For the 2-step firing, the pots must be pre-heated.
One method is to build a circular fire around the pots and gradually move the fire in until finally the pots are actually in the fire itself. Eventually building the fire up with larger pieces of wood and letting it burn for a couple of hours, or instead of using large pieces of wood, constantly replenishing with smaller twigs spaced around/between and above the pots. Another method is to build a fire and to move the pots closer, turning and rotating them until well pre-heated, then scraping the fire away, laying the pots in the coals/ashes or on an insulated layer of pre-heated rocks or old pottery shards. Let the pots sit a short bit to allow gradual heating up before adding more fire.
Another method, pre-heat, then set on coals for a bit. Then add pine needles etc to make for slow heating. Then begin adding small woods, and then finally larger woods. When burned to coals, re-build. Twice for smaller pots, Three times for larger pots. Avoid wind or rain threatening days to fire. A cool draft or drop of rain can ruin your efforts.
A more complicated method is to build a primitive kiln. This allows more control in heating gradually. Simply, the kiln is dug into a hillside, a firing chamber below it. The heat and smoke of the fire is tunnelled through the kiln. The ancient Chinese used this method extensively, often stairstepping kilns up the hillside, heating from one fire chamber.. You must ensure that the required temperature (12-1600 degrees F, color will be from dark red at the lower temp, thru cherry red, bright cherry red, to orange at high temperatures). This temperature should be maintained for 2 to 4 hours. The chimney escape should ideally be through the bottom of the kiln (so as to retain as much heat as possible, which rises) and then up, and the opening needs to be large enough to allow enough draw for fire to reach the high temperatures required.
The final test is to cook with the pots. Don't pick them up by the rim to avoid breakage. A well constructed and sound pot will get better with age.
Offered by Steve.
Source: Primitive Wilderness Living & Survival Skills by John & Geri McPherson, $24.95.