Low biopersistence inorganic fiber free of crystalline silica

A method for making the inorganic fiber comprising:

• combining raw material ingredients to form a molten melt, the raw material ingredients comprising
• i) 15 to 50 mol percent silica,
• (ii) 10 to 28 mol percent alumina,
• (iii) 16.7 to 35 mol percent of one alkali metal oxide, and
• (iv) 15 to 35 mol percent of:
• (a) at least one transition metal oxide;
• (b) at least one lanthanide series metal oxide;
• (c) a combination of at least one transition metal oxide and at least one lanthanide series metal oxide;
• and
• producing fibers from the molten melt;
• wherein the amount of silica+alumina+total alkali metal oxides is 80 mol percent or less;
• wherein a molar ratio of alkali metal oxide to the alumina is from 1.25 to 2;
• wherein the amount of alumina+total alkali metal oxides is 30 mol percent or greater;
• and
• wherein the inorganic fiber exhibits a linear shrinkage after exposure to 1260° C. for 24 hours of 5% or less.