GEC 26W SOX-E Super SOX Low pressure sodium lamp

this is a lamp I got a while back but never got round to uploading pictures of it

I finally decided to so, not only because its a GEC SOX-E lamp which is quite rare (so I was very pleased to have gotten it!, and its also my first GEC sox lamp :) I have always liked the construction of GEC SOX lamps so was pleased to tick that box as well)

but mainly because at the time I fired it up I noticed it exhibited an usual phenomena where for the first id say few minutes, there was a rather extreme pretty much incandescent hot spot on one of the cathodes

which continuously sputtered some emissive material into the discharge (hence the blue glow) and as a ring of material on the arc tube wall, eventually it settled down it was quite strange! normally you only get a brief flash of blue as the cold hard start of a SOX lamp sputters some emissive material from the cathode into the discharge, but this one was constant and unchanging until it eventually died down, like someone had frozen in time the first start of a SOX lamp :)

and so im quite curious to know what the cause of this was?

its interesting to note that James had a GEC 60W SLI/H lamp also exhibit a similar phenomena which can be seen in his Gallery