About 75KV. Like an xray machine! Also, 2mA, which is quite hot.
Here are a couple pages. Did better this time on the size.
The bleaching was referred to as being the 'repair' or thermal heating process by which the darkening was removed.
What causes darkening in this tube? good question.
Can x-ray or electron beam heating-related darkening of glass be reversed by raising its temperature but not so high as to destroy the metallic backing or phosphor adherence to the tube face? The tube is air cooled by a blower.
Can heating of the P4 phosphor well above its normal operating temperature somehow 'reflow' it or realign the crystalline structure of it to remove a darkened scan pattern?
I sure don't know those answers or what causes the darkening. I guess I wanted to assume 'burn in' as we know it from being caused by stationary patterns on TV sets.
The skiatron, or dark trace tube, is erased by IR and heat. it's not a P4 family of phosphors, but P10, maybe potassium chloride, making a dark trace on white.
One of those is the 4AP10. There is much info about it in
volume 22 of the MIT Radiation Laboratory series.
Makes me wonder if some 'regular' phosphors can be renewed by the method described for the 7WP4.