When do you put out the fire?

I have been making my biochar in a TLUD for years.  It has a chimney. After it has been burning, you need to put out the fire, so that it doesn't all turn to ash.  In my first attempts, I think I let it go on too long.  I got a lot of ash and not so much biochar.  Obviously, I'm not talking about retorts, because they put the fire out by themselves. Eventually, I settled into quenching the fire with water when the flames were about 5-8 inches above the char.  I would get lots of char, almost no ash and some wood that wasn't completely burned.  Not a problem.  I would just save it and burn it in the next biochar burn.

However, yesterday, I was reading something interesting and didn't get to it until the flame was about 1 inch above the char.  I got tons of char.  There wasn't much more ash.  How much unburned wood did I get? None. Absolutely none.  This seems to be a better outcome than previously, but it makes me want to ask you people out there.  How do I know if that outcome is better or worse?

When do you quench the fire from your biochar?

John S
PDX OR

I'm not very particular about it because I'm not really trying to optimize the output, I'm usually pit burning to process brush piles so I just go with the flow of burning what I need to, letting it cook down just a bit further at the end, and quenching when I'm ready to move on. Usually that means a bit of ash and a few larger chunks of uncooked leftovers, neither of which bothers me since I'm just putting it all in the ground anyhow, not processing further.

If you are cooking batches in a dedicated setup, it sounds like you're on the right track of observing and adjusting based on your circumstances and aims. I'm sure you can get fairly scientific and try to reach a certain temperature for a set time...if that's important? What do you want to use the char for, may I ask?

I make biochar in a fireplace, by scraping the coals once the wood disintegrates into chunks (coals) and dumping them into a bowl of water. So there are no flames present at that point. I think as soon as the flames die back and it is just coals is my signal.

I am improving my soil.  It holds more water during our dry summers. It drains better in our wet winters.  The microbes hold onto more nutrients in the winter when the frequent rains wash out nutrients.  It enables more communication electrically, through the mycelium for nutrients to be shared among members of the soil food web.  It also tends to neutralize our naturally very acidic soils.
John S
PDX OR