[Tom_Ryan]

Charles,

Glass frit (you spelled it correctly) is essentially an extremely fine ground glass that flows into the microstructure of any adjacent surface (metal or glass). The frit is usually a lower temperature glass that has the ability to flow so that coalescence occurs during heating and bonds chemically to the adjacent surfaces. Temperatures as high as 500 degF (371 degC) not only helps the frit to flow but allows for rapid outgassing of the tube (mainly drive off water vapor - which sticks like hell to everything!). During manufacture high temperatures also help to anneal the glass structure. However, it could warp or buckle internal metal parts. As I recall, excess sealant exists during manufacture, so that during high temperature baking the sealant does not all pump out of the joint. However, reheating an old 15GP22 to extremely high temps would not be necessary since the original structure is already outgassed and frit seal created. If only the electron gun is replaced then outgassing may be accomplished at a lower temp, 392 degF (200 degC), within the limits even of any externally applied sealants, e.g. Varian 'Torr Seal'. The trick is to pump the tube for a long time!

'73
Tom Ryan