Experimental Firing 5th November 2016

This page records the experimental firing undertaken on 5th November 2016 at the site of the Upwey Village Bonfire. It is an archive of photographs taken before, during and after the firing.

Bill Crumbleholme worked with local archaeologists Peter Woodward and Greg Rochfort, together with wife Diane and a Special Guest visitor Neil Wilkins who had given an excellent talk about Bronze Age Gold at Dorset County Museum the evening before, based on his work in the British Museum's Bronze Age department.

Jennie Hanrahan, potter from Bridport, together with her and Bill's pottery class members used the main bofire to heat up and toast some of their ceramics, this is described in the separate "Pit Firing" section below.

While demonstrating with the Ancient Wessex Network, Peter had produced a pair of urns as replicas of those found at Hambledon Hill. These, together with a couple of Bill's replica urns and beakers were fired in a small kiln constructed of turf.

Bill, Diane and Greg tended an open bonfire firing of a collection of Bill's replicas.

Menu for sections on this Page:-

Making Pots

Firing Pots

Unloading Pots

Pit Firing

Report

Making Pots - 2016

 
Bill and Peter Making Pots

 

Bill & Peter making pottery replicas during an Ancient Wessex Network event.

Bill Beaker making

 

Bill decorating a beaker - using a comb to impress lines of dots.

Bill resting

 

Bill really looking more ancient than usual!

Firing Pots

 

TurfKiln Start

The turf kiln was made by excavating a circle of turf and building up the walls, adding more turf from the bonfire area.

 

Turf Kiln

 

A fire box was dug leading into the chamber, in which timber was burnt.

The pots were laid on a bed of dry ash at the bottom of the chamber.

Turf kiln burning

A timber lid was built across the top of the turf walls to hold in the gentle initial heat, while the pots dried out.

 

Bonfire preheating

Meanwhile a trench area had been de-turfed and pots laid on a bed of dry ash.

Lines of smoldering timber branches were stoked along each side and end, gently heating the pots.

Bonfire preheat

The gentle heat gradually dries out the pots.

If the wind suddenly gusts, a lick of flame can cause the pot to heat too quickly and the face of the pot will "spall" with a skin of the pot breaking away as steam is generated inside the wall, causing a rapid expansion.

 

Archeologists

The archaeologists look on as the fire is gradually built up.

Bill tending the fire

 

The timber surrounding the pots is carefully tended to maintain an even heating.

Bonfire building up

Gradually the fire is built up to increase the temperature.

 

Lidded turf kiln

The lid on the turf kiln is replaced with new timbers at intervals, as the wood burns away.

The lid traps the heat inside the kiln and the burning underside works like a grill to toast the pots from above.

Bonfire blaze

 

When dried out sufficently the bonfire is stoked up with more timber between the pots and the outside wall.

Bonfire Blaze

Then branches are placed across the walls, to make a lid over the pots, that is not resting on them, but keeps the heat in and grills the pots.

As they burn away the embers fall amongst the pots to heat them more.

Blazing

 

While the blaze continues fresh branches are stolen from the main bonfire and there is time to chat!

Bonfire blaze

Close up of the blazing bonfire pottery.

Still blazing

A glowing pot can be seen in the embers. That indicates the pot has turned into hard ceramic - about 700C.

The fire may reach 850C, but that is hotter than needed - ancient pots were often tempered by adding crushed seashell - which turns to quicklime above 820C, thus ruining the pot.

Turf Kiln glowing

A view into the turf kiln, where the pots are glowing nicely.

The kiln is stoked by adding timber down through the top of the chamber, the firebox becomes the air intake for the fire.

glowing

Another view of the inside of the turf kiln.

Glowing

Cannot have too many images of flames!

General view

General view of the site.

The "pit firers" are starting to load the containers of pots into the main bonfire.

Unloading Pots

 

Turn kiln unloading

After a short period of cooling down, the pots are withdrawn from the turf kiln, carefully using long tongs.

Turf kiln unloading

The hot pots are rolled about to even out the rapid cooling, once out of the chamber. Any embers or ash is emptied out of them to stop localised heating.

Just out

One of Bill's urns out of the turf kiln.

Cooling

Peter's urns cooling as the fire dies away.

Bonfire cooling

 

The bonfire pots are left until the timber has all turned to embers and ash.

Several pots have spalled during the rapid heating. A very sickening noise as they pop apart!

Bonfire unloading

Then the pots are removed using the tongs.

Bonfire unloaded

All the pots taken out of the bonfire, cooling down.

Peter's Urns

Peter's Urns out and cooling and looking very impressive!

Unloaded turf kiln

All of the vessels removed fronm the turf kiln.

Peter's urn

Close up of one of Peter's urns. Each has a pair of lug handles

Fired Urn

One of Bill's urns from the turf kiln

Results

Bill's pots lined up ready to be scrubbed clean.

Spalled urns

Typical spalling on an urn, not dried out by pre-heating enough and maybe made a bit too thickly, without enough grog added to the clay.

Spalled urn

More of the same!

You win some, you lose some!

Beaker & urn

Getting better, but still not fit for purpose!

Urns

But these are better results.

More OK urns!

Beakers

 

beakers

beakers

Several of the beakers came out very well.

"Pit Firing" in Main Bonfire

 

"Pit Firing" is the term often used by potters to describe treating pottery by submitting it to the effects of heat while contained within a saggar - a technical term for a sturdy surrounding vessel that protects it from the fierce flames and traps the results of burning substances placed around the pottery. This is often done underground in a pit. However the Upwey Bonfire was so huge that a pit was not needed!

 

Containers in position

These are the collections of containers nested in their hollow within the main bonfire.

The pottery has been burnished smooth while drying and some has been covered with slip to provide a much smoother surface and also to whiten it. This is known as terra sigilata, used from Roman times.

Containers are usually metallic, they range from dustbins, cooking pots and biscuit tins.

Combustable materials are packed around the pots inside the containers, to create interesting flashes of colour on the surface of the pots. These materials can be mixtures of vegetable matter, preferable oily plants such as bay leaves, oregano, rosemary. Other things such as seaweed provide salt and iodine, with other trace elements.

Metallic oxides used in glazes can also be sprinkled in.

Copper wire of various thicknesses is wrapped around pots.

As they vapourise, the trapped atmosphere holds them close to the surfaces of the pots.

Black Burnished Ware

This is a dustbin full of Bill's replica Black Burnished Ware. Made some time ago and never properly blacked, the aim was to smoke the vessels to turn them jet black.

However most of them stayed cloudy, some actually got lighter coloured than before and several cracked under the stress of the extreme heat.

So not a good result!

Binned globes

Bill's larger globe shaped vessels had been fired in an old dustbin.

An assortment of twiggy herbs had been packed around the pots, all that burnt away leaving subtle marks on the surfaces.

Globe

Globe

Globe

These are some of Bill's larger globes, after cleaning up.

Tinned

Tinned

Tinned

These are a few of Bill's small globes, revealed when the lids of the biscuit tins had been removed.

They had been wound with copper wires, packed with seaweed, herbs, bay leaves and wood shavings.

Also a lock of Diane's hair! Nothing to do with Voodoo!

Small Globes Cleaned up

These are the smaller globes cleaned up.

The copper wires seemed to act as a resist and leave a pale trail, rather than a mark.

Some of the twiggier herbs, such as Rosemary, left distinct marks.

Report and Conclusions

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Bill and Peter were very happy with most of the resulting pottery.

The turf kiln provided better results because the heating is more controlled and evenly distributed, without the cold spots that can happen with an open bonfire.

The bonfire also worked well, apart from a higher rate of spalling damage, the pre-heating should have been done better, perhaps with the pots being turned around after a while to enable all the surface to be preheated.

Bedding the vessels in ash stopped them getting damp from underneath, but also maybe stopped them getting hot enough. Some had darker areas outside where they were buried in the ash and the embers formed during firing. Peter's vessels were slightly under fired on the bases because of this effect. Next time we will try placing the vessels on old shards to raise them and allow the heat to get underneath.

Firing items on their sides, rather than upright can lead to them distorting into an oval shape.

Taking the pots out of the fire as it died down seems to work well, previous firings have produced cracked rims - as they cool down and shrink, while the bases in the embers remain very hot.

Bill has a laser temperature measuring device, the gun provides an indication of the temperature of objects it is pointed at. That gave highest readings up to 900C, but that could have been the burning wood, rather than the actual pottery, which measured at least 750C (which can be estimated from the colour by any serious pyromaniac!)

Some of these pots are destined to be displayed at subsequent events. Bill's during his appearance at his Local Ship Inn on 13th November, support the Baking Bird's festive sale, with a collection of his pottery for sale. Both Peter and Bill are participating in the Dorset Pottery Group's annual exhibition at Bridport Arts Centre in December 2016.

Comments and additional images are very welcome - email Bill Crumbleholme

 

Thanks to :-

The Upwey Society for hosting the bonfire event.

The Foot Farming empire for allowing the use of their field.

Greg for his help firing.

Neil for his encouragement.

Diane for her stoking skills and for putting up with being called Diana!

Pottery Class members for their enthusiasm and willingness to allow their ceramics to be subjected to the flames.