So, those of us who have roasted a piece of pork have eventually run into the problem of crackling. What you're aiming for are thin, crunchy strips of pork skin with a sticky underside, in which the skin has bubbled up at some point so that the crackling itself has an aerated and frangible texture, making it possible to eat it without the need of major dental reconstruction afterwards. Without crackling, roast pork runs the risk of being bland and unexciting. (The French typically pot-roast their pork with aromatics and vegetables and so do without crackling altogether, and although I think that the French can do very little wrong in the food line, their low opinion of the possibilities of crackling does them little credit.)
However, a lot of the time we end up with crackling that hasn't crackled. Either it's rock hard, or else it's bendy, leathery and chewy and almost impossible to eat. Why should this be? What did we do wrong? There are theories. Nigel Slater's is that the quality of the crackling depends a lot on the quality of the pig, and there's a lot to be said for this idea. Roast cheap supermarket pork and you will probably not get good crackling. For some time now, I have been scoring the skin on my own roast pork with a Stanley knife kept for that sole purpose, rather than asking the butcher to do it (my experience is that they usually score it too widely). I then rub some salt into the skin before roasting. But sometimes it doesn't make any difference, and when I bring my (moderately) expensive piece of butcher pork out of the oven and once again my crackling has gone all floppy, I want to hit Nigel Slater over the head with a copy of Appetite, because we can't all afford to buy meat from, you know, posh London butchers.
However, because I am in my modest way a man of science, I was determined to figure out in the spare moments that my pressing humanitarian duties afford me, why, in the name of Jesus, my crackling was so inconsistent. After a while I began to see a pattern.
When I roast pork I usually cook pork belly, because a.) it's still cheaper than most other cuts and b.) it's harder to get wrong. But when pork belly roasts, something happens to it; the skin shrinks, and causes the belly to flex into a vaguely concave shape, the top surface (the cracklingy bit) forming a shallow bowl. This in turn traps juices in the bowl, and crackling will not crackle when it's in any way damp. When I have cooked pork loin the outside surface is convex, and so juices run down and off the skin; the grooves between the crackling form natural drainage channels. So the crackling on my pork loin is always better than the crackling on my pork belly.
Here is the secret reason why crackling sometimes doesn't crackle; meat juices are seeping into the skin from underneath. You can partially solve this by not scoring the meat too deeply and also by making sure that any juices can run out of the meat - perhaps by cooking it on a tilt. The other thing you can do is remove the skin before cooking and cook it separately, but the only problem there is that the underside of the crackling tends to crisp up, and so you end up with a great crunchy wodge of skin and fat that lacks the delectable combination of crunchy and gooey that's part of the point of good crackling.
It so happens that the best crackling I've had lately was from a half pig's head that I'd roasted in the oven, partially submerged in a bath of stock and madeira. The skin on the pig's cheek was much thinner than the skin from loin or belly, didn't need scoring at all and broke off in crisp and juicy little pieces. From a half pig's head that cost me £1.75, I got enough skin, meat and lusciously moist neither-skin-nor-fat-nor-meat (i.e. the snout) to feed at least four people. But I don't think that pig's head will become the pork belly of tomorrow. Recession or no recession, I doubt that enough people want to get that intimate with the animal that died for their dinner.