A permeable nozzle is made from MgO and MgO—Al2O3 spinel and has a nonservice backpressure of about 0.5-50 psi at gas flowrates of about 5-20 liters/minute. In one embodiment, the permeable nozzle comprises about 50%-80% of about a 20-48×100 mesh MgO, about 5%-20% of about a 325 mesh MgO and about 5%-40% of about a 48×200 mesh MgO—Al2O3 spinel. In another embodiment, the permeable nozzle comprises about 10%-20% of about a 20-48×100 mesh MgO, about 0%-10% of about a 325 mesh MgO, about 30%-60% of about a 28×48 mesh MgO—Al2O3 spinel, about 10%-40% of about a 48×200 mesh MgO—Al2O3 spinel, and about 10%-20% of about a 325 mesh MgO—Al2O3 spinel.