Kerstin Gren: Raku and Pit firing

Notes from the week

1.  Bisque your pieces - Any clay can be used, so long as it is grogged. The grog helps the clay withstand the thermal shock ie. dramatic change in temperature involved in this firing process. It also helps to reduce shrinkage. There are some formulated ‘raku clays’ which contain the mineral kyanite. Although, you can use any clay and add molochite to the clay. The addition of this granular material opens up the clay body, which allows for strength, durability and quicker drying time. A trade-off of this though is that the plasticity of the clay is lessened, but overall the clay becomes far more versatile and tolerant given the addition of this material. (Long story short, any clay but porcelain.) Typically, you can bisque to around cone 07 for Earthenware and 04 for Stoneware. 
2.  Glazing   Same same but different: Glazing raku pieces are different to typical stoneware pieces. Particularly, the fact that unglazed areas are exposed to carbonisation in post-reduction. In other words, the raw clay becomes black. (without going into it, there is naked raku too…). Raku is ultimately a low-firing process, therefore whilst the pots are glowing hot, in the scheme of pottery they are not that hot. The aim is to get the kiln hot enough so that the glaze has melted, which is followed by various post-firing techniques. Therefore, the glaze just needs to be low-firing. Kerstin has given me some of hers (enchantingly in Swedish), but there are plenty online like these ones. Copper glazes given the deep metallic, lustre-looking ones. (Note to explore - sacrificial slip here)
3.  Firing the kiln  Raku kilns run MUCH quicker than typical ones, their aim is to get the pots up to temperature as quickly as possible. The kiln itself is relatively low-tech, as detailed in this DIY Raku Kiln here. Ultimately, it is an oil drum or steel bin, insulation material as a ceramic fibre, a propane torch and a few kiln shelves/props. It’s really important that the air can flow because this is the oxidation part of the firing, so there must be a vent of some kind at the top. Unlike other kilns which are stacked, there is simply one layer in raku kilns. This is because at top temperature, there is no way you would be able to handle the shelves out as they would be too hot at 1000°C. The firing takes between 1-2 hours, Kerstin swears by 45 minutes for earthenware pieces. This is dramatically different to typical 24-hour firings!
4.  Post-firing reduction Once the top temperature has been reached, it’s time to take the pots out and let the post-reduction effects begin. A lot of decoration can be prepared beforehand in the glazing stage, such as masking patterns, wax resist, latex, slip etc. In typically western raku, the pots are buried in huge quantities of sawdust. Although, any combustible material can be used. There are mixed theories as to how long pots should be buried in the smoke bins, but generally, 6 minutes is a minimum because most of the reduction happens then (absence of oxygen). Although, the longer the better it seems online. If you don’t use the smoke bins, you can dry carbonaceous materials to make smoke patterns and carbon trails. Here’s another method that uses some acids too.

1.  Make your pieces Like raku, a grogged clay is best for pit firing to aid with the thermal expansion. The trick to strong pit-fired pots is polishing, which involves spending a lot of time rubbing a smooth hard rock across the surface of the clay. You want to do this on the harder side of leatherhard, erring towards bone dry. Once you have done 2/3 polishes, you can add some sunflower oil and do a final polish. Only then will you apply the terra sigililata. 3 layers can be a good amount, and seeing that you would apply the terra sig at bone dry, it will dry immediately. I wanted my pots to be super shiny, so I did a polish again…
2.  Bisque so as to make the pots more durable and able to withstand the weight on top and thermal shock.
3.  Building the pit and firing best demonstrated in the image below. There are 9 key stages identified here: (1) Filling with sawdust (2) adding dried coffee (3) Scattering copper carbonate and ruffling it into the first few layers of the pit. It is the addition of this carbonate that provides the wonderful reds that we see in the post-firing (4) bury the pots in the pit, carefully. Very submerged parts will be smoked black, the exposed areas - who knows what will happen! Depends how the flames lick it. (5) More copper carbonate, but NOT touching the pots themselves as it fuses to the pieces and goes all bubbly and horrid (6) Seaweed is added, which will release salt when burnt and hopefully add some more shine to the pieces. It was also just very fun harvesting seaweed to dry out for the firing. (7) Driftwood and other scraps go on top of the pots, getting ready for the fire. Driftwood is used for the same reason as seaweed, so as to release salt. Then (8) fire time! You light the pit after shoving newspaper in all the nooks and crannies between the wood. You want to get the whole pit started as soon as possible to ensure a uniform firing. Be warned, the heart of the pit is the hottest - it is cooler on the outside. Finally, (9), wait for all the wood to burn down. Ideally, let the pit sit overnight to allow the colours to mature before removing them from the pit. 
4.  Unveiling the pots and finishing means fishing your pots out! It is at this point that you realise it might have been useful to have kept track of how many went in beforehand… Please remove them carefully! They are fragile! Once removed, it is a question of wiping off any debris with a damp cloth. The final touch is a soft polish with some bees wax, then, voila.

FINAL THOUGHTS