In this lab our combined group of 8 came together to attempt a bonfire firing of 17 pots made from a variety of clay bodies that we formed and dried over the past week. We selected 4 of those pots for specific attention and data gathering: two made from a pure clay dust and two made from clay dug out of the Carleton Arboretum.
Here you see the four pots slated for observation, one and two are the fine clay, three and four are arb clay, and yes, we did reorder them, but only after we’d taken this picture.
Multiple other participants in the lab commented on out aesthetically pleasing ‘pot summoning circle’ which in reality was meant to slowly warm the pots up to avoid norma shock. We progressively turned and moved the pots closer as time went on.
After warming the pots and reducing the internal fire to coals, we made a pyramid structure of the unfired pots and surrounded them in a teepee of kiln wood.
The pots sat inside the teepee until it caught fire.
Unfortunately, not all the pots passed 100 degrees Celsius unscathed and we heard the telltale pops of pottery fragmenting inside our bonfire kiln.
Over the course of this process, we attempted to take temperature data to the best of our abilities. In the section at the bottom of this post you can see a chart of the (somewhat erratic) data from the start of the experiment until we pulled the pots out of the fire.
After the kiln wood to burned down we began smothering the fire to make a clamp kiln in the hope of producing a reducing environment.
After piling on straw (our chosen smothering agent) we waited in the rain to see if we would need to add more.
As the fire burned through, we continued adding straw eventually smothering the fire, but would find the next day that the straw had reduced almost fully to ash.
It was quite satisfying to see all but one of our pots survived intact, even if some lost major structural features. We then proceeded to weigh and measure all of the pots. Our data is below. Note is is the same data as our partner group, Group F.
| Width at lip | Width at base | Height | Wall Thickness | Weight | |
| Pot 1 (standard clay) | 3.25in | 2.5in | 2.5in | ±.25in | 219g |
| Pot 2 (standard clay) | 2.5in | 3.25in | 2.75in | .25in-1in | 313g |
| Pot 3 (arb clay/no temper) | 2in | ±1in | 1.75in | .375in | 77g |
| Pot 4 (arb clay) | 3in | 2.75in | 1.25in | .5in | 145g |
| Width at lip | Width at base | Height | Wall Thickness | Weight | |
| Pot 1 | 3.75in | 3in | 2.75in | ±.25in | 182g |
| Pot 2 | 2.25in | 3.5in | 2.5in | .75in | 210g |
| Pot 3 | 2in | ±1in | 1.5in | .5in | 73g |
| Pot 4 | 3in | 2.75 | 1.25in | .5in | 139g |
| Δ Width at lip | Δ Width at base | Δ Height | Δ Wall Thickness | Δ Weight | |
| Pot 1 | +.5in* | +.5in* | +.25in** | 0 | -37g |
| Pot 2 | -.25in | +.25in* | -.25in | -.25in | -113g |
| Pot 3 | 0 | 0 | -.25in | +.125** | -4g |
| Pot 4 | 0 | 0 | 0 | 0 | -6g |
These results require some explanation, particularly the instances when a measurement increased. Both of the standard clay body pots chipped or lost material which drastically changed their size and especially their weight. For all the data points marked with a singe asterisk (*), we identified a major structural change that affected where we could take our measurement from. In the case of the base width, the pots flared out and the loss of their base caused a new, wider base to be exposed. However, such an explanation is not possible for all of the increases and all of the data with two asterisks (**) is likely measurement error.
But these numbers only tell half the story, each of the pots looked different from the unfired clay than went in and even from the other pots in the same firing.
Each of the pots had interesting and distinctive colorations across their surface. Pot 1 turned uniformly black on the inside and was a lighter grey/brown on the outside It cracked on both the bottom and the lip, but is otherwise intact.
Pot 3 surprised us with an interesting two tone coloration. The interior was much lighter than the exterior. After discussing with ceramics professor Kelly Connole we determined the fact that the pot was fired upside down may have cause a difference in air between the two. As was the case with many other pots, an irregular pattern of blackening and lightening occurred on the outside.
Unfortunately for pot 2, its thick walls meant most of the bottom cracked off. Other than this, the pot was impressively intact. Its colors ranged from almost white, to tan, to black on one edge.
Pot 4 had the same lighter interior, with one strong black mark on its side and a lighter body throughout. Despite its rough lip we did not observe any cracking
Conclusions
This lab yielded a number of interesting conclusions. The first of which was related to the physical appearance of the pots. Ours were much lighter than the groups who used sawdust or bark to reduce their wares, perhaps an indication of the straw’s permeability to are making it a poor choice for a reducing environment. We were also surprised by the Arb clay pots’ resistance to shattering, a factory we discussed and attributed to the natural temper provided by the organic materials in the processed clay.
Appendix 1: Temperature Data
The time in bold text represents the start of the kiln wood, the time in italics represents the start of the second day. All temperatures were taken at the center of the vessel.
| Time | Pot 1 | Pot 2 | Pot 3 | Pot 4 |
| 2:13pm | 30-33°C | X | X | X |
| 2:23pm | 43.5°C | 57.4°C | 41.3°C | 45°C |
| 2:31pm | 60.9°C | 67.3°C | 61.8°C | 62.8°C |
| 2:40pm | 67.5°C | 63.8°C | 77.1°C | 77.9°C |
| 2:48pm | 84.8°C | 76.6°C | 70.3°C | 69.8°C |
| 3:01pm | 223°C | X | X | X |
| 3:10pm | 812°C | 813°C | 905°C | X |
| 3:22pm | 763°C | 700°C | 689°C | 805°C |
| 3:34pm | 665°C | 622°C | 725°C | x |
| 11:20am | 119.1°C | 397°C | 162°C | X |
| 11:30am | X | 204°C | 85.7°C | X |
| 11:40am | 50.6°C | 96.5°C | 76°C | 162°C* |
| 11:50am | 40°C | X | X | X |
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