You ask an interesting question.  Making bone broth is nothing new and people have been making it for centuries but now it has become the in thing and there are so many recommendations that it is confusing to people.  IT is one of those things there is not a standard recipe to follow since bones are all different.  Example, you would probably cook chicken bones less time than large beef bones.  Fish stock generally cooks faster because the bone tissue breaks down faster and for vegetable stock it will cook faster as well because the tissues break down faster.  

Food safety is an issue.  You need to keep the temperature high enough to prevent spoilage.  Most experts like to keep the bones at a simmering temperature 180-185 degrees F. for many hours. You could go a little lower if you plan to cook for a long time but NEVER drop below 140 degrees F.  Most bones 8-10 hours would be sufficient to break down the bones to release the collagen, gelatin and minerals.  It could be less for a smaller pot full or finer bones and longer for larger quantities or big bones.  Some people like to add vegetables to their meat stock and a little vinegar added will help break down the bones and release calcium and other minerals from the bones.  Whether you roast the bones or not is a personal preference.  Some people like the flavor of bones roasted, especially red meat bones and it also tends to make a darker richer color.  It is not a safety issue rather a personal preference.  You can make stock from both raw bones and cooked bones.  The more of the interior of the bone that is exposed the more of the minerals and protein is released.  

The longer you cook the broth the more concentrated it becomes.  The important step to remember for food safety is that once your broth is cooked to your liking it needs to be cooled quickly and not left set on the stove for hours.  You can set the pot in an ice bath of water and stir to cool.  I personally like to use water bottles that I have frozen and put them into the stock pot to cool.    It works great and have never had a bottle break yet.  After cooled, refrigerate. It can be frozen or canned for long term storage.  You can season to taste to your liking.  

As for equipment. There are many appliances out there that you can use.  Instant pots (large size work well), large slow cookers (low temperature setting) , electric pressure saucepans/cookers.  If you don't want to invest in another appliance all you really need is a large stock pot, preferably one with a heavy bottom to more evenly distribute the heat.  It needs a tight-fitting lid.  

It is recommended that you start the bone stock with cool or warm water and then heat to boiling, turn down the heat and keep it at a simmering temperature for however long you want to cook your bones.  Usually 6-12 hours is enough to produce a rich stock.  Then the broth is strained, cooled and refrigerated.   

Hope that helps.  

Thanks for using Ask Extension
Nellie Oehler  

Dear Nellie,

Thanks for the very thorough and practical response!!

I'll pass this along to my wife and get back with you with any ques she might still have.

Background:
We've been looking for a way to keep cooking chicken bone broth, without need to monitor, between 180 and 200 degrees F for 24-48 hrs.  The best we've found is an expensive Lab Hot Plate.  She's found that our stove top temps, while set at one setting, meander far from this range.  Also, on the web we find little information about temperature stability of slow cooking appliances of various types. 
I've thot that since people made broth over open fires for centuries, surely we don't need that much precision.  Carol wants to make sure we're safe, plus she wants to get the most nutrients for all the work.  I'm glad you emphasize the need for rapid cool-down and share your personal technique.  

Your personal techniques - that brings to mind a question I had while reading your answer.  About the frozen water bottles, what are they made of?  Do you think unopened bottled water bottles would be OK?  Perhaps personal sized to maximize heat exchange surface area? 

Assuming you're somewhere near OSU, I hope you can get out to enjoy a nice spring-like weekend - looks like we have some winter-like weather coming next week,
Randy

I was a beautiful day here in Corvallis.  I live on a farm and had fun watching the new lambs and moms out in the field eating and playing.  Flowers popped open.  Maybe spring is on the way.

As for water bottles, I use a variety of sizes that are food grade.  The ones that juice, water, energy drinks etc.  come in work well.  I personally like the heavier plastic one.  That said I have used the lighter plastic ones as well and they have never broken.  The smooth plastic ones are easier to clean and refreeze than the ones with ridges.  

When I do the turkey carcass from Thanksgiving in a large pot, I put in a gallon size bottle or a couple half gallon juice jugs in to cool quicker.  I use them all the time to cool foods before refrigerating.  

I wouldn't worry about the safety of
slow cookers on low setting.  It will keep the temperature in the safe range.  I tell my volunteers that life begins at 40 degrees and ends at 140 degrees F.  bacteria don't multiply very fast outside that range.  

Good luck making stock.  I do it all the time.  Have a big bag in my freezer were I stash bones and veggie trimmings and when I get a good supply make batch of broth and then freeze or can it.  Never buy broth from the store.  

Nellie