Wednesday, April 11, 2012

Firing the Thule Pots

Whole and exploded pots
I fired the Thule and Choris/Norton pots yesterday, with generally positive results.   Laurie let me use one of the fire pits on The Compound for this phase of the project.  Of the six pots that started the firing, one was reduced to a ziploc bag full of barely recognizable pottery sherds by the end of the day.  The remaining five stayed in more-or-less one piece, although two had sizeable heat spalls pop off in the second hour of the firing.

Looking good on the outside
I fired them for just under three hours, turning them every 10-15 minutes.  I'm not sure exactly what the firing accomplished.  I wouldn't say that the pots are fired like ceramics in a kiln.  I don't think there was a significant change in the chemistry of the clay, but it got hot and dry and hopefully a little harder and more durable.  They should be a little more stable now, although I didn't get all of the colour change that I was hoping for.  A couple of the pots have a good colour on the outside, but the clay inside is still very light coloured.  I need them to be almost black in cross-section so I'll have to antique them a bit more once I crack the sherds into shape.

The  six pots before the firing. 

The five survivors at the end.

This one lasted about a minute
I had originally planned to put two pots directly in the fire and heat the remaining four around the edge.  These are pretty fresh pots, with a lot of moisture in them.  I made half of them five days before the firing and the other half four days before the firing.  In pottery terms, I'm sure I would have had much safer results if I had waited another week or two for the pots to dry.  But, even stretching the drying time to four or five days was pretty generous for this style of pottery which would traditionally go from raw clay to firing all in one day.  The plan to put two pots in the fire ended pretty abruptly when the first pot that I put in started exploding almost immediately.  It kept popping like popcorn until it was rubble.  The two video clips below show the pot popping.  Based on that, I didn't bother putting a second pot into the fire.

At the end of the firing, I fished these and a dozen other fragements from the exploded pot out of the coals.

They hold water without reverting to mud
When I got the pots home I wanted to see if they would hold water.  They were still warm from the fire and I didn't want to risk thermal shock so I filled them to the brim with warm water.  They all held water, so I let them sit for about 15 minutes to see if the water would slowly seep through them or if they'd turn back into mud.  When I checked on them again, the two blood coated Thule pots had developed big cracks around the rim.  I'm guessing that they started to reabsorb the water and tried to expand, which led to the cracking.

Cracks formed in the two thicker pots with the best seal blood coating.  There was one thick pot that didn't crack, so I don't think it was thickness alone that caused the problem.

Surface heat spalls
So at the end of the day, I really only have one pot left intact.  Two survived the firing, but are now cracked from the water, two have surfaces pitted with heat spalls, but can hold water, and the sixth pot is in inch sized fragments.  As far as making pots go - this would have to fall into the "learning experience" rather than "howling success" category.  However, for making sherds, I think I'm still on track and can continue to work with these vessels.  Even the surface heat spalls might work out for me.  The sherd that I'm trying to match has a couple areas where the outer rind of the pot has flaked off in a similar size and shape to the heat spalls.

The interesting things that I want to remember for next time:

  • Don't put the pots in the fire - heat them around the edge of the flames.
  • Pots coated with seal blubber only were the ones to experience surface heat spalls after 2 hours of firing.
  • The seal blood adheres to the pots best if it goes on while they are still cool and barely dry.  Blood smeared on the pots on the hot sunny day flaked off in the firing.
  • The two thickest pots with seal blood coating were the two that cracked from the added water.  Thinner pots and those smeared with seal blubber only, did not crack.

The three pots in the foreground had seal blood on them at the start of the firing.  The two on the left had the blood applied on a cool day, while the pots were barely dry.  It stayed caked on and was cooked onto the surface - although they are also the two pots that cracked when I filled them with water.  The taller pot on the right had the blood coating applied on a hot sunny day and it never really adhered like the other two.
Photo Credits: Tim Rast


  1. Is there any archaeological evidence about how the Thule fired their pots? I ask because digging half to a full meter down into the soil would help contain the heat and increase the temperature reached.

  2. Where did the Thule get enough wood to fire pots in this way? I remember the Inuit on Baffin Island did not even know how to light a fire on the beach when I was there in the early 1990s. I thought this was really funny until I realized that we archaeologists were burning ALL of the really valuable diftwood in the entire area.

    I guess there was a lot more driftwood and small trees in North Alaska and the Bering Sea than in the East Arctic. Still it seems an awful lot of wood for just a few containers - gratuitous use of wood which must have been somewhat of a valuable resource. Patrick

  3. I'm not aware of any pits like that. The archaeological evidence for the firing process seems to be indirect. Pot sherds seem to range from low fired to not fired at all. Some of the organic tempers like grasses and feathers are preserved in tact in the pots, suggesting that they were never brought to high enough temperatures to burn out. From what I've read, sintering may begin to occur in some vessels indicating that they were exposed to higher temps, but that may be the exception rather than the rule.

    Ethnographically, the two methods of firing that I've heard for Alaska are firing the pots around a fire like I did or putting flammable material inside and around the pot and heating them directly in a fire.

  4. I didn't keep track of how much wood we used in the fire. Its wasn't alot - maybe the equivalent of an 8 or 10 foot log with 3 or 4 inch diameter and some kindling. The impression that I get from the literature and the artifacts is that firing the pots wasn't really a priority and they weren't expected to have a great lifespan. In excavations, the many of the sherds dry out and crumble quickly and the fracture patterns on those that survive look more like hard caked mud than a vitreous ceramic.

    After seeing how completely the pot crumbled in the fire it makes me wonder about some weird clay concentrations that I've seen in hearths here in Newfoundland. I can imagine a low fired pot returning to a puddle of clay if left to time and the elements.

  5. Cool stuff! Certainly a bit mind-bending for me, used as I am to more, ah, permanent pottery...

    If you ever do this again, you could try letting the pots warm up (and, therefore, dry out) a bit around the edges of the fire before moving them directly into the flame. Without the water trying to evaporate out, you'd have less risk of explosions.

  6. I did try coaxing one of the seal fat coated pots a little closer to the flames, but that's when the heat spalls started popping off of the surface. It might have been because I was moving it closer to the fire, but the second fat covered pot started heat spalling at about the same time, so it might not have been related specifically to me relocating the pot. I think I might try spreading the firing time out a bit more. Maybe fire them all day rather than just a few hours. But then you run back into the problem of consuming an unrealistic amount of fuel.

  7. I have a piece of authentic Norton Culture Pottery,the vessel was dug this year. I am looking for someone to stabilize the base and restore the rim/lip. Any suggestion?

    1. I'm not sure - I haven't done fieldwork in Alaska. I'd start with the folks that issued your archaeology permit and ask them if they can provide a list of conservators who've done work in the area.


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