DE1496074C - A transparent glass object with a partially crystalline surface layer having a mechanical strength of at least 2800 kg / cm 2 and a method for its manufacture - Google Patents

Description

The invention relates to a transparent glass object with a partially crystalline ^{surface layer with a mechanical strength of at least 2800 kg / cm 2} , which is characterized in that the glass (3) consists of at least 96 percent by weight of the following oxides:

Na ₂ O
O Weight percent until 70 Mole percent SiO,. 45 until
until
until 8th
20th
35 56 to 73 TiO ₂ 5
4th
18th 4 to 7
&, 5 to 20
11 to 24 Li ₂ O and / or
and / or K ₂
Al ₂ O ₃

the ratio of Li ₂ O to the total amount of alkali oxides not more than 0.8 (based on the weight) or 0.9 (based on mol) and the ratio of Al ₂ O ₃ to the total amount of alkali oxides from 1.1 to 5.0 (based on weight) or 1.0 to 1.5 (based on mol), the partially crystalline layer (2) of / i-spodumene and / or / i-eucryptite crystals dispersed in a glass matrix exists and it differs from glass only by a higher Li ₂ O and correspondingly lower content of the other alkali oxides, but in it the total molar content of the alkali oxides is the same, and a process for its production, which is characterized by, that a glass set of the above composition is melted, the melted glass is simultaneously deformed and cooled below its transformation point, this glass form is ^{exposed to a temperature of 600 to 750 0} C for 5 minutes to 5 hours, with a molten substance containing lithium salts until an O is achieved Surface crystallization brought into contact at 800 to 900 ° C and then cooled to room temperature.

US Pat. No. 2,779,136 discloses a process for the production of glass objects with high mechanical strength which receive an essentially uniform compressive stress in the surface and parallel to it. This method is based on the observation that when a _{glass body containing Na 2} O or K ₂ O is brought into contact with a lithium salt having a low melting point, an exchange takes place between the alkali metal ions of the glass and the molten salt and that the lithium ions are exchanged migrate or diffuse with the sodium or potassium ions into the glass. Experience has shown that in the case where the molten lithium salt had a temperature below the lower stress point of the glass, such a tensile stress arose on the surface of the glass body due to the ion exchange, which is due to the small size of the lithium ions compared to the sodium - Or potassium ions, that very small cracks or cracks appeared in the surface, which eventually caused the breakage. Laboratory studies have shown that this tension and breakage can be prevented if the object is kept at a temperature above its lower tension point during the ion exchange and if the amount of lithium exchanged is equal to the molar equivalent of the amount of sodium and potassium displaced by it. The weight of lithium introduced into the glass in this way, calculated as Li ₂ O, was about half the weight of the displaced Na ₂ O plus a third of the weight of the displaced K ₂ O. This percentage displacement and reduction in the amount of alkali metal oxide in the glass results in one Compared to the original coefficient of thermal expansion of the glass, the coefficient of thermal expansion is low in the part of the glass in which this exchange has taken place. Since the ion exchange is of a progressive nature and takes place from the surface inwards, the glass body receives a layer on the surface with a lower coefficient of thermal expansion than on the inside. This process thus resulted in a body with a surface compressive stress layer which - as has been recognized in the ceramic industry - significantly increases the mechanical strength of the body.

In the cited patent it is further described that the lithium ions in silicate glasses containing alumina, when diffusing into the glass, react under certain conditions with the Al ₂ O ₃ and SiO ₂ contained in the glass and there very small / i-spodumene crystals (Li ₂ O -Al ₂ O ₃ · 4SiO ₂ ). As a result, the coefficient of thermal expansion in the layer on the surface that is subject to conversion is further reduced considerably, since the ß-spodumene has a coefficient of thermal expansion that is almost zero, and the alkali metal oxide content of the glass (base) structure of the surface layer is again considerably reduced by the conversion will. The main requirement in the process of the cited patent is therefore the formation of / i-spodumene in the pressure layer lying on the surface.

It has been found that the breaking strengths that can be achieved are between 1050 and 5250 kg / cm ² . The formation of / i-spodumene in the surface of _{glasses containing Al 2} O ₃ occurs spontaneously. However, its subsurface development requires the presence of a crystallization accelerating or nucleating agent. TiO _{2 was found} to be an ideal nucleating agent and, when contained in an amount of 3 to 15 percent by weight, gave good nucleation when the glass was cooled. / i-Spodumene crystals formed and grew around these nuclei when the lithium ions from the molten salt diffused into the glass, provided that the ratio of alkali metal oxide to Al ₂ O ₃ and to TiO _{2 was} not too high. This reservation had to be made because the TiO ₂ cores are soluble in a sufficiently high alkali metal oxide concentration and then do not form. Also, instead of / i-spodumene and / i-eucryptite, large crystals such as lithium silicates are formed. So if the percentage by weight of the alkali metal oxide in relation to the TiO _{2 is} too high, the inside of the glass remains transparent and the β-spodumene crystals only form on the surface. If the ratio of alkali metal oxide to alumina is too high, crystals other than ./f- spodumene and ß-eucryptite are formed.

The products which can be manufactured according to the process of US Pat. No. 2,779,136 have high degrees of strength of 2100 kg / cm ² and more only when the finished body is cloudy and translucent. These bodies are very suitable for use as projectile cones, tableware, electrical insulators, and for other purposes where haze is not an obstacle.

The glass article according to the invention, which has a well-defined composition and after a certain process is produced, has not only an extremely high strength, but is also see-through, making it suitable for use with panes of glass for airplanes, automobiles, schools and Spaceships are suitable. ,

The mixture suitable for glass production according to the present invention consists essentially of 56 to 73 mol percent SiO ₂ , 4 to 7 mol percent TiO ₂ , 8.5 to 20 mol percent of at least one alkali metal, such as Li ₂ O, Na ₂ O and K ₂ O, wherein the ratio of Li ₂ O to R ₂ O is no more than 0.9, and 11 to 24 mole percent Al ₂ O ₃ , wherein the ratio of Al ₂ O ₃ to R ₂ O is at least 1.0 but not more than 1 .5 is. Glasses with compositions falling within the stated ranges are made from glassware, which may be oxides or other compounds which, when melted together, are converted into the desired oxides in the required amounts. Depending on the quantity and size of the desired objects, the glass sets are melted ^{in glass pans or tanks at about 1400 to 160O 0 C for about 4 to 16 hours.} The components of the glassware are preferably ground in a ball mill and the melt flow stirred to ensure the uniformity of the glass. The molten glass is then processed into various shapes by conventional glass working methods such as pressing, drawing, rolling, blowing and the like. The mold is then preferably cooled in a cooling furnace and cooled to room temperature.

Table I gives examples of compositions that have exceptional strength associated with each other with transparency, in the finished products made by the method of the invention are, result, with the information about the corresponding glass sets in mol percent on the The basis of the oxides has been calculated after deducting the impurities contained in the glassware could be.

Table I.

Na ₂ O

Li ₂ O

Ai ₂ O ₃ ....: ..

SiO ₂

TiO ₂

K ₂ O.

A1 ₂ O ₃ / R ₂ O ...
U ₂ OfR ₂ O ...

6.9

6.9
15.1
66.2

4.9

1.02
0.50

14.3

16.0

64.0

5.6

1.12
0

11.8

2.4

15.7

65.0

5.1

1.11 0.17

9.2

4.7 15.4 66.0

5.0

1.11 0.34

9.2

14.9 65.6 6.3 3.6 1.17 0

7th 8th 9 U) 11th 12th N / A, 4.8
8.1
14.4
69.0
4.3
1.11
0.63 4.9
7.0
13.3
70.0
4.7
1.12
0.58 4.5
6.7
14.1
69.9
4.9
1.25
0.6 9.0
3.2
14.5
68.2
5.0
1.19
0.26 5.5
7.7
14.9
67.3
4.5 '
1.12
0.58 5.1.
8.3
14.4
67.9
4 3 Li ₂ O 1.08
0.62 AUO, SiO ₂ TiO ₂ A1 ₂ O ₃ / R ₂ O
Li ₂ O / R ₂ O

In general, three stages are required to achieve the To process glass objects according to the preferred embodiment of the invention and the desired To achieve properties. First, the object is preheated in air, which is known as the "heat history", abbreviated as WVG. Here, the object becomes one for about 5 minutes to 5 hours Exposed to temperatures of around 600 to 750 ° C, the duration being shorter when temperatures near the upper range are used; then lets one cool the object to room temperature.

Second, the object is heated again, and to a temperature approaching that of the salt bath, about 800 to 900 ⁰ C and maintained at this temperature for several minutes. Third, the article is then immediately immersed in a bath of molten salt until the desired surface ion exchange with subsequent crystal formation develops, generally about 3 minutes to 1 hour.

In order to speed up the work process and save fuel, the shape of the glass needs according to the Melting and molding stages not to be cooled to room temperature in a cooling furnace. Of course you have to the vitreous body cooled to at least below its transformation point before the start of the WVG stage will. The transformation point is considered to be the temperature from which it can be presumed that the liquid melt flow has converted into an amorphous solid with her; he lies in general close to the cooling temperature of the glass (about 700 ° for those in accordance with the invention Types of glass).

It has been found that the WVG greatly affects the clarity of the glass article. A WVG at too low a temperature makes the glass translucent in the salt treatment, while a A lack of WVG means that only translucent or cloudy objects are created during the salt treatment. A WVG at too high a temperature leaves the surface in the subsequent processing will generally crack (hair) in the bath of molten salt.

The WVG can be carried out by giving the object a reasonable length of time on each characteristic Heating temperature within the prescribed range or by making it different Exposing to temperatures within this range, e.g. B. by making the object uniform Speed heated within this range. However, it has been found that shorter heat treatments result in more suitable products, since opacifying phases have less opportunity to occur, so that the glass remains transparent. The preferred WVG is therefore that the glass object

stand for about 5 minutes at a temperature of 75O ⁰ C is exposed.

The duration of the WVG can vary considerably, but too long a WVG generally leads to: that the glass object becomes cloudy or milky during the heat treatment or that during cracks form in the surface during the subsequent salt bath treatment. A relatively easy attempt to determine the correct duration of the WVG for a certain glass composition and a . special temperature consists in placing a number of rod glass samples in an oven of the desired temperature Bringing temperature and taking the individual specimens out of the oven at certain intervals and then subject each specimen to the salt bath treatment. The specimens for which the duration of the WVG was insufficient, will become cloudy or milky in the salt bath treatment, while the Specimens for which the WVG was too long, become cloudy or milky before the salt bath treatment or show cracks on the surface when cooling after the salt bath treatment.

The influence of the WVG cannot be fully explained, but it is assumed that it causes a phase separation in the glass, with an extraordinarily large number of TiO ₂ nuclei being formed which are uniformly, albeit randomly, dispersed in the amorphous phase, and that these nuclei cause the crystallization of other crystalline phases during the subsequent ion exchange process. These other crystals are so small that they have very, very little ability to refract or scatter visible light and therefore cannot adversely affect the transparency of the glass. It is also believed that inadequate or lack of WVG leads to the formation of large TiO ₂ crystals and other phases that cause the body to be cloudy.

According to the WVG, the object does not necessarily need to be cooled to room temperature. When this happens, it is generally therefore to put the goods in the molten state before treatment To be able to subject the salt bath to a visual inspection.

The stage at which the item immediately before being immersed in the salt bath in air to the im Salt bath is heated to the prevailing temperature, is not absolutely necessary, but serves two purposes Purposes. Breaking or chipping of the body due to thermal shock, which is a problem can, if the object is cooled to room temperature after the WVG, can be prevented will. This preheating also brings the item to the right temperature, see above that the salt bath treatment can be carried out uniformly. This preheating stage is usually only carried out by that the glass body is placed in an oven maintained at 800 to 900 ° C. and for about 2 to 5 minutes left in this.

Any lithium salt or mixture of such salts is suitable whose melting point is below the softening point of the glass (about 950 ° C for these types of glass) and that is not different at the treatment temperature decomposed to such an extent that the molten salt becomes so alkaline that it considerably affects the glass etches. In general, the organic lithium salts are not suitable according to the invention. To the inorganic Lithium salts whose melting points are low enough and which are sufficiently stable at the treatment temperature are included other lithium nitrate, lithium sulfate, acid lithium sulfate, lithium chloride, lithium bromide and lithium iodide.

If the melting point of a salt is above the softening point of a glass, it can be advantageous be mixed with another lithium salt, either being a eutectic mixture arises or one that has a medium melting point. The decomposition of the melted Salt can be suppressed by keeping it in a closed vessel and using the total partial pressures of the gaseous decomposition products during the treatment of the vitreous body to 1 at or holds higher.

Lithium sulfate is the preferred main component of the salt bath according to the invention. This salt melts at 860 ° C, however experience has shown that it should be used at 880 ° C or more. Lithium chloride (5 to 15 mole percent) was added to lower the melting point to 820 to 850 ° C, see above that only the lithium ions were available for the sodium-lithium exchange. after the After baths had been used for a short time, annoying corrosion occurred due to the formation of lithium hydroxide of the glass. The addition of ammonium sulfate helped little. The further work led to the Add lithium sulfate instead of lithium chloride 5 to 15 percent by weight sodium sulfate. The preferred bath, in which 5 percent by weight sodium sulfate is used with lithium sulfate, can be in the range from 850 to 900 ° C are used. It has been found that the addition of this small amount of a non-lithium containing salt affects the efficiency and rate of the ion exchange reaction not noticeably affected. Experiments showed that already a few percent by weight of lithium salt sufficient to carry out the exchange; however, the implementation is very slow because in the Reaction zone on the surface of the object less lithium ions are available. It leaves recognize that, although in the preferred practice, in the case of a concentrated Lithium ion mixture is used, an impregnation time of about 3 minutes to 1 hour for a satisfactory Ion exchange and the surface crystallization is sufficient, different lithium ion concentrations and efficacies require different soaking times. Choosing the right one Concentrations and periods of time can easily be determined by the person skilled in the art in the context of the above description to be hit.

The following flow sheet is provided to illustrate the various process steps of the invention.

A distinction is made between the levels for the invention is necessary, and those that - albeit optional - are often carried out, to visually inspect the product, to protect the body from thermal shock or from others Establish.

Required levels
Melt

Optional levels

Cooling and deforming

Cooling and testing.

_ ". Reheating

WVGi '

~~ "-» cooling

-Reheating

Salt bath * "'

Cool

After the glass set has been melted and then deformed by cooling below the transformation point of the glass, the body can be ^{fed directly to a furnace maintained at 600 to 750 0} C, and the WVG can be carried out, or it can be cooled in the cooling furnace and then cooled to room temperature to check it visually or simply to store it until the actual heat treatments are to be carried out. When the body is cooled to room temperature, it must be reheated to the temperature range of the WVG. This can be done by introducing the body into an oven maintained at this temperature range, or by bringing it into the oven at room temperature and heating it to the WVG range.

After the WVG stage, the glass mold can be placed directly in the salt bath or put back on for inspection Room temperature. If the body is cooled to room temperature, then one should The preheating stage can be switched on in order to warm it up in hot air to the temperature of the salt bath, so that the risk of breakage due to the thermal shock is eliminated.

After the treatment in the salt bath, the body is cooled to room temperature, such as clinging Salt is washed off with water whenever possible.

In practicing the present invention, the salt was melted in clay pots and crucibles, which have a coating of 96% silica. Also other fabrics that have good resistance Against corrosion by the lithium salts, such as stainless steel, can be used.

The analysis of the X-ray diffraction patterns showed that the crystals in the printed layer on the surface were /? - spodumene (Li ₂ O-Al ₂ O ₃ -4SiO ₂ ) or jS-eucryptite (Li ₂ O-Al ₂ O ₃ · 2SiO ₂ ) acted. The already mentioned displacement of sodium and / or potassium ions by lithium ions is greatest on the glass surface and decreases progressively towards the inside. The crystal formation of / S-spodumene and /? - eucryptite is reduced in the same way, but does not extend as far into the interior of the vitreous as the penetration of lithium. The limit up to which the lithium penetrates cannot be drawn sharply and cannot be measured precisely, although it can be estimated from the resulting stress image. The limit of crystal growth can be measured microscopically, and the penetration of lithium beyond this can be determined by a visible change in the refractive index of the glass. The average depth of penetration of lithium into articles made from the compositions listed in Table I is usually less than 1 mm.

It is clear that this crystal formation and penetration is directly related to the length of time that the object is immersed in the molten salt bath. Too short a treatment does not give the optimum difference in the physical properties of the surface and the untreated Core. As already mentioned, ß-spodumene has a low coefficient of expansion while /? - eucryptite has a negative expansion coefficient has. Too long treatment would therefore reduce the difference in the properties of the surface and the core. A treatment which takes a long enough time to completely replace lithium for sodium or potassium, would result in a finished glass-ceramic which, although considerably stronger than the original Glass, however, would not be as strong as that obtained by surface exchange, coated glass. These facts required strict observance of the period of about 3 minutes to 1 hour with the actual ion exchange process, if that is the preferred Composition comprising salt bath is used.

The molar ratio of Li ₂ O / R ₂ O is another critical factor in the manufacture of an article that is to have high strength. In glasses with a high lithium oxide content, immersion in a salt bath of lithium ions would not have much effect because there are few sodium ions that can be replaced by the lithium. If the ratio of Na ₂ O and / or K ₂ O to Li ₂ O in the original glass increases, the fixing effect of the lithium treatment also increases, since the inside of the glass then has a progressively increasing coefficient of expansion, while the outer layers become spodumene or eucryptite crystallize and expand very little. It proved to be possible to form by appropriate choice of glass composition and salt bath body whose strength is 2800-8400 kg / cm ² and the different coefficients of expansion of -10 to +70 x 10 ^-7 / ° C. The higher strength values - occur together with higher expansion values.

It was found that. the molar ratio of A1 ₂ O ₃ / R ₂ O must be kept between 1 and 1.5, or only translucent or cloudy bodies result, regardless of which WVG and salt bath treatments are used. It is believed that this haze is due to the formation of crystal phases other than eucryptite and spodumene. TiO ₂ concentrations of more than about 7 MoI-

⁶ ½ percent cause the precipitation of rutile or anatase during deformation of the glass object. Higher percentages of TiO ₂ increase the risk that during a

309 627/1 S3

ίο

future processing of the substance in the open flame, z. B. when fire polishing, a cloudiness occurs. The presence of less than 4 mol percent TiO ₂ leads to insufficient nucleation in the thermally induced phase deposition, so that a translucent or milky glass is formed during the salt treatment. This inadequate nucleation also results in less extensive crystal bonding, so that the fixing effect is very poor.

The amount of silica should not exceed about 73 mol percent, otherwise it will be excessively viscous Melt flow results, which requires higher melting temperatures than with conventional refractories Materials is practically accessible.

It has been found that the presence of small amounts of other oxides can be tolerated and even be beneficial. Such oxides include: Rb ₂ O and Cs ₂ O, which can partially replace _{R 2 O;} B ₂ O ₃ , which is suitable as a flux; ZrO ₂ , which can improve the durability of the glass; UUdAs ₂ O ₃ and Sb ₂ O ₃ , which are suitable fresh agents. The amount of any single or more such oxides should not exceed about 4 mole percent.

Table II gives details of the WVG and salt treatment, as well as a visual description, coefficient of expansion and modulus of rupture of the articles made from each of the mixtures listed in Table I, with melting and deformation in the manner described was carried out. The modulus of rupture was determined by the conventional method using rods ground with carborundum (30 grit). The method for determining the linear thermal expansion coefficient is also known; the values listed in Table II represent the expansion coefficient between 25 and 30O ⁰ C. In Examples 1 to 6, a molten salt bath of 85 mol percent Li ₂ SO ₄ and 15 mol percent LiCl and in Examples 7 to 12 a bath of 95 mol percent Li ₂ SO ₄ and 5 mol percent Na ₂ SO ₄ .

Table II

Glass WVG Salt treatment Be
writing expansion Modulus of rupture (■ 107 ⁰ C) (kg / cm ² ) 1 650 ° C - 2 hours 800 ° C - 15 minutes clear 5,334 650 ⁰ C - 2 hours 800 ⁰ C - 30 minutes clear 7.175 700 ° C - 1 hour 820 ° C - 30 minutes clear 5,292 2 600 ⁰ C - 4 hours 820 ⁰ C - 30 minutes clear 4,706 600 ° C - 4 hours 84O ⁰ C 30 minutes clear 7,630 3 625 ° C - 2 hours 860 ° C - 5 minutes clear 67.9 5,607 65O ⁰ C - 30 minutes S60 ° C - 5 minutes clear 69.8 6475 655 ° C - 1 hour 860 ° C - 5 minutes clear 71.6 6671 700 ° C - 1 hour 900 ° C - 10 minutes clear 72.2 8400 4th 600 ⁰ C - 4 hours 840 ⁰ C - 30 minutes clear 8812 600 ° C - 4 hours 82O ⁰ C - 30 minutes clear 5880 700 ° C - 2 hours 840 ° C - 10 minutes clear 6720 5. 600 ° C - 5 hours 800 ° C - 1 hour clear 5800 , 650 ° C - 2 hours 840 ° C - 1 hour clear 6160 750 ⁰ C - 10 minutes 85O ⁰ C. - 30 minutes clear 6580 6th 600 ° C - 4 hours 82O ⁰ C-! Hour clear 4760 750 ° C - 15 minutes 860 ° C - 30 minutes clear 5180 7th 650 ⁰ C - 2 hours 840 ° C - 10 minutes clear 43.7 4088 650 ° C - 1 hour 86O ⁰ C - 5th minutes clear 27.6 4382 8th 650 ° C - 1 hour 860 ° C - 5 minutes clear 3640 750 ° C - 1 hour 86O ⁰ C - 5 minutes clear ' 3990 650 ° C - 2 hours 860 ° C - 10 minutes clear 4452 650 ° C - 2 hours 86O ⁰ C- 15 minutes clear 30.6 '4774 600 ° C - 4 hours 900 ⁰ C - 10 minutes clear , 15.0 3500 9 700 ° C - 1 hour 86O ⁰ C - 10 minutes clear 36.1 4186 700 ° C - 1 hour 88O ⁰ C 5 minutes clear 44.1 3920 700 ° C - 1 hour 880 ° C - 5 minutes clear 44.1 3920 700 ° C - 4 hours 86O ⁰ C 10 minutes clear 47.2 3234 10 700 ° C for one hour 860 ° C - 15 minutes clear 44.4 6118 700 ° C - 1 hour 86o C ⁰ 25 minutes clear 37.5 6055 650 ° C - 1 hour 860 ° C - 25 minutes clear 40.7. ·. 6496 700 ⁰ C - 1 hour 900 ° C - 10 minutes - clear 47.3 4277

WVG continuation Be
writing expansion Modulus of rupture Glass Salt treatment (· 10 ⁷ / ° C) (kg / cm ² ) 700 ° C - 1 hour clear 48.7 4704 11th 700 ° C - 1 hour 840 ° C - 10 minutes clear 42.5 5978 650 ° C - 1 hour 860 ° C - 5 minutes clear 3493 12th 650 ° C -! Hour 860 ° C - 4 minutes clear . 33.9 5166 860 ° C - 5 minutes

The table above clearly shows the extraordinary strength of the glass objects, which were prepared from the mixtures listed in Table I, according to the invention Procedure is awarded. Example 3 is the preferred blend used if only emphasis is placed on the mechanical Firmness was laid. However, it becomes excellent in strength and strength against thermal shock, example 8 is a body with a very low coefficient of thermal expansion, the preferred mix.

In all the examples given, / formed S-eucryptite the predominant part of the in situ precipitating crystal phase.

In A b b. 1 shows the composition ranges in which the constituents of the glassware were used according to the invention. In order to explain the invention as a system of three components, the alkali metal oxides were combined as R ₂ O and the relatively small amount of TiO _{2 was combined} with the silica. The range within which the compositions can be used is very small, since the ratio of Al ₂ O ₃ to R ₂ O should be at least 1.0 but not more than 1.5. A b b. 1 also emphasizes that eucryptite and spodumene are the crystal phases that precipitate in glasses of these compositions.

A b b. 2 shows a time-temperature curve for the WVG heat treatment required according to the invention. It has already been said that the duration of the heat treatment is between about 5 minutes and 5 hours, with the higher temperatures requiring a shorter duration. At 750 ° C z. Legs 5 minutes of treatment, and a much longer treatment would cause haze. Through Experiments have shown that the relationship between time and temperature is at the maximum applicable Treatment time, which gives a transparent body, is not exactly linear, but in general the one in A b b. 2 follows the hyperbolic curve shown.

A b b. Figure 3 shows a time-temperature curve for the required salt bath treatment using concentrated lithium ion salt baths in accordance with the preferred embodiment of the invention. The use of a salt bath to promote alkali metal ion exchange is described in U.S. Pat. No. 2,779,136 mentioned above. This graphic representation has only been included in order to be able to better explain the entire method according to the invention. The heavy line represents the salt bath treatment of a specific example of the invention, namely Example 3 listed in Tables I and II, wherein the article is deformed in the lehr after the glassware has been melted by heating in a crucible to about 1600 ° C for 16 hours had been cooled and cooled to room temperature, was treated according to the method of the invention. WVG treatment of the body was carried out by placing it in an oven maintained at 700 ° C. and leaving it there for 1 hour, then removing it from the oven so that it was allowed to cool to room temperature. The body was then placed in an oven maintained at 900 ° C., left in this for 5 minutes in order to prevent breakage in the salt bath through this preheating, immediately thereafter in a molten salt bath of 85% by weight Li ₂ SO ₄ and kept at 900 ° C. 15 percent by weight LiCl immersed, left in this for 10 minutes, then removed from the bath and cooled; the attached salt was washed off with water. The letter A in the A b b. 3 represents the duration and temperature of the preheat stage, while the letter B represents the duration and temperature of the salt bath treatment. The areas delimited by the dashed lines indicate the areas of these treatments. The use of a preheat stage is optional and is included in this curve to complete the description of the invention.

A b b. 4 shows the plate of a dinnerware made according to the invention was produced. The plate shown as a side view in cross section at 1 consists from a crystalline surface layer 2, which lies over a glass-like body part 3.

Claims (5)

Patent claims: 1. A transparent glass object with a partially crystalline surface layer with a mechanical strength of at least 2800 kg / cm ² , characterized in that the glass (3) consists of at least 96 percent by weight of the following oxides: Weight percent Mole percent SiO ₂ 45 to 70
5 to 8
4 to 20
18 to 35 56 to 73
4 to 7
8.5 to 20
11 to 24 TiO, Li ₂ O and / or Na ₂ O
and / or K ₂ O ...
Al ₂ O ₃ where the ratio of Li ₂ O to the total amount of alkali oxides is not more than 0.8 (based on the weight) or 0.9 (based on mol) and the ratio of Al ₂ O ₃ to the total amount of alkali oxides is 1.1 to 5 , 0 (based on the weight) or 1.0 to 1.5 (based on mol), the partially crystalline layer (2) consists of / S-spodumene and / or jS-eucryptite crystals dispersed in a glass matrix and they only get through the glass distinguishes a higher Li ₂ O and correspondingly lower content of the other alkali oxides, but in it the total molar content of the alkali oxides is the same. 2. A method for producing the transparent glass object according to claim 1, characterized in that a glass set, the composition specified in claim 1 melted, the molten glass simultaneously deformed and cooled below its transformation point, this glass form 5 minutes to 5 hours at a temperature of 600 to 750 ⁰ C exposed, with a molten, lithium salt-containing substance until a surface crystallization is achieved at 800 IO brought into contract to 900 ⁰ C and then cooled to room temperature. 3. The method according to claim 2, characterized in that the glass ^{mold is exposed to the temperature of 800 to 900 0} C for 2 to 5 minutes before it is brought into contact with the molten lithium salt-containing substance. 4. The method according to claim 2 or 3, characterized in that the glass mold is exposed to ^{a temperature of 750 0 C for about 5 minutes.} 5. The method according to any one of claims 2 to 4, characterized in that the glass shape with the lithium salt-containing substance is brought into contact for 3 minutes to 1 hour. 1 sheet of drawings