Sooner or later most honey goes from clear and runny to thick and pale. It hasn't gone off. This is called setting, or crystallising, and it happens to almost all real honey.
What's going on
Honey is mostly two sugars, glucose and fructose, dissolved in a small amount of water. There's far more sugar than the water can comfortably hold, so the honey is supersaturated. Over time the glucose comes out of the liquid as tiny crystals, just like salt forming in a drying rock pool. Fructose stays dissolved much more easily.
So the more glucose a honey has, the faster it sets:
- Oilseed rape honey is very high in glucose and can set within days, even in the comb. That's why spring honey from rape country is often sold soft-set.
- Ivy honey sets hard, sometimes too hard for the bees to use in winter without water.
- Lime and clover honey stays runny for longer.
- Heather honey is different again: it's jelly-like (thixotropic) until stirred, and is usually pressed out of the comb rather than spun.
Cool temperatures, around the temperature of a cold larder, speed setting up. So do bits of pollen and wax that crystals can start growing on.
Set honey is fine
Set honey tastes the same and keeps just as well. Many people prefer it on toast because it doesn't drip. If crystals form slowly they can be large and a little gritty. Beekeepers make smooth soft-set honey by stirring in a little fine-grained honey as a "seed".
Making it runny again
Stand the jar, lid loosened, in a bowl of warm water, around bath temperature, and leave it for an hour or two, stirring now and then. Gentle warmth is the key. Very hot water, or a microwave, can spoil the flavour and darken the honey. It will set again eventually, and you can warm it again.
Keep honey in a cool, dry cupboard with the lid on. Honey is unsuitable for infants under 12 months.