I'm having quite a lot of fun this evening playing with clay. It's difficult to call it anything but play -- I have minimal clay training or experience. I think it will enrich my master's degree thesis, however, if I construct a medieval-style kiln to understand that experience and process.
I've only found historical information on two kilns -- One is the kind documented in Theophilus and which has a recreated example in the Centre International du Vitrail just next to Chartres Cathedral, in France. The other is a late Gothic/early Renaissance brick kiln from Amsterdam, documented in a manuscript in Antwerp which was transcribed in 1974 and analyzed by Joost Caen in his doctoral thesis and related publications. Of the two styles, the earlier is more appropriate for my formal research but I'm fairly sure I'll have to try the later model for SCA purposes before too long.
"Chapter 22. The Kiln in Which Glass is Fired
Take some flexible canes and fix them in the ground in a corner of the building with both ends aligned to form a series of arches, a foot and a half high, the same in width, and a little more than two feet long. "
The skeleton of this project involved a number of necessary modifications. I was building this kiln in my garage, and while 10-cane could probably handle the impact, I am just not strong enough to get them into the concrete floor. Further I'd love to be able to take this to SCA events or similar educational activities, though at this point I probably can't lift it alone. From the start some kind of platform was planned. I used hardwood/birch plywood as the base and attached four corner brackets on the bottom. At the time I thought I could use the brackets' holes to string rope for lifting, and now I laugh at the folly of youth. They do serve their secondary role of table legs quite well, with no apparent strain yet.
The top surface of the board I covered with thinset (cement) and then covered it with stone tile. I'd like to avoid doing all this work again for a couple of years at least, and I hope the stone level between the kiln and wood will add some longevity to it.
I have some limited experience working with clay through the Chicago Mosaic School's fantastic classes [Go check them out, no I am not paid to say this], so initially I threw a bit of clay down, tried to practice wedging, building clay, really playing with it to get the sense of how strong it is in different ways. This wasn't related to the project in any way, but I mention it because it inadvertently gave me observations later along that I would have missed.
I asked friends for suggestions where to buy basketweaving canes, and this lead to the generous Donna Weeks coming way out of her way to give me a few hours of basketweaving education AND spare materials. I knew nothing when I placed my own order before this demonstration, and figured size '10' canes (the largest) would be the most appropriate choice. Every basketweaver who asked, I told this to, and universally everyone was shocked. I understand this now, I'm pretty sure size 10 cane is produced by folks from the rebar plant in their free time. Much good came from this lesson, however, and several hours of wasted time avoided.
I had intended to stick the ends of the canes into clay, and allow the clay to hold it in place. "Allow" as though I have any real say in the matter, when in fact the 10-canes were giving the orders. The clay was not going to hold the canes in form (yes, soaked). I continued with the idea of making a "basket upside-down" and made a wooden shape to hold the ends of the canes, which stood in perfectly for packed-dirt in my glazier's shed outside Sées... oop, sorry, I'm back. Here came an important lesson: When I said I made a wooden shape, that's probably barely legally defensible. I used a chunk of MDF I had in the corner of the garage, without thinking about it at all. Cut an oblong horseshoe-shape out, drill holes just big enough for 10-cane. Those canes, which had been soaking in water. Onto a bed of clay, fresh out of a sealed bag. That's a lot of moisture hitting thin MDF from all sides.
So it began disintegrating very quickly and I had to start tying reeds together to try and maintain the shape.
So yes, use real wood.
"Then vigorously knead some clay with water and horse-dung in the proportion of three parts of clay to a fourth of dung. After kneading this very well, mix with it some dry hay and make long loaves out of it. "
I'm sure I'm not alone in that every once in a while I stop to examine my life and try to figure out what specific mis-step lead to voluntarily spending a nice fall Saturday trying to track down horse excrement for my latest mad engineer experiment. After contacting local horse ranches/therapy facilities, most of whom were more than willing to let me cart of their refuse, my friend Kate came through with an offer to assist a few days sooner than the horse facilities could, which meant a whole weekend more I could get done. I took a plastic tote (15 gallon size, I think) lined with an industrial garbage bag, got to meet the horses, and drive off into the distance with a considerable amount of road apples. Trunk apples?
Three parts of clay to a fourth of dung. The interesting thing is when measurements are given in "parts" it's usually because volume and weight measurements are interchangeable (think baking, where many ingredients are so similar). I could not say the same about pottery clay and horse leavings. I have a possible side-project of testing both volume and weight compositions, if I can figure out the best way to do such a test.
I'm going to talk a bit about making and working with this clay mixture. However, since I know the subject can cause a visceral reaction to some people with sensitive stomachs, everything from this paragraph to the next picture should be black-on-black text. You can read it, if you want, just by highlighting it.
Edit: Ok, that didn't work. FROM HERE:
The before-mentioned Donna came up with the great term cloop, a portmanteau of clay and poop that is much less evocative to the imagination. Some people seem to consider this a form of cob (as in cob ovens, cob buildings) but I'm not finding highly consistent usage. Cloop, it is.
If this whole idea is leaving you with doubts, let me assure you it is not remotely as bad as you would expect. The faint smell in the close vicinity of the tub disappeared in a day. The first several days I worked on it, I wore latex gloves which inevitably tore up within ten minutes. Eventually I gave up and took relief that I had filled a bucket with water beforehand I could use to scrub a millimeter off my hands before going back inside. Nothing about it is reminiscent of dog, cat, human, etc. waste, and really it feels and looks like something you'd buy at the garden center, like a fine mulch.
I went with the idea to go by volume. It seems widely-accepted that the organic additions of dung and clay are to build pockets of air to change the clay's thermal properties. Volume seemed more natural for that choice. I did not use any modern measuring tool and just did it by visual approximation.
For the longest time I would tear out a fist's worth of clay, flatten it into a flat disc (it took me seven tries to find a phrasing that was in no way connected to food). I then added the equine component... and started mashing it. I think that the most effective technique I found, at least initially, involved rolling the cloop into tubes, folding them in half, and repeating the process in order to get many layers. Then just a bit of random squishing. I didn't decide this until later, but the easiest way to add a reasonable amount of hay was to put hay down on a workbench and just dredge the cloop like breading (Oops, sorry). Kneading it in with no particular technique yields a sturdy ball or log of material quickly. I noticed that after an hour of making these I stopped really measuring it all out. The cloop has a certain visual quality to it, and the hay adds so much rigidity, you can quickly just get a feel for your normal.
I did try, one time, to mix it en masse in a bucket. PLEASE, do not try it. The only problem with hand mixing it as above is that your hands will get tired quickly. I did find that taking larger (say double) portions and kneading it on a covered workbench like dough (sorry). It felt like it went quicker without straining my weak nerdy limbs.
Thanks to my initial goofing off with the clay, I realized how different the two are in properties. When I made a slab of plain clay, it dried while I worked with it and was not easily indented hours later. When I made the base slab of cloop, it was easily indented 36 hours later (both were uncovered).
To HERE.
While I was working on the base, I was plagued by a beast which is famous for having plagued monk-scribes.
"Cover the arch of canes inside and out to the thickness of a fist and leave a round hole in the center of the top, big enough to put your hand through. Also make three iron bars a finger thick and long enough to reach through the width of the kiln. Make three holes for these on each side of the kiln so that you can put them in and take them out whenever you wish."
At this point, it's a building montage.
The walls are between 2" and 3" in thickness everywhere I measured. The cob oven bakers in my life said they aim for 2½" to 3½", and beadmaking furnace friends said at least 2", so I feel like this is a solid range.
I'm trying to let this boulder dry naturally for a few days, occasionally hit with a heat gun, before I follow Theophilus' last step:
"Then put fire and wood into the kiln until it is dried out."
For Part 2, I plan to write-up firing the kiln (including its door) and tests on how long it takes to heat up, likely using various fuels, etc. I'd like to have a fairly predictable firing idea even without modern measurements, and I don't have 4-6 free years to fake an apprenticeship and learn the timing by trial and error. I have 4 high temperature thermoprobe sensors I can use to take some measurements.
For Part 3, I plan to document firing a piece of glass in it, or several sample chits using period formulas which I'm incorporating into my MA thesis.
