We prefer to do nothing but conserve our puzzles; however occasionally more than conservation is required. We never recommend restoration by amateurs and it is something you undertake at your own risk.
Many old water-filled dexterity puzzles develop bubbles of air which can completely spoil their appearance and playability. The problem is that the plastic allows water vapour to pass through; so they gradually lose the water over a long time. If we need to replace the water we drill a 1 mm hole at an unobtrusive point where it will be possible for all the air bubble to escape, then we use a syringe to inject previously boiled (but now cold) distilled water. We block the hole with a plug of transparent silicon bathroom sealant; then if in a few years time another bubble forms, the silicon sealant can be pulled out without having to drill a hole in a new place.
It is important not to use tap-water or algae will grow inside the puzzle. If the puzzle is going to be exposed to the light continuously it may be necessary to be more aggressive in the prevention of algae.
Some "Aquabatics" were filled with an oil and do not suffer so badly; however even they lose the liquid over a longer time. We believe the oil was basically unscented "baby oil" but we are not certain.
NEVER be tempted to polish acrylic plastics with methylated spirit or similar solvents if the plastic has been in touch with water for a long time. The water will have produced minute invisible faults within the plastic and the solvent will instantly find them and within seconds your clear plastic will be filled with fracture lines.
Some very amusing puzzles have been produced using mercury as the moving object. Mercury is now recognised as a poison and as such we can only advise that you have nothing to do with it.
The Puzzle Museum has some fine puzzles containing mercury which were made between 1890 and 1990. Our senior curator has had much fun playing with mercury puzzles, which explains why he has gone bald, those of his teeth that have not dropped out have gone black, and he is also becoming increasingly eccentric.
Mercury if spilt is nearly impossible to pick up. It falls through carpets and between cracks in floor-boards and slowly evaporates giving off poisonous vapours which you, and future occupants of the room, will inhale without realising it. It is a cumulative poison, and over a long period the mercury in your body will build up and you and your family will be able to enjoy black teeth, madness, and other worse disabilities. So keep cracked and damaged puzzles in a sealed container which will catch the falling mercury and limit any evaporation. If a puzzle is badly damaged consider removing all the mercury (If asked nicely, your local pharmacy or chemist shop will probably dispose of mercury for you).
Some well made and well sealed puzzles have retained all their mercury for nearly 100 years. If you are considering "topping-up" a mercury puzzle, consider how, why, and from where it has lost its original mercury; then think if you are just going to spread more around.
How are you going to get the mercury in?
How are you going to seal it in?
Will your seal last for 100 years?
Why do you want to do it anyway?
Do you want your teeth to drop out?
Do you want to go mad?
Are you, like our youngest conservator, already nearly completely mad?