Homebrew Pottery - Firing local clay

SO! I have now found some local clay and I have figured out the best amount of tempering to use. I also mixed up my pure clay with some different additives for the purposes that I wanted.

Now it's time to fire the clay into true ceramic! Several processes are involved, but mainly, the firing process mainly drives out water (only some water evaporates, and the rest is physically trapped or chemically bound), burns off organic material, changes the crystal structure of quartz components ("quartz inversion"), and sinters particles together.

My "kiln" at the moment is nothing but a Weber grill. The temperatures on the rack where you would normally put food only get up to maybe 600-800 degrees, but by completely burying clay wares in charcoal, the temperatures can get up to the roughly 1800+ degrees needed to begin to make true, low-fire earthenware ceramics (bisque temperature). The whole heating process needs to proceed SLOWLY. For more information, google "firing schedules." Details are beyond the scope of this blog, unless people are interested.

I'm firing simple bricks for now, to be used later as kiln components for a proper homemade ceramic kiln. The orange bricks in the photos are ones I fired earlier, and have painted with a homemade ash glaze as test pieces for glazing. This is simply part potash and part local clay, mixed with water and painted onto the pieces then left to dry in place. You can see the grayish ash on top of them (see future blog posts for more details).

First, I heat up the bricks around the outside of the grill, as the coals are lighting. This very gently brings them up to ~150 degrees over the course of half an hour to an hour, driving out as much water as possible without creating expanding steam that might crack the pieces:

Next, I move the bricks into hotter and hotter zones, eventually leading to a pile in the middle. This brings them closer to maybe 400 degrees, driving out the last bits of chemically bound water.

I remove the grill and very quickly (lest the pieces start cooling off) add more fresh charcoal to the fire to make a nice bed. I pile the pieces back on top of the new charcoal bed and leave them there for maybe another half an hour or so. You can see in one of the photos below that open flames are visible from organics burning off, and the bits of grass are charring away. this is somewhere between probably 600-1000 degrees. 

I did this part too quickly still and caused some black charred-looking areas, which is a sign of too little oxygen and too quick of organic fuel consumption (it has to do with the iron in the clay body reducing), but oh well, lesson learned for next time.

Once most of the organics looked burned off, I covered the pieces entirely with a layer of lump charcoal. This decreases oxygen access, so AS SOON AS POSSIBLE they need to have a forced airflow added to them at this point, or further "black coring" will occur, which is bad for your clay. I had a box fan pointed at the fire when I did it at this point, but this was probably not enough.

The upper charcoal is lighting, and you can see the red-hot core of the inside part of the grill where the pieces are. Open flames are still visible, but at this point I wouldn't be too concerned about it being organic material, it's probably just the charcoal itself. This is dull to cherry red glow around 1500 degrees maybe at this point:

The firing continued into the evening hours. The internal temperature is now sufficient to create bright red to orange hot ceramic pieces inside, as you can see in the photo where I briefly lifted the lid off to show a brick inside. This bright orange to almost yellow glow is probably somewhere around 1800 (+?) degrees. During this time, I was blowing air into the fire from a squirrel cage electric powered fan, positioned underneath through the ash collection hole, firing oxygen up through the fire. I got lazy and was inconsistent with this, but it really should have been going continuously, all the way until the fire burned down, if I wanted the strongest pieces. Since I let it go without forced air for awhile, it further reduced more of my clay's iron and made the pieces more brittle than they should have been.

This temperature was held for maybe another hour or so, adding charcoal as necessary. After that, I let it die down on its own all the way until cool (several hours). Then I removed the bricks and took the photo you see below. The bricks have been successfully made into true ceramic, with their oxidized iron components giving them a familiar brick hue. The especially light color is no doubt due to the sand mixture I added.

The gray/black spots you see are reduced iron from me being impatient with my firing schedule and not using enough forced oxygen. These reduce the strength and effectiveness of the bricks and are to be avoided.

Generally, the bricks were very brittle, probably too brittle to use. Not just from black coring, but from probably using too much sand and grass clippings in the mixture. Just from touching them, it is OBVIOUS though that they have huge insulation value. The bricks are very lightweight and very porous from the burned away grass, almost like pumice. All those tiny air pockets make them more effective at insulation, at the expense of fragility. These need a higher percentage of clay overall, because the balance is too far toward fragility, but it's a minor recipe tweak.

The other three pieces were ones that were fired earlier, and simply had glaze applied to them. You can see how they hold specific shapes much better and have little or no porosity, because these pieces were 100% clay in composition (on the inside/main body). You can also see how they are redder in color due to higher iron content when sand and grass are not mixed in.

The glazing experiment was a partial success. From left to right in the image below, glaze mixtures with a lower ratio of clay slip to potash are shown. The highest clay content on the left strangely seemed to have burned away completely, or perhaps got scraped off by accident, this one is confusing. The other two are as expected: the medium mixture in the middle looks like I just painted more earthenware right onto the block. This is no true glaze, and won't help to keep water out, etc.  But the high-potash content piece on the right shows signs of JUUUUUST beginning to form an actual glaze in the upper left, and a near-glaze (white) elsewhere. The black corner is actual glass, and is proper, waterproof glazing!

Getting the whole piece to be consistently covered will require a combination of just a slightly thicker coat of glaze painted on, a couple hundred more degrees of temperature, and probably more potash in the recipe. The more potash you add, the lower the glassifying temperature, but the more fragile and likely to crack (and harbor bacteria) the glaze is, so this is a careful balancing act, but I feel confident now that an actual waterproof glazed mug is within my abilities!

Next steps include making a more proper kiln with insulative lining to get hotter temperatures, adding better airflow and oxygen control throughout the process, and fixing the glaze and brick recipes.

Creating Homemade Refractory Bricks

"Refractory" is just a word that means "very resistant to high temperatures." In the context of pottery and glassmaking, I need refractory material for the lining of my kiln(s) I am going to make. Regular, pure clay doesn't cut it. I can also use refractory for making bricks to hold up pieces in the kiln or to use for making the actual walls of a kiln, if I want to make one out of entirely my own materials.

For either of these purposes, I need a substance that is actually not only refractory, but also insulating. That is, it should neither break down at high temperatures nor should it let heat get past it. Tungsten metal is highly refractory but a poor insulator. Cotton is not refractory (will burst into flames at maybe 400 degrees F) but is a great insulator.

Pure earthenware clay by itself is mediocre at both of these things by itself--it will slump and melt at moderately high temperatures, and it isn't actually that great of an insulator (Think about how quickly your mug gets too hot to touch on the outside when you pour coffee in it). So we want to improve these qualities for furnace materials.

Refractory recipes can get very complicated, but for mine, I want only locally available ingredients. I am going to have 3 ingredients: dry grass, quartz sand, and some of the clay I prepared earlier here.

• The clay forms the main body of the material and holds everything together. It is also, of course, reasonably heat resistant.
• The sand raises the melting point of the material significantly, since quartz melts about 500 degrees celsius higher than earthenware clay does (note: sand is locally available, but this particular batch used hardware store sand just for now). 
• The oven-dried grass helps hold everything together at first, since the clay mixed with sand is crumblier than pure clay (see here [link]). Later, the grass will burn away in the kiln and leave lots of tiny air pockets that will help insulate against heat transfer.
• ...another ingredient I could have added is wood ash. WASHED wood ash. I.e. the stuff left over after you have soaked new ashes in water and drained off the potash a few times (this will be the subject of an upcoming blog post, since potash is used for glass making and pottery glazes). Washed ash stuff is much more heat resistant than quartz, so replace a little or a lot of the sand with it, if it's available. It must be washed ash, though: raw ash contains potash, which is a chemical that actually lowers the melting point of quartz, and thus is counterproductive.

The ratio I'm using is about 50/50 clay and sand by volume and then about another equal part as the sum of the first two ingredients in dry grass by volume, loose (not packed!). In reality, it's probably something like 70/27/3 by mass for clay/sand/grass, but I don't know. It's easier for me to measure volume, so I'm just using that. Mix it all together, and you get something like this:

This is essentially adobe in it's raw form. You can build some pretty strong buildings out of this in dry climates if you let it bake in the sun for awhile, although you'd probably want more sand and grass in it.

The adobe is much harder to work with than pure clay, but making things like bricks is still easy. Below are some examples of bricks that I made out of this material and then fired (I'll cover firing in a later post). Click for a larger size. As you can see, the grass has burned away and left them porous. These things are super lightweight and feel like pumice, and their insulation is probably excellent. Unlike the raw adobe, though, the fired bricks are also extremely brittle and easy to break, since the binding strength of the grass has been removed and now actually hurts the strength of the material by leaving air behind instead. 

The examples below actually used more like 60/65% sand, 35-40% clay by volume, and the fact that they are so brittle is why I'm now suggesting a 50/50 mixture instead (I also toned down the grass slightly from these bricks):

Notice the gray spots all over -- there was not enough oxygen and/or not enough time during firing. This is the same issue as that cheap hardware store pot I showed in an earlier post. It's not the end of the world, but it does increase brittleness even further, and I need to fix it. I'll discuss this more in my post(s) about firing clay.

You can also see a crack in the middle brick that formed just from roughly piling them in this stack! That's how brittle these are with the sand-heavy recipe... Again, use more clay than this (which I will be doing in the future).

HELPFUL LINKS

Probably the most popular recipe for homemade refractory online is this one:
A version using perlite for insulation and cement as well.
It does not use local materials though. Even less local and not homemade are commercial versions:
An example of hardware store refractory.

My own version is adapted from commercial versions and the backyard metal casting website, but substituting locally plausible materials (the sand and clay are literally locally gathered) to fill all the same roles in about the same ratios.

Homebrew Pottery - Finding and Refining Local Clay

In this long term project, I'm going to make a drinking mug completely from scratch. I will dig my own clay, make my own additives, refine my own high temperature fuel, fire it all in a homemade kiln, and apply homemade glaze then fire it again. In the process, I hope to get more fully in touch with all of the items and the effort that go into a single, simple tool in my life. This is a project about attaining a broader perspective.

Also... I get to play in the mud.

SO LET'S BEGIN! FIRST WE NEED TO FIND SOME CLAY

 

There are many kinds of clay, and you should be able to find some sort nearly anywhere you live:

"Earthenware" clay is easy to find most places. This is clay that has a significant number of impurities, notably iron oxides. As a result, it falls apart at lower temperatures than higher grade clays do. It also turns red when fired from the iron content (or black in an oxygen-starved environment). Earthenware won't vitrify (turn to a glass-like state during heating), so the products aren't waterproof unless you glaze them.

"Stoneware" clay (also called fire clay) is a purer clay that can be fired to much higher temperatures, where it can vitrify and be watertight naturally. You CAN also glaze it if you like. It is white or light gray in color and is usually deeper underground.

"Porcelain" clay (or china clay) is an almost pure "kaolinite" mineral of silicon and aluminum and oxygen, which allows for the highest firing, whitest, slightly translucent pottery wares. This is rarer than fire clay, harder to work with, and you're very unlikely to find any on your own.

Personally, I was only able to find local earthenware clay. Not too pretty right out of the ground:

For earthenware, just look around your neighborhood or yard for areas that LOOK like clay when wet (pretty technical stuff, I know). A good time to notice this is after a storm. Also check any bluffs/cliffsides/ravines that show you a cross section of soil. River or stream banks are also good bets.

Another method is to go to this website (http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm), hit "start WSS," highlight your neighborhood as an area of interest, then go to the "Soil Data Explorer" tab and run a map of "clay content" - it asks you for a depth first, so choose something like 0-12 inches.  You will see a colored overlay of your neighborhood with % clay of nearby soils. Find a spot where you might get permission to dig that has a lot of clay, and start there!

Once you find what you think is a high clay content soil, get it sort of wet, and roll a "snake" between your hands about half an inch thick. Try wrapping it around a finger. If it wraps without breaking apart, then you have a good amount of clay in that soil. You can still work with lower content soils, it will just take longer.

NOW TO REFINE YOUR CLAY

Chances are, if you dug up some local clay, it's going to be impure. The process of purifying is pretty labor intensive, but also entertaining! You get to explore your inner child and go play in the mud a lot.

You will need: A couple of 5 gallon buckets (if you used one to dig clay, that counts as one), some twine or rope, straining cloth like muslin, a sieve or loose burlap, an old large spoon, and one or two nice wide wooden boards or a piece of plywood for drying.

1. With the clay in a 5 gallon bucket, pour in about 2 parts water for every 1 part soil you dug.
2. Use your hands to break up any big chunks underwater. You want the smallest possible sized bits of clay. Be careful NOT to compress clay together! This will will slow everything down, because water won't get to your clay particles as easily. Just loosely break up clumps, and swish water into all the nooks and crannies.
3. Let this sit for a full day (you may want to put a lid on it if it's outside, due to bugs and leaves and stuff). The smaller the bits you crumbled up, the faster the clay will "slake" into fully wetted clay water.
4. The next day, stir up the mixture just a bit to get settled clay off the very bottom. The idea is to get tiny clay particles in suspension again, but then wait just a couple of minutes for sand and silt to settle out again.
5. What you're going to do is pour the clay into some muslin or similar cloth to let it drip out excess water. A small scale version I did looked like this:
   
   
   
6. But it's more efficient to do everything at once. Since then, I've changed to 5 gallon buckets with muslin tied around their rims, with the water dripping into the bottom of the bucket. So tie off some muslin VERY SECURELY (4-5 tight wraps of twine, it needs to hold heavy weight!) around the rim of a bucket, leaving about a 1/3 bucket height worth of room at the bottom for dripping. Then position a strainer and pour your clay water through it into the cloth, to remove the floaters. As soon as you see anything other than clean, smooth looking clay water coming through, STOP pouring:
   
   
   
7. Discard everything left in the first bucket, which should be a bit of clay, lots of sand, rocks, etc. Also discard whatever the strainer caught (usually remaining floating things). From my small scale versions, this is what some of the leftover material in the bottom looked like:
   
   
   
8. You may want to repeat steps 4-7 if you still feel sand or sediment at the bottom of your bucket, but really it's not that big of a deal.
9. Around 1-2 days later, check on your clay. A lot of water will have dripped off, but it should still be fairly wet. If it is dry enough to spread onto a board without running off the side, then you're ready for the next step. Usually a "milkshake" consistency is enough to make it not flow off the board. If it is still too wet, let it sit another day. Note: the clay will compact a bit on the bottom near the cloth. It may look like pure water on top, but be milkshake consistency or thicker underneath still, and it might be ready. Check the actual clay.
10. Now take a spoon or your hands and glop your clay onto your wooden boards or plywood, and spread it maybe 1/2-1 inch thick. Scrape off as much as you can from the cloth.
11. Set the boards outside in full sun if available, and they will dry out the clay rapidly. A fan blowing over them works pretty well, too, at night, but sunlight is much less tedious. In sun, check every 10 minutes or so and remix if necessary to make sure that the edge clay isn't drying too quickly. With a fan, every 20-30 minutes. You don't want it to ever get crumbly-dry.
    
    
    
12. When the clay is of a good strong pottery building consistency, put it in a plastic bag or wrap in plastic wrap or similar, so that it stops losing moisture. Now you should be able to store it and treat it like any store bought earthenware clay. When it's all dry, it works just like commercial clay! Here's some freshly sunned clay:
    
13. Be sure to "wedge" your clay before building with it, like you would a commercial clay. If you don't know what I'm talking about, wedging means just mixing it together to make it an even consistency. There are many ways to wedge clay, and I can't explain them as well as online tutorials can. Check out several methods here: wedging/kneading methods. Then build stuff:

    
    (I know, I'm a masterful potter, eh?)

IN FUTURE POSTS

I will go over the remaining steps to making pottery wares (assuming I can get them to work!):

• Tempering the clay: adding crushed pottery bits, shells, or sand to the clay to make it less likely to crack. This requires methodical testing to find the right amount to add for a given clay.
• Making some sort of kiln: I will be attempting a wood or charcoal fired kiln to continue with the "from scratch, if possible" theme. In another post, I may also attempt making my own charcoal. Ideally, the kiln would be made out of homemade refractory bricks, but I might need a source of fire clay first, if I'm to do that.
• Firing the clay, and possibly glazing it usefully somehow, although I understand glazing is difficult to do with low temperatures and with non-exotic, local materials. We shall see.