Black Interior On Terracotta?

[GavJ]

I am firing some locally dug earthenware clay in, well, currently a Weber grill... which might have a lot to do with my situation. But I'm only firing tiny test pieces and tiles just to get a feel for the clay for now. I am warming up my greenware on the food rack while charcoal heats up, and then when it gets to about the temperature you'd cook at, I remove the grill and carefully nestle each ware in coals and then cover them above and on the sides with more charcoal. This gets them fairly evenly bright orange hot for maybe a 20 minute soak, which I'm guesstimating is around cone 02.

 

They are definitely fired when I'm done. They are light orange color (started out chocolate brown), they sound different when pinged, and they don't melt in water anymore. However, they are still much more brittle than i would expect even for earthenware. Also, when a piece breaks, it looks just like the picture at the bottom of this post (this is not my pot, but it looks the same).

 

Is this bad? Does it signal some inefficiency or non-ideal firing conditions that I could fix, like not hot enough or too hot or not long enough of a soak? Does it indicate lower strength than I could have with a better process? I know that not all terracotta looks like this when broken, since I've seen a lot of broken roof tiles that were orange throughout the thickness. I have seen this on cheap hardware store pots, though.

[Tyler Miller]

Welcome!

 

I'm curious about where you go the idea to use a grill to fire your pots.  You're like the third person in a month to think of using a cooking device to do so.  Is there a youtube video on it?

 

As for your dark cores, the photo you've posted shows carbon coring--the organic material hasn't fully burned off.  A common problem in clays you dig yourself, but given that you're firing so short and in a grill, I'm wondering if it's not something else.  My guess is you're closer to cone 012 than cone 02 in your grill.  That would explain the brittleness.  Cone 02 is very hot indeed--at least when talking about a barbecue.

 

I recommend saving the grill for grilling and joining a pottery club to fire your work, even pit firing benefits from a bisque.

[bciskepottery]

Welcome!

 

I'm curious about where you go the idea to use a grill to fire your pots.  You're like the third person in a month to think of using a cooking device to do so.  Is there a youtube video on it?

 

I recommend saving the grill for grilling and joining a pottery club to fire your work, even pit firing benefits from a bisque.

A while back CAD featured Sumi Van Dassow doing a pit fire in a Webber kettle grill as an alternate firing method.

 

http://ceramicartsdaily.org/firing-techniques/grilling-season/

[Tyler Miller]

Thanks for the link.  Creative use of a grill!

 

I do still think a non-pit (or grill) bisque is a good idea, higher success rates and tougher ware.

[GavJ]

It may very well be cone 012, I don't have any official means of measuring on hand. I was just going off of pyrometric charts online that equate different levels of glow/black-body radiation with cones.

 

Note that these are not just sitting on top of the barbecue or even just on top of the coals, though. The wares were completely surrounded in all 3 dimensions with brightly burning lump charcoal, and I was directing a steady stream of air at them from a blower made for small pipe organs. They go bright orange-hot all over, somewhere between orange and yellow, which is anywhere from 04 to 1 depending on what chart you look at, so I hedged at 02. Could be very wrong.

 

The organic matter still in the interior makes a lot of sense with such a short firing, though. I can definitely try upping it to an hour or an hour and a half without going through too much charcoal. I'll see if that more fully burns off the organics, and might help on the strength too.

 

I can't for various reasons use a proper kiln just yet, but may be able to soon (lack of a location to legally fire one on for now. Grill is incognito. Also, certain self-imposed artistic constraints)

 

 

 

 

Also, use of a grill is just coincidental, I guess. It was just a natural choice as the only place I can make hot fires at the moment without risking drawing too much attention, that's all.

I also used a similar setup back in college to cast some brass, which melts at about 1700 and casts easily around maybe 2000, so I remembered that and figured it should be enough for earthenware, too.

[Evelyne Schoenmann]

GavJ: welcome to the forum! I often heard of doing firings in Webber (or other) grills. My mentor, Bonnie Staffel, did grill ceramic firings way back. I never had the courage to put greenware onto or into a grill. I'am with Tyler there: always bisque your piece before alternative firing. Maybe you won't get the black inside then. The "going by looking at the glow level" is too arbitrary IMO. But I encourage you in experimenting with diff. alternative firing methods.

 

Evelyne

[Denice]

Judging the temp of a firing by color takes many years of firing, I have been firing for forty years and just about got the hang of it.  In a college Anazai research group we fired a large trench firing supposedly like the Anazai's would of done.  Used enough wood to heat a house for the winter and still only got it up to C010.  I stuck all of my pieces in a bisque firing because they were so fragile.  I would get some  raku clay that is a known low fire cone, what your working with could be a much higher Cone Clay than you think.  Even the raku clays can have quite the temperature range.  Denice

[GavJ]

I'm not sure what people are referring to here by "bisque firing" since AFAIK the term, bisque firing is what I'm already doing (firing at sintering temperatures without glazing)? Sorry I'm very new to this.

 

If by just higher temperatures, I can't do that yet, because my project is to only use local materials that I have made or at least have made before (grill itself doesn't count, because it is just standing in for a dirt pit that I don't have a yard to make, and it confers no other advantages. Air stream will be replaced by an actual homemade bellows by the time I do my final pieces eventually). Some day I will have gathered or sourced enough materials and friend's land to build a larger kiln that can hold more heat and will certainly be using it at such in the event that I just need higher temperatures.

 

Anyway, until I can upgrade, there might just not be anything I can try other than a longer soak. I just didn't know if it might have been TOO much heat for earthenware. So thanks all!

 

(By the way, note that I am using charcoal, not wood, and using a large amount of forced air, which would burn hugely hotter than an unventilated wood trench.)

[JLowes]

I recall reading that Sumi was using IMCO Navajo Wheel clay, which is a Cone 5/6 red clay.  Part of achieving a certain cone is the heat work over time to achieve the maturity of a given cone, so short firings, although hot, may not give the same body maturity as a longer firing to the same terminal temperature.  This blurb on black coring gives insight and an example of black coring even at cone 10: 

 

http://digitalfire.com/4sight/glossary/glossary_black_coring_black_core.html

 

Using red clay introduces iron and impurities.  To burn out the carbon, oxidation is necessary, and our clay surrounded by burning fuel is unlikely to provide the needed amount of oxidation in a fast firing.

 

On a side note, I bought a charcoal grill after reading the article, and added this type of firing to my long (growing) list of things to try. Sadly, the grill is still shiny and new.

 

John

[GavJ]

Ah, nice. So that link suggests it may have as much to do with my cage of fuel causing reduction then as it does temperature and time. I may just have to suck it up and find some way to build an actual kiln then to fix it, where the wares will have full airflow.  Or perhaps really upping my game on the strength and directness of my airflow line (which gives two things to try immediately!)

[Colby Charpentier]

Ah, nice. So that link suggests it may have as much to do with my cage of fuel causing reduction then as it does temperature and time. I may just have to suck it up and find some way to build an actual kiln then to fix it, where the wares will have full airflow.  Or perhaps really upping my game on the strength and directness of my airflow line (which gives two things to try immediately!)

 

Consider how much heat is being transferred to the wares. Take note that in a kiln, the surrounding air temperature at peak firing is above the melting point of the clay and glazes, however, the load temperature is fixed during the phase change. I have questions about how well you're firing your wares if they're brittle. Yes, the black coring will contribute to a weaker body, but I am more questioning the quality of the melt. Achieving temperature isn't all that counts, you need to supply enough energy to the wares to obtain a good melt. The reason everyone is mentioning bisquing prior to your atmospheric firing is because bisquing will ensure you have a moderately strong body, as most pit, smoke, and low-fire atmospheric processes do not. Also, you should probably test your locally dug clay in an ideal setting before subjecting it to your primitive firing practices.

 

In response to Denice, color charts were commonly used to assess color, before cones were widely used. The color charts were a good "crutch." Most will agree that color memory isn't very strong, as color perception tends to be comparative. In a kiln situation, just observing the glaze surfaces would be more than enough.