Sunday, November 9, 2014

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.


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  2. Hi, can I know what type of charcoal that been used and how many that needed for this firing?