My friend and fellow deshi of Shimaoka sensei asked me an interesting question the other day.
"How do you keep your black glaze black in a Soda Firing?"
There are indeed lots of ways of getting a black glaze, but you have to decide what kind of black you want. Many black glazes use an overcharge of several colouring oxides to give a dense black. In my case, I am looking for the warm translucency one finds in lacquer ware. Lacquer is in fact brown, but layer upon layer make it so dense it appears black.
In order to achieve a similar brown based black I turned to the Tenmoku style iron glaze. The base glaze is clear, using only four ingredients; Feldspar, Silica, Whiting, Kaolin. To this I add FeO, or Black Iron Oxide.
Using Black Iron Oxide gives you a head start, as the Iron is already reduced. Using Red Iron Oxide, Fe2O3, will require more work to get rid of the unnecessary oxygen.
There are a plethora of glaze recipes available which are excellent Tenmoku in a gas kiln.
Within any plant there are minerals which have been drawn from the ground, and when burnt these minerals either become gaseous and volatise off into the atmosphere or remain as a solid residue, ash. When firing a wood kiln the gaseous minerals, like sodium, will flow through the kiln with the flame and exhaust gases, combining where it contacts with free silica in the clay or glaze surface to form a soda glass glaze. Ash is blown into the kiln with the flame as a powder, settling on the pottery and glaze, adding a coating of a variety of minerals depending on the plant. At a high enough temperature, usually about 1300 centigrade, these minerals, silica and fluxes, will melt and form a glass coating all on their own. The colour and surface will be affected also by what minerals are contained in the vessel surface and mix into the glaze matrix. At the end of my wood firing I also add some Soda Ash , NaCO3, which becomes a gaseous flux inside the kiln in the same way as the natural vapours from the wood.
So, in a wood and soda firing the glaze gets watered down. The ash adds an extra wash of relatively clear glaze and the gaseous flux Sodium makes the glaze melt even more. You end up with a variety of oatmeal and honey glazes. You can see how the "hidasuki" markings from the "Igusa" straw have left golden veins in the black glaze on this plate. Too much straw and it all turns to golden toffee. That can be nice, but it's not black, and it might not be what you want.
So, you need to make the glaze harder, so that it is a bit short of flux and a bit resistant to soda, raising its melting point. By reducing the Feldspar, for example, it raises the melting point, but also raises the ratio of Iron in the glaze. Iron is resistant to soda. Effectively you adjust the glaze recipe to make allowance for the extra flux which is floating around as a gas in the kiln atmosphere.
Glaze application is also important, as no matter how good your glaze is, if it is too thin the ash and soda are going to win. I apply my glaze at a viscosity of 60 by the hydrometer. Tenmoku is very sensitive to variations in texture underneath, so also be aware that the finished glaze surface and colour are only as good as the finished surface of the vessel.
It has taken me years to get my clay body, glaze recipe, glaze application and firing right, and all of those variables affect the finished result. The same glaze will be different in a different firing or on different clay, so it is necessary to experiment with your own materials and firing. In the end it becomes another inimitable expression of your own aesthetic. Hopefully these principles will help you along the way.