A jar candle is not simply a candle in a jar. Container wax is formulated to be softer than pillar wax, because it never has to hold its own shape — it only has to cling to glass without shrinking away from it. Pour pillar wax into a jar and it contracts as it cools and pulls away from the sides.
Soy and paraffin, without the marketing
Soy burns cooler and slower, holds less fragrance per gram, and is more prone to frosting — those pale crystalline patches on the surface, which are a natural property of vegetable wax and not a defect. Paraffin throws scent harder and gives a brighter flame, and it is a petroleum by-product, which is the honest reason people avoid it rather than any established health difference at domestic exposures.
Blends exist because each wax fixes the other's weakness. A shop telling you one is simply better than the other is skipping the trade-off.
Tunnelling is set on the first burn
Wax has a memory. The melt pool on the first lighting sets the diameter for the life of the candle, so a jar candle blown out after twenty minutes will burn a narrow well down the middle and leave a ring of wax stuck to the glass. In a wide jar that can be half the candle.
So the first burn is the one that matters: leave it until the pool reaches the glass all the way round, which in a standard jar is one to two hours. If it has already tunnelled, wrapping foil round the rim to reflect heat inward will sometimes recover it. Sometimes it will not.
Signs it is done
Stop when there is about 10mm of wax left in the base. Below that the flame is heating the glass directly rather than the wax, and a jar that cracks does so at the bottom, usually while you are in another room.
Getting the jar back
Kilner and similar jars are worth keeping. The freezer method is the least messy: put the finished jar in for an hour and the wax plug shrinks and lifts out, often whole. What is left comes away with hot soapy water. Pouring boiling water directly into a cold glass jar will sometimes crack it, so warm it first or do not bother.
The metal wick tab is glued down; a bit of leverage with the back of a spoon takes it off.