This week’s experimental blog is courtesy of Rachel Brewer, Bradford Kaims Assistant Supervisor.
Following on the heels of the beer brewing experiment, our Week 5 experimental archaeology project was an effort to make a variety of pottery vessels using only raw clay sourced from our prehistoric site here at the Bradford Kaims. We knew from previous seasons that the trenches and test pits often turn up natural clay deposits of varying colours and quality. We’ve also had a few examples of possible Neolithic and Bronze Age potsherds surface during excavation; so this summer’s experimental archaeology program seemed like a perfect opportunity to test out our prehistoric potting skills! Altogether, processing the clay, forming the pots, and the subsequent firing turned out to be a messy, fun and educational experiment for all involved.
Step 1: Gathering the clay
While digging a series of shovel test pits earlier in the season, we hit upon a substantial deposit of clay about 50cm below topsoil. Seeing a source of raw material for our pottery experiment, I dug out a bucket’s worth to begin processing. Though the clay was mostly light orangey-brown, there was a thin layer of grey overlaying that; it also gathered a good amount of silt and peat on its way out of our 20cmX20cm shovel test pit. Through processing, these colours and textures blended together as shown in the later photos.
The raw clay.
Step 2: Processing the clay
This was the longest part of the process, due in part to my own inexperience and also to the wet conditions on site. I knew from research that the best way to process raw clay is often to dry it out completely, grind to a powder, sieve, and slowly reintroduce water until the clay reaches a workable consistency. I also knew how unlikely it would be that we could completely dry out that amount of clay in a timely manner, particularly when it was raining almost daily. So I opted instead for wet processing, which involved the help of several pairs of hands pulling all of the clay into small lumps and mixing/mashing it up with added water in a large plastic box. This part worked better than expected, and after a couple of days of minimal stirring, nearly all of the clay was liquefied.
Slaking the clay on site.
At this point we poured the slip (liquid clay) through a sieve to remove the largest inclusions, mostly small stones and twigs. We could have used smaller screens and sieved multiple times for greater purity, but I chose not to since examples of prehistoric pottery found at this and other sites indicate that prehistoric people were not processing their clay to a high degree.
Sieving the slip.
For about a week I attempted to do a daily pouring-off of the water that would accumulate on the surface, hoping that between evaporation and pouring off that the clay would thicken a bit every day. The couple of days I was able to let the boxes sit out in the sun did help, but it wasn’t working quickly.
The slowly thickening slip.
On one sunny day I cut open a bin liner and laid it out on the grass like a small tarp, then I poured the thickened slip out on the plastic. This increased the surface area the sun could reach and it was noticeably thicker by the end of the work day, but it still wasn’t drying out fast enough. We had to rearrange the experimental schedule and move pottery back a week – I had only a week to get some workable clay and I was running out of ideas!
Spreading out the clay to increase the surface area for evaporation.
My final effort involved pouring the clay into an old pillowcase, tying it closed with string and hanging it from a tree to allow the water to drain out with gravity and air. The better part of the week passed before I noticed much of a change, but much to my relief, the night before we were scheduled to make the pots we had somewhat sticky but relatively workable clay!
The last resort.
The final product.
Step 3: Forming the pots
Before beginning our pot making, the students used rocks to crush up a few soft pieces of sandstone sourced from Trench 6; we used this sand as temper.
Crushing the sandstone down.
The coarse sand used as temper.
Since wheels were not used by prehistoric potters, the students learned to use the two most common methods of building pottery without a wheel: the pinch method (formed by pinching a solid ball of clay into the desired shape) and the coiling method (rolling out rings of clay, stacking the rings, and smoothing them together). A small amount of temper was added by each individual to their own allotment of clay. Of course we had some creative minds in the mix who ventured beyond the utilitarian forms like bowls and jars, and by the end of the day we had quite a collection of unique creations! We set everything we made on two log disks that would be easily moveable and would absorb moisture. After that we just had to let everything dry out completely to prepare them for firing.
Step 4: Firing
When it was time for firing, we began by building a small fire in our fire pit on site. Pottery has to be heated very slowly, so we began by placing the dried pots around the edge of the pit and then slowly moving them in close to the fire. Once the pieces were against the central fire, we began placing larger branches over and around the pots, completely covering them and creating a kiln effect. We kept a large fire burning for about an hour and a half, then allowed it to die down to coals. Since our time on site was limited to 5 hours and the pots needed to be cooling before we left site, we weren’t able to keep the pots firing for the ideal amount of time, which for our purposes would have been around 4 hours.
The final step of firing is allowing the pots to completely cool before removing them from the pit. Before we left site for the day, we dug the pots (none of which had broken!) out of the coals, stacking them against one wall of the fire pit and shoveling the coals to the opposite wall. We then covered the pots with a layer of grasses and sedge, placed a couple of metal sheets over the pit to protect the pots from rain, then left for the night.
The fired ceramics hot from the fire.
Upon examining the cooled pottery the next day, I was pleased to see that none of the vessels had cracked or exploded during firing. Additionally, the pieces had fired, if not completely through, then most of the way through despite the shortened firing time. The fired pots are noticeably brittle and not completely water tight, but with a little more practice we could probably produce vessels that would be more serviceable. Since we accomplished our goal of using only raw materials from site and a fire to create prehistory-inspired pottery – and we had fun doing it – I’m calling this experiment a success!