Week 12–Slip Ups

Lauren Aldoroty
I am a sophomore studying physics with a minor in mathematics. I started doing pottery at age 8, and picked it back up again in high school. I am currently a member of the Johns Hopkins Ceramics Club.

kilntalkThis week, we had two objectives: to present our work at the first annual JHU Undergraduate Research Day, and to analyze the results of our firing. At URD, everyone in the class took turns manning our class’s poster and answering questions about our project. It was exciting that so many people were interested in our work!

In class, we returned to the museum after several weeks away at Clayworks to inspect our fired cups and tiles. Some pieces showed the signature red and black of Athenian pottery we were aiming for, while others did not turn black where they were supposed to, and others turned black almost everywhere. We observed a variety of defects, including scorching, warping, overfiring, underfiring, cracks, and discolorations from debris. These problems are reflective of the production process—from its very earliest stages all the way up to the firing process—and we have a lot to learn from each pot!

A major factor in the variation in color of our pots was the approximately 250°C temperature difference between the top and bottom shelves of the kiln. Our kiln had a total of four shelves that stacked one on top of another.  In order to track the temperatures at different levels in the kiln, we had placed pyrometric cones at the lowest shelf (the one just above the firebox) and also the third shelf up from the firebox, in addition to a digital pyrometer at the very top of the kiln’s firing chamber.  Pyrometric cones are small, pyramid-shaped clay objects that bend when subjected to a certain amount of heat.  Matt and Cami created two “cone packs” for our kiln shelves by grouping cones designed to melt at 896°C, 929°C, 949°C and 991°C.  Based on our measurements, the pyrometer at the top of the kiln recorded approximately 950°C, the cone pack on the next shelf down suggest we got to 991°C, and the fully melted conepack on the lowest shelf in the kiln suggests that we got close to 1200°C!  The result of these temperature differences was obvious. The slip on the pots on the top shelf was barely vitrified, whereas the slipped areas on the pots near the bottom of the kiln which was the hottest, turned black but were likely overfired.

Another major factor in how black (or not black) our slip fired was how fully we managed to create a reducing (i.e., oxygen-poor) environment inside the kiln around 950°C.  Since the heat in our kiln climbed very quickly to 950°C, and we did not carefully control the placing of bundles of wet twigs far back into the firebox, we likely did not starve the inside of the entire kiln of oxygen.  As we know from our readings, this reducing step is critical for changing porous red iron oxides into vitrified black iron oxides.  As it seems that we only managed to create a reducing atmosphere primarily in the lowest chamber of the kiln, and perhaps just along one side of the kiln, pots and tiles in other areas did not turn black as we’d hoped.

Matt Hyleck discusses firing flaws on our cups with us.

Matt Hyleck discusses firing flaws on our cups with us.

Another common problem is that our cups and tiles were covered with white flecks. These were a result of ash and other debris from the firing process landing on the surfaces of the pots and tiles and shielding these areas from the kiln environment. We also had numerous black scorch marks on the sides of cups and tiles that were along the pathway that the kiln’s flames took as they moved through the firing chamber and out of the chimney.  The solution to these problems would be a protective enclosure for the pots to rest in—such as other pots—during firing called a saggar box.

The most exciting part of examining our results was piecing together the chain reaction caused by a tile that exploded in the kiln because it hadn’t been fully dried before firing. By comparing our results with the photos of the loaded kiln, we figured out that when the tile exploded, it knocked two other tiles into each other. They did not vitrify where they were in contact, and had horizontal cracks as a result of the impact. The explosion also knocked off an adjacent cup’s handle!

After looking at our own results, we examined the museum’s ancient vases for flaws of their own. Examining them up close was more impressive than the first time, especially since we’ve tried to replicate them and now know how difficult it is! At the beginning of the semester, they seemed flawless; today, we were able to notice defects, including uneven firing and painting errors.

11 Responses to Week 12–Slip Ups

  1. Nice title Lauren! It was great to finally return to the museum this week to better understand what happened to our objects. Yes, there are many aspects that could have been changed or improved, but I am still very happy with our results. Although there are several scholars that precede us in attempting to recreate kylikes and similar objects, we are still apprentices and learned the most from our own attempt. Producing perfectly slipped objects from our first firing would have been an almost impossible task. Our success is only partly in darkened slip and intact objects. We are able to look at our objects and see specifically where more slip should have been applied or where the kiln was too hot. We certainly haven’t answered all of our questions, but I think we are off to a pretty good start.

  2. In addition to all the flaws that you have mentioned, it was fascinating to see that one cup in particular warped considerably from its pre-firing shape. This specific cup “ovaled”, that is, the bowl of the cup warped into an oval from the perfect circle that it was before it was fired. Interestingly, the cup was one of Matt’s demonstrations which he had handled excessively, picking up the bowl in his hands immediately after throwing it. He warned us then that clay has a memory, and indeed, we can see the result of that memory in this cup which warped to the shape it took when being held many weeks ago.

  3. In terms of a refiring, I would definitely like to try using a saggar box. Constructing the saggar box would probably be the biggest challenge. It would have to maximize volume (fill out as much of the kiln as possible), and we would probably use fire clay to make the box and have to build it in sections just like how we built the kiln walls and dome. While definitely a beneficial addition in terms of creating aesthetically pleasing paint jobs, we still have to be concerned with kiln temperatures and making sure they are even/consistent at all levels of the kiln, which is something we certainly struggled with in our initial firing.

  4. What a clever title! But even though we didn’t end up with perfectly recreated tiles or cups, I consider the firing to be a success. We ended up with results all over the spectrum of vitrification, especially when you compare Ross’s tile (which vitrified well) to other tiles that didn’t. When we discussed the methods by which we went about applying the slip, a lot of it made sense. Like on my tile, the portions that had better vitrification had more layers of slip. And in regards to a saggar, I think that would be a great idea. My group’s kylix was covered in ash and now looks speckled as a result. It’s very cool looking, but its not exactly the design we were looking for.
    Although the most eye-opening part of this past class was looking at the actual Greek kylixes. Before this course, I wouldn’t have noticed how pronouced the contour lines were or how there were small “slip ups” on them as well.

  5. Great post! It was rather surprising to me that the temperatures differences in the kiln were so drastic between the top and bottom shelves. One limitation of the pyrometric cones was that they were unable to detect the fluctuations in temperatures whenever new fuel was added. Judging from the thermometer positioned at the top of the kiln, those temperature changes were rather huge. It might be possible that the fluctuations in temperature at the bottom, which is closer to the source of fire, would be lesser. (Or, as Matt has told us that the greatest heat of the fire is at the tip of the flames, then the temperature fluctuations at the bottom of the kiln should be even greater than that at the top.) It still remains unclear if the cups and tiles at the bottom fired better (had more blackened slip) due to a higher temperature or a smaller fluctuation in temperature. Definitely looking forward to a second firing in summer!

  6. I completely agree that, after having this experiences, I will never look at these vessels the same way again. A cup that looked effortless at the beginning of the semester, now just looks impossible. Every time I look at a super-thin relief line on one of the ancient vessels, I get frustrated because I found it impossible to render one as smooth and effortless as the ancients did.
    The results of our firing, though far from perfect, has given us a good understanding of the dynamics of our kiln, and I can’t wait to see how what we’ve learned will translate to the second firing!

  7. I was amazed at just how hot the kiln got. Even after reading about kilns, I always wondered if and how the kilns actually reached the temperatures necessary for vitrification. Obviously based on our results, ancient kilns were easily capable of vitrification, and Matt said our kiln could even be used effectively as a glaze kiln. At 1200 C we could even fire stonewear, which is less porous than earthenwear/terra-cotta, and was not made in the ancient Mediterranean due to its difficulties with firing. The fact that our model of kiln could get so hot is incredible.

  8. Hey Lauren, great post, title included. I think you’re right about the reducing environment not making it’s way up to the top of the kiln. That coupled with the fact that the pyrometric cones did not droop further up means that we really had a circulation problem in the kiln. Like you said, we also noticed that ceramics placed next to one another influenced their firing. A perforated platform would help mitigate this. But what I wonder is, if we add a sagger box, wouldn’t we decrease circulation further?

  9. Great post! One of the most intriguing aspects of unloading the kiln for me was seeing how the objects around each piece influenced how it was fired. It was pretty easy to tell where an object was in the kiln based on the parts that vitrified, and the parts that were “protected” by other cups or tiles. Looking at ancient examples, we can sometimes see where this occurred, but not nearly as much as our attempts. Perhaps after years of firing, they knew how to load their kilns just so that none of the pieces would be affected by the surrounding objects.

  10. After putting so much work into working on trying to replicate the ancient designs, it was so an eye-opening experience looking back at the ancient pots. Some of the lines look like they could not have been painted with anything more than a single hair. Many of the raised relief lines were nowhere near as raised as the one that we made were. I’m left wondering, again, how could they have gotten the slip thick enough to stick to the hair enough so that it can be then applied, het not so thick that it creates dramatically raised lines?

  11. Thanks Lauren!

    Reinvestigating our pots for firing mistakes was almost like a treasure hunt for certain scorch marks and burns. Since we had such a fantastic record of where all of the ceramics were placed in the kiln, we were able to attribute certain marks to location of tiles in respect to bricks, walls, and other pieces. Looking at ancient pots once more, it was fascinating to see how some of the ancient firing mistakes mimicked our own. For example, our cup only vitrified for several lines where we used a different consistency of slip than the rest of the cup, which may have meant that vitrification only occurred at a certain temperature for that particular slip. One of the ancient pots also had an entire section that did not vitrify, but may have also been applied with a different consistency slip than the rest of the pots. The only strange thing is that the other pots on our layer somehow vitrified with the same type of slip we used, so maybe the temperature or smoke exposure on our half of the level – the side further away from the firebox – was in some key way different than the rest of the kiln.

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