What you are seeing is probably grease that evaporated once the oven got hot, and condensed on the case.
Brass Annealing from Handloader’s Manual
Early Naramore, Major Ordnance Dept Reserve, Small Arms Publishing , 1937
“The best and surest way of drying cases is with the use of artificial heat, but care must be taken not to overheat them, as too much heat will soften the brass and may render it incapable of withstanding normal pressures. Most modern stoves, whether electric, gas, or coal, have oven thermometers that are, at least, fairly accurate. For stoves sold in the United States, these thermometers register degrees Fahrenheit and brass can be heated up to 428 degrees Fahrenheit without undergoing any change in its grain structure. For drying cases it is best to keep the temperature as low as 300 degrees. This heat is amply high for the purpose and offers a liberal allowance for any inaccuracy of the thermometer. If you oven has no thermometer, one can be purchased at small expense in almost any department or five and ten cent store. The thermometer should be placed near the cases as the temperature will not be the same in all parts of the oven. It is also well to place the cases on one of the sliding shelves or racks, away from the bottom of the oven, or the heating element if it is an electric stove.”
I doubt anyone has coal fired ovens anymore; this was written in 1937.
Major Naramore worked in an era when the Army actually made cartridges and had research labs. Today everything is contracted out and data sharing just does not exist between contractors or anyone else. But then, he could call up an Army buddy and find out within the system whom had material data on cartridge cases.
I put my oven on low, and in a half hour or so, my “five and dime” store thermometers read 212F, and doesn’t go any higher. Since water boils at 212F (at sea level) I know my brass is dry. I don’t set the oven any higher than warm because all the grease in the oven evaporates on my brass.
Basically, keep temperatures low and you won't anneal the brass. Annealing the brass is bad. If the case head gets soft, it will blow, and that could ruin your pistol or your rifle action. Might put out an eye
I have been using the toaster oven more of late, I pour rifle brass into a steel colander ( just the right size for the toaster oven) and turn the thing to warm. Which is about 150 F on the dial. A half hour later, all the brass is dry, and too hot to handle. So I pick up the colander and empty the brass into something that won't melt.
These are freshly annealed cartridges, dropped in water because they will melt carpet if they hit the floor. To dry them, I put them on a toaster oven sheet, and put them in the toaster oven, set the thing on warm, and wait. They get about 200 F.
