October 3, 2016
Based on archaeological finds in Halder,
Noord Brabant, The Netherlands
Prepared and executed by:
Leo Moonen (writer of this report)
(all volunteers at the “Hunebed Centrum”)
Cisca made and dried various pots and spoons of clay.
Leo searched the web for archaeological information about Roman kiln remains.
Cisca, Andre and Leo started in mid July and continued the construction work in August with assistance from visitors. A final air-insulation layer was applied on Sunday, October 2
Cisca, Andre and Leo did the firing on Monday, October 3, also assisted by
Bep and Rob van Hees (BEPPIE’s POTTERY), at the “Oertijdpark” , Hunebed Centrum, Borger.
Alun Harvey for checking this document for grammatical and spelling errors.
I) Preparation and orientation
II) Building the kiln:
III) A weekend firing the pottery:
Preparation on Sunday
Firing on Monday
Opening on Tuesday
IV) Next “Roman kiln” project?
I) Preparation and orientation.
Translated from: “NEDERLAND IN DE PREHISTORIE”, chapter 27:
… an advanced type of pottery kiln, in which the fire chamber and firing space are separated by a vent-holed floor . The oldest remains of vent-holed floors in the “Lower Rhine area” date from the end period of the “early iron age”. Except for the rather small dimensions of the kilns, a diameter of at most 4 feet, nothing is known of the construction … (ISBN 90 351 248 47)
In spring, during a meeting about the “Oertijdpark”, Cisca came up with the idea of firing pottery in a late Celtic/Roman type kiln. Firing pottery in the ground, “fire pit pottery ” in 2015 and 2014, resulted in many (~50%) broken pieces. Remains of a Roman kiln were found in 1973 at Halder, Noord Brabant, the Netherlands. The museum in Halder has a model on display.
The “fire chamber” is the lower space of the kiln where the wood burns and generates heat,
The “firing chamber” is the upper space where the pottery is placed, covered by a dome.
The “vent-holed floor” separates these two chambers.
During the excavations in Halder in 1973, only the remains of the fire chamber were recovered and nothing of the dome. The fire chamber, the vent-holed floor and (probably) a large part of the dome were below ground level. They did not find any remains of the vent-holed floor and the dome.
I found more information on the web about such a vent-holed floor (or grid, grate) in Germany where it is called a “Lochtenne”.
A hand-written report by “Brother Celestinus” is at http://www.romeinshalder.nl/wp-content/plakboek/halpagx.html
II) Building the kiln: – Design
Deviation from the drawing:
The height of the firing space, the distance between the vent-holed floor and the large entrance on top of the dome became 60 cm and not 40 cm as in the drawing.
The total height of the kiln became ~150 cm.
In the first drawing the dome was higher and the total kiln ~ 180 cm. It included an extra platform (grid) inside the firing space, no removable lid on top, but a large entrance “door” at the side: a top-to-bottom segment over 30 degrees along the dome circle.
The functions of the fire space are: (1) the area must be a closed space from which enough heat at high temperature can be delivered to the firing space above, (2) it must support the vent-holed floor and the dome and (3) withstand also the lateral force component of the weight of the dome.
The construction is based on the find in Halder: the same height (50 cm) but a diameter of 80 cm instead of 120 cm ( ~70%).
The functions of the vent-holed floor are: (1) the floor supports the pottery and (2) lets the heat pass into the firing chamber. This has to be done gently – over time and equally spread over the surface. It is not desirable that the temporary high flame temperatures (1000 to 1500 °C *) reach the pottery. I made a floor with fifty holes after the example of the “Lochtenne von Osterhofen-Schmiedorf” .
(*) the theoretical adiabatic flame temperature of ~ 1900 °C will never be reached.
The dome stands on top of the vent-holed floor.
The functions are: (1) the shell has to maintain a high temperature inside, stable and equal distributed over the whole volume and (2) must be strong enough to protect the pottery – never collapse during operation. There are no archaeological data for the construction. We constructed a double walled curved shell. A strong inside wall, 7 cm thick, and very thin outside wall, ~0.5 cm thick. In between is a layer of air, ~ 1.5 – 2,5 cm thick.
On top of the dome is a hole of 40 cm diameter for entering the pottery, to be closed by a large curved lid. The flue gas escapes through a hole on top. A stone may close the exit partially.
The total volume of the firing space in our kiln is ~330 liter.
“Keileem”, fire proof building bricks, osier branches and twigs are used as building material. Straw and even some chicken wire for temporary support.
“Keileem” is the same as boulder clay, “Blocklehm or Geschiebemergel ” in German, “argile à blocaux” in French and “Moränlera” in Swedish.
Straw was also used as padding (filling) for the dome, and chicken wire to give the dome the initial raw shape and to hold some plaster.
We used fire proof materials where necessary.
“ORDINARY” STONES, BRICKS, ARE DANGEREOUS AT VERY HIGH TEMPERATURES AND COULD EVEN EXPLODE! Loam or clay containing too much chalk may already start to melt at ~1000 °C and the construction may collapse. Our bolder clay (Drentse keileem) maintains its form up to ~1300 °C
Building the fire space:
At the end of July we started with cleaning and flattening a few square meters of the surface. Some 12 garden tiles (1×1 feet) formed the base on top of which the fire chamber wall was built with fireproof bricks. Because the construction is exposed to bad weather conditions (rain, wind) we decided to experiment with adding cement to the boulder clay (1 part cement to 5 parts clay). A few weeks later this turned out not to be so successful, as it didn’t give a strong hard weatherproof joint between the stones. The entrance of the fire chamber somehow became a little higher than planned (original it was half this height). Less heat would have escaped through the entrance on the pottery firing day…
The walls of the fire chamber in Halder were slightly “waved” horizontally and approximately 7 cm thick. In the excavation there was a narrow small wall in the middle of the fire chamber for supporting the vent-holed floor, one side against the surrounding wall and the other side sticking halfway into the chamber. Our vent-holed floor is strong enough and does not really need support, but we left a pile of bricks in the middle of the chamber.
Building the vent-holed floor
In Halder, the chamber has a surface area of ~ 1.1 square meter (diameter 120 cm)
Our vent-holed floor area is ~ 0.5 square meter (diameter 80 cm), half of the original area in Halder. (We took 70 % of the width measured in Halder)
Our vent-holed floor rested directly on top of the fire chamber wall, covering the stones. It started with a thin layer of plywood (a 2 cm rim rested on the bricks of the wall) followed by a layer boulder clay (2 -3 cm thick) and a simple very coarse metal tube grid. The floor was supported in the middle, by a pile of bricks. I poked 50 pieces of osier branches (over one inch thick) into the wet layer. Then on top of all this (branches sticking out) a bucket of concrete (1 part cement -2 parts sand-3 parts gravel). Finally on top of this layer of concrete (still wet) another inch of boulder clay. A few days later I made a small fire in the chamber and the plywood took fire in a few minutes, the branches needed a few hours to turn to ashes. The remaining holes looked very similar to the “Lochtenne” in Germany.
The Celts and Romans probably made the vent-holed floor entirely on the ground and that is how I would do it next time:
Create a flat surface with a diameter of 120 cm, for example with a 60 cm rope around a centre stick.
Put two thick straight branches just under the surface for later lifting the floor and moving to and on top of the fire chamber. Put a few buckets of boulder clay on the area. Add a few branches horizontally on top of the clay and add again some clay. Pierce ( 80 – 100?) branches through the thick layer/cookie down into the subsurface. Wait a few (sunny) days or make a fire on top of the wet plate for drying.
Building the dome:
In the summertime there are always many visitors at the park – certainly in the afternoon.
This means that after talking with the visitors, not much time was left for construction work, only a few hours a day. At certain stages the public was invited to participate in the building of the kiln -and they really enjoyed it. For example helping to apply extra layers of clay on the dome – and on the inside. Some pictures and film clips are on Facebook.
The dome was double-walled, the strong inside wall was constructed as follows: A half-sphere of chicken wire was put on the wall/edge of the fire chamber and filled with straw – as much as possible for stiffness and to prevent distortion of the shape. Boulder clay, mixed with small pieces of straw, was smeared on and in the wire. Tip: cutting straw into fine pieces can be done with a large A4 paper cutter. We simply had a big bag of straw pieces at hand. The next day we made the dried outer part soft again with some water and applied a 1-2 cm thick layer of mortar, together with several pieces of wire, to the clay-dome.
Before this mortar layer was hard (a few hours) another layer of clay was applied onto the mortar.
The shell was now hard and stiff enough to withstand some pressure such as pushing against it. After a few days we burned the straw away and heated everything for a few hours.
Again a few days later – the shell was now hard and strong but only 3 to 4 cm thick – the inside of the dome was plastered with an extra clay layer. And then again on the outside, until the total thickness was ~ 7 cm.
Also for this work we asked visitors to help, both young and old, and they all loved playing with clay.
By the end of August the kiln was “finished”.
Before heating and a total drying, Cisca modelled a face with a smile on the front of the dome – giving the kiln a personality.
The pottery had to be entered from the top. The depth is equal to “one arm length”.
To close the wide opening, I made a lid with the same mix of construction materials as the rest of the dome: a layer of clay, a layer of wire, a layer of mortar, a layer of loam. All shaped on top of a nice big curved BBQ fire bowl.
On top of the lid was an exit for flue gas – a tiny “chimney”, 12 cm wide and ~10 cm high. On top of the chimney was a small plate slate with a stone which could be pressed to regulate the gas flow.
To make sure the lid on top was a close fit, we first placed a layer of plastic on top of the dome rim. On this plastic a roll (abundant layer) of clay and then on top of the clay we placed the lid – first wetting the edge to make it soft again and then waiting for a few minutes. The correct position was marked with a large vertical scratch.
III) A weekend firing pottery
We waited a number of weeks before bringing the kiln “to life”.
We needed good weather conditions and a minimum of three people, not only for the firing itself but also for talking to visitors.
Preparation on Sunday, October 2
The prepared pottery was put into the dome. Only the strong inner wall was finished and we started now with a thin outside wall and an insulating air layer in between.
With a few osier branches we made a very simple arch-shaped construction around the kiln. One end of the branches stuck in the ground, the other end was tied to another branch. We wrapped the branches with a few large cotton sheets. And then placed a clay slush on top of the sheets.
The idea was to get a final layer of clay on top of the sheets which was stiff and 1 cm thick.
The combination of these three heat resisting layers together comes down to one single layer of loam, ~ 40 cm thick.
I did some rough basic calculations and in theory the outer sheet temperature could reach 100 -150°C at an inner firing temperature of 800 °C using this insulating layer of “dead air”.
At the bottom, in the fire chamber, we started a gentle fire, just hot enough for the wet outer layer to begin to evaporate – steam. Inside the dome the temperature was somewhere around ~ 200 °C (not measured), warm enough to slowly dry the tuyeres (small tubes through which air is blown into an iron smelting furnace) and the body of the thermo couple without causing cracks. This also applies to other objects where some moisture may still be present (trapped) in clay. As soon as moisture in a bubble is changed into vapor (gas), the increase in pressure will cause cracks.
The shell coated only in clay got harder every minute and another layer of slushy clay went on top. A minimum wall thickness of 1 cm would be fine but we only reached a few mm.
Even if the osier branches inside lost strength and converted to coal, the outer shell would be stiff enough not to collapse. And apart from some wind, no external force was applied on the shell.
We could not put the pottery directly on the vent-holed floor, it would close many holes and partially block the heat flow. So we found a few BBQ grids to put on the bottom first. And a piece of bent wire to cover the remaining area of the floor.
Then we put the pots and bowls in the kiln, on top of the grids, upside down (I don’t know why)
And also four “large” tuyeres, still wet, a firing experiment for our next “iron smelting” session.
At the bottom of the dome, inside the wall, just above the vent-holed floor, there was a rectangular hole for placing a device inside the kiln to measure the temperature. We placed a 30 cm “thermo couple” (type K: nickel-chromium , up to 1200+ °C) horizontally inside the kiln. We didn’t want to measure the temperature of the heat gases coming from beneath, but the actual temperature of the pottery clay. So we made a “tube like” body (1 cm thick wall) stretched next to the pottery, on a brick, 10 cm above the floor, and put the thermo couple inside this tube. The actual bi-metal point at the end of the thermo couple tube now reached 20 cm inside the kiln.
We were almost at the end of the day and decided to start the next day with a low fire, a very gentle warming-up. The kiln should not become too hot too quickly. We also wanted to apply a few extra layers of slushy clay to the hull.
Firing the pottery on Monday October, 3:
We started by inspecting the outer insulation layer – dried clay slush on cotton – it felt brittle and thin and was still warm. There were no holes, so the air was fully enclosed: a “dead” air layer. Up until lunchtime we kept adding more clay slush to get a thicker and stronger shell.
At 11 we started the fire with straw and small pieces of wood – a few minutes later the kiln came alive. After a few more minutes the first thin and flat pieces of wood were thrown in, later followed by larger chunks. Slowly the temperature rose. But we wanted to keep it low until the afternoon.
After lunch we mounted a simple electric blower ( and covered it with a sheep skin). We stuck the end of a 2 meter long pipe into the middle of the fire chamber. With only 120 Watts power, the area around the end turned to yellow and later to white-hot in a short time. (at the end of the day the pipe was a lot shorter).
Remark: most of the air (78%) is nitrogen which does not help in burning wood, but which still has to be heated to a high temperature – what a waste. With only oxygen as input, the fire would be much hotter…
The “chimney” on top of the kiln was only a “small 12 cm hole”, 10 cm high, not a pipe, and did not give a boost to the airflow .
Despite the extra airflow due to the blower, there was no characteristic “humming sound” coming from the kiln, as there would be in an iron smelting furnace. A lot of flames escaped from the fire chamber into open air through the upper part of the large entrance door – this meant a large heat loss.
It would be ideal if the heat could be kept as much as possible inside and only allowed to flow through the 50 holes in the floor into the dome, where the hot gas put heat into the pottery and could then escape through the chimney …
So far only wood and charcoal was burned with an excess of air. Later in the afternoon we closed the fire chamber entrance almost completely and restricted the air supply – this meant that a lot of CO was formed (incomplete combustion) and present in the flue gases.
As you cannot smell CO, AND this gas is dangerous, this is something to keep in mind together with the wind direction. CO prevents the blood absorbing and transporting oxygen in the body.
At ~ 2:30 in the afternoon it seemed that the temperature in the pottery space had reached a limit of approximately 550 °C. There was now a balance between the heat entering the pottery space through the Lochtenne and the sum of the heat leaving through the chimney plus the heat going through the wall of the kiln. After passing the wall, most of the heat was radiated as T to-the-power-4 and lost due to convection (warming up the passing air). Also the heat flow through the top of the entrance to the outside was completely wasted energy.
Because the outside of the insulation layer could still be touched by hand for several seconds, we decided to put a few extra hides (sheep and cow) as a second insulation layer over the kiln (not the chimney) and see what would happen. The temperature inside – at pottery level – started to increase and stopped at approximately 650 °C.
And again the temperature stayed at this level.
We decided not to use extra charcoal for heating anymore and we even considered for a moment to stop here, “call it a day”. (Our initial “goal” was :700 °C.)
We removed the skins – they were now quite warm.
At the bottom of the entrance of the fire chamber we piled up some stones and blocked roughly one third of the entrance height. We set the blower at a low speed.
Surprise: the temperature started to increase, slowly but consistently! The column stones inside the fire chamber supporting the Lochtenne were now clearly glowing deep red. We decided to put all the remaining wood into the chamber and to continue adding stones at the entrance, reducing further the input area of the fire chamber.
As soon as the entrance was two-thirds blocked, and it became difficult to add the remaining fuel blocks with a spade, the fire started to burn more quietly and the flames remained inside the chamber instead of leaking along the top of the entrance to the outside. The column stones supporting the vent-holed floor changed color and now glowed bright red. The blower was shut off. Just before 4:30, the temperature stopped increasing again: slightly over 700 °C. Over the whole area of the chamber the fire burned high and steadily.
A few spots of the insulation layer (clay over sheets over branches) were showing signs of damage. Smoke started coming out and we saw a few small holes appearing.
We decided to stop adding wood to the chamber and blocked the entrance “completely” with bricks – as much as we could. Air could now only enter the fire room through the slits between the bricks. A rough guess would be a total replacement area of a 2 inch pipe.
The chemical process of burning wood was already changing into its final phase, converting the (char)coal to gases, ashes and heat. All that was left at the end was a thin layer of white ashes.
The temperature now started to increase again! Up to an estimated maximum value of 810 °C (+/- 20 °C) around 5 o’clock.
The insulation layer was now really suffering and started to catch fire in a few places. After 5 o’clock the temperature started to drop very slowly – and we decided to conclude the experiment.
The insulation layer caught fire and I pulled it away. The front of the kiln now showed a black smiley face!
The whole kiln was now much too hot to work on, the charcoal (carbon) slowly burning, and we decided to let it cool during the night.
We decided to wait to open the kiln until the next day… all that remained for now was to clean the area and go home.
Firing the Roman pottery kiln on Monday, October, 3 2016
10:00 Nice weather, dry, sunny, light breeze from NE. ~16 °C
The outer insulation layer is hand-warm and feels as stiff as a thin stone cloth.
11:00 Kiln: we put woodblocks in the fire chamber, light them – fire starts.
Two wheelbarrow loads of wood are waiting, we use five wheelbarrows in total (~200 kg)
12:00 After half a “wheel barrow” of wood in the chamber, T = 130 °C
12:30(~) T= 250 °C
13:06 T= 390 °C
We have two 10 kg bags of charcoal – part of one bag goes in the chamber.
The blower is switched on.
13:45 T = 480 °C
14:20 T = 550 °C
The temperature increases /seems to level out over time.
15:30 T= 600 °C
15:45 T= 650 °C
The pile of bricks in the fire chamber supporting the Lochtenne, is now glowing cherry red.
15:55 T = 640 °C temperature is dropping!
A few holes appear in the insulation – hot air escapes. We stop adding wood.
We close the lower part of the fire chamber entrance with bricks.
Up to now, 4 wheel barrows of wood have been burned in the chamber
16:00 (~) T = 653/ 656/ 659/ 663/ 669/ 673 °C – increasing.
16:12 T = 716 °C
The last wheel barrow of wood goes into the fire chamber – – block after block.
16:20 T = 736°C
The blower is switched off and we close the chamber entrance “completely” with bricks.
Temperature starts to increase again!
16:32 T = 753 -> 759 °C
16:45 T = 776 °C 16:50 T = 783 °C 16:55 T = 799 °C
17:00 T = 806 °C
17:03 T = 813 °C maximum value
17:05 T = 799 °C the End.Opening the kiln at Tuesday October 4, – the result.
Tuesday morning, about 10 o’clock, the weather was lovely, the sun was shining and the kiln was still warm – hot. You could put your hand on it for only 5 seconds or so. We removed the lid and after a first glimpse inside, Cisca and I were very happy to see that everything, all the pottery, seemed to be in one piece. The color of everything inside the kiln had changed into a warm soft red. I invited some other guests to look inside the kiln and admire “the Roman style fired pottery”. The pieces were still too hot to hold them in your hand for more than a few seconds. One object had a horizontal crack. All the other objects were in one piece.
IV) Next “Roman kiln” project?
After this successful result we decided we would like to use this kiln again – in spring next year.
The construction – mostly clay – is vulnerable, some water in the cracks followed by freezing would certainly ruin the kiln. We decided to build a small flat roof just above it so it will survive the winter period in good shape.
If the second pottery firing session also gives good results, and we are satisfied with our firing experience with this type of kiln, we may attempt to construct a full-sized Roman kiln (copy of Halder?). The floor area for the pottery would be twice as big (1m2) and the internal height for the pottery could be ~ 150 cm, split in 2 levels. A large amount of pottery could be fired – even some tall amphora’s … and that would be beautiful … fired the way the Celts and Romans did …
The construction will be different:
– Perhaps a thick solid wall made from mud/clay-bricks, dried for weeks in the open air, and built with the help of interested visitors (both young and old),
– Perhaps again a double walled curved construction (less weight and lateral force on the supporting wall). A large opening for the pottery in the side of the dome.
This might perhaps be a subject for school – a “hands on” history project.
– You will find some extra pictures and short film clips on the Facebook pages of the Hunebed Centrum and “Leo Ironsmelter” time lines.
– I would appreciate your constructive remarks about this report on “Roman kiln building and firing”
– END –