Reduction, as you may know, is the combustion of fuel (a combination of hydrogen, carbon and oxygen, usually found in the form of gas, oil or wood) in a relative lack of oxygen. When this happens in your car engine (too much gas, not enough oxygen), it is called incomplete combustion and it is a bad thing for your car and the environment. But when it happens in a potter's kiln, it is a good thing! And here is why.....
It's all about that periodic table we saw in high school. Yes, art really is about science! Remember the low part in the centre of the table given the group name "transition metals". One thing they have in common is they all have several oxidized states (i.e., the electrons on the outer surface of the atom move). And when the metal changes oxidized states, they change colour! Now I have the attention of the artists!
All clay, be it earthenware, stoneware or porcelain, contains iron. Most typically in the form of ferric (or iron (III)) oxide, or more commonly called rust. It is a red-brown colour and is most typically found on your car! This is the most commonly found type of iron oxide because the world around us is plentiful with oxygen.
If we are able to remove the oxygen around us, as we can in a reducing kiln, we convert the ferric iron (III) oxide (red) to ferrous iron (II) oxide (black). So, this little bit of chemical magic seems intriguing, but why do potters care about this change of colour? Can't we simply change the colour of clay with stains and glazes on the surface? Sure we can. But there is a key issue that make reduction an essential element in functional tableware production...it is porosity of the clay body (underneath the surface decoration)!
Potters, who make tableware, as opposed to cooking ware, want the lowest porosity possible in their finished pots. This makes them dishwasher and microwave safe. Porosity is reduced to an almost insignificant level when the clay body is well fluxed (the silica in the clay is turning to glass). And here's the bottom line....black iron is a flux, red iron is not. That little transitioning electron makes a huge difference to the nature and quality of the fired clay.
Pots fired in an electric, oxidized kiln will seldom have the same low porosity as pots fired in a fueled, reduced kiln. And you will be less likely to get the crystal ping sound when you flick it with your finger. And your customers will not be disappointed by pots that break in the dishwasher or microwave!