Andy,

According to Peter Keller, in his book "The Cathode Ray Tube", the ceramic-like material you mentioned is used to make glass-to-glass seals. It is called "frit" or solder glass seal. It was developed by Corning, and was a major advance in color tube manufacture, making possible the 21CYP22 in 1957. It eliminated the need for a metal flange in color picture tubes.

When a metal flange is used, separate halves of the flange are sealed to each piece of glass with a glass-to-metal seal. Then, the two metal halves are welded together.