glass having fine particles of 
crystals and good homogeneity can be easily obtained. 
The oxide composition of crystallized glass of this invention is expressed 
by the oxide composition of the above raw glass, and the reasons for which 
the composition of each oxide constituting raw glass is limited to the 
above range will be described as follows. 
As to SiO.sub.2, the amount is limited to 52 to 65% by weight because the 
crystal size of resulting crystallized glass tends to become more coarse 
and cracks on surface are often formed during crystallization when the 
amount of SiO.sub.2 is less than the lower limit. Also, when the amount of 
SiO.sub.2 is more than the upper limit, clarification in melting is hardly 
obtained, and the resulting crystallized glass becomes poor in 
homogeneity. 
As to A.sub.2 O.sub.3, the amount is limited to 8 to 15% by weight because 
the formation of the .beta.-spodumene solid solution tends to be difficult 
during crystallization when the amount of Al.sub.2 O.sub.3 is less than 
the lower limit. Also, when the amount of Al.sub.2 O.sub.3 is more than 
the upper limit, melting characteristics become poor, and the resulting 
crystallized glass tends to be impaired in homogeneity. 
P.sub.2 O.sub.5, ZrO.sub.2, and TiO.sub.2 function as nucleating agents, 
and the presence of P.sub.2 O.sub.5 and/or ZrO.sub.2 together with 
TiO.sub.2 induces precursors of crystals, and crystallization at lower 
temperatures can be realized. The reason why the ratio of P.sub.2 O.sub.5 
and/or ZrO.sub.2 to TiO.sub.2, that is, 
(P.sub.2 O.sub.5 (weight %)+ZrO.sub.2 (weight %) )/ TiO.sub.2 (weight %) is 
more than 0.1 and less than 0.8 is that, when the amount is more than the 
upper limit, the crystal size of resulting crystals becomes larger, and 
exerting a bad influence on surface characteristics at the stage of 
precision polishing, and that, when the amount is less than the lower 
limit, it is impossible to obtain the desired effects of this invention, 
that is, the size reduction of crystal particles owing to crystallization 
at lower temperatures. 
As to P.sub.2 O.sub.5 and ZrO.sub.2, the amount of each constituent is 
limited to 0 to 4% because, when the amount is more than the upper limit, 
the grain size of resulting crystals not only becomes larger, but also 
instability of glass state is likely to occur. Preferably, P.sub.2 O.sub.5 
is not less than 0.2% for the purpose of low temperature crystallization. 
As to TiO.sub.2, the amount is limited to 3 to 8% by weight because, when 
the amount is less than the lower limit, no sufficient nucleating effect 
can be obtained, resulting in difficulty in formation of crystals having 
the desired composition. On the other hand, when the amount is more than 
the upper limit, the devitrification resistance of raw glass is 
significantly reduced. 
ZnO, MgO, and BaO are all effective for an increase in low temperature 
melting characteristics of raw glass and homogeneity of resulting 
crystallized glass. The total amount of these constituents of ZnO, MgO, 
and BaO is limited to 3 to 15% by weight because, when the amount is less 
than the lower limit, it is difficult to melt the raw glass. On the other 
hand, when the amount is more than the upper limit, the crystal phase is 
adversely affected. 
As to ZnO and MgO, each amount of such oxides is limited to 0 to 10% 
because, when the amount is more than the upper limit, the devitrification 
resistance of raw glass material is significantly reduced, and each amount 
of 2 to 8% by weight is preferable, and 3 to 8% by weight is more 
preferable. As to the BaO component, the amount is limited to 0 to 5% 
because, when the amount is more than the upper limit, the devitrification 
resistance of raw glass material is significantly reduced, and 0.5 to 4% 
by weight is preferable. 
Li.sub.2 O is one of major constituents of the .beta.-spodumene solid 
solution together with SiO.sub.2 and Al.sub.2 O.sub.3, and the amount is 
limited to 3 to 12% by weight because, when the amount is less than the 
lower limit, melting characteristics of raw glass is not only impaired, 
but also it becomes difficult to obtain the desired composition and 
structure of resulting crystals. On the other hand, when the amount is 
more than the upper limit, the devitrification resistance of raw glass 
material is significantly reduced, and furthermore it becomes difficult to 
obtain the desired composition and structure of resulting crystals. 
K.sub.2 O is effective for increasing the melting characteristics of raw 
glass. The maximum content of K.sub.2 O is 4% by weight and the preferable 
content thereof is in the range of 1 to 4%. 
As to As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3, either one or both of them can 
be added as a clarifier at the time of the melting of raw glass. The total 
amount of 2% by weight or less is sufficient to be effective as a 
clarifier. 
The raw glass of the composition described above is melted and formed, 
cooled, and then subjected to heat treatment for obtaining crystallized 
glass. One example of the heat treatment conditions is described below. As 
the first heat treatment, the temperature is raised to a specified range 
of 500 to 650.degree. C. at a temperature increase rate of 50 to 
300.degree. C. per hour. The glass is maintained at that temperature for 1 
to 20 hours. Then, a second heat treatment follows the first heat 
treatment. The second heat treatment is conducted as follows. The glass 
temperature is raised to a specified range of 700 to 900.degree. C. at a 
temperature increase rate of 5 to 200.degree. C. per hour. The glass is 
maintained at that temperature for 0.5 to 20 hours. 
It should be understood that each condition of the first and second heat 
treatments are not limited to the conditions described above. The first 
heat treatment may be any temperature if it permits nucleating. Further, 
the second heat treatment may be any temperature if it is capable to 
crystallize and is higher than the first heat treatment temperature. It is 
unnecessary to say that any preliminary heat treatment may be applied 
before the first heat treatment, and that any subsequent heat treatment 
may be properly performed after the above first and second heat treatments 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, by giving concrete Examples and Comparison Examples, the 
characteristics of the crystallized glass and the method for producing the 
same according to the present invention are clarified. 
First, raw materials such as oxides and nitrates were prepared and mixed to 
obtain 2500 grams of glass. Raw glass materials of ten different 
compositions were charged into platinum melting pots in a furnace. The 
temperatures were controlled to deal with different melting temperatures 
attributable to different glass compositions. Agitation was also conducted 
steadily. During operation, visual observation was made to confirm that no 
bubbles were present in the molten glass. After the molten glass liquor 
was formed for desired shapes, they were cooled by reheating and heat 
treatment was applied. Thereby the crystallized glass of ten different 
compositions were obtained. 
The compositions of obtained ten kinds of crystallized glass (Samples No. 1 
to No. 10) are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
COMISON 
EXAMPLE EXAMPLE 
SAMPLE NO. 1 2 3 4 5 6 7 8 9 10 11 12 13 
__________________________________________________________________________ 
COMPOSITION 
SiO.sub.2 
55.0 
64.0 
52.0 
57.0 
63.0 
55.0 
60.0 
57.0 
58.5 
57.0 
68.0 
70.5 
62.8 
(WEIGHT %) 
Al.sub.2 O.sub.3 
13.0 
10.0 
9.0 
15.0 
12.0 
10.0 
13.0 
12.0 
13.0 
14.0 
19.5 
17.2 
22.3 
P.sub.2 O.sub.5 
1.5 
2.0 
3.0 
1.0 
1.0 
-- 2.0 
2.0 
-- 1.0 
-- -- 0.8 
TiO.sub.2 
5.0 
5.0 
7.0 
5.0 
3.0 
5.0 
4.0 
6.0 
6.0 
5.0 
4.5 
4.7 
1.8 
ZrO.sub.2 
1.0 
-- 2.0 
-- -- 2.0 
-- -- 2.0 
1.0 
-- -- -- 
ZnO 5.0 
5.0 
6.0 
6.5 
6.5 
8.0 
3.0 
6.3 
7.0 
9.0 
1.5 
0.8 
-- 
MgO 8.0 
4.5 
6.0 
3.5 
4.0 
4.8 
6.5 
4.0 
7.0 
3.0 
3.5 
1.8 
0.8 
BaO 2.0 
1.0 
3.0 
1.0 
1.0 
3.0 
1.0 
1.0 
1.0 
2.0 
-- -- -- 
Li.sub.2 O 
7.3 
4.0 
10.0 
7.5 
6.0 
12.0 
6.0 
8.5 
3.0 
5.0 
3.0 
3.5 
4.7 
K.sub.2 O 
2.0 
4.0 
1.5 
3.0 
3.0 
-- 4.0 
3.0 
2.0 
2.5 
-- -- 0.5 
As.sub.2 O.sub.3 
-- -- 0.5 
-- -- -- 0.5 
-- -- -- -- 1.0 
2.4 
Sb.sub.2 O.sub.3 
0.2 
0.5 
-- 0.5 
0.5 
0.2 
-- 0.2 
0.5 
0.5 
-- -- -- 
B.sub.2 O.sub.3 
-- -- -- -- -- -- -- -- -- -- -- 0.5 
1.2 
F -- -- -- -- -- -- -- -- -- -- -- -- 2.4 
__________________________________________________________________________ 
Also, as to each glass ceramic of Samples No. 1 to No. 10, the melting 
temperature of raw glass, treating temperature and holding time at a heat 
treatment for nucleating (the first heat treatment), and treating 
temperature and holding time at a heat treatment for crystallization (the 
second heat treatment) are all shown in Table 2. In addition, the 
temperature was increased from ambient temperature to the nucleating 
temperature at a rate of 114.degree. C./hour while the temperature was 
increased from the nucleating temperature to the crystallization 
temperature at a rate of 25.degree. C./hour. 
TABLE 2 
__________________________________________________________________________ 
COMISON 
EXAMPLE EXAMPLE 
SAMPLE NO. 1 2 3 4 5 6 7 8 9 10 11 12 13 
__________________________________________________________________________ 
MELTING TEMPERATURE (.degree.C.) 
1400 
1450 
1400 
1450 
1450 
1450 
1450 
1450 
1450 
1450 
1600 
1600 
1500 
NUCLEATING 
TEMPERATURE 
600 
500 
550 
650 
600 
650 
550 
550 
550 
650 
750 
800 
800 
HEAT (.degree.C.) 
TREATMENT 
TIME (HOUR) 
2 5 4 2 5 5 5 5 2 2 2 1 1.5 
CRYSTALIZING 
TEMPERATURE 
780 
780 
780 
850 
800 
800 
750 
780 
780 
850 
1100 
900 
1050 
HEAT (.degree.C.) 
TREATMENT 
TIME (HOUR) 
2 5 2 5 5 2 5 2 2 2 1 2 2 
HARDNESS (Hv) 620 
640 
640 
680 
650 
650 
600 
620 
640 
670 
720 
720 
740 
SURFACE ROUGHNESS Ra (.ANG.) 
15 6 12 15 12 10 7 12 10 18 25 26 30 
__________________________________________________________________________ 
X-ray analysis (XRD) on each of the obtained crystallized glass of Samples 
No. 1 to No. 10 was conducted to identify the crystal structure. It was 
confirmed that all samples contained the .beta.-spodumene solid solution 
as their predominant crystal phase. Also, the crystal sizes of the 
crystallized glass of Samples No. 1 to No. 10 were found to be not more 
than 0.5 .mu.m. 
Furthermore, measurements were carried out to obtain Vickers hardness (Hv), 
and surface roughness (Ra) for each of the obtained glass ceramic of 
Samples No. 1 to No. 10. The results are also shown in Table 2. It should 
be noted that when the surface roughness was determined, the specimens of 
the above crystallized glass (Samples No. 1 to No. 10) were subjected to 
lapping treatment with sand particles having an average size of 9 to 12 
.mu.m for 10 to 20 minutes, and then were subjected to polishing treatment 
with cerium dioxide particles having an average size of 1 to 2 .mu.m for 
30 to 40 minutes. Then, those Samples were finally sent for measurement of 
surface roughness. 
For purposes of comparison, samples of the .beta.-spodumene type 
crystallized glass of three different compositions known conventionally 
prepared in the same procedure as described in the above Examples. Those 
three different compositions of resulting samples of crystallized glass 
(Samples No. 11 to No. 13) are shown in Table 1. However, as to each glass 
ceramic of Samples No. 11 to No. 13, the melting temperature of raw glass, 
treating temperature and holding time at heat treatment for nucleating, 
and treating temperature and holding time at heat treatment for 
crystallization are all shown in Table 2. Also, the temperature was 
increased from ambient temperature to the nucleating temperature at a rate 
of 120.degree. C./hour while the temperature was increased from the 
nucleating temperature to the crystallization temperature at a rate of 
25.degree. C./hour. Other conditions were the same as the above Examples. 
X-ray analysis (XRD) on each crystallized glass of Samples No. 11 to No. 13 
was conducted, and it was confirmed that the Samples contained the 
.beta.-spodumene solid solution as their predominant crystal phase and 
that the crystal size was not more than 0.5 .mu.m. Also, measurements were 
carried out to obtain Vickers hardness (Hv) and surface roughness (Ra) for 
each Samples No. 11 to No. 13 of crystallized glass, and results are also 
shown in Table 2. 
It can be seen from Tables 1 and 2 that the melting temperatures of all raw 
glass of the above Examples are not higher than 1450.degree. C. while the 
melting temperatures of all raw glass of the above Comparison Examples are 
in the range of 1500 to 1600.degree. C. Thus, it can be concluded that, by 
comparing with those Comparison Examples as described above, the above 
Examples have such advantages that the evaporation of volatile components 
such as Li.sub.2 O and the like is few during the melting of raw glass and 
that the raw glass has good homogeneity. Also, in the above Examples, 
temperatures for a nucleating heat treatment and crystallization are 500 
to 650.degree. C., and 750 to 850.degree. C., respectively, and are lower 
than the temperature for a nucleating heat treatment (750 to 800.degree. 
C.) and the temperature for a crystallization heat treatment (900 to 
1100.degree. C.) of the above Comparison Examples. Hence, in the above 
embodiments, crystallization can be conducted at lower temperatures than 
in the above Comparison Examples, and they have advantage for mass 
production in industrial scales. In addition, crystallized glass of the 
above embodiments have adequate degree of abrasion on account of fine 
particles of crystals, and have excellent workability according to on 
their hardness. Further, crystallized glass of the above embodiments have 
small values of surface roughness (Ra) and accordingly good surface 
properties. 
Needless to say, the present invention is not limited whatever to the above 
embodiments. For example, those crystallized glass compositions of Samples 
No. 1 to No. 10 are merely illustrative, and the content of each oxide 
constituent can be suitably selected from the range claimed in the present 
invention. In addition, each temperature of heat treatment for nucleating 
and crystallization can also be suitably selected from the range claimed 
in the present invention. 
As described above, according to the crystallized glass of the present 
invention, raw glass thereof can be melted at lower temperatures during 
processing and that both steps of heat treatments for nucleating and 
crystallization can also be carried out at lower temperatures. Therefore, 
crystal grains thus formed are fine enough to provide good homogeneity. 
Accordingly, the crystallized glass of the present invention possesses 
adequate degree of abrasion to provide good polishing workability, and 
hence they are suitable for those various raw materials that require fine 
and precision processing, for example, materials for electronic 
components, substrate materials for electronic components, substrate 
materials of magnetic disks, and mechanical member materials. 
Also, according to the method for manufacturing a crystallized glass of the 
present invention, all the steps of melting of raw glass, forming of 
nucleus and crystallizing can be conducted at temperatures lower than 
conventional ones. Hence, the clarification in melting of the raw glass is 
improved, and a crystallized glass having fine-grained crystals and good 
homogeneity can be easily produced.