What is the best way to preserve it?

I had some i collected and never added anything to it. It's probably a year and a half old and it really hasn't changed much. It was a small sample and your results may vary but that's my limited experience. Great base, I just wish there was an easier way to get it in bulk. I really think marrow is why K9s chew on bones. They are willing to work to work pretty hard for such a small return.

Best eaten raw.

Have u really done that?

YS, do you feel it needs to be refridgerated? Thanks.

My has just set on the shelf in the shed

Marrow is a high percentage of fat mixed with stem cells. It will sour / change odor and develop mold over time if kept at room temperature for a period of time.

Soup bones give the best flavor to soups and broth. Particularly the large sliced leg bones. Yes I have eaten my fair share of bone marrow. Mostly cooked. I have eaten some fresh just to get an idea of the flavor and taste.

I would think some type of Low Heat is necessary to allow the preservatives to blend into the fatty material?

My bone marrow after a year in a half in and old grainery. Might have soured bit as it has a bit more odor to it but I wouldn't be surprised if it doesn't make it better. Guy would have to do some comparative testing to know one way or the other

Dark stuff is just some foreign material that got in while collecting not mold or bacteria

I believe bone marrow if high percentage cholesterol similar to brains. The difference between true fat and cholesterol is one is not very digestible my K9s and one is. Seems to make a difference in their attractiveness based off my testing.

Brain tissue is not near as dense or fatty as bone marrow. Brain matter is very high moisture tissue and watery in nature once processed/aged. It will get very runny over time if not handled properly. Used fresh will give much better texture or mixed with other materials to hold it together better.

Marrow is very close to bear grease from my experience and would likely serve as a good boot grease and leather treatment material as well as a good water proofing material if used as needed. Warmed up leather and marrow penetrates nicely.

But back to the question..

What is the best way to collect and render and save marrow?

I collect mine from deer carcasses after butchering and butchered beef. Take a hammer and cracked open the bones and scrape out the marrow.