Alkali-free glass compositions having high thermal and chemical stability

1. A glass comprising:

67≦SiO 2 ≦70

11≦Al 2 O 3 ≦13.5

3≦B 2 O 3 ≦6

3.5≦MgO≦7

4≦CaO≦7

1≦SrO≦4

0.5≦BaO≦3

0.02≦SnO 2 ≦0.3

0≦CeO 2 ≦0.3

0.00≦As 2 O 3 ≦0.5

0.00≦Sb 2 O 3 ≦0.5

0.01≦Fe 2 O 3 ≦0.08

F+Cl+Br≦0.4

wherein all oxides are in mol % and

1.05≦(MgO+BaO+CaO+SrO)/Al 2 O 3 ≦1.25

0.7≦(CaO+SrO+BaO)/Al 2 O 3 ≦0.9

0.3≦MgO/(CaO+SrO+BaO)≦0.6

where Al 2 O 3 , MgO, CaO, SrO and BaO represent the mol percents of the representative oxide components.

2. The glass of claim 1 wherein Beta-OH is less than about 0.55.

3. A sheet of the glass of claim 1 made using a down-draw process.

4. A sheet of the glass of claim 3 wherein the down draw process is the fusion process.

5. A process of making the glass of claim 1 wherein the oxidation state of the batch materials used to make the glass is manipulated via nitrate additions to achieve an oxidation state such that 20 consecutive Gen 4 sheets are produced using a down draw process having gaseous inclusion levels of 0.2 defects per pound or less.

6. The glass of claim 1 wherein the glass is essentially free of alkali metal oxides.

7. The glass of claim 1 wherein the glass exhibits an anneal point of between 750° C. and 770° C.

8. A glass according to claim 1 wherein the glass comprises in mol percent on an oxide basis 68-70 SiO 2 , 12-13 Al 2 O 3 , 3-5 B 2 O 3 , 4-6 MgO, 1 -2.5 BaO, and 5-6 CaO.

9. The glass of claim 1 wherein the glass exhibits a Young's modulus of from 10.8-12.1 Mpsi.

10. The glass of claim 1 wherein the glass exhibits a liquidus viscosity of greater than 100 kpoise.

11. The glass of claim 1 wherein the glass exhibits a density of less than 2.6 g/cc.

12. The glass of claim 1 wherein the glass exhibits a temperature corresponding to a 35 kpoise viscosity of less than 1275° C.

13. The glass of claim 12 wherein the glass exhibits a temperature corresponding to a 35 kpoise viscosity of less than 1250° C.