How do low mercury fluorescent become mercury deprived ?

Does it get soaked in the phosphor or someting ? combines with something ?

Can they be restored by heating (in oven etc) or other way to get the mercury back ?

During lamp operation, mercury slowly absobed into the phosphors, electrodes and glass.
Low mercury fluorescents have usually not enough mercury to last the life of the lamps, and so the lamp goes mercury starved before the electrodes becomes depleted emitter.

Sometimes you can cook it out with a neon sign transformer, but the very old Sylvania Daylight lamp I have only improved marginally after doing this several times. It will get VERY hot.  Not sure about heating in an oven..might be an interesting experiment.

How do low mercury fluorescent become mercury deprived ?

Does it get soaked in the phosphor or someting ? combines with something ?

Can they be restored by heating (in oven etc) or other way to get the mercury back ?

Yep, they bind to glass and phosphor coating and exhibit symptoms when the mercury vapor pressure inside the lamp drops below saturated vapor pressure of liquid mercury at room temperature.  Once it binds, its semi-permanent to permanent.

Semi-permanent binding will cause the lamp to start pink/dim in the middle and take many minutes for linear fluorescent lamps to warm up. The cycle starts all over once the lamps cool down. 

Permanent binding will cause the lamps to never warm up even when they reach normal working temperature. 

Big part of low mercury lamp development is the coating that goes with phosphor blend to inhibit mercury from getting absorbed by it.

Permanent binding ususally means the mercury has reacted with something and will stay that way. It will most likely form some kind of mercuric salt with parts of the phosphor coating. Elemental mercury (Hg) will oxidize to form mercury (II) ions (Hg2+). Heating the lamp might reverse the chemical reaction (which would reduce the Hg2+ into elemental mercury, Hg), however, it might take more than heating. Other methods would require opening the lamp and isolating the mercury, which is obviously undesirable in this situation, not to mention very dangerous. For the chemical process to be successful, you would need to crush a large number of tubes. That is part of what happens at a lamp recycling facility. Mercury (II) ions will not behave chemically the same as elemental mercury and are worthless in producing light in the lamp.

P.S. : I would guess that the temperature you would need to try to reverse the reaction would be about 350 degrees Celcius, which is about 660 degrees Fahrenheit. It would be quite dangerous, however.

P.S. : I would guess that the temperature you would need to try to reverse the reaction would be about 350 degrees Celcius, which is about 660 degrees Fahrenheit. It would be quite dangerous, however.

And possibly smash the glass, I think.

Best way I've found to brighten them up is to use a hair dryer on max after the lamp is running! I tried it once on a 34 watt ALTO (I love torturing them more then fixing them!  ) it got a little brighter from that, but not much!