I recently went to Izu, Japan for a Rakuyaki camp with some co-workers at the pottery studio. None of my bowls and cups turned out as I had hoped, but it was nonetheless a great learning experience. It’s true, Rakuyaki epitomises unpredictability in the world of pottery – control freaks beware!
We often get unexpected results when we try something for the first time – experimenting with a new form or trying a new glaze adds excitement to the process as we anxiously anticipate results. Unfortunately, the mystery quickly fades after the bisque firing or initial glaze tests are done. Replicating results with an electric kiln is easy since the other factors of influence, airflow and temperature, can be controlled. On the other hand, the Raku technique is fast and intense. The firing takes merely one hour and post-firing reduction takes an additional hour. It’s during this last stage that the magic happens – Raku-wares are removed from the kiln once the glaze begins to melt into a partially liquid form and placed in a reduction chamber full of combustible material.
Unfortunately, such high exposure to thermal shocks is inevitable in the Raku process so it’s better suited to clays with more opening agents (materials that are refractory and expand or contract enough so the ceramic ware can withstand sudden thermal shocks without cracking). To this end, Raku clays have c. 20% to 30% of opening agents, such as sand and grog, mixed into them.
Similar to other pottery making processes, the greenware is bisque-fired once it’s bone-dry so that it becomes strong and porous enough for glazes or underglazes to be applied and absorbed. The bisque-firing will also remove any organic matter, carbon and interlayer water molecules trapped in the clay particles.
Raku firings typically fall in the range of 800 to 1,000 degrees celsius, much lower than high temperature firings that can reach 1,400 degrees celsius. Naturally, the fluxes used in Raku glazes reduce the melting point to the appropriate temperature. If the Raku firing temperature does not reach the glaze melting point, the glaze components will not fuse completely and form a glassy coating.
From the firing onwards, it gets a lot less controllable. Raku kilns can take many forms and sizes but typically need to be structured in a way so that the ceramic works can be removed quickly and safely. We used a simple makeshift kiln made by fixing six square-shaped barbecue wire meshes at the corners and lined with high temperature insulation wool on the inside. The kiln was then placed on a portable charcoal stove that holds burning lump charcoal used as fuel for the fire. A blowing hair dryer was aimed at the burning charcoal to ensure there was sufficient oxygen supply (for combustion).
Since the firing temperature is dependent on uncontrollable factors such as the consistency and rate at which the fire interacts with oxygen and the fuel, the time required for it to reach 800 to 1,000 degrees celsius varies greatly. Without thermometers, the color and sound of the flames are the best indicators that it’s burning efficiently – a blue flame means there’s complete combustion and the “roaring” sound is indication that the flames are turbulent, which aids in the blending of the gaseous fuels and oxygen. Periodic checks on the appearance of the glaze is needed – the Raku-wares are ready to be removed from the kiln when the glaze has fully melted and has a glossy appearance.
There are typically two ways to deal with the Raku-ware post-firing: submerging it in water (oxidation) or covering it with combustible material to start a series of redox reactions (reduction).
The spectrum metallic colors characteristic of Western Raku-ware are achieved by the reduction method. The ceramic pieces are first removed from the kiln when the glaze melts and covered with combustible material. A metal can or bin is then placed on top of the mix to choke off further oxygen supply, after which it’s left inside until the glaze is set.
Since the temperature of the red hot ceramic ware is higher than the kindling point of the combustible material, it will catch fire. When fire burns in shortage of oxygen, as in the case of a reduction environment, carbon monoxide and water is produced (instead of carbon dioxide and water). Carbon monoxide will naturally seek out oxygen so it can form carbon dioxide, which has a much stabler bonding structure. Since there is already shortage of atmospheric oxygen, carbon monoxide molecules will bond with oxygen in the metallic oxides of the glaze or clay. The colour of the glaze will change depending on the degree of oxygen transfer from the metallic oxides to the carbon monoxide. For example, copper sulphate (CuSO4), copper carbonate (CuCO3), copper (I) oxide (Cu2O) and copper (II) oxide (CuO) are all different colors despite being derived from copper. The observed color differences in metallic oxides of the same metal is due to differences in oxidation states, which absorb different coloured light. Another by-product of incomplete combustion is carbon in the form of soot. It will typically attach to any unglazed or waxed parts and result in a matte black charcoal-esque look.
Once the Raku-ware has completely reacted with the combustible material and the glaze has been set, it is removed from the reduction chamber and washed with cold water to remove any soot or ash on the surface.
As the finished pieces show, no two Raku-wares can be exactly the same and you never know exactly how they will turn out. It’s this element of surprise that has kept Japanese and Western potters captivated for so long. Who doesn’t like a bit of unpredictability and spontaneity?
Useful sources: https://digitalfire.com http://pottery.about.com http://ceramicartsdaily.org