I am just starting to clean skulls with beetles. My colony is large enough to clean a deer skull in about 5 days. I started with about 5 mule deer and a dozen bear skulls. After beetle cleaning and soaking in peroxide (about 15%) 4 of the 5 mule deer skulls and about half of the bear skulls have dark spots. I usually soak the skulls in the peroxide solution for 12 to 24 hours. These spots are rather large. They are not all in the same spot on each skull. They are on the forehead, nasal bones, back of the skull etc. Some worse than others. When I get the skull fresh, I remove the eyes and as much meat as I can. I remove the brains by water injection. I then put them in the freezer and beetle them one at a time. Some may be in the freezer for 3 to 5 months. I understand getting an occasional skull soaked with blood in the bone but 50 to 75%of the skulls I brought in? I am going to try the "Iron out"product that has been mentioned her before. Is it best to heat the iron out solution to the degreasing temp of 120 F? Cold soak? Looking for help as to why so many of these skulls have the dark spots that looks like blood soaked bone? Need some advice!!