"Why is my caramel crystallising?"
Lisa Featherby, Gourmet Traveller Senior Food Editor, writes:
The usual cause of crystallisation in caramel is impurities in the sugar or, more commonly, less-than-clean pans, so make sure your equipment is clean. Caramel can be made from superfine, fine or granulated sugar. For caramel made without water, superfine is best because it will melt quickly and evenly, but shouldn't be stirred.
This can be tricky, though, because making caramel without water increases the chances of caramel burning faster than expected. If you're making it with water, the sugar needs to dissolve evenly and this can be helped along by stirring the syrup while bringing it to the boil.
Once it's boiling, however, it's important not to stir at all. Brush down any crystals that collect on the sides of the pan with a clean, wet pastry brush or they'll crystallise other sugar particles in the syrup, and once this happens it's all over. You can salvage it with a little vinegar, but that's unlikely to be the flavour you're after: best thing is to cop it sweet and start over.