Black Burnished Ware Experiment at Ancient Technology Centre March 2019

In March 2019 Bill Crumbleholme undertook a firing in a kiln made of turf at the Ancient Technology Centre at Cranborne to fire Black Burnished Ware, the type of pottery that the Poole Harbour area was famous for in the Iron Age and Roman times. This page documents that event.

Return to Webpage about Black Burnished Ware Experiments.

A batch of jars were constructed for the firing.

Clay Preparation

The clay came from a building site in Wyke Regis, Weymouth. Thought to be Oxford Clay in the same seam as the old local Chickerell brickworks. It came from 6 feet down, an undisturbed geological deposit. A small piece was pinched into a pot shape and fired in an electric kiln to check it survived and fired satisfactorily, which it did. It was not very plastic or elastic, so not clay of choice for pot making, but very good for brickmaking!

It contained small nodules of gravelly stuff, which needed to be removed. So the clay was put in a plastic dustbin and soaked with water so that it turned to slip. An electric plaster stirring device was used to beat up the slip into a smooth consistency. It was left to settle and the water that came to the surface was thrown away. The slip was then passed through a garden riddle to remove anything larger than a couple of mm, leaving behind the lower layer into which larger lumps had sunk. It was left to settle again and the water removed from the top. It was then decanted into plaster containers and dried out to a soft clay state.

Various inclusions were added :-

Some commercially prepared ground up flint and quartz powder, which should make it a bit more able to stand the firing stresses.

Builder's sand - the stuff without seashells.

Grog - made from crushed and ground and sieved fired pottery - both some of Bill's rejects and a special batch of "real" Roman BBW that came from the dig at Bestwall in very small shards.

No organic type inclusions were used.

The inclusions were added to the damp clay, the flint and quartz powders were tipped into the clay still in the plaster containers. The rest was added by cutting and forming the clay into slabs, which were stacked up with handfuls of dry stuff in between, somewhat like a lasagne! The lumps were then flatted out by dropping them on a board, cutting into 4 quarters which were stacked up and flattened again, and repeated several times, so the layers gradually became thinner. More inclusions were added when required. The clay was then mixed by wedging, which blends the clay into an even consistency. It was then left to rest, as was the potter!

The inclusions helped to dry out the clay further, so it ended up about the right stiffness, in balls ready for making into pots.

Pot Making

The pots were fashioned by pinching the lumps into jar shapes, the taller jars were made by making a extra ring of clay that was fitted onto the bowl shape, with a tongued and grooved joint. After a bit of drying to stiffen up they were pinched again to a better thinness and shape. As the clay was "Short", i.e. not very plastic or elastic and tended to crack over the surface and around the rims, it was brushed with water to dampen so the cracks could be squeezed back together and the surface smoothed out by scraping with a piece of wood. After a bit more drying they were burnished by rubbing a pebble over the outside and rim. After more drying they were burnished again. Some had horizontal lines incised around the necks. Some had the shoulders roughened with a stiff brush and then a lattice decoration was made by diagonal strokes of the pebble.

Unfortunately the weather was not good for drying, so the pots had to be dried off in an electric kiln. That swift drying managed to open up a few cracks, so the number fit for proper firing was reduced (actually useful because the kiln could be reduced in size with only one layer of pots, rather than having to stack them inside the kiln.

Firing

The kiln was constructed using turf in the shape of an igloo, with the entrance tunnel serving as a firebox for the preheating period. The turf was put down in rings, each ring being slightly smaller than the one below, so that the structure was corbelled inwards like a dome.

The fire was started in the tunnel using dried bay leaves with twigs and some glowing charcoal from the bronze furnace nearby. It was gently fanned to get it going. Branches were placed in to keep the fire stoked and producing hot air that flowed into the kiln and out of the hole in the top of the chamber.

The hole was damped by placing small planks across it to cut down the flow of air, keeping the heat in the chamber longer.

After a few hours the pots had dried out sufficiently and so the main firing could begin.

The branches were threaded down into the kiln through the top opening and the fire built up in the tunnel, then that was pushed in to set the main chamber alight.

The turf of the tunnel was taken off and placed on top of the kiln wall to build it higher and act more as a chimney to pull the hot air through the chamber from the hole in the base.

When the branches had burnt away somewhat another load was put in, some over a period of time and some as a mass at once.

After half an hour or so, the chamber started to glow red and the pots looked hot enough to have turned ceramic. Stoking continued for about an hour.

The kiln was stoked one last time and then the top of the chamber was closed over with more turf. The air inlet hole was also blocked up. The aim was to create a smoky reducing atmosphere inside the chamber to achieve the blackness of the original ware.

The kiln was left smoking overnight. It was checked occasionally and if gaps had started to open up they were stopped up with turf or earth.

Unloading

At midday the next day the turf was deconstructed and the pots removed.

There was some charcoal unburnt in the bottom of the chamber, together with some burnt earth that had fallen off the inside of the wall, as the turf was dried and then cooked.

The pots looked pretty good, mostly dark grey, with some blackness and a few spots of red.

Cleaning Up

The pots were taken home and washed, then filled with water to check if they were fully ceramic.

Worries began when some of the pots appeared to have a slightly muddy surface when cleaned with water.

The pots looked generally OK....

But it turned out that some of the bases had not turned properly ceramic and were decaying back to damp clay.

Conclusions

The demonstration of firing was very successful in terms of explaining the possible method of firing this type of simple kiln to the audience, who never knew they were somewhat under fired!

The blackening was partly OK, but was not as complete as wished for. The kiln walls, which were still fairly pale colour internally when taken down, were probably slightly porous, allowing some oxygen to enter the chamber during the cooling period, thus burning away the sooty surface and reoxidising the iron oxide content of the clay, which might contribute towards the darker colour.

Bill's pet theory about achieving true blackness is that the original pots were fired in a kiln structure to turn them ceramic, then placed in a charcoal burning clamp to completely infuse them with sooty black carbon.

The uncooked sections of the pot may have been caused by the pots being placed directly on the earth base of the kiln, rather than on shards of previously fired clay, which would have raised them up and enabled the heat to penetrate more easily into all the pot. Those sections were also the palest colour - protected from the extreme heat by being slightly buried.

A future firing of the nature would be done with a longer period of stoking to get the pots hotter for longer and to enable the heat to spread downwards. The pots would be placed on shards to raise them. If the pot walls were pinched a bit thinner, they might fire more satisfactorily all the way through.