Wood-firing is a process that uses the flame and ash inside the kiln to glaze the work during a protracted firing at very high temperature. Alkaline earths and other elements in the flame react with the clay to form glaze, in greater or lesser degree according to the amount of exposure. Fine ash carried by the flame settles on the work where it melts to create glaze as well, forming drips if enough has accumulated. These glaze surfaces develop slowly as the kiln is stoked with firewood around the clock for multiple days. I typically fire my anagama kiln for three days to create the surfaces that you see in this website.
Most of my work goes into the kiln unglazed on the exterior, as you can see in the photo of the kiln load ready to be fired. The interiors of closed forms such as bottles and vases are glazed prior to firing because the flame and ash will not get inside to create a natural glaze there.
I occasionally use an applied glaze on a pot's exterior prior to firing in order to extend the range of potential surfaces. Some clay and glaze combinations do very interesting things in wood-fire, reacting with the flame and ash to create unusual beauty.
in a typical firing the kiln is warmed slowly at the beginning to protect the work from uneven heating and thermal shock. After the kiln has reached the temperature at which quartz crystals change size (quartz inversion), I stoke the kiln vigorously to achieve a high temperature in the front near the firebox of about 2400F.
When the pyrometric cones that have been set in view of the stoke door indicate that a desired level of heat work has been achieved (cone 12 down & cone 13 softening), I back off on stoking the front in favor of stoking the back half of the kiln through ports in the sides. This "side-stoking" will bring the temperature up in the rear half of the kiln more effectively than if only the front firebox were stoked.
I maintain the high temperatures in the kiln long enough to give the work what amounts to a day and a half "soak" in this very hot environment of flame and ash. Then, after stoking to bring cone 13 completely down in the front of the kiln, I do a reduction cooling down to 1650F, because cooling the work in a reduction atmosphere gives the color palette and textures that I prefer. After eight days of cooling the kiln is opened and unloaded.
The colors of the work are determined by the clays used and by the quality of kiln atmosphere. The patterns are significantly determined by how the pots are loaded in the kiln -- the loading being a painting process wherein visualization of the flame path is essential. However, the currents and eddies in the flame cannot be precisely controlled, and subtleties in the chemistry can have profound and not-anticipated effects on color, texture, opacity and so on. Firing a wood kiln is, then, always a dance between the potter's intentions and the magic of natural process.