which can be used include cardboard suitably sized or coated for example with baryte cellulose acetate propionate cellulose acetates cellulose propionate cellulose acetate butyrate poly vinyl chloride or vinyl acetate polyamide metal sheeting for example aluminum and poly ethylene terephthalate the latter being a preferred support the support may have for example a thickness of about 0 00025 inch 0 00064 cm to about 0 30 inch 0 76 cm with films having a thickness up to about 0 02 inch 0 05 cm being preferred without further elaboration it is believed that one skilled in the art can using the preceding description utilize the present invention to its fullest extent the following preferred specific embodiments are therefore to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever all fired samples prepared in the examples were fired in fused silica boats in a belt furnace b t u inc mode tlh 22 32 4n12y north billerica mass having three heating zones with an effective heating length of 24 inches 60 cm the nitrogen flow or 2 oxygen containing nitrogen flow through the furnace during the firing steps was maintained at between 20 liter minute and 25 liter minute to minimize possible contamination with silica all of the apparatus used to prepare and mix the bafbr slurries was made of or coated with teflon u00ae fluoropolymer in the foregoing and in the following examples all temperatures are set forth uncorrected in degrees celsius and unless otherwise indicated all parts and percentages are.