Overview of Bronze Age Firing Techniques

There is a great deal of interest in how the Bronze Age folk (and others) fired their pottery.

Bill has been investigating the possibilities by trying various methods and gradually learning the pros and cons of the different approaches.

This document sets out an overview of Bill's methods of firing, explaining the basic techniques and what he has learnt. Some of this may be useful to other people wishing to try their luck at ancient firing.

Follow this link to see the archive images and explanations of a firing using a turf construction at Down Farm, Martin Green's farm at Sixpenny Handley.

Turf clamp kiln at Down Farm

This is the turf clamp kiln at Down Farm, beginning to warm up a bit!

Follow this link to see details and a video of a firing session making replicas of the Amesbury Archer Beakers.

Close up of beakers

Close up view of fired beakers

Description of Firing Experiments.

I used to follow the perceived wisdom that wares should be dry before firing, but a couple of potters have shown that it is not necessary if the clay is suitable (maybe a particular type or with masses of inclusions like grog (crushed fired pots - recycling wasters)). During a recent firing at Corfe Castle I successfully fired some pieces made the day before which were still damp, they were heated very slowly and so dried out before the flames got at them.

My early attempts involved building a small fire and putting the pots in a ring around it, but the heat rises up in the middle and sucks in fresh cold air from around the base - so the pots are cooled, not heated by the passage of air.

I then modified that by having a fire in the middle, putting the pots round it -

circular fire

thus being able to position the pots in a ring, the diameter of which depends on the number of pots. Then a ring of fire was built around that - see more of the images here

One preferred method is to start with double line of pots, rather than a ring. The pots are stacked bases outwards and small pots can be put inside larger ones. With a circular formation the centre does not get hot enough and any pots put there tend to suffer spalling (see below). If it is windy the line is at right angles to the wind, so the heat is carried more evenly towards the wares.

Pots in a Row

Urns fired in a double row - luckly it rained after they had cooled, so they got a wash.
More images of the firing at the Festival of History at this link

I position the pots on their sides, resting on greenwood twigs or willow wands. These keep the pots off the damp cold ground and allow air to circulate under them. These rests must be almost fireproof or at least burn by slow smouldering, if they burst into flames too soon that will damage the pots.

I build small fires at least a foot away from the pots to start with, to encircle the pots in a ring of fire (the "ring" may be rectangular). The fire is kept slowly burning all the way round for 3 or 4 hours, gradually moving closer in and turning the firewood over so that the radiant heat from the glowing charcoal hits the pots.
The pots can cope with radiant heat but not direct flames licking over their surfaces - a flame produces too high a temperature early on. If the pot still contains water (either dampness or chemically combined water) and is heated too high too quick, then the water turns to steam and that sudden expansion blows the pot apart - often "spalling" which is where the outer surface comes away. Heavy grogged clay is more porous and allows the steam to escape, some ancient pots were tempered with organic material - i.e. straw - which forms escape routes for the steam.

The pots turn colour as they get hotter and drier and slightly smoked.

Ring of hot pots

Knowing when they are ready to be taken hotter is tricky. Often a gust of wind will fan a flame over a pot and it will spall - so you know it is not ready! On windy days a barrier up wind might be useful to control the forced air (which could be directed onto the fire when it is at its hottest).

After 4 hours or so, the pots should be hot and dry enough for the fire to be built up. Try moving the fire closer in one corner first and leave it for a few minutes. If the pots do not spall then move the fire closer all the way round, turning the wood round to face the glowing parts inwards. Leave it for a while to let the heat get into the middle of the heap of pots and to reach the small pots inside. At this stage the twigs under the pots may start to burn, if possible stamp such fires out with a stick (but this is a bit risky!)

Just before the blaze

After say half an hour the main ring of fire can be built up into a wall - as high as the pots, this is where larger logs are useful to form a stable structure which will not burn away too quickly. Then cover the whole area with timber - trying not to rest heavy pieces onto a single pot, but spreading the weight out and using the wall to support the ends. English Heritage supplied cut logs some of which were selected as being like shingles and they formed a fairly tight overlapping roof, which kept the heat in better. Less authentically a closed boarded pallet or sheet of ply can be used as a temporary roof - which will hold the heat in until it burns away.

Small branches have lots of gaps between them and so the heat can escape and they burn quicker. Fill in any large gaps that appear as the fuel burns away.
If enough fuel is available it is best to put a second layer over, when the first layer has partially burnt and turned to charcoal, that will be trapped inside and will help raise the temperature. The aim is to get the pots to glow red - which indicates they have reached about 800 degrees, the temperature required to turn clay into ceramic.
Let the fire burn down naturally, it will turn to ash fairly quickly and sink down to reveal the pots. Do not remove the pots from the fire too soon, as the cooling may crack them. However to avoid patches of smoke remaining on the pots where they rest in the ash and the carbon does not burn away, they can be turned over (still in the firepit) - that carbon should burn away if they are hot enough. Some people prefer the patterns of carbon smoke on their pots, if so then keep them buried until cool.

I sometimes remove the turf and replace it afterwards for cosmetic reasons, but I understand from archaeologists that sites were used repeatedly and tended to form a shallow pit, sometimes several meters across, found by them as a burnt earth lining, often mixed with charcoal. These are very hard to prove to be pottery firing areas, but may have been used for metal working, cooking or charcoal production.

Digging a pit may help protect from wind, which was certainly a favoured method by the iron age, but perhaps moving hurdles would be better - they can also be used as an umbrella if the weather turns damp.

Clamp firing is described on this linked webpage

Monkton Clamp firing lid

That was a lot of work and only worked partially as some of the pots were underfired and many broke. It was performance art setting it up!

I have done pit firings - using a hole like a large grave - but we were just smoking already biscuit fired pottery to get interesting patterns and flashes from oxides and salts. That involved putting the pots in the bottom of the pit on a bed of sawdust, covering with timber (offcuts from the local timberyard) up to ground level and then covering with corrugated iron roof, with gaps at the ends to light it from and let the air in and smoke out. Light the blue touch paper and stand back as it conflagrates in a couple of hours.

A large bonfire can be used (Nov.5th is useful!) - put the pots in metal boxes packed with sawdust and close the lids, bury them in the centre of the fire before it is set alight. Retrieve the next day. This can work with raw unfired pots, as the sawdust insulates them from extreme heat to start with and also the closed box traps the steam and they heat up in a damp atmosphere, so the steam generated does not have the urge to escape from the pots in the normal way. As the fire progresses the sawdust turns to fine charcoal, but may not burn due to the lack of oxygen. If the main fire is hot enough they will be cooked and turn ceramic.

There are many Health and Safety issues to consider when doing these firings. I tend to risk only the health of myself and immediate family (my wife is now chief stoker - as a Guide leader she misses camp fires, but enjoys fire and smoke!). I exclude everyone else from the firing area, unless they are sensible adults known to me as unlikely to sue!
I have lost some hair from my arms a couple of times, but with common sense the risks can be minimised. Buckets of water and crowd barriers etc etc.

This type of firing undoubtedly helps people to understand the processes and is very valuable to bring history to life and to show simple technology.

The montage of images below was prepared for a display, follow the links at each of the hotspots on the images.

Festival of History Fire Corfe Castle Firing Festival of History Firing New Year's Eve Bonfire Monkton Clamp Firing Soda Beakers 28 Soda Beakers 31 Beakers July 2005