GB2379659A

GB2379659A - Making a transparent glass-ceramic armour

Info

Publication number: GB2379659A
Application number: GB0122124A
Authority: GB
Inventors: John George Darrant; Christopher Thompson
Original assignee: Alstom SA
Current assignee: Alstom SA
Priority date: 2001-09-13
Filing date: 2001-09-13
Publication date: 2003-03-19
Also published as: WO2003022767A1; GB0122124D0

Abstract

A process for making transparent glass-ceramic armour comprises :- <SL> <LI>(a) performing a nucleation heat treatment step on a vitreous prescursor material to produce a nucleus population density in excess of 10<SP>20</SP> per cubic metre, preferably by heating the vitreous material to a nucleation temperature in the range 520{C to 580{C, and maintaining said nucleation temperature for a time period in the range 10 to 170 hours, <LI>(b) performing a crystallization heat treatment step (optionally time-separated from the nucleation heat treatment step) by further heating the vitreous material to a crystallization temperature, preferably in the range 710{C to 770{C, and maintaining the crystallization temperature for a time sufficient to achieve a desired degree of crystallization, preferably 15 minutes to two hours; and <LI>(c) cooling the material to room temperature at a rate such as to avoid differential temperature damage. </SL> The armour may be used in visors, vehicle observation ports and helicopter windscreens.

Description

PROCESSING OF TRANSPARENT GLASS-CERAMIC This invention relates to the processing of transparent glass-ceramic, and in particular to an optimum heat-treatment regime for a known transparent glass-ceramic material useful for its resistance to ballistic impact.

Patent No. GB 2 284 655B, with a priority date of 21 September 1993, describes and claims an armour system comprising transparent glass-ceramic, e. g. , for use in visors, observation ports, windscreens or the like, having good transparency at visible wavelengths and improved ballistic performance, characterised as made from lithium disilicate based material and including silica as a majority constituent, lithium oxide as a next major constituent and nucleating agents comprising zirconium dioxide in major part and phosphorus pentoxide in minor part. To prevent damage or injury from flying fragments of the glass-ceramic itself, it is preferably used in combination with a transparent backing material which has sufficient strength, compliance and ductility to capture, and prevent penetration of, the ceramic fragments and eroded projectile.

The above-mentioned patent should be consulted for complete details of the transparent glass-ceramic compositions tested in connection with that invention and the manufacturing techniques adopted at that time. However, the invention glass-ceramics were further characterised as comprising transparent glass-ceramic made from a vitreous precursor material comprising a lithium disilicate based material, including silica as a majority constituent iprefs-rshly 67-75 wt. %, most preferably about 71.8 wt. % Si02) ; lithium oxide as a next major constituent (preferably 9-14wt. %, most preferably about 11 wt. % Li20) ; and nucleating agents comprising zirconium dioxide in major part (preferably 6-12 wt. %, most preferably about 8 wt. % Zur02) and phosphorus pentoxide in minor part (preferably 1.0 to 3.0 wt. %, most preferably about 2.0 wt. % P205) ; balance other oxides (most preferably about 4.5 wt. % alumina, A1203, about 0.5 wt. % zinc oxide, ZnO, and about 2.2 wt. %"potash", pure K20). The above most preferred composition is that of Glass NK2/4163, the preferred precursor glass for the ALSTOM proprietary transparent armour glass ceramic called TRANSARMTM.

Successful manufacture of an article comprising such a glass-ceramic requires controlled devitrification of the glass composition with careful control of the chemical composition and thermal processing such that on conversion from the pre-cursor glass to the final glass-ceramic, good transparency is retained, as well as the improved ballistic performance. According to the above patent, manufacture includes the steps of : (a) mixing together appropriate quantities of the constituent oxides, or raw materials which decompose to give those oxides, to give the desired chemical composition in the final material; (b) heating the mixture in a suitable container or furnace to form a homogenous melt, the mixture being maintained in the molten state for sufficient time to allow the clearance of bubbles from the bulk of the melt ; (c) cooling the melt at a rate sufficient to retain the material in a vitreous condition to a temperature (usually room temperature) which is below the softening temperature of the vitreous material, the material if required being subjected during cooling of the melt to various forming operations to give particular shapes, and if required being cooled to suitable temperature schedules to limit residual stresses arising from excessive thermal gradients; (d) heating the vitreous material to a crystallization temperature, such that during heating to the crystallization temperature the material is allowed to dwell at an intermediate temperature (the nucleation temperature) for sufficient time to allow a high population density of nuclei to form-upon these nuclei crystallization cop orr,,-when the material is subsequently heated to the crystallization temperature; (e) after any required further forming during heat-treatment, cooling the material to room temperature at a rate which avoids differential temperature damage; and (f) finally shaping the glass-ceramic by normal ceramic machining operations such as diamond grinding, and restoring full transparency by surface polishing if required.

As noted, particularly important for retaining good transparency is the part of the heat treatment which achieves nucleation, wherein the nucleation temperature is maintained for a period sufficient to allow a high population density of nuclei to form, preferably a nucleus population density in excess of 1020 per cubic metre, ideally in excess of 1 021/m3. Upon these nuclei, crystallization can occur when the material is subsequently heated to the crystallization temperature to give a microstructure whose constituent grains are substantially smaller than the wavelength of visible light, say of the order of one tenth to one half the wavelength of visible light. To achieve such a nucleus population density for the most preferred glass-ceramic composition, Glass NK2/4163,

patent no. GB 2 284 655B recommends a nucleation stage in which the vitreous material is maintained at a temperature of between 580 and 650 C for between 10 and 100 hours. Following this, the nucleated material is maintained at a temperature of between 750 and 770 C for at least two hours to achieve crystallization.

An object of the present invention is to reduce the cost of the manufacturing process for producing the finished transparent glass-ceramic, preferably without substantially deleteriously affecting its properties.

Another object is to achieve a lowered nucleation temperature and/or a shorter i crystallization time while improving or at least not substantially degrading the properties of the finished transparent glass-ceramic.

A firtber Ohject is to enable more convenient or optimised prod' finished glassceramic articles.

In accordance with one aspect of the present invention, a process for the heat-treatment of a vitreous material to obtain a transparent glass-ceramic includes the steps of : (a) performing a nucleation heat treatment step by heating the vitreous material to a nucleation temperature in the range 520 C to 580 C and maintaining said nucleation temperature for a time period in the range 10 to 170 hours ;

(b) performing a crystallization heat treatment step by further heating the vitreous material to a crystallization temperature and maintaining the crystallization temperature for a time sufficient to achieve a desired degree of crystallization; and (c) cooling the material to room temperature at a rate such as to avoid differential temperature damage.

Preferably, the nucleation temperature is in the range 530 C-570 C, most preferably approximately 550 C, and is maintained for a period in the range 50-170 hours, most preferably 100 hours. Such a lowered nucleation temperature relative to the prior art, when maintained for a suitably long period, achieves improved transparency in the

~ ; U A I 4 finished glass-ceramic article at a reasonable process cost.

LJLAU it According to another aspect of the invention, a process for the heat-treatment of a vitreous material to obtain a transparent glass-ceramic includes the steps of (a) performing a nucleation heat treatment step on the vitreous material to produce a nucleus population density in excess of 1020 per cubic metre; (b) performing a crystallization heat treatment step by heating the vitreous material to a crystallization temperature is in the range 7100C to 770 C and maintaining the crystallization temperature for a time period in the range of about 15 minutes to two hours; and (c) cooling the material to room temperature at a rate such as to avoid differential temperature damage.

Shortened dwell times for the crystallization heat treatment step reduce the cost of the manufacturing process for producing the finished transparent glass-ceramic, and with judicious choice of parameters as taught in this specification, this can be achieved without substantially deleteriously affecting the material's properties.

Preferably, the crystallization temperature is approximately 750 C.

In either of the above aspects, after the nucleation step, the vitreous material may be cooled to room temperature at a rate such as to avoid differential temperature damage and then stored and/or transported before being reheated to the crystallization temperature. The vitreous material may also be formed to a desired shape during the crystallization step. These measures facilitate optimisation of mass production of finished glass-ceramic articles by enabling the two heat treatment stages to be performed by different persons or at different facilities.

In yet a further aspect, the invention provides a process for the manufacture of a transparent glass-ceramic article in sheet form, including the sequential steps of : (a) heat treating a precursor sheet of vitreous material in a nucleation heat treatment to produce a nucleus population density in excess of 1020 per cubic metre; (b) cooling the nucleated sheet to room temperature at a rate such as to avoid differential temperature damage; (c) at a later time heat treating the nucleated sheet in a crystallization heat treatment to produce crystal sizes less than one half of the wavelength of visible light, while simultaneously forming the sheet to a final shape of the glass-ceramic article; and (d) cooling the glass-ceramic article to room temperature at a rate such as to avoid differential temperature damage.

In the above processes, the vitreous material preferably comprises a lithium disilicate based material, including silica as a majority constituent, lithium oxide as a next major constituent and nucleating agents comprising zirconium dioxide in major part and phosphorus pentoxide in minor part.

Other aspects of the present invention will become apparent from an inspection of the following description and the appended claims.

Several examples of processing of glass-ceramics in accordance with the invention will now be described, by way of example only, with reference, inter alia, to the accompanying figures, in which: Figure 1 is a graph illustrating the linear thermal expansion characteristics of Glass NK2/4163 nucleated at different temperature for periods of 100 hours; Figure 2 is a graph illustrating the linear thermal expansion characteristics of Glass NK2/4163 nucleated at 550 C for different times; Figure 3 is a graph illustrating the linear thermal expansion characteristics of TRANSARM after variation of the time spent at crystallisation temperature; and Figures 4 and 5 are electron micrographs of the microstructure of TRANSARMTM after heat-treatment to two different schedules.

Unless otherwise noted, the examples to be described are for glass-ceramic armour which essentially has good transparency at visible wavelengths and which has a high ballistic performance. In all of the examples given, the production of the glass melt from the original constituents has been done as a preliminary step and is not included in the heat treatment schedules given.

Known heat-treatment schedule for the preparation of TRANSARMTM Following is a summary of the continuous heat-treatment process disclosed in patent no. GB 2 284 655 B in respect of Glass NK2/4163 :

(a) heat to 580-650 C, preferably 590 C or 610 C, and maintain that temperature for 10-100 hours stage) (b) heat to 650-780 C, preferably 750-770 C, and maintain that temperature for at least two hours (crystallisation stage) (c) allow to cool to room temperature at a rate such as to avoid differential temperature damage.

New heat-treatment schedules for the preparation of TRANSARMTM The following examples show that in distinction from the disclosure of patent no GB 2 284 655B, nucleation temperatures substantially below 580 C will give improved

transparency of the glass-ceramic after crystallization if such nucleation temperatures are maintained for a suitably long period, preferably in excess of 50 hours. However, no benefit was seen in increasing nucleation times beyond about 170 hours and the optimum nucleation temperature was found to be in the region of 550oC.

Furthermore, again in distinction from the disclosure of the prior patent, the following examples also show that the crystallization stage of the heat-treatment can be effectively accomplished with dwell times well below two hours.

In the following examples, variations in nucleation density and crystallisation were gauged by reference to the dilatometric softening temperature (Mg) of bars of Glass NK2/4163, because Mg increases with increasing nucleation density. It is estimated that Mg was measured with an accuracy of3 C.

EXAMPLE 1 Variation of the nucleation temperature Bars were cut to the dimensions necessary for thermal expansion measurements from annealed blocks of Glass NK2/4163. They were heat-treated for 100 hours at 530,550, 560 and 570 C respectively to achieve nucleation. Note that 100 hours is the maximum nucleation time disclosed in patent no. GB 2 284 655B, and is believed more advantageous than substantially shorter times disclosed therein. Thermal expansion measurements were carried out on all of the bars.

Table 1 details the dilatometric softening temperature (Mg) of each bar. Figure 1 displays the linear thermal expansion characteristic of each bar from 500 C to its Mg.

Table 1: Dilatometric softening temperature (Mg) of Glass NK2/4163 bars after heat-treatment to different nucleating temperatures for 100 hours

Heat-treatment Dilatometric softening temperature C temperature (Mg) C 530 610 550 660 560 650 570 669 As can be seen from Table 1 and Figure 1, heat-treatment to 570 C for 100 hours gave the highest Mg at 669 C. However, the glass bar became opaque during this heattreatment. None of the other nucleating temperatures gave the glass an Mg as high as that give by a treatment temperature of 550 C (Mg = 660 C). Therefore, the 550 C value is indicative of the likely optimum nucleating temperature.

EXAMPLE 2 Variation of nudeation time at 5500C Bar samples were prepared from Glass NK2/4163 as above. These bars were heattreated separately to 550 C for periods of 41,60, 84,100, 125 and 168 hours respectively to nucleate them. Thermal expansion measurements were carried out on all of the bars.

Table 2 details the dilatometric softening temperature (Mg) of each nucleated bar. Figure 2 displays the linear thermal expansion characteristic (percentage linear change versus temperature, C) of each bar from 500 C to its Mg.

Table 2: Dilatometric softening temperature (Mg) of Glass NK2/4163 bars after heat-treatment to the nucleation temperature of550 C for different times.

Time held at 550 C Dilatometric softening (hours) temperature (Mg) C 41 622 60 642 84 645 100 660 125 664 168 654 As can be seen from Table 2 and Figure 2, the overall effect of increasing the time spent at 550 C is to increase the Mg of the samples. However, dwell periods of 100 and 125 hours gave Mg's of 660 C and 664 C respectively which are higher than that obtained from the nucleating dwell period of 168 hours (Mg = 654 C). The maximum dwell period of 168 hours was chosen on the basis of previous experimental work.

If the Mg's of the glass bars are very similar after heat-treatment to 550 C for different times then it follows that their nucleation densities are probably very similar. The nucleating dwell period of 100 hours at a temperature of 550 C represents the best compromise between limiting time spent on the nucleation heat treatment schedule while maintaining good nucleation density and is therefore indicative of the optimum nucleating conditions.

EXAMPLE 3 Variation of the Crystallisation Time Five blocks of Glass NK2/4163 were all heat-treated at a crystallisation temperature of 750 C, but in each case the dwell time at 750 C differed. In the first of these heattreatments, this dwell time was omitted and the furnace was allowed to cool on reaching the crystallising temperature. In the four remaining heat-treatments the dwell

time at 750 C was for periods of 15 minutes, 30 minutes, 1 hour and 2 hours respectively. Bar samples were then cut to the dimensions necessary for thermal expansion measurement from all of the heat-treated blocks.

Table 3 details the dilatometric softening temperature (Mg) of each heat-treated bar.

Figure 3 displays the linear thermal expansion characteristic of each bar from 500 C to its Mg.

Table 3 : Dilatometric softening temperature (Mg) of Glass NK2/4163 bars after heat-treatment to crystallisation temperature of 750 C for different times

Time held at 550 C Dilatometric softening (hours) temperature (Mg) C 0 721 0.25 792 0.5 790 1.0 792 2.0 797 As can be seen from Table 3 and Figure 3, with the exception of heat-treatment to the crystallisation temperature without a dwell time, there is surprisingly little experimentally significant variation in the Mg's of the glass-ceramic bars in the heattreatments. The dwell time at the crystallisation stage of heat-treatment can therefore be reduced below the Known minimum of two hours to enable more economic production and can tolerate variation down to about 15 minutes.

In accordance with patent no. GB 2 284 655B, it is known that satisfactory crystallization can also be achieved at lower temperatures, but with shorter dwell times than in the prior patent, we prefer a range of 710-770 C.

Preparation of TRANSARMTM in two time-separated heat-treatments In distinction from the technique reported in our prior patent, the examples reported below perform the nucleation and crystallisation of the glass in two time-separated heat-treatments. This will facilitate large-scale manufacture, in that during volume production the long nucleation heat-treatment could take place in bulk as the glass was worked from the melting furnace, the nucleated glass stock so produced then being held in storage and/or transported to customers or subcontractors before performance of the shorter crystallisation heat-treatment to produce the finished glass-ceramic.

Furthermore, since the crystallisation heat treatment is at a temperature above the softening temperature of the nucleated glass, forming of, e. g. , nucleated glass ceramic sheets to finished shape could conveniently be done during the crystallisation heat treatment.

The following examples show that good results can be achieved by accomplishing the nucleation and crystallization stages in two time-separated heat-treatments.

EXAMPLE 4 Nucleation heat-treatment A 3.55 mm thick polished plate of Glass NK2/4163 in the shape of a visor for a protective helmet was placed on a bed of alumina powder spread evenly on a refractory

batt. The refractory batt and its load were placed in the furnace and heated at 5 C/min to 550 C (nucleation temperature) for a period of 100 hours. Thereafter, cooling took pj a rate not-exceeding 5 C/min. The visor pla ? ' ?" dusted to remove any excess alumina powder when removed from the refractory batt. It had not undergone any change in appearance as a result of being subjected to a nucleating heat-treatment.

EXAMPLE 5 Crystallisation heat-treatment with reforming A polished 16% chrome-iron sheet 350 x 200 x 1.5 mm was rolled to produce a former having a radius of curvature of 130 mm, this being different from the above-mentioned nucleated visor plate. It was coated with graphite release agent (ROCOLTM). The

nucleated visor plate from Example 4 was placed on the rolled former and heated at 3 C/min to 7500C (crystallisation temperature) and allowed to dwell at that temperature for a period of 2 hours. Thereafter cooling took place at a rate not exceeding 5 C/min.

It was found that the visor plate had reformed to the required shape, and with an overall superior appearance compared with previously produced plates. This is believed to be because the visor plate was only on the chrome-iron former for heat-treatment at the crystallisation temperature, whereas in the known continuous heat-treatment, the visor plate was in contact with its chrome iron former for a longer period of time, including both the nucleation and crystallisation stages. Small blemishes can be

deposited on the contact & ce of the glass-ceramic plate due to oxidation of the former FM A & % 4 % JAI L I & LaCt A LIJL 61 z,-% lux all" when it is held at the heat-treatment temperatures for lengthy periods.

EXAMPLE 6 Microstructure of TRANSARM with time-separated heat-treatments Figure 4 shows the microstructure of TRANSARM heat-treated to a schedule comprising nucleation at 550 C for 168 hours followed immediately by crystallisation at 7500C for two hours. Figure 5 shows the microstructure of TRANSARM heattreated in a two-stage schedule in which the nucleation and crystallisation stages were at the same temperatures as mentioned above, but with cooling to room temperature interposed between the two stages so that they were time-separated. Furthermore, the dwell time at the nucleation temperature wac redd to 100 hours.

Comparison of the microstructures of Figures 4 and 5 shows that there is no substantial difference in TRANSARMTM produced by the two routes with respect to optical transparency and physical appearance. The crystallite size and packing density, though not identical, are similar and it is believed that the time-separated heat-treatment stages and the shorter nucleation dwell time have no substantial deleterious effect on the important properties of the material.

As an example of the advantages obtainable by the production of TRANSARMmM in two time-separated heat-treatments, take the manufacture of curved sheets of TRANSARM, such as visors or windscreens/windshields. The nucleation could be achieved on flat section glass with subsequent reforming to the required curved shape performed in a separate crystallisation heat-treatment, thereby facilitating an increase in the rate of manufacture of such articles and lowering costs.

As discussed in our prior patent GB 2284 655B, it has been found possible to increase the static strength of such transparent glass-ceramic materials significantly by surface ion-exchange, two benefits of which are improved handling characteristics and resistance to accidental damage.

Patent no. GB 2 284 655B discloses that such ion exchange strengthening can be carried out on the glass-ceramic in its final shape. In this operation, the glass-ceramic article is immersed in a bath of molten salts of appropriate composition at a temperature below the crystallization temperature of the material for a time which is sufficient to allow exchange of various ions in the surface of the article with ions in the salt bath. The article is withdrawn from the molten salt bath, cooled to a suitable temperature and washed to remove residual salt deposits.

Claims

CLAIMS 1 A process for the heat-treatment of a vitreous material to obtain a transparent glassceramic, including the steps of (a) performing a nucleation heat treatment step by heating the vitreous material to a nucleation temperature in the range 520 C to 580 C and maintaining said nucleation temperature for a time period in the range 10 to 170 hours; (b) performing a crystallization heat treatment step by further heating the vitreous material to a crystallization temperature and maintaining the

crystallization temperature for a time sufficient to achieve a desired degree of crystallization ; and vrt.tTtt. tte materit to rMT. tmTfainf of a < *ate such as to avoid differential temperature damage.
2. A process according to claim 1, in which the nucleation temperature is in the range 530 C-570 C and is maintained for a period in the range 50-170 hours.
3. A process according to claim 2, in which the nucleation temperature is approximately 550 C and is maintained for a period of approximately 100 hours.
4. A process according to any preceding claim, in which the vitreous material is heated to the nucleation temperature at a rate of about 5 C per minute.
5. A process according to any preceding claim, in which the crystallization temperature is in the range 7100C to 770 C and the crystallization temperature is maintained for a time period in the range of about 15 minutes to two hours.
6. A process according to claim 5, in which the crystallization temperature is approximately 750 C.

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7. A process according to any preceding claim, in which the vitreous material is heated to the crystallisation temperature at a rate of about 3 C per minute.
8. A process for the heat-treatment of a vitreous material to obtain a transparent glassceramic, including the steps of : (a) performing a nucleation heat treatment step on the vitreous material to produce a nucleus population density in excess of 1020 per cubic metre; (b) performing a crystallization heat treatment step by heating the vitreous material to a crystallization temperature is in the range 710 C to 770 C and maintaining the crystallization temperature for a time period in the range of about 15 minutes to two hours; and (c) cooling the material to room temperature at a rate such as to avoid differential temperature damage.
9. A process according to claim 8, in which the crystallization temperature is approximately 750 C.
10. A process according to claim 8 or claim 9, in which the vitreous material is heated to the crystallisation temperature at a rate of about 3 C per minute.
11. A process according to any one of claims 8 to 10, in which the nucleation temperature is in the range 520 C to 580 C and is maintained for a time period in the range 10 to 170 hours
12. A process according to claim 11, in which the nucleation temperature is in the range 530 C-570 C and is maintained for a period in the range 50-170 hours.
13. A process according to claim 12, in which the nucleation temperature is approximately 550 C and is maintained for a period of approximately 100 hours.

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14. A process according to any one of claims 8 to 13, in which the vitreous material is heated to the nucleation temperature at a rate of about 5 C per minute.
15. A process according to any preceding claim, in which after the nucleation step, the vitreous material is cooled to room temperature at a rate such as to avoid differential temperature damage and is then stored and/or transported before being reheated to the crystallization temperature.
16. A process according to any preceding claim, in which the vitreous material is formed to a desired shape during the crystallization step.
1 1 ï. A process for the manufacture of a transparent glass-ceramic article in sheet form, including the sequential steps of : (e) heat treating a precursor sheet of vitreous material in a nucleation heat treatment to produce a nucleus population density in excess of 1020 per cubic metre; (f) cooling the nucleated sheet to room temperature at a rate such as to avoid differential temperature damage; (g) at a later time heat treating the nucleated sheet in a crystallization heat treatment to produce crystal sizes less than one half of the wavelength of visible light, while simultaneously forming the sheet to a final shape of the glass-ceramic article; and (h) cooling the glass-ceramic art1cle tn rnom temperature at a rate such as to avoid differential temperature damage.
18. A process according to claim 17, in which the nucleation heat treatment is adapted to produce a nucleus population density in excess of 1021 per cubic metre and the crystallization heat treatment produces crystal sizes of the order of one tenth of the wavelength of visible light.

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19. A process according to claim 17 or claim 18, in which the nucleation heat treatment is carried out at a nucleation temperature in the range 520 C to 580 C and the nucleation temperature is maintained for a time period in the range 10 to 170 hours.
20. A process according to claim 19, in which the nucleation temperature is in the range 530 C-570 C and is maintained for a period in the range 50-170 hours.
21. A process according to claim 20, in which the nucleation temperature is approximately 550 C and is maintained for a period of approximately 100 hours.
22. A process according to any one of claims 17 to 21, in which the crystallization heat treatment is carried out at a crystallization temperature in the range 710 C to 770 C and the crystallization temperature is maintained for a time period in the range of about 15 minutes to two hours.
23. A process according to claim 22, in which the crystallization temperature is approximately 750 C.
24. A process according to any one of claims 17 to 23, in which during the crystallization heat treatment, the sheet is formed to a final shape on a stainless metallic former.
25. A process according tn claim 24, in which the metallic former comprisses iron.
26. A process according to any preceding claim, in which the vitreous material comprises a lithium disilicate based material, including silica as a majority constituent, lithium oxide as a next major constituent and nucleating agents comprising zirconium dioxide in major part and phosphorus pentoxide in minor part.

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27. A process according to claim 26, in which the vitreous material comprises from 67 to 75 wt. % silica, from 9 to 14wt. % lithium oxide, from 6 to 12 wt. % zirconium dioxide as a nucleating agent, from 1. 0 to 3. 0 wt. % phosphorus pentoxide as a nucleating agent, and other oxides to total 100%.
28 A process according to claim 27, in which the vitreous material comprises about 71. 8 wt. % silica, about 4. 5 wt. % alumina, about 2 wt. % phosphorus pentoxide, about 11 wt. % lithium oxide, about 0. 5 wt. % zinc oxide, about 8 wt. % zirconium dioxide and about 2. 2 wt. % potassium oxide.
29. A process according to any preceding claim, in which the vitreous material is cooled to room temperature at a rate not exceeding about 5 C per minute.
30. A process for the heat-treatment of a vitreous material, substantially as hereinbefore described.
31. A process for the manufacture of a transparent glass-ceramic article in sheet form, substantially as hereinbefore.
32. A transparent glass-ceramic article produced by a process according to any preceding claim.