Video of the process below! Raku firing lends its name from a family saga of Japanese ceramicists, who I don't think are too happy about the name borrowing. From what I understand, the techniques used are only similar in a very superficial way, but here we are.
In western "raku" we fire in a gas oven (although electric, or I suppose wood fire, can also be used), and remove the red hot pots to do a variety of treatments, most often involving a "reduction bin". This is very spectacular, and the results can also be, although they're a bit unpredictable so it can also be disappointing.
These two pieces I made got fired during ing @ceramicartclass's yearly (pandemics allowing) raku party. The specific technique in this case is naked raku, so called because the end result is actually the bare surface of the clay. So how do you get those cool black cracks?
Raku glazes often fire a bit too "small" for the pot, so they contract as they cool down and this creates the crackled surface. This is accentuated when the red hot pot is put in a fireproof bin with combustible material inside, which catches fire. The bin is closed so the fire searches for oxygen elsewhere (in the pot or the glaze), this is what we call a reduction atmosphere: the oxygen is reduced, and can create cool color shifts and lustre effects in glazes. It also generates a lot of smoke and soot and this colours the clay behind the cracks that form a deep black.
So naked raku is a further exploration of this. The pot's surface is often burnished or covered in a layer of terra sigillata (very fine, lustrous liquid clay) - I used the latter on very smooth pots. The pot is then bisque fired (fired to "low" temperature, around 1000°C) which transforms the clay into a porous ceramic, and which will make the pot resist the following treatments and temperature shocks. To begin the raku process, first a layer of liquid clay is applied, and on top of this, a layer of raku glaze. After firing the pot in the raku oven, it is taken out at red hot temperature, just as the glaze is beginning to melt. We wait a few moments until we start to hear the glaze crack, put it in the bin, and wait for a few minutes for the soot to be absorbed between the cracks. The pot is then taken out of the bin and shocked with water. This thermal shock causes the glaze to flake off the pot, thanks to the intermediary layer of clay we applied earlier. The result is a naked clay surface, white (or whichever colour the clay or terra sigillata had before) on the parts that were protected by the glaze, and black where this skin cracked.
Thanks to several of my classmates for snippets of video or photos that allow me to better illustrate the entire process, as I didn't have those details from my own pieces.
These two pots are moss planters, as you can see towards the end of the video and on the class exhibition photo at the top. Here's one with visitor:
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