I have seen lanterns running red all night. What would cause this?Low pressure sodium lamps have always been at the more interesting end of the spectrum (excuse the pun) and in some respects could be regarded as living chemistry demonstration. Their method of operation is simple and the resulting electrical and physical changes as the lamp warms up can be viewed perfectly over several minutes, until the lamp has reached full output.
The history and development of the low pressure lamp has been recorded elsewhere including some excellent websites, so I needn't duplicate it here. Others may be able to explain this better than me in any case!
However, your observation of some lanterns running red all night is valid one and is something everyone here will have come across too.
Without waffling too much, the reason for the red glow is down to one thing - a loss of vacuum. If you are fortunate enough to check out a SOX lamp, it comes in two main sections - a sealed glass "U" tube which contains the metallic sodium, some neon and argon gas and two electrodes which are connected to the lamp cap. There is also a larger outer glass vacuum bulb or "envelope" which is coated on the inside surface with an infra-red reflective coating to provide thermal insulation. Even without the envelope, the bare lamp could be plugged into the lantern and it would glow the characteristic red colour you have described. However, it would struggle to go much further than this because without the outer envelope and its vacuum, the heat from the discharge would be lost and the sodium wouldn't get hot enough for the lamp to run up properly (as I discovered to my cost after dropping my first ever 35W lamp).
When a SOX lantern is exhibiting the behaviour you describe, it will be found that the vacuum in the outer envelope will have been lost, usually down to a failure of the main seal or an actual crack in the glass envelope. The only option is to replace the lamp.
Whilst a marvel of technology, the vulnerable vacuum of a SOX lamp can be its Achillies heel, as it can be compromised by vibration, impacts and even the quality of the lamp itself. Interestingly, the loss of vacuum won't turn a lamp red straightaway, unless its loss is very dramatic. You will often see early signs of this condition in that the lamp won't quite run up to full or you'll see a slight red tint in one area of the lamp. Over time, the condition gets worse. If you look at a unlit SOX lamp which has lost its vacuum, you will generally see a characteristic white / grey clouding on the inside of the envelope usually near to the lamp cap.