I am one of hundreds of people who consider it a holiday to dress up as a Tudor and pretend to be living in the 16th century, and carrying out 16th century activities in a way that they would have done. This year I was supposed to be doing and talking about land surveying and improvement, but the slightly more interesting stuff that went on in the evening included helping the potters fire some pottery the old fashioned, but very effective, way.
This is the kiln, note the chains on it to help keep it all tied together when hot, although it would matter more on the full sized one (Without the chains and some repairs, it might open a big crack in the side letting all the heat out) :
A 1/3rd scale replica of a 16th century kiln dug up a few miles away. It has 3 fireboxes which you feed with wood, starting with logs 3 inches or so in diameter and working down to smaller ones as everything warms up. These are usually more like an inch or even half an inch in diameter, but of course burn through quicker, giving more heat more quickly later in the firing when you need it. This is a natural draft kiln, so called because the insides are arranged so that the hot air and flames from burning wood passes through the inside, heating the pottery there and then passing out the top. Because hot air is less dense than cold air, it expands and rises out the top, drawing much more cold air in at the bottom which then keeps the wood burning well. This draft effect makes the fire more intense, and is the same principle used in reverberatory furnaces of the period for melting metal and in glass kilns, which is why I am interested in it. That and it’s cool and spectacular – sometimes it looks like an upside-down rocket, or it has escaped from a period picture of hell:
The flame comes from the unburnt hydrocarbons and carbon monoxide and stuff that come off the wood, and rise up through the furnace, burning as they go. If you have enough of a draft through the kiln and enough wood at the bottom lots of unburnt stuff passes out the top, forming the flame. Ideally you want to balance it so that there isn’t much of a flame, with all the combustion taking place inside the furnace. Later on that doesn’t matter so much and you just push as much wood in as possible to increase the amount of combustion inside the furnace next to the pottery, but you have to start slowly to avoid cracking it. It’s also more effective to separate the fuel and the things you want heated, so that the combustion can take place and the hot products and vapours still burning can transfer heat more easily to the heated stuff.
The first firing this summer was difficult, because someone had left the tarpaulin off it over winter and the kiln had soaked up a lot of water. So the potters took it slowly, and managed to complete a firing of biscuit, needing about 800C.
The second firing went better, and we had some useful discussion about how the kilns worked and how the fire in the box tried to rise through the kiln. The kiln just wasn’t drawing as well as it should have done, and flame was coming out the front of the fireboxes, indicating that the draft through the kiln was not very strong; in this case you can see there’s no flame out the top yet flames out the bottom, and you can also see this in the first photo in this blog post:
One of the old hands pointed out how the rebuild of the kiln a couple of years ago had installed a narrower chimney tube at the top of it, which was throttling the draft through the kiln. This caused the flames from the wood to blow back out through the openings of the fire, rather than being sucked up the inside of the kiln. We agreed that the solution was to rebuild it with a wider throat at the top of the kiln so that the hot gases could exit more easily. There will of course be a sweet spot, with the correct ratio of area of firebox cross section to chimney at the top, bearing in mind the diameter of the wood, which affects how quickly it will burn, and how damp it is. Which is of course why there are 3 or 4 removable bricks at the top near the chimney which can be used to fine tune the draft through the kiln.
It is, as far as I am aware, the only large wood fired pottery kiln in the entire country, and you can really get a feel for how the technology works by watching it in action. It is very simple in general concept but a bit harder to get working properly; as usual with technology from this period, it’s more a matter of experience than of precise adjustment to a pre-arranged schedule. But you can quickly learn to do it, most of the people helping don’t have much experience of using it, as long as there’s someone there who can tell them to watch out for and judge the heating rate and wood size. It also shows how good the kiln technology was by this time, far superior to that of a few thousand years earlier.
Other requirements and skills for using the kiln include the use of clay, straw and horse dung treaded together, to seal holes and then the entire furnace when finished. There is danger of sparks and burning yourself on hot bricks so you need heatproof gloves and clothes not made of plastic, and the final moments of sealing the fireboxes with bricks and daub can be fraught. The firing is rather dull at the start, you just sit there pushing another stick in now and then, although there’s enough time spare for a good chat, some drinking etc, although alcoholic drinking and fire don’t mix. It’s the end that is busy. And of course gathering the wood, of the right size, preferably left to dry for weeks, because how damp it is affects how well the furnace works. A lot of man hours goes into firing the kiln and it makes you appreciate how much quicker and easier it is nowadays. On the other hand it is much more relaxed than modern efficient production. Although a lot of work was hard back then, there were also many times when people could relax and not strain themselves in what they were doing.
Here’s the sealed up kiln, it takes 3 days or so to cool down enough to unpack: – Photo to be inserted when I find a usable one.
So a medieval- Tudor clay jug can go from production to finished object in a week or so, with the other 3 or 4 days mostly used for it to dry naturally and wait for enough pots to be produced to fill the furnace.