Stoking

This is a method used both in a firepit and in the kettle. The methods are slightly different. Kettles you simply add wood to, while firepits requires careful timing to stoke the fire with a Sharpened Stick or an Iron Poker. No extra fuel is used by the firepit when stoking.

Stoking a Firepit

Once a fire has been lit, you have the option of stoking it if you have a Sharpened Stick or an Iron Poker. If you have both, it will let you choose which to use. Both seem to act identically other than that the poker does not seem to have a chance to burn up. Stoking a fire correctly keeps it burning longer, which increases its charcoal, ash, and lime yields. Stoking does not affect Grilled Food yields.

A firepit runs through the following stages:

A very dim warmup stage, immediately after lighting. Stoking the fire at this stage will put the fire out immediately and you will recover all materials except Tinder unburned.
Burning merrily: a normal stage where the fire is orange. Stoking the fire at this stage will put it out, but not immediately - it will go to the "smouldering out" stage first. This phase can last for anywhere from 10 to 45+ seconds.
Periodically, the fire will brighten then dim (orange -> yellow -> white -> yellow -> orange) over a period of 14–15 seconds. This period is a stoking phase. You must stoke the fire exactly once during this entire brightening sequence, and preferably before it goes from white -> yellow. If you correctly stoke the fire, it will continue burning and return to the previous "normal" stage at the end of this stage. Be careful you stoke the fire only once over the entire brightening sequence, otherwise it will quickly smolder the fire, decreasing the fire burn time by 6 minutes than if it slowly smoldered out. If you do not stoke the fire in time, it will slowly "smouldering out" over a period of 6 teppy minutes.

If you look at the center of the fire with a color picking program it is trivial to know when a stoke phase occurs. The Hue of a fire will be 60 during the no-stoke phase, and then the Hue will drop to 0 for the stoke phase.

Smouldering out: The fire has gone out and the firepit is cooling. This stage takes approximately 6 Teppy Minutes (About 6:34 real time). You will get a message in Main ("Your firepit has smouldered out") when the fire finishes cooling. At this point, the final products are produced and can be taken from the firepit.

Stoking a Kettle

In general terms, it works as follows:

Once the kettle is ignited, it runs in a series of 30-teppysecond ticks. At the beginning of each tick, it consumes one deben of wood. At the end of the tick, it evaporates one deben of water. (Meaning that most of the time, except for the very beginning and end of the batch, it will consume one wood and one water simultaneously every 30 tsec.)
The first three ticks after igniting are "warm-up" ticks... wood is consumed, but no water is evaporated.
If at the beginning of a tick there is no wood left under the kettle, the fire goes out. The batch isn't ruined, but the fire will have to be reignited, and will go through the warm-up process again, wasting time and wood.
You can stoke the kettle at any time to add wood, but it will only hold up to 5 at once.
Takes 15 Teppy minutes to complete.