sub 2 for sio sub 2 465 479 yes26934 151 5 a 2 5 cds for cdf sub 2 448 461 yes27934 151 6 a 1 tio sub 2 for sio sub 2 445 458 yes281002 55 5 b 3 cdcl sub 2 for cdf sub 2 440 460 yes29______________________________________ a chemical analysis was conducted on base glass b the batch therefor was melted for 30 minutes at 1000 u00b0 c in a platinum crucible ______________________________________element batched weight melted weight ______________________________________si 6 9 5 3al 3 3 3 6cd 26 7 28 9pb 28 9 30 6y 2 9 3 3zn 2 7 3 0f 17 8 12 8______________________________________ those data suggest that essentially only si and f were lost during melting inasmuch as the f si ratio loss approximates 4 it is believed logical to conclude that sif 4 comprises the principal volatilization product under the present melting conditions the great resistance of the inventive glasses to devitrification permits the production of large preforms thus crystallization at the interface between the core and cladding during the production of the preform is avoided in the most commonly used methods in preparing an optical fiber waveguide that attribute led to investigations to discover cladding glasses exhibiting lower refractive indices than the inventive glass ceramics but demonstrating linear coefficients of thermal expansion transition temperatures and viscosity characteristics compatible with the inventive precursor glasses and glass ceramics so as to permit the fabrication of optical fiber waveguides we have found a family of glasses in the k 2 o pbo b 2 o 3 sio 2 system which exhibit the necessary properties thus they.