In terms of charcoal, there are a number of arrangements that can be used to achieve a desired result. If your goal is to achieve "low and slow" barbecue temperatures, one of the most popular methods to use is the Minion Method.
In essence, the concept behind the Minion Method is to place a small number of hot coals on top of a charcoal chamber of unlit charcoal briquettes. The intake damper is then adjusted in order to control the amount of oxygen entering the smoker and subsequently, maintaining low and slow barbecue temperatures (225 F - 250 F).
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The inventor of the Minion Method is Jim Minion. Jim discusses how he discovered the method on The Virtual Weber Bulletin Board forum - here.
To quote Jim:
"I was cooking in a competition and on the morning of the turn-ins I had my wife go to a shop and pick up my first WSM. I put it together filled the ring with charcoal and needed a way to light it off (I never did read the directions). I decided to do what is today called the [Minion] Method.We took a 1st in chicken and 2nd in ribs that day.
I got home and was reading Ray's forum and the question was asked on how to control the temps on a WSM, I answered and the rest is history.Jim Minion - June 4th, 2002
The only real debate was the fact that you were putting unburnt charcoal in the ring and it was lighting off as you go, knowing a little about Jedmasters [A discontinued charcoal rotisserie smoker] I knew this was not really a problem and the results answered that question
These days, Jim answers lots of barbecue questions on Quora. If you're after some extra barbecue insight, chances are, he's likely answered your question.
The minion method is used in order to maintain low and slow barbecue temperatures - between 225 - 250 F. Using the Minion Method reduces the risk of your smoker running too hot.
The reason the Minion Method is so against the grain is because traditionally charcoal users are instructed to light their briquettes and to wait until all the coals haved ashed over - turning from black to grey. While this method still holds true for grilling temperatures(325 F+), it is not conducive to smoking.
As you might expect, it's much easier to work with a few hot coals and to come up in temperature than it is to work with a full charcoal chamber of all hot coals and to come down in temperature.
While you can certainly control the temperature through vent adjustment - intake and exhaust dampers. This is sort of where the problem lies, you're going to have to choke the fire by starving it for oxygen in order to come down in temperature - closing the intake damper.
If you're smoking meat, closing the intake damper will result in "thick white smoke" and acrid flavored meat. A charcoal smoker functions based on the fire triangle or "combustion triangle" Meaning, in order to ignite a fire you need oxygen, fuel, and heat. When any of these elements is removed, fire can't be present.
The goal for smoke color is "thin blue smoke" which implies an equilibrium of the fire triangle and the burning process being deemed efficient.
While Jim founded this method using the Weber Smokey Mountain (WSM), the concept can be applied to all charcoal smokers. Whether you use a WSM, Weber Kettle, Drum Smoker, or a Kamado smoker.
This method is even suggested by companies like the Pit Barrel Cooker (PBC) - Granted, they don't market it in that way.
PBC states to fill the charcoal chamber with charcoal briquettes and then remove 1/4 of the briquettes and light them with a chimney starter. You then re-add them to the unlit charcoal to feed the fire throughout the cook. The PBC is known for running a bit hotter - between 275 - 310 F. To get lower, you could technically add less lit charcoal.
This is hard to estimate and dependent on factors like how big the charcoal chamber is (how much charcoal it can hold) and ambient temperature (the outside temperature).
With that said, it's not unrealistic to see a Weber Smokey Mountain run at 225 F for 12-16 hours. For longer cooks, expect to fill the water pan every 3-4 hours.
The PBC can run for 7-8 hours with a full charcoal basket.
Note: If your meat finishes and you still have unlit charcoal left, pull the meat and close your vents. This will deprive the fire of oxygen and you're then able to reuse the unlit charcoal.
The benefit of the WSM is that it inherently uses a water pan. I have an entire article on the usage of water pans, however to echo some thoughts from that article that are related to smoking temperatures:
From testing, yes. However, there are some important points to note.
The main reason briquettes are used is because they're uniform. Lump charcoal - regardless of brand - is not. Lump charcoal is a natural product produced in a kiln. Where-as briquettes are made of several materials (wood particles, saw dust, mineral char, and additives) that are compressed into one briquette.
With that said, Kamado-style smoker Manufacturers will often recommend using lump charcoal - less ash production that stifles the oxygen/intake damper.
Unlike the other types of charcoal smokers you can simply build the fire within the smoker:
There have been a few additional ways people have suggested to do the minion method, however, they are entirely unnecessary in my opinion. The most popular alternative is the "can" method.
Essentially, hot coals are placed in a bottomless tin can (like a can of beans) in the center of the charcoal basket. Unlit charcoal is placed around the can. The can is then removed from the charcoal basket (with pliers) and you're left with lit charcoal in the direct center of the charcoal chamber.
The above method was thought of by Bruce Kennedy - or as a User referred to it as "Kennedy's Kan" - on Virtual Weber Bullet Forums.
Again, the above is super nuanced in order to achieve the same end result. The main reason this idea was proposed is that it requires you to refresh the coals less often. During long cooks (10+ hours) the ash may accumulate which will require you to stir the coals up.
I've had great success over the years with simply dumping lit charcoal on top of unlit charcoal - the standard Minion Method. However, try both and see which you prefer. I personally am apt to be as lazy as possible; The saying of KISS or "Keep It Simple Stupid" has never applied more.
The Minion Method is one of the most efficient ways to maintain low and slow temperatures and getting it setup is a fairly simple process. If Jim can make it happen based on pure happen-stance, you can easily apply it too.