Maceration is the process of purification to remove the remaining flesh from bone. It is the easiest & least expensive way to do this while still leaving bones in good condition. Note that this process is typically best for animals larger than a rat. Smaller bones can turn to mush during maceration as they themselves start to break down.
To macerate you take your animal remains that have very minimal flesh on them (never a whole animal!), add them to a plastic container & fill with plain tap water then replace the tight fitted lid. The lid is important because you don't want insect larvae in your maceration water only the bacteria that will grow and eat away the flesh. You also don't want to leave it out in the rain without a lid or you'll get algae growing. Insect larvae & algae with both start to break down and possibly even stain the bone.
The more flesh on the remains the longer this process will take. Somewhere around 2 weeks the bones should be coated with a film that's pink, red, rust, brown or black. That's normal & means the bacteria is working to remove the flesh. The bacteria may however be dead now though so time to change the water if still fleshy and start the process again, or time to start peroxide bath if done. Pour out only half on the nasty water and add fresh water to fill the rest of the container if the maceration is not done yet.
These deer bones were covered with a bright red film because they were left for too long in the maceration process, on accident. They were completely flesh free though. Thankfully I just hosed the film right off then started the peroxide step to get rid of the awful smell.
The odd colored film you see on your bones is a residue of the bacteria or the bacteria itself coating the bone. It does this when the water runs out of oxygen for the bacteria to survive. Referred to as the bacteria going anaerobic. This anaerobic bacteria in maceration replaces the microbes that were doing the maceration work of defleshing the bones. And therefore the maceration process has come to a halt.
It can be caused by too many microbes using up all of the oxygen in the water or it can be from an oily film on top of the water that keeps oxygen from entering the water. Some ways to prevent this are - larger quantities of water, changing the water or a bubbler. It commonly happens to marine mammal remains during maceration because of the high oil content in their bones rising to the top of the water.
This is a potbelly pig skull that started off lightly mummified that soaked undisturbed for 2 weeks. The maceration water was still fairly clear and yet the skull was turning black. I've found that when there is little flesh to work with or very old flesh this can happen. Sometimes the black film easily hoses off or goes away once the bone dries. In this case though it took a peroxide bath to remove the black coloration.
Raccoon bone that was pulled out of maceration. It was dropped into peroxide and pulled out an hour later to show how fast the peroxide can get rid of the staining. It can in most cases happen instantly.
"Before" of diamondback rattlesnake bones straight out of maceration.
"After" of same diamondback rattlesnake bones after a peroxide bath.
The maceration water needs to be kept at a certain temp to work. It will not work in the Winter in most locations because it is too cold for the bacteria to survive. It will also die off if it gets too hot in warmer months. For exact temperatures & more extensive info about this see this blogspot blog.