Process for producing alkali-free aluminosilicate glass

1. A process of producing aluminosilicate glass containing more than 13.5% by weight, based on oxide content, of aluminum oxide and free of alkali metals, apart from inevitable impurities, said process comprising the steps of:

a) formulating a batch of raw materials;

b) adding at least one fining agent to said batch;

c) melting the batch to form molten glass, said melting including fining and homogenizing; and

d) after the melting, hot-forming the molten glass formed from the batch;

wherein the batch contains from 0.005% by weight to 0.6% by weight of at least one sulfate, based on SO 3 , but wherein the batch does not contain any evaporating fining agents; and

whereby said aluminosilicate glass is formed containing more than 13.5% by weight, based on oxide content, of said aluminum oxide and free of said alkali metals, apart from said inevitable impurities.

2. The process as defined in claim 1 , wherein the batch contains from 0.03% by weight to 0.2% by weight of said at least one sulfate, based on SO 3 .

3. The process as defined in claim 1 , wherein the batch contains from 0.015 to 1% by of Fe 2 O 3 .

4. The process as defined in claim 1 , wherein the batch contains from 0.03 to 0.45% by weight of Fe 2 O 3 .

5. The process as defined in claim 1 , wherein the batch has a composition comprising, in % by weight, based on oxide content:

SiO 2

 46–70,

Al 2 O 3

>13.5–27,

B 2 O 3

 0–15,

MgO

 0–10,

CaO

 0–14,

SrO

 0–15,

BaO

 0–25,

with MgO + CaO + SrO + BaO

≧8,

ZnO

 0–10,

ZrO 2

 0–5,

TiO 2

 0–3,

P 2 O 5

 0–9.

6. The process as defined in claim 1 , wherein said batch contains at least 0.5% by weight of said B 2 O 3 .

7. The process as defined in claim 6 , wherein said batch contains at most 20% by weight of said MgO+CaO+SrO+BaO.

8. The process as defined in claim 1 , wherein the batch has a composition comprising, in % by weight, based on oxide content:

SiO 2

 46–64,

Al 2 O 3

>13.5–26,

B 2 O 3

 0–5.5,

MgO

 0–7,

CaO

 3–14,

SrO

 0–11,

BaO

 6–25,

with MgO + CaO + SrO + BaO

≧8,

ZrO 2

 0–5,

TiO 2

 0–0.5,

P 2 O 5

 0–9.

9. The process as defined in claim 8 , wherein said batch contains, in % by weight, based on oxide content, >65–64 of said SiO 2 , >13.5–18 of said Al 2 O 3 , 5–14 of said CaO, 0–8 of said SrO, 6–17 of said BaO, 0–2 of said ZrO 2 .

10. The process as defined in claim 9 , wherein said batch contains, in % by weight, based on oxide content, 59–62 of said SiO 2 , >13.5–15.5 of said Al 2 O 3 , 3–5.5 of said B 2 O 3 , 2.5–5 of said MgO, 8.2–10.5 of said CaO, 8.5–9.5 of said BaO, 0–1.5 of said ZrO 2 .

11. The process as defined in claim 9 , wherein said batch contains, in % by weight, based on oxide content, >58–62 of said SiO 2 , 14–17.5 of said Al 2 O 3 , 0–1 of said B 2 O 3 , 5.5–14 of said CaO, 0–8 of said SrO, 6–17 of said SaO and 0–1.5 of said ZrO 2 .

12. The process as defined in claim 11 , wherein said batch contains, in % by weight, based on oxide content, 15–17.5 of said Al 2 O 3 , 0.2–0.7 of said B 2 O 3 , 0<1 of said MgO, 6–10 of said BaO and 0.05–1 of said ZrO 2 .

13. The process as defined in claim 8 , wherein said batch contains, in % by weight, based on oxide content, 46–63 of said SiO 2 , >13.5–25 of said Al 2 O 3 , 0–5 of said MgO, 6–15 of said BaO, 0.1–5 of said ZrO 2 , 0.1–9 of said P 2 O 5 and wherein MgO+CaO+SrO+BaO ≦25 and SrO+BaO ≧10.

14. The process as defined in claim 1 , wherein said batch contains from 0 to 2 percent by weight, based on oxide content, of each of Sb 2 O 3, CeO 2, MoO 3, WO 3 and V 2 O 5 ; from 0 to 1 percent by weight, based on oxide content, of each of As 2 O 3 and SnO 2, and wherein Sb 2 O 3 +As 2 O 3 +CeO 2 +SnO 2 +MoO 3 +WO 3 +V 2 O 5 is contained in said batch in an amount between 0 and 3% by weight, based on oxide content.

15. The process as defined in claim 1 , wherein said at least one sulfate is selected from the group consisting of CaSO 4, ZnSO 4, MgSO 4, Na 2 SO 4 and SrSO 4 .

16. The process as defined in claim 1 , wherein said batch contains BaSO 4 .