Soda firing is a process where the ceramic material is heated to temperature in a (usually) gas-fired kiln. Towards the end of the firing, a super-saturated solution of sodium carbonate (and sometimes bicarbonate) is sprayed into the kiln with a garden-sprayer. The sodium carbonate separates into sodium (Na) ions and carbon dioxide. The CO2 leaves the kiln, while a portion of the Na attaches to the silicon dioxide in the clay. You end up with a texture called "orange peel," because it feels like the skin of an orange. When the Na reacts with the glaze, you end up with running, color changes, and changes in surface qualities (i.e. matte to glossy variations).