I've no idea where the goo is coming from but the fact that its green to me indicates contact with copper conductors. On a 50 yr old property thats never been rewired sounds like it needs some urgent attention. I'd get it checked out by an electrician who's a member of one of the competent persons schemes pretty quick.
I have heard of plasticiser leaching out of old cables before. It will in the end result in the cables insulation becoming brittle and if moved cracking. But my understanding is if not disturbed it does not cause too much of a problem.
Although if it were in my house I think I would want to renew cables before next time it was decorated.
I have also seen this before when working in victorian terrace houses and i decided it was caused by penetrating or rising damp or condensation reacting with and oxidising the copper cable conductors in the switch back box, especially in cold weather such as this. If the cover fixing screws are of the brass type these also can show green. The steel screws fastening the box to the brick may also be very rusty and even rusted through.
Hope this helps...[/b]
I found this but I will warn I can't verify the information it may be wrong.
Green exudate from PVC
Draft BCA statement (April 2001)
PVC comes in two main grades, plasticised and unplasticised PVC.
Unplasticised PVC (UPVC) is used for example in double glazing window frames where a rigid material is required.
The PVC used for manufacture of cables is a plasticised PVC that conforms to the relevant British Standard for the cable type in question.
2) Ageing effects
As a cable ages (at temperatures above normal ambient) the elongation to break decreases (also the Insulation Resistance increases). The life expectancy of a cable is arbitrarily considered to be when the elongation to break of the PVC is 50%.
A lower elongation to break value could be considered suitable especially for a fixed wiring cable. Therefore, providing the cable is not subject to movement or when moved due to inspection of socket outlet or the like, the PVC does not crack, a much lower value of elongation to break is considered by some as suitable.
Greening is the appearance of a wet green substance that is a product of an adverse reaction between certain types of plasticiser and the copper conductors. This greening, which is a rare occurrence, can happen either after a long period of time for some cables, or if the cable has been severely overheated.
The plasticiser itself is a clear oily liquid that is non conductive. The green substance is a combination of copper oxide and plasticiser which may become conductive under certain adverse conditions.
4) Action if Greening is found
Therefore whenever this green substance is found at socket outlets etc. initially it should be removed and the terminations cleaned (gloves should be used) otherwise it is possible that tracking/overheating may occur. It is strongly recommended that rewiring should be carried out as soon as possible.
If further information is required, the original cable manufacturer should be contacted.
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