You might read this article:
http://www.lasc.us/CartridgeCaseAnnealing.htm
Here's an excerpt: (Personally, I'd clean 'em and use 'em)
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Brass is an excellent conductor of heat. A flame applied at any point on a case for a short time will cause the rest of the case to heat very quickly. There are several temperatures at which brass is affected. Also, the time the brass remains at a given temperature will have an effect. Brass which has been "work hardened" (sometimes referred to as "cold worked") is unaffected by temperatures up to 482 degrees (F) regardless of the time it is left at this temperature. Remember, water boils at 212 degrees (F), and oil heated in a frying pan easily reaches 500(F) or more degrees. (All temperatures will be in Fahrenheit).
At about 495 degrees (F) some changes in grain structure begins to occur, although the brass remains about as hard as before -- it would take a laboratory analysis to see the changes that take place at this temperature.
If cases are heated to about 600 degrees (F) for one hour, they will be thoroughly annealed -- head and body included. That is, they will be ruined. (For a temperature comparison, pure lead melts at 621.3 degrees (F)).
The critical time and temperature at which the grain structure reforms into something suitable for case necks is 662 degrees (F) for some 15 minutes. A higher temperature, say from 750 to 800 degrees, will do the same job in a few seconds. If brass is allowed to reach temperatures higher than this (regardless of the time), it will be made irretrievably and irrevocably too soft. Brass will begin to glow a faint orange at about 950 degrees (F). Even if the heating is stopped at a couple of hundred degrees below this temperature, the damage has been done -- it will be too soft. From this discussion we can see that there are four considerations concerning time and temperature:
1> Due to conduction, the amount of heat necessary to sufficiently anneal the case neck is great enough to ruin the rest of the case.
2> If the case necks are exposed to heat for a sufficient period of time, a lower temperature can be used.
3> The longer the case necks are exposed to heat, the greater the possibility that too much heat will be conducted into the body and head, thereby ruining the cases.
4> The higher the temperature, the less time the case necks will be exposed to heat, and there will be insufficient time for heat to be conducted into the body and head.
