RSS FEED / SITE SYNDICATION

This site uses SharedRSS

To keep informed of new content as it is added, just right-click on the XML icon below and select 'copy link' -- then insert that link into your favorite RSS Aggregator.

Prehistoric Pottery Replication Experiment

Prehistoric archaeological sites in the Lake Chapala basin are often first recognized by the abundant fragments of broken pottery that are found in and on the ground. Indeed, since most of the towns around the lake are situated on sites that were occupied during the prehistoric period, one also finds potsherds well above ground -- in the adobe walls that are still common throughout this area.

Archaeologists have described and classified the pottery types discovered in the area, and have a rough idea of the time sequence particular styles represent. One question that has not received much attention so far concerns the origins of these pots. Were they all locally made, or were some or all imported from other areas? To identify the origin of the pots we need to find what clays they were made from, and learn how widespread particular types of clay are. Archaeologists with access to well equipped laboratories may do this some day by analyzing the chemical makeup of the sherds and clays in the area.

I checked the tianguis in Chapala, and they didn't have any affordable HPGe detectors available for Neutron Activation Analysis, so I had to resort to more primitive means of investigating this question.

My plan is to find, process and fire clay samples from local clays, and see if I can learn anything in the process. This is a report on my first attempt. My total previous experience in making pottery consists of one Community College class in pottery making nearly thirty years ago, and a single attempt to collect clay and fire a pinched-pot (ash-tray size, nothing grandiose) in a dung-powered bonfire. The class, of course, used commercial stoneware clay and most projects were wheel-thrown. As I recall, the little piece I created then was very similar in character to the ones from this experiment, except it was light gray in color after firing. I was living in Wyoming back then, and used clay from a dried playa that was highly bentonitic.

Gathering and Processing Clay

For this experiment, the most obvious first choice for clay is Lake Chapala itself. So one day last September I took our big plastic bucket down to the lake, waded in to a fine mucky area full of lirio here in San Antonio Tlayacapan, and dug up the plants to get them out of the way. Then I scooped the black, slightly sewage-smelling, muck into the bucket. Not a very promising start -- I wasn't sure how much was clay and how much was decayed plant material, sand and other contaminants.

Lugged the bucket, now alarmingly heavy, back to the house -- which seemed to have moved much further away during my absence. Added a little clean water to the bucket of muck and stirred it up well, then let it sit for about two minutes for the stones to settle to the bottom and plant material to float to the top. Scooped off as much plant material as possible, and then poured the remaining slurry into another bucket, stopping before the stones at the bottom came out. Discarded those stones and the sandy muck that surrounded them, then left the slurry to settle overnight.

The next day it didn't look much better, so I covered the first (now empty) bucket with window screen, and poured the slurry into it, discarding all the rocks and plant fragments caught in the screen. Let that settle overnight.

The third day it was still totally opaque looking, but I decanted from one bucket to another, leaving the heaviest sludge in the first bucket. It felt very sandy when I rubbed some between my fingers, so I discarded that sludge. Left the remainder to settle overnight again.

Late the next day, after it had been sitting for about 35 hours, I poured the water off the top, leaving just the material I hoped was mostly clay. The water I poured off was murky, but thinly so -- I could see my hand (barely) six to eight inches under the surface. Poured as much of that off as I could without losing too much of the clay, which of course was becoming stirred up by the pouring process. Then I mixed up what was left to make it as runny as possible without adding more water, and poured it into a large plastic bowl.

Making Pots

Now I have since read on the Internet that it is common practice to let the clay dry out completely, then pound it to a powder and add water for use. Guess that would be more like the powdered commercial clay we used in class, but I didn't do that. Instead I checked the clay each day as it dried out. Three days later (six days after the clay was gathered) it seemed about the right consistency to begin working with it. I formed it into a solid ball, just over four inches in diameter, and weighed it -- 2 lbs. 5 oz. or about 1.05 kilograms. It is very black at this stage.

Next I added some sand for 'temper' (a controversial term that, but I have seen no adequate substitute, so I'll use it). I added a half-cup (5 ounces or 142 grams by weight) of fine, mostly black sand but with lighter specks of tan and white. I didn't have any locally gathered sand -- my wife and I picked up some of this black sand on the shores of the Coahuayana River, near its mouth at the Pacific Ocean (that is the river that forms the boundary between the states of Colima and Michoacan there). It was the only sand we had that looked fairly sharp-edged, which I have read is preferable to rounder particles. I didn't add more than the half-cup because the clay was not very plastic, and I feared more sand would make it crumbly.

Began by making a flat 'pancake' of clay we will call The Disk. Then I made three little pinch-pots, which we call Lazarus, Crude and Base. Base, of course, I used as the base on which to build a coiled pot, so Base ceased its independent existence and became Coiled. I had some clay left, which I set aside to use the next day. I put the ball of left-over clay, Coiled and Crude in back in the bowl and covered with a damp cloth. Crude and The Disk I left in a shady spot, but exposed.

A note on my coiling technique to produce Coiled. I rolled chunks of clay between my hands until they were just a little thicker than a pencil and about four inches long. Then I wetted the top of the pinched-pot Base and looped the coil on top, smoothing the edges by pulling clay downward on the inside and upward on the outside of the pot. After each coil was added I let it sit an hour or so before adding the next coil -- that gave it time to get a little stiffer, but still left it plastic enough to work.

The next day The Disk and Lazarus, which had been in the shade, were leather-dry and light brown in color. Coiled and Crude, along with the remaining ball of clay, were dark brown and softer. Lazarus was badly cracked, so I added water and melted it back down and mixed it with the clay ball. With added water it became black again. Crude already had hairline cracks along the rim, but it didn't look like it was going to fall apart so I tried to seal over the cracks with a damp finger.

I drew a crude figure in The Disk with a pointed stick (OK, so it was a toothpick, that's a pointed stick isn't it?) I smoothed the outside of Coiled by rubbing over it with a wet finger. Probably should have used a smooth stone (hindsight is 100%) but it looked OK, though the surface felt a bit gritty from the sand. Left things that way for three days while I looked for cattail fuzz for added temper. Found plenty of cattail plants but none had flowered. Finally got some flower-pods from a plant in my mother-in-law's yard in Guadalajara that had seeds attached to fluffy parts -- kind of like milkweed, only the pods were small Chinese-lantern shaped affairs.

Took the remaining clay and added some water (it was too dry to work by then) and the fluff, and it seemed easier to work than it had been. By this time the earlier pots were light gray and looked and felt dry. I used the remaining clay to make a pinched-pot with attached features that I like to call Tlaloc (it was inspired by the pots thrown into Lake Chapala prehistorically to appease Tlaloc, the rain/water god) though it looks more like Al Lewis (Grandpa Munster, may he rest in peace). There was a tiny bit of clay left so a fashioned it into a small ring-shaped band and pierced two small holes in it. Placed the new pieces in the damp-cloth covered bowl to dry slowly.

Next time I'll use 'grog' -- ground up potsherds -- for my temper. I have read that is a much better choice than sand or organic materials, and we have big ceramic bowl from Tonalá that broke recently. Any temper reduces the hardness of the fired clay, but it reduces shrinkage and in some cases may help make the clay easier to work.

Firing the Pots

I let everything dry thoroughly. We don't have easy access to wood here, nor a place to build a good bonfire, so I decided to try firing the pots in our grill using charcoal. Figured I couldn't make as large a fire as with wood, but the charcoal burns hot so a small fire might suffice. So twelve days after making Tlaloc and The Ring (22 days after gathering the clay) I stoked up the grill.

After the steaks were done (you didn't think I was going to waste that opportunity now did you?) I turned the grate sideways (so it only half-covered the grill) and put the pots near the edge to warm gradually. I'd started with 1/4 of a five-kilo bag of charcoal. I scatted those coals over the bottom of the grill, and added another 1/4 bag, gradually moving the pots directly over the coals. Half an hour later, I put the pots directly on the remaining coals and covered them with the other 1/2 bag of charcoal. An hour and half later, everything was still glowing but the pots began to show through the ash (they also glowed red-hot) so I added another 1/2 bag of charcoal.

Had planned to add the other half of the second bag to the fire, but after two more hours it began to rain slightly, so I just covered the grill with a sheet of tin and ducked inside the house. An hour later it was raining ... not the tropical downpour we get during the rainy season, but a steady light rain. Two more hours and the rain had stopped, but so had the fire. I left the pots in the grill overnight.

The next morning I took the now cool pots and washed the ash off them. The Ring was so fragile it broke while I was cleaning it. Crude was badly cracked, but in one piece. The Disk, Coiled and Tlaloc all looked OK, mostly orange in color but with black areas in some places.

Characteristics of Fired Pots

Of all the pieces, The Disk had the least black showing, just some swirls of gray on the bottom side. I broke it in half to see if there was a black 'firing line' inside, like so many prehistoric potsherds have ... but no, it was the same buff-orange all the way through. The sand temper was clearly visible. It didn't take much pressure to break, about like breaking a hard cookie in half.

I forgot to measure the dried pots before and after firing, to see if they shrank further from that process. The Coiled pot was about four inches high when I finished it, and now is three inches -- I assume most of that shrinkage was during drying -- next time I'll measure to be sure.

The fired pots do not have any odor when dry, but when wet they have a musty smell, not sewage-like as the raw clay, but still unpleasant. They are fired enough that water does not soften them, but the pots are highly porous. Normal stoneware should 'sweat' a little, but these sweat a lot. The Coiled pot holds about 3/4 cup of water. When dry, if you fill it almost an ounce is immediately absorbed into the pot. Then it sweats -- if you hold it water drips (slowly) off. Put it on a surface, like a tile counter-top, and a little puddle develops around it. It takes about two hours to empty out. I was really surprised it took that long when I timed it -- the drips begin within 30 seconds of filling it, and the first ounce absorbed into the pot makes it look like it is emptying fast -- but those drops are small and there are a lot of them in six ounces. I'm sure a little fat heated in it would be absorbed and further slow the leakage, I may try that in the future.

I broke the Crude pot, just because it was so ugly. Maybe because of the shape, or because it was thicker than The Disk, it was a little harder to break, but I still had no problem breaking it into three parts just by inserting two fingers from each hand inside and pulling it apart. For most parts the outside color continues into the interior, black surface has black interior, orange surface orange inside -- but one part has a very thin orange layer on both exterior surfaces (and top rim) but the inside is entirely black [see scan].

Discussion

I have continued to read about primitive pottery making techniques, and am still learning new things. I was under the impression that the black areas were due to a 'reducing' atmosphere (lack of oxygen) during firing -- but given my open grill method of firing this seems unlikely. Another source of black, apparently, is firing that is too short, or incomplete. Since the rain forced me to cut short the planned firing process, this may be the cause of the black areas on these pots. I'm still looking for references that make clear the difference between insufficient firing black and reducing atmosphere black.

The bad smell may be due to sulphur compounds in the clay -- that may be due entirely to pollution in the lake, or it may be that the rocks that form the surrounding mountains have sulphur that is weathering into the lake sediments.

The broken edges of The Disk and Crude pot have some small bubble-like pockets in them. Since these were not the pots that had the fibrous temper, I'm thinking perhaps I didn't work the clay enough -- or properly, to get all the air out. That would no doubt contribute to the porosity.

All in all I'm very satisfied with this as an initial experiment. I want to go into the mountains and see if I can find clay deposits left from Lake Jalisco (though I don't look forward to carrying samples down the mountain ... may need a porter!), and see if I can find other clay deposits buried on the flats between the mountains and lake. I'd like to try other firing techniques, like a pit with wood bonfire, and maybe even a crude adobe kiln. Also, I'd like to 're-fire' some of the shards from my broken pots and some prehistoric shards with a modern kiln, to see what they look like when fired to a known standard (this is a well-known technique in archaeology). I'm also thinking of going over the mountains to Sayula Playa Seca and the Guadalajara valley to seek clays for comparison.

Please use our Feedback Form if you can help educate me on any of these matters, or if you know of native clays or other resources I might use.