Iron Age Pottery Firing at Butser Sept 2020

This is a record of images from a workshop Bill Crumbleholme ran at Butser Iron Age Farm in September 2020.

The photographs are a series of random shots taken while constructing the kiln, firing and unloading the pots.

Many thanks to fellow Ancient Wessex Network member, Mark, for his construction and fire tending. Thanks to Butser for hosting the event and the good folk from the Chichester District Archaeology Society for attending the pottery workshop during which these firings took place.

Initial simple bonfire with pots being dried out. Bill had pinched out and decorated the shapes the week before and let them dry in the sun, but they remain slightly damp and chemically combined water needs to be driven out by slow heating.

The bonfire was first built in the middle, the pots were arranged around the outside, but not on the downwind side (because sometimes gusts of wind can sweep flames that way and overheat the pots). Then when burning well, sticks were placed on it and when they were alight they were moved to around the outside of the pots to form a burning ring. The fires were then kept stoked gently to heat up the pots slowly. Care was taken to avoid flames licking the pots, as that would result in spalling (see below). The pots dried out and when tapped with a stick they would ring rather than have a dull thud (as at the start of the firing). They were turned over a few times to even out the heating.

Meanwhile Mark dug a hole and mixed up the clay-rich soil with water and straw/hay (one day I'll remember the difference!) which was rolled into fat coils and used to build up the walls. Turf was used to shore up the sides. Rolled turves with branches inside were used to form rooves over the two firebox entrances which had been excavated leading into the main chamber. A small fire was built in each firebox and the chamber gently heated.

Pieces of broken fired pottery shards were laid out over the base of the chamber, on which the pots were laid, upsidedown. The shards kept the pots away from the cold damp base and encouraged the heat to circulate under them. The bowls that had been pre-heated in the bonfire for a couple of hours were transfered to the kiln.

The remaining jars were pushed into the middle of the bonfire. More branches were placed around and top of them and left to blaze.

Sadly the noise of exploding pots started to be heard. It was then apparent that they had not been sufficiently preheated for long enough at a high enough temperature to dry them out. The initial drying opens up the pores and allows steam to escape more easily as they get hotter and the chemically combined water is released. If the steam cannot escape quickly the pressure pops the outer skin of the pot off - which is known as spalling.

The bonfire was restoked a couple of times and the temperature built up until the pots could be seen glowing red in the embers below the fire. After about 45 minutes of heating the pots were dragged out of the embers to reveal their state - sadly spalled badly. A plus was that when they were washed the pots had turned ceramic throughout and did not turn back into mud when wetted!

The bonfire was raked apart and bits of spalling picked out.

Meanwhile the bowls were being warmed by the occassional flames passing into the chamber, but they were above the level of the pots, so the pots heated up.

Unfortunately, before they were dried out enough, a few gusts of wind blew the flames into the chamber too fiercely and three of the pots spalled.

As time was running out we went for the burn! Branches were lowered into the chamber from above to form a teepee shaped fire over the pots. They burnt down to produce a bed of embers which just about covered the pots. The fireboxes were kept stoked to introduce heated air into the chamber where it helped the embers produce more heat.

The fire was stoked a couple more times to build up more heat.

Eventually we took the plunge and reached in with the tongs and pulled out the spalled bowls from their nest of embers.

They were nice and black and took the shock of rapid cooling with water very well. All completely ceramic.

After a bit longer the rest of the bowls were drawn out and cooled. These had reoxidised somewhat and turned mottled rather than dark black from the reduction and smoke. The ash had distressed the surface, but that would clean off later with a bit of a scrubbing.

The bowls were fashioned in the style of Dorset Black Burnished Ware, with a foot ring and latticed decoration. Others were inspired by Somerset's Glastonbury Ware, with more rounded bases and decorated with more a Celtic style of curved incisions.

A pleasing day's firing! We know where we went wrong with these activities and learnt some new ideas. Mark's kiln was all the right dimensions and aperture sizes to get a good heat generation and control.

The small jars were made with clay that did not have very much added to the mix of "found" clay with some commercially prepared clay - more "inclusions" would have opened up the body and allowed it to breath as it dried out. They were also not preheated for long enough and a couple of times they cooled down a bit rather than steadily got hotter - as the branches burned away and were not replaced soon enough - trying to save fuel and just being forgetful!

The bowls were made with more added grog - a variety of ground down fired pot shards and a mix of reclaimed clays which had all been stored in a dustbin from previously failed or dried out lumps of clay and mis-fired pot wasters.

The blackened pots were blacker than any others ever produced by Bill using this sort of firing. Leaving the embers to build up without being disturbed helped them to produce more heat over a longer time. Getting preheated air flowing into the chamber through the fireboxes by keeping some fire burning there helped and promoted a more reducing atmosphere in the chamber, which promotes darkening. Some folk (not Bill!) think the iron oxide in the clay turns much darker when it is heavily reduced by lack of oxygen. Drawing the pots hot out of a nest of embers seems the trick. The carbon soot does not have time to burn away and iron oxide does not reoxidise? Those left for longer while the chamber cooled a bit had time to reoxidise as the nest of embers was turning to ash and no longer kept the oxygen out.

Thanks again to everyone involved.

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