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The fire box serves as a temperature control device and indirect heat source. Fuel burned in the fire box produces heat that flows into the barbecuing chamber. This happens because the air in the firebox is always hotter than the air in the cooking chamber. The hot air trys to cool off by moving toward the cooler air in the chamber. The hot air convects, (moves). This is why a barbecue pit with an offset fire box is sometimes referred to as a convection cooker. A side benefit of this air movement is it will also carry the natural moisture present in the air with it. This helps keep the food from drying out.
The most important reason for keeping the heat source separate from the food you are barbecuing is.. So you can control the temperature. Adjusting the air damper on the front of the fire box determines how fast the fuel burns. The more air you let the fire have, the hotter it will burn. The talent lies in knowing how to adjust the damper opening just enough to produce the barbecuing temperature you want maintained in the chamber.The amount of fuel in the fire box determines how long the fire will burn. The amount of air you let the fire have determines how hot it will burn. Some of the things that affect this control are outside temperature and wind.
Two of the reasons for wanting to have control of the barbecuing temperature is taste and texture of the food.
First taste: The main reason food slow cooked in a "pit" tastes distinctively better, in addition to the smoke, is because fresh food is composed of nutrients, carbohydrates, proteins, fats, minerals, and vitamins all dissolved in solution that make up the natural moisture content and flavor of the food. In other words, its true, red meats are made up of about 65% water and water boils at 212° for most Texans. It is also widely accepted that a cut of red meat is well done when its' internal temperature reaches 160°... It follows that a barbecuing temperature that doesn't get much above 212° will allow the food to cook in its' own flavor, without barbecuing the flavor out to the air over your neighbors backyard. Where did you think that great taste-tempting smell was coming from? Keep that taste where it belongs. Another benefit of not boiling all the flavor out is that you won't have to use all that high salt content artificial seasoning to try and get it back.
The second reason, texture, is simple. High temperatures tend to dry the food out and make it tough.
One last point about low barbecuing temperatures. There's a trade-off. It takes longer for the food to barbecue. But, in Texas, getting the barbecue right is nearly always preferable to getting it right now.
It may take as long as 12 to 14 hours to finish barbecuing a brisket at low temperature. You will have to add more fuel to the fire box to maintain the temperature for that length of time. How do you know when it's time to add more fuel? I suggest you watch the temperature gauge. Once you have adjusted the draft on the fire box to get the chamber temperature where you want it, check the gauge every hour or so. When it starts to drop, say 15-20°, you know the fire is going out and it's time to add more fuel. It's easy to see that if you just throw another log on the already dying fire, or add fresh charcoal, two things will happen. The barbecuing time will increase dramatically, because the new fuel needs time to get going, and the draft opening will have to be increased or readjusted to get it going. If you're using wood as your fuel source, it will also produce smoke while it is turning to heat producing coals. This might cause the food to be over smoked, taste bitter, and turn black. One solution is to have the additional fuel already burning by using a charcoal starter like the one pictured.
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Also, charcoal won't add any extra taste you might not want. As soon as you notice the temperature drop, light the starter up and about 15 minutes later you have 4 pounds of hot coals ready. This is enough for 3- 4 more hours of a 225° temperature, depending on wind and the temperature outside.
Smoke is a seasoning like salt and pepper. Too much of any seasoning will ruin the taste for most people. A good rule to start with is about one hour in the smoke per one inch thickness of the meat. This will produce a red smoke ring about 1/8 th of an inch around the outer edge of the meat that you can see if you cut a brisket in half. This is generally all the smoke flavor most people want. You can see why keeping a brisket in smoke for 14 hours will probably be to much. Try rolling your favorite wood in a piece of heavy aluminum foil, keeping the ends open. This will keep the wood smoking longer; because it won't burn up as quickly. Keep adding wood for the recommended time. Watch the smoke stack "If the smoke is white, the fires right. If the smoke is black, add some draft".
When wood first starts to burn the initial smoke that is produced is what I call bad smoke. This is because the wood in trying to reach its kindling temperatue has to get rid of all the moisture. The first smoke you see coming out of the stack is made up of soot or creosote. It will coat your food with a black and bitter tasting crust. Wood smokes two ways. When its starting to burn and when its fully burning as a coal. What you want is the smoke from the wood after it is fully burning, this is where the flavor is. You won't see a lot of smoke coming from the smoke stack, but it's there.
The thickness of the metal has absolutely nothing to do with the pit burning out or not. Because the fire is kept from directly touching the pit by the fuel racks, it's impossible (since you are barbecuing not forging horseshoes) to burn out the barbecue pit. The fuel support grills act as heat sinks that deflect and absorb the high temperatures away from the pit itself. An example would be pouring boiling water into a drinking glass. The glass will break unless you put something in it like a spoon before adding the boiling water. The spoon acts as a heat sink. It absorbs and deflects the high temperature so the glass doesn't break. Also, anyone that has ever repaired an electrical circuit board with solder will attest to the value of having a heat sink between the molten solder and a delicate transistor. So, don't let anyone tell you the pit needs to be made of tank armor to last. That's a story concocted by so-called "custom" pit builders who don't own the equipment necessary to roll a piece of steel. So they buy it already rolled. It's called drill pipe, or drill casing, and it's usually sold by the pound. This makes their pits more expensive and cumbersome. It doesn't make them last longer or barbecue any better.
What most people confuse as burn out damage is really rust out. The biggest reason for rust out damage is not removing the ashes regularly. Letting them stay in the bottom of the fire box or the bottom of the chamber where they get wet and hold moisture causes rust. Ashes also contain alkaloids, an acid, that will eat through any thickness of steel. Granted, the thicker the steel the longer it will take. Don't let it happen at all. Clean out the ashes.
"Custom" pit builders tell everyone that the thicker the metal, preferably the thickness of drill pipe, the better; because it makes the barbecuing temperature more even and holds the heat better. This is pure rubbish. Any thickness of metal will lose heat to the atmosphere based on the amount of area exposed to the cooling effect of the air surrounding it. An 18 inch diameter, by 36 inch long pit like the Hondo™ or Black Diamond™, made of 12 gauge steel, (about 3/16" thick), will cool off at the same speed as a 1/4" or 1/2" pit the same size while you are barbecuing. The fact of the matter is, it will take more fuel to maintain a given temperature in a pit made of thicker metal because it takes more energy to get the thicker metal up to the desired temperature in the first place. For barbecuing purposes, a 12 gauge steel thickness is more fuel efficient. Not less. The only time the thicker steel will stay hotter, longer is when your finished cooking. The thicker steel will take longer to cool down ... I wouldn't pay extra for this.
You can prepare foods that cook fast like hamburgers, steaks, chops and hot dogs directly over the hot coals in the fire box on the upper grill. The fire box is also handy for small get-togethers. Instead of piling cooked food on a plate (only to get cold), transfer it to the attached chamber. Heat from the fire box will keep it warm as you continue barbecuing or join the party. Everybody eats hot off the grill! The new flat cover on the fire box makes an excellent tortilla warmer, too.
Need a large barbecuing area for a Texas size party? Build the fire in the cooking chamber on the lower fire grates, place the food on the upper grills and barbecue to your hearts content.