Pit-Firing Success

Just had my first run at pit firing, which was a huge success. By that, I mean that I went in to this expecting everything to break, and only half of them did. And the ones that did break are still largely usable. They just look like they went through a minor battle.

I’ll gather up the photos and notes below and use this as a space to figure out my timing. The broadest lessons learned so far are:

  1. Start slower
  2. Finish way slower
  3. Pile on more wood and get it hotter

Fire got started a bit after 2pm. Mix of wood, charcoal, and lighter fluid. Fire stayed relatively small – in retrospect, I should have built it up hotter from the start. I’ll get to that in a bit.

2:25pm – The pieces have all been added to the fire pit. They’re starting to warm up. They’re staggered a bit, but packed in quite well. On average, I think they were roughly equal distance from the heart of the flame.

2:52pm – Moved the pieces all closer to the fire by a few inches. The actual burning portion was rather small, and it didn’t seem to be putting out much heat. This is probably good for slowly warming the pieces, but it seemed remarkably slow to change anything. There wasn’t any color shift, and the damp pieces weren’t drying out in any way.

3:06pm – Moved everything a little closer to the fire again. Now only inches away at the closest pieces. The damp pieces on the wings still aren’t showing any change, but the closest pieces are turning a darker sort of grey. Though I didn’t know it at the time, the ashen ground underneath the pottery was heating up to quite a degree. This, as much as anything, was warming them up.

3:24pm – The closest pieces and a few inches back are definitely showing some color change. The very closest pieces are touching the fire and turning black. Some are resting on colas, some on wood. This is likely where I should have gone more slowly, rather than at the earliest movements (or both, now that I think about it). I heard some popping and cracking here, but it seemed to be coming from the wood more than anything.

3:35pm – I’ve started covering some of the closest pieces with charcoal and chunks of wood. They seem to be able to take the weight without issue, but some of the pieces are starting to break apart. They don’t do damage to nearby pieces, but some of the chunks can fly a few inches. The damp little scoop shape with thick walls was one of the first to break. It was still relatively damp when it was getting near the wood – I’m not sure why it was so stubborn about drying out. I’ll make sure to air dry more thoroughly in the future.

4:00pm – Starting to really cover the pieces up. It seems like the breaks are happening in thicker pieces, areas where joined by slip. or in pieces that started the process while still damp. I’m definitely rushing this portion of things, especially with the fire so weak. It doesn’t project much heat, so the transition to direct contact with charcoal must be very intense. I should have had a much larger fire, and moved to rotate and cover the pieces more slowly. I was pushing everything with a stick, so I didn’t have a solid means of actually turning them around.

4:21pm – The pieces are all touching charcoal or wood right now. By no means can I say they’re all completely covered – there’s plenty of gaps up to open air. I really should have brought more fuel, but I didn’t quite anticipate the room I’d need to cover. I also didn’t expect having so much room to work with. I’ll be sure to plan out the preheating portion, maybe form the wood into a crescent to heat around a pile of ceramics. Use some tongs to rotate them.

4:46pm – The fire is definitely dying down, and all the charcoal is pumping out heat from under ashen exteriors. Many corners are starting to come up in open air. This was likely not enough time for everything to properly cure (sinter, I guess?). The colors are tending towards white, though I later found out this was simply due to the ash. Underneath, they were turning red and black and grey.

5:22pm – the pieces are still mostly covered with charcoal. For the most parts, cracks seem to have stopped forming. If it was going to break, it would have done so by now. There’s still a pretty good amount of heat coming from the pile, but even from 3 feet away it is nearly imperceptible. There’s just too little burning stuff to fully cover the pile. Lesson learned!

5:23pm – The pieces are starting to be revealed. This is mostly my fault, as I’m poking things to uncover them more quickly. I figure there’s not enough heat present here to be worthwhile, so it’s best to just let things cool off. I don’t want to be here all night!

6:31pm – I’m pulling pieces from the fire as they become cool enough to hold. I’m being pretty cautious about holding my hand over the pieces and tapping them gently. The ground is holding a ferocious amount of heat, and the pieces need to be pushed to the sides bit by bit. They seem to be cooked well enough, and they’ve hardened well. I’ll test them later by soaking them in water.

Deflocculant Notes

Gathering up all my deflocculant notes here ahead of a slip casting run… There’s a lot of learning to be done.

I’ll start by noting that I haven’t used a deflocculant for casting yet, though I’ve been casting simple 1.5″ cubes. They’ve largely turned out odd, with variable thickness in the walls and plenty of clumping. I’ve also been using a rather thick slip to do this, so I could start by thinning it out.

What is a Deflocculant?

Particles in the clay body tend to attract each other. The electrostatic charges of the particles can be really attractive in an acid environment. They bond together, and readily enjoy forming clumps when they’re suspended in water. Also, when they’re suspended in water, the particles like to settle out. If you can make everything be the same charge (alkaline), they’ll try to repel each other. This leads to a more even mixture with less settling.

Think about it this way: ideally (what you’d get with a deflocculant), the particles should repel each other. They’re also going to try to settle out of the water, since they don’t actually dissolve. So, some clay settles in the bottom of the mix. After a time, more clay begins to settle down to the bottom too. It would very much like to reach the bottom of the container, obeying gravity and density and such. Unfortunately, there’s already clay particles down at the bottom of the container that don’t want to touch other clay. They’re pushing up against the clay trying to settle down on them. They’re also pushing left and right against the other particles on its same level. Still, there’s only so much water, which means there’s only so much room. And everything is pushing equally. Inevitably, the clay is going to be hanging in the water quite evenly; everyone wants space, and they all want an equal amount of space (i.e.: “as much as they can”).

I guess you could say – clay particles like to flock together on their own. A deflocculant makes the clay not-flock. Deflocc. (-ulant)

Casting

To make a slip liquid enough to pour into a mold, there needs to be water. Too little water, and the slip will bunch up. More complex mold shapes (or even just the corners in a cube mold) won’t get the kind of wall-to-wall coverage that makes for a good casting. Normally, this means you’d need to add a lot of water to make a slip thin enough that it’ll fill the mold.

Meanwhile, the plaster mold is working to absorb water from the slip. This means that the slip touching the plaster will harden and settle against the plaster. Over time, this clay wall thickens as more water is drawn out. (Side note: clay bodies that are too plastic won’t be permeable enough for water to be drawn through this wall). The more watery the slip, the more water that the plaster mold needs to absorb. This can lead to the mold becoming waterlogged and needing extra time to dry out. A large amount of water in relation to clay also means the clay will tend to settle to the bottom, leading to an uneven mold. As the mold dries and more clay is absorbed, the cast piece will shrink away from the walls of the mold – more water to remove means more shrinkage and more chance to crack. And, finally, the clay body will simply clump up at the entrance to the mold, making pouring the slip in and out impossible.

So let’s toss the time constraints. I only work on ceramics in the evening, after my day job. I can only use molds once per day as is, so it doesn’t do anything for me to speed up the process. I’ve also found that pouring in a rather thick slip, shaking the mold, then leaving it on the windowsill for 24 hours actually yields a leatherhard piece that is fully workable. I’ve also found that the intense shrinkage makes removing a simple shape very easy. If there is an underhang of some sort, I could certainly imagine that portion cracking off.

It seems that having a deflocculant could help the overall process. Suspend the clay in a lesser amount of water – less shrinkage, less settling, more even pouring, more homogenous mix. I don’t much mind the time side of things.

Making a Deflocculant

Clay shops aren’t readily available nearby. That means I’d either need to ship some deflocculant, source it from a non-ceramic shop, or make it myself. Much like how I’ve been pit firing, I like the idea of doing it at home.

So what can be used as a deflocculant? Looks like they’re meant to turn the mixture alkaline, so that’s the first bit to look out for. It’s meant to make the clay repel itself by being the same charge. Alternatively, the deflocculant could cause the flocculants to settle out of the mix, or plug up the particles so the attractive bits aren’t available on the outside.

There’s quite a few organic and inorganic substances that can serve this purpose. Seems that most of them have sodium in some degree. Potassium in more than a few. Phosphates are very popular too. Not stuff that I can really find at home. One compound has some potential, though: sodium silicate. I don’t explicitly have this substance, but I have the stuff to make it:

  • silica gel beads, crushed
  • sodium hydroxide (caustic soda or lye)
  • water

Side note: Apparently epsom salts make for a good flocculant. Exactly the opposite of what I’m going for, but an interesting note for the future. If I ever need something to not be runny, I’ve got something to work with.

To Do

It isn’t until I’ve reached the end that I realize shortening ‘deflocculant’ would be a good idea. I’ll refer to it as De-F from now on.

  • Cast a ball with and without De-F
  • Cast a cube with and without De-F
  • Cast a complex shape with and without De-F

Then I’ll cook them all and see what happens!

Raku and Pinching

It’s time to start gathering my notes for a Raku firing – my very first, and done the old fashioned way. I figure that humans have been firing ceramics in fire pits for longer than most modern countries have been around… so it can’t be impossible, right? I just have to be willing to accept potential losses.

The Firing Line

The Clay

Texture

This clay is sandy. There’s just no getting around that. Even looking at the dried greenware of this clay compared to the EM-100 clay I’d used previously, it’s easy to see how gritty this clay is. And it’s absolutely thrilling. The other clay, while wonderfully plastic and able to take any bend or shape without splitting, didn’t speak to me. It had no life.

This clay has a texture that helps me feel the walls as they shape. I follow the ridges of thickness to find where I can next press and slide clay. The material lends itself well to shifting weight all around. I’ve had great luck forcing a ridge down the wall of a new cup and into the growing foot. Where I once made round-bottom cups out of necessity, I can now pinch and pull the bottom of the vessel (indirectly, even) into broadening out. Then I press it into the palm of my hand to level the base out further. It’s all very organic and peaceful. Feeling the grit and the weight, there’s a real piece of earth taking shape in my hands.

Moisture

It helps that the clay body can’t retain much moisture. I start a batch of 5 cups by wedging up one large lump of clay and chopping off little bits to use. I use all the clay that I chop off for each piece, so I don’t have to re-wedge little pieces back into the pile.

As the clay takes shape and the walls thin out, the steady rate of drying lends a bit of rigidity and strength. The thicker portions stay moist and pliable, while the thinner walls can be left alone. By the time I’ve gone from the lip to the foot, the lip is ready for the next stage – burnishing.

To burnish, I stiffen up a thumb and massage the walls of the vessel. Due to how I pinch, the inside of the pot remains even and fairly smooth. It’s the outside that needs the most effort. The lip also develops a light surface cracking, but a mild pinch and poke is enough to smooth that out as well.

When the clay dries, it shrinks quite astoundingly. The first teapot I made had to be set aside at night before I could get to the lid. The next day, I went about making the lid to the perfect size. I didn’t care to notice if the teapot had shrunk overnight, as the lid would be based off the teapot’s mouth. The lid was made and fitted perfectly. The next day, I poked at the lid to make sure it hadn’t fused to the pot. Not only had it not fused, it also slid precariously. I’m afraid that by tomorrow, the lip of the lid may not entirely cover the opening of the pot. We’ll see…

Making Some Pot(s)

This is my first raku clay firing, and I don’t know much about this clay body. To combat the potential emotional toll I’ll take if all the pots break, I’ve decided this is entirely a test firing. If every piece breaks, I’ll be fine with it as long as I can learn something. (Though I’ll be very disappointed with the loss of the teapot) (Okay… I’m not actually fine with all of them breaking in the fire)

With the pinching that I’m using right now, I’ve gotten fairly swift at forming an even rim and then pushing the rest of the cup down and away from that rim. When I’d first started pinching, I began in the center and worked my way up and out. Turns out I was thinking in the wrong direction (though I’m still quite happy with the cups I’d made with that method). The only real limit of my pinching is the height or length of the walls – they can’t be much longer than the distance from my knuckles to the tips of my fingers. This limits me to reasonably sized tea cups and handheld bowls. This is a size I find particularly enjoyable.

I like the idea of easily holding what I’ll consume. No massive, gut-wrenching portions.

To further the test, I’ll be trying different thicknesses of the walls. Right now most pieces are between 1/8″ – 1/4″, though I should make a few sculptural pieces that are thicker to see what happens. A happy little monolith.

Try to be Primitive

You just can’t be a savage in a modern city, lighting a fire in the ground or gathering food from shrubbery. Living in a city makes natural living pretty tedious, all things considered. Especially when I’m trying to find a place to mix fire and earth to make a vessel that holds water.

This is possibly the most eye-opening part of this experience. I can understand the fears of forest fires and leftover waste… but that doesn’t help me fire clay. The hunt for a fire pit continues, though I have a few promising leads.

The alternative is always to rent out space in a kiln. That’s less likely to break the pieces, and more likely to ensure they’re properly fired. It’s also absolutely not what I’m trying to do.

Firing Plans

firing tests:
Object markings
Over Fire
(metal grate?)
In Fire
Bare (slow)LeafTwo Leaves
Foil-wrapped (slow)CircleJoined Circles
Bare (fast)Flower BudOpen Flower
At least one piece from each group will go to Raku post-firing, so there needs to be at least 3 pieces in each group (Raku, thin, thick). A total of 18 pieces to start.

Below is my expected list of steps and the new materials I’ll be introducing for each step.

  1. Arrive, lay out all materials
    1. Fire Pit, Cart or Box to carry stuff
  2. Place all pottery in the firing pit
    1. Greenware pottery, Foil
  3. Start coals/wood near the pots
    1. Charcoal, Wood, Lighter, Lighter Fluid, Kindling
  4. Allow fire to build and pots to dry out thoroughly
    1. Snack for the downtime
  5. Remove quick-firing pieces
    1. Tongs, Resting Stone
  6. Move above-fire pieces to the grate
    1. Steel Grate, Stacking Bricks
  7. Shuffle coals and wood
  8. Position in-fire pieces
  9. Wait (for how long? No idea)
    1. Crossed fingers, Hope
  10. Add quick-fire pieces
  11. Begin removing pieces to cool nearby
  12. Remove some pieces to test Raku bucket firing
    1. Bucket, Flammable Materials
  13. Allow fire to go out
  14. Remove and examine all pieces

The Pieces

So far, there’s:

  • 1 teapot
  • 1 handled cup
  • 1 thick (textured) cup
  • 4 thin round cups
  • 2 thin bowls

This has used up around 6-8 lbs of clay. I’m not keeping good track of that, admittedly. Certainly less than half of a 25 lb block of clay, but close to a third. And there’s still some wedged clay to count against that.

I’m also thinking about just tossing in the EM-100 Cone 06 stuff with this load if there’s enough room. It’s probably all going to break, but I’m dreadfully curious. I’m hoping the jug will survive, even if nothing else does.

Other Ideas

I might want to make a big chamber out of the EM100 clay to insulate the pottery while in the pit. I’m concerned that it will break and then collapse on the still-soft greenware. It’d likely work much better than foil.

I’d also like to begin decorating some of these pots a bit more. And make a larger, 1/2″ thick-walled pot. Projects for the weekend?

Actually, fun idea – make a larger pot out of the EM100 clay. If it breaks, no big deal. If not, then I have a fun pot that survived pit firing. Either way, it should help insulate the inner pots. However, if it breaks, there might be a sudden shift in temperature (at the critical temperature where the break occurs). Maybe not the best idea. Maybe a little Kintsukuroi afterward. We’ll see.

EM100 and Plaster

Picked up sand at two locations. I think they’re more likely a sort of chalky, diatomaceous substance instead of real sand… but that’s what these initial firing experiments are for!

Started digging into some sand research (pun intended), and came across this lovely article. Turns out it’s possible to predict (to some degree) whether a sand is likely to have high or low feldspar content based on the type of weathering it’s undergone.

Where fragmentation is rapid, granite crumbles before its feldspar has fully decayed and the resulting sand contains more feldspar. If fragmentation is slow, the resulting sand contains less feldspar. Fragmentation of rock is enhanced by exposure to fast-running water, so steep mountains are often source areas for feldspar-rich sands and gentler terrains are often source areas for feldspar-poor sands.

Jrank

I’ll be testing three mixes in the the sand test firing, to see how this clay survives being cooked in a regular or charcoal fire.

Test 1: Plain EM100 #10 clay. Wedged into a workable texture, then pinched, rolled, scored and slipped. I have three test pieces for this clay type: an espresso cup, a rocking bird, and a mannequin. They’re each worked at different stages of drying.

Test 2: EM100 clay mixed with 25% (by volume) bone-dry sand from location 1. This sand is redder than the other, and has been sifted through a medium kitchen strainer. I think it’s meant for soup. The mix was wedged with water slowly being added, and then worked immediately. There were a few moderate grains of sand, but most of it was an immensely fine powder. It mixed pretty easily, but I probably should have let it settle and wedge it again to really mix the sand in. I made two pieces from this mix – a rough pinch-formed face set with slip onto a plain square. I’m worried about bubbles in it. The second piece is more of a miniature modern-art sculpture. It’s still drying into a leather hard state as I write this, but I think it’ll join together quite well.


Slight change of plans…

Actually, a severe enough change that I’m not sure where to start.

I’ll start at the end?

While moving a bag of plaster, I wasn’t paying attention to how I was swinging it. This, plus gravity and bone-dry greenware, led to the mannequin and rocking bird being crushed. The sculptures with the sand mixed in have been knocked over and horribly misshapen as well. There’s enough damage that I’ve decided to turn the EM-100 pieces into slip for slip casting, while the sandy sculptures will still just be test-firing pieces.

Making the slip seems like it’ll be easy. I just threw the dry clay in with some water and waited. There was minor fizzing and bubbling for about 15 minutes as the pieces began to break down, and I had to add more water to cover the clay again. Checking on the bucket the next morning, it looks like all the clay has turned into a nice slurry and settled in the bottom of the cup. I’ll give it a good stir later and see how it’s faring.

And in other news… Plaster!

Plaster

Back in college, I hadn’t gotten around to slip casting. I’d use the molds for press-molding, especially on the wall of a large pot. Never with the slip, though.

I’ve been trying to make pour molds for the past few days, and it’s proven to be more difficult than expected. I’m starting with geometric shapes. No serious contours or undercuts. The first experiment was with an egg. So far I’ve successfully broken two eggs and have three halves of a mold. For the first mold, I simply did not mix and pour the plaster properly. It’s a sponge on the surface. I didn’t both creating the second half.

For the next mold, I got half the egg shape properly done (just one or two bubbles). It has a perfect opening up to a pouring shelf, so I can overpour and let the slip settle in. Unfortunately, for the second half, the egg shell bonded to the plaster. I’ve cracked the egg and drained the fluid, but the shell simply won’t release from the plaster. In the course of trying to release it, I’ve scratched through to the plaster beneath.

That’s when I realized I’d need a sturdier object to mold, and potentially one that I wasn’t afraid to break during removal. That’s when I settled on a simple wooden block. I bought a bag of these 1.5″ wooden cubes a while back, and have steadily been using them up for one project or another. Looking through the dozen or so remaining, I found the smoothest one possible. I set it up to cast from one corner to the other, almost like I’m casting a diamond shape.

The first half of the mold cracked during drying. Either the wood swelled or the plaster shrank (or both), and it split one wall away from the other three. The two pieces still fit together perfectly, so I’ll count it as a success.

The boba straw I used to create a pouring channel floated a bit away from where it was supposed to be, but not enough to be a problem.

Casting the top half of the mold was considerably quicker, but not as perfectly-shaped. After smoothing out the top of the lower half and carving a few alignment nubs, I double-soaped (with liquid hand soap) and double-oiled (with canola oil) every side of it. Then the same to the wood block. Looking back, I skipped this step on the boba straw. Over all this, I poured some more plaster and let it settle for an hour or two. When the plaster was coming down from its peak drying heat, I tried removing it from the other mold. The complete two (or ‘three‘) part mold dropped out quite easily. The two main halves separated with a sharp hit on a cloth-protected surface. The straw basically fell out on its own. The wood block was a little tougher, needing some serious wiggling to remove it from the mold. The wood felt to be in the process of swelling, but not enough to crack the upper half of the mold. One of the alignment nubs stuck to the lower mold and had to be carved out.

If the egg was a 50% success, the cube was more of an 80%. Some surface issues, a bit of misalignment on the position, and the cracked lower section. Now I have to pour the slip and test out the mold in a realistic way – once it dries completely.

Next Steps

Handbuilding

  • Nothing ready at present. I need some sort of shelf space or some such to leave the greenware out of the way of swinging bags of plaster.
  • Things to build – a face, a small full-body statue

Slip Casting

  • Half of a ducky mold with many bubbles and no pour channel
  • A full cube mold (cracked) that needs some smoothing
  • Three halves of an egg mold, unlikely to be saved

Ready for Firing

  • Small slab-built cup in greenware stage
  • Remnants of sandy clay slab sculptures in greenware stage

EM100 Clay Building Tests

First attempts building pieces with Laguna Clay’s EM-100 #10 clay – an awesome thing to win during their Grand Opening celebration.

Catalogue

I picked up a couple more 25-lb bricks of this stuff too. It seems like a basic clay to work with for some of the firing I have planned. My eventual goal is to fire this clay to bisque and then to a glazed finish, all in a fire of one sort or another. Getting my hands on a kiln is a little difficult at the moment, so I’ll be trying to get this fired up the old-fashioned way.

The first few projects are tests of this clay in an unmodified state (no grog added to help with the firing process). I’ll probably have to add some sort of grog or other material to help deal with the temperature shifts I’ll come across when firing this on the grill or in a bonfire. Side note – figure out how I’ll be handling temperature and time readings. Flame and coal color, maybe?

Projects

I’ve started with a few projects for the base clay tests.

The slip has been made from dry and semi-dry flakes of clay left over from forming the first few ceramic pieces. Just dump some water over the flakes and mix thoroughly.

The clay bodies for these pieces are the above-mentioned EM-100 #10, cut into 1/2 lb chunks and spiral-wedged. Each spiral-wedged chunk was stored in a separate plastic bag from the main brick until completely used. I added a few drops of water here and there to keep the humidity high in storage. Before shaping, I re-wedged the chunk to work the surface moisture back into the body. Few air bubbles appeared at any time, but I may have been wedging too poorly to see.

It’s been a while.

The mannequin is focused on pressure-joining pieces that have been scored and slipped. Each of the joints were pressed very firmly into one another with a fair amount of slip between. I scored all the contact surfaces using a narrow blade a bit before the material became leatherhard. It was soft enough to still be shaped somewhat without any stress cracking.

Things to watch out for: splitting at the joints due to trapped air, failure to join separate parts, warping or slipping of any parts.

The second project is a short espresso cup. The base is a simple circle cut from a 1/4″-thick rolled slab (using my water bottle), while the walls are slightly thicker. The walls were left to dry a bit more than the base, and then all contact points were scored and slipped. Rather than pounding pieces into place, I scraped and poked the damp clay of the base to join with the walls, then smoothed it all over. Then more slip, rubbed in to fill in the gaps. Then more scraping, especially on the outside.

Once the whole cup dried to leatherhard, I scuffed and trimmed the edges and corners to get a comfortable round drinking lip. If this cup survives, I intend to use it.

Notes

This clay shapes easily. It stays wet and workable for quite a while – at least 4 hours in open air at room temperature and 50% humidity, left open on a clipboard with moisture-wicking wood. The surface squishes more than it cuts, though that may be the edges and angles of my cutting tools as much as the firmness of the material. Side note: let the cup dry more and try working on the edges again.

The clay starts from a rather dark gray when wet. As it dries, the color lightens severely when going to the leather state. However, sufficiently wet slip is an even lighter shade of gray that stands out against the base clay body.

From reading and memory, it’ll be a good idea to bake all this stuff in the oven before I try firing it. I need to get as much water out as I can – which is about the only preventative measure I can take right now. I’ll also have to slow-heat the pieces before throwing them in the fire, then maybe let the fire die out naturally to keep the cooling slow. Will work out a method based on firing location/grill type later.

Next Steps

Raw Materials

  • Find a suitable natural grog from the surrounding area. How well does desert sand work?
  • Make a groggy clay and test pieces
  • Pick up charcoal

Greenware

  • Make a few more pieces with the EM-100 base for test firing. Maybe some pieces with walls thicker than 1/4″, and one with a segment at least 1/2″ on the cross section
  • Finish the hollow-body bird shape – focus on pinching the corners together, and make sure to make holes for outgassing. Maybe at the tip of the beak, for design? Then, put some serious texture on the outside walls. Will use this piece to test how texture is retained
  • Allow the cup to dry a bit more before reworking the surface for smoothness. Maybe rub it with a smooth-ish rock to seal, in case I can’t use a glaze later

Going to bisque

  • Slow-dry the mannequin before letting it finish air drying. This may keep the joints strong while the moisture of the slip settles