Kerstin Gren: Raku and Pit firing

In Spring 2023, I had the joy to go spend a week with Kerstin and James in their pottery in Banff, Aberdeenshire in Northern Scotland. Here, I was to learn about raku and pit firing, but the inbetween bits seeing how Kerstin works was even more compelling to me. Better yet, as a train lover I was staying in a cabin on their farm, which is a renovated LNER cabin!

Kerstin Gren’s bio from Birch Tree Gallery

“I am working with colour and form. Clay is the material. For many years I have been throwing though recently I am getting closer to abstract painting with all that this involves. I am foraging and preparing all paints and pigments I am using. Being Swedish has important value to me, the natural foraging and looking in nature, which also have a sustainability aspect, I am brought up with. Only take what you need. I am firing in a method that corresponds with my idea, and that is usually Raku or Smoke Firing.

Kerstin was born in Helsingborg, Sweden, where she got her foundation training in ceramics, and later attended Carl Malmsten School of Arts And Crafts. In 2004 she moved to Scottish Highlands, where she had built her steading and established her studio. She continues to exhibit in Sweden, UK and various juried ceramic shows, such as Oxford and York
Ceramic Fairs. “

LNER cabin I was staying in!

Vessel with circles and white bands (photo: Shannon Tofts). The pot was thrown, decorated with slip then uses a variation of smoke firing.

Notes from the week

Day 1

We began by chatting about where I want to be in ceramics, and perhaps the crossroads of where I find myself now: somewhere between drawing, ceramics and technology. I am wanting to unite these practices.

Subconsciously I think I had expected to throw because typically throwing is what I do. However, given the quick turnover time of firing, I found myself slabbing for this workshop. Slabbing… and polishing. For pit firing, no glaze will be applied. Therefore, to achieve a high-shine surface I will have to polish my pieces. This seals the clay (although not food safe), bringing out both the colour and natural shine in the clay.

In the meantime, I made my terra sigillata. Technically, “Terra sigillata is an ultrarefined clay slip that can give a soft sheen when applied to bone-dry wares and, if polished or burnished while still damp, may give a high gloss. The ancient Greeks and Romans used this technique in lieu of glaze.” (Ceramics Arts Network, 2023). To make it, you need clay, a deflocculant and water. The deflocculant we used was sodium silicate, and this separates the clay into its different-sized particles. The terra sig is the finest layer of particles. The settling period (ie. time taken for the finer particles to rise to the top and heavier particles sink) varies from clay to clay, we left ours overnight. I was honoured that Kerstin let me use her ‘Red Californian Clay’ from her incredible collection of clays foraged from around the world.

White and red terra sigillata, just as it begins to seperate

Kerstin’s incredible collection of clays collected from around the world.

Red Californian clay

Day 2

Day two began with finishing off the test tiles, notably slipping and beginning for them to dry out so that I could begin polishing them before the bisque firing. Today felt like a day to hold the space. Typically I think more more more to enable more experimentation and exploring. However, being constrained by the single bisque firing is perhaps the collar I need. Instead, it has erred me to spend more time with the pieces that I have made, whilst still not getting too attached and fixed with preconceived notions.

Earthenware vase, grogged with molochite, polished with agate stone.

Perhaps the most useful thing from today was to really wrap my head around the different firing processes of raku and pit firing, which I will consolidate here.

RAKU FIRING

“Raku generally refers to a type of low-firing process that was inspired by traditional Japanese raku firing. Western-style raku usually involves removing pottery from the kiln while at bright red heat and placing it into containers with combustible materials. Once the materials ignite, the containers are closed. This produces an intense reduction atmosphere which affects the colors in glazes and clay bodies. The drastic thermal shock also produces cracking—known as crackling since it is deliberate. The original Japanese style of raku is an outgrowth from Buddhist influences in life and especially in the tea ceremony.” - Beth Peterson, The Spruce Crafts

That’s a summary, the process goes as follows:

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.
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)
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!
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.

Snap of the pots in the kiln just before you take them out. You want them to look shiny, which is an indication that the glaze is beginning to melt.

PIT FIRING

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…
Bisque so as to make the pots more durable and able to withstand the weight on top and thermal shock.
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.
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.

The stages of prepping the pit fire kiln, firing and then retrieving the pots thereafter.

Day 3

The day began with more polishing and the application of terra sigillata, followed by more polishing. Therefore, it came as an absolute relief when we decided to go the beach to forage some clay!

Sandend Beach clay foraging

When you know what you are looking for, it is incredible how easy the clay is to spot in the landscape. Kerstin explained some major giveaways are 1) the colour. It is more orangy reddy, 2) the cracks , in the case of dry clay and 3) its near water or a river bank just underneath the topsoil. It just so happens that on our walk across the beach, we walked directly towards a clay pit and stumbled upon 2 more! (The Wild Clay book was a great reference for this to identify what is below the topsoil).

Clay reserve found at the beach!

Clay found at Portsoy Beach

Day 4

Building a raku kiln

Today was straight in from the onset. The morning constituted of building a raku kiln (in this case repairing theirs). To do so, we had to rip out the old ceramic fibre blanket and replace it with a new one. It’s nasty business, with the fibres being nearly pure silica and alumina you do not want to breathe that stuff in.

Putting the blanket in was the easy bit, the challenging part was attaching the ceramic buttons to hold the fibre onto the lid of the kiln. Once on, she is good to go! We will be firing her tomorrow.

Kick Wheel

They are very different, and I am not sure if I like it… Beyond being much mroe phsyical, you have to anchor your body in a different way and also work with an inconstant speed. The moment you touch the clay, the wheel slows down. It is a much more meditative process in many respects.

Some good tips I got from Kerstin today was 1) the pre pull, 2) minimal contact and 3) minimal water. The pre-pull is after opening up and before the first pull, the aim is to softly pinch the clay with your inside hand so as to even out the walls to a uniform consistency before pulling. The second trick Kerstin gave me was to touch the clay less. More touch + more water = weaker clay + more mess. Try and be more considered, and for me more forceful pushing with my hands, so as to reduce the number of times you need to touch the clay. For example, Kerstin combines pulling and collaring when she comes up - moving inwards the entire time. Finally, less water the better…

Raku glazes

It felt weird only using two ingredients in a glaze, but much easier too! Raku glazes actively craze and crackle to give that iconic effect. It it sht post-reduction smoking that chars the clay black. Staying true to my colour palette this week of white, brown and terracotta reds, I chose a rich iron glaze and a transparent (comes out white) glaze only. I am curious to see how the wax resist behaves on the bowls. I am fearful that the clay won’t go black-black, more a grey, due to the resist. Tomorrow we will see…

Day 5

FIRING TODAY!

Raku is fast, FAST. Once the kiln has got to top temperature, you are turning around pots quickly. Having revamped the raku kiln yesterday, we were excited to see how the kiln would run (and hopefully much more efficiently!). It is important with the propane gas going into the raku kiln at the base that 1) it is pointed in at an angle because it helps to circulate the gas around the kiln. And 2), the lid is open slightly to allow airflow and oxidation of the glaze.

We began with a few pots, placed into the kiln using tongs that look like they belong in the Victorian era. After 20 minutes or so, you can see the insides of the kiln begin to glow orange. What we are looking for is the glaze itself to look shiny and of a honey consistency. Some (likely most) potters use cones, but Kerstin goes on look from so many years of experience. Somehow, I align more closely with this approach as it can be so easy to get caught up on the numbers you loose sight of what’s at hand. When this point is met, we are ready to begin.

Turn the gas to very low.

Take the lid off.

With our Victorian tongs, remove the pots piece by piece. At this point, it is very useful to have 2 people involved: one tonging the pots out, the other smothering the pots with sawdust before dunking in the water for a scrub. Pot by pot, we work our way through the kiln - only to go again. This raku firing and the various rounds of pots constitute about an hour or so. Fini.

…The pit firing on the other hand was much easier on the final day. It was simply a case of removing the pots from the fire once the fire had all burnt down. MOVE WITH CAUTION, for they are still fragile. I picked my bowl out by the rim and it snapped immediately. Well, at least it compensated for the lack of destruction in the firing itself as bizarrely, nothing broke. All that was left to do was clean in water and polish up with beeswax to give a lovely shine.

Final test tile pieces. Whilst a lot of the colour came from the terra sigillata, a lot too came from the carbonate in the pit and where it was positioned in the saw dust (which would make it black).

Final raku pieces, fresh out of the kiln!

FINAL THOUGHTS

Well, what a week. Looking back, my biggest takeaway was exploring unglazed ceramics through these alternative firings. This is definitely something that I want to explore further, particularly the method of ‘smoking’ your work to give that lovely, charred black effect. In contrast to a shiny glaze - wow. I love combining the matte and the shiny, the new looking and the old - which is what this week opened up my eyes to and made it a reality I can reach.

Beyond the teaching and ceramics itself, I loved seeing the lifestyle at Durn Hill Farm. Kerstin potting and James pottering about in his boat workshop, which is a beautiful space made by the boats suspended from the ceiling. Together, the two of them coming together to share their progress and their thoughts over lots of coffee and even more chocolate before returning to their own worlds again. It was truly a joy joining these two makers for the week, I am sure to return soon.

A test piece, whereby I mounted some of the pit test tiles on a smoked sheet of clay. I must admit… I do like…

James’ boat workshop

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