Method for making porous acicular mullite bodies

A process for producing an acicular mullite body or bodies, comprising:

• (a) heating at least one green body to a temperature of 650° C. to 800° C. in an enclosed region of a furnace, in the presence of a process gas containing a fluorine-containing compound flowing through the enclosed region of the furnace, such that a source of aluminum and a source of silicon in the green body react with the fluorine-containing compound to form fluorotopaz, wherein the furnace has a volume of the furnace defined by an exterior shell and the enclosed region of the furnace is defined by an enclosure, wherein the enclosure is spaced from the exterior shell and the enclosed region of the furnace constitutes 20 to 95% of the volume of the furnace;
• and then
• (b) heating the body to a temperature above 800° C. such that the fluorotopaz decomposes to form a porous acicular mullite body,
• wherein, in step (a), the process gas is introduced into the enclosed region through multiple inlets distributed over at least one side of the enclosed region and the process gas is withdrawn from the enclosed region through multiple outlets distributed over at least one other side of the enclosed region to establish a flow rate of the process gas through the enclosed region from 0.25 to 2 times the volume of the enclosed area of the furnace per minute.