Patent ID: 8028544

Claim:
An isopipe for making a glass or a glass-ceramic comprising a body for use in a fusion process, said body comprising a silicon nitride refractory material that: (a) contacts the glass or glass-ceramic in a molten state during use of the isopipe, and (b) has a flexural creep strain rate at 1250° C. and 1000 psi that is less than 1×10 −6 /hour, wherein: (i) said body comprises inwardly sloping sides which meet at a root; (ii) the silicon nitride refractory material is produced in block form in an atmosphere having a pO 2 of less than 0.1 using less than 10 weight percent of one or more sintering aids; (iii) the block form of silicon nitride refractory material is machined into the body; and (iv) the machined block of silicon nitride refractory material is treated by exposure to a partial pressure of oxygen equal to or greater than 0.1 for a period of time and at a temperature sufficient to form a SiO 2 layer which exhibits substantially only a passive oxidation mechanism, said SiO 2 layer serving as a protective barrier for further oxidation of the silicon nitride during use of the isopipe.