Annealing is the process of heating brass necks so that it loses its rigidity, becoming pliable. To the reloader, this is desirable at the cartridge neck because it allows the bullet to be uniformly gripped all around the case mouth. It is undesirable anywhere else in the case, as it would weaken the brass and create the risk of case rupture and even catastrophic failure of the gun being used. The benefits of annealed cases have been fairly well documented by precision shooters, however your average shooter is unlikely to notice a difference.
Physical Properties of Brass
Brass is an alloy generally composed of copper and zinc. Brass can also be made of copper and lead, but the brass used in cartridges is usually a 70/30 copper-zinc alloy. Unlike steel, brass is hardened only by 'work-hardening'. This is, for lack of a better term, the process of hammering and beating at it until it becomes harder and harder. As opposed to steel, which gains significant hardness when heated and then quenched, brass becomes far more malleable after having been heated. The long and short of it is: heat makes brass soft.
Brass cases must be hard and soft
The neck of every brass case is meant to be soft and malleable. The reason for this, is that softer brass allows for uniform neck tension when seating a bullet into the case. However, the remainder of the case should be hard and rigid. A softened case body is more likely to rupture, and can potentially damage or destroy the gun it's being fired in.
The challenge, then, becomes: how do you anneal the neck, without annealing the body?
There are a large number of ways to heat brass to accomplish the annealing process. Heating the brass to 600 degrees for an hour will anneal it, but thermal conductivity will have annealed the entire case as well. What's necessary, is a temperature which can anneal the brass in a very short amount of time, before conductivity has a chance to anneal the rest of the case. There are two temperatures to keep in mind here: first, as long as the temperature stays below 482 degrees, no annealing will take place. Another temperature to keep in mind is 750 degrees, at which point annealing will completely take place within a couple of seconds.
One of the significant challenges here, is that copper (the main component of cartridge brass) is an excellent conductor of heat. When heating the neck enough to anneal the case, enough heat can be transmitted along the metal to anneal the case body. If the neck was annealed within a couple of seconds, there is no continuing reason for the brass to stay hot, is there? As mentioned earlier, quenching brass does not enhance its enhanced strength - when you quench brass, all you're doing is shedding the excess heat before the annealing progresses past where you want it to go.
There are a few significant methods of heating the brass neck, but not the body. Most of them involve open flame, but the other one personally excites me a little.
Held propane torches: A lot of annealers achieve results which they claim are fairly reliable when carefully heating the mouth of the brass with a blowtorch, then quenching it as soon as the very mouth glows a dull orange. It is difficult to get it 'just right', and the results can be no means be rigorously reproduced. Additionally, it takes forever.
Automated propane torch assemblies: This class of machine uses a propane torch and uses some kind of mechanical action to ensure that each piece of brass receives uniform exposure to it.
Ken Light BC-1000:, Priced at $400 as of 2/4/2013, this device can purportedly anneal 1000 cases per hour.
Dipping into lead: A lot of lead smelters top out around 800 degrees, which is ideal for annealing the case neck. One could simply heat up the molten lead, then dip the case neck into the lead for a few seconds, take it out, and quench it. The only problem with this approach is that it leaves lead buildup on the case neck, which is laborious to clean. Another hazard is the toxic fumes generated by the molten lead.
Dipping into salt:This is untested!!! I personally intend to try using a lead smelter to heat salt up until it reaches 800 degrees. This would be well under its boiling point, but it should still anneal case mouths just as readily as dipping into lead. However, the only residue or buildup which might be left would be salt, which is water soluble and should be readily dissolved during the quenching process. And salt doesn't let off any particularly nasty vaporous by-products in a molten state.