Charcoal burning

Start of clamp constructionA new venture for the 1999 season was a campaign of charcoal burning. The first clamp was fired during weekend of 30th April-3rd May (the museum's May Fair). This initial clamp is burned oak. A second oak clamp, part sectioned to show the construction technique, was built during the May Fair (but will not be fired) and a third clamp was built on the weekend 28th-31st May (coinciding with the Woodland Fair) and is also single species, but of birch.

Not only will the clamps provide a focus for displays (temporary and permanent) on the woodland industries, but will also allow for analytical studies of charcoal composition, as well as (hopefully!) fuel for the on-going iron making studies.

Clamp 1

The clamp has been constructed by Ian Smith (MWL), under the guidance of professional charcoal burner Peter Richards, and assisted by Berwyn Thomas. TY and KB provide the night watches.

The site was prepared by lighting a brushwood fire first. The initial layer of the clamp comprised radially arranged logs, supporting concentrically arranged small diameter branches; a small amount of straw was scattered into the base of the clamp. These materials allow the basal layer of the clamp to draw air. Air flow was also assisted by the provision of series of eight brick-supported air passages around the base of the clamp.

Start of clamp construction

The woodpile, approximately 2.5m in diameter and 1.5m tall, was constructed from 2 tiers of inclined split oak logs. The wood was typically of 200mm diameter, and was cut to 600-800mm lengths. The centre of the pile was built around a 150mm diameter stake, which was removed to provide space for a fire, kindled from glowing charcoal and twigs, to be lit.

The completed woodpile

The cover comprised a layer of straw 250-300mm thick, sealed with a thin layer of bracken mould, and covered with a further 200-300mm of turf and soil. This clamp did not have the usual low bounding earth bank, so a slight screen of hazel stakes and withies was positioned to provide some support for the cover, with a row of turves at the base between the straw and the ground. The clamp was lit at 16:30 on 30th April.

Lighting the clamp

During the first 12-15 hours the clamp produced white smoke and steam. Temperatures just below the cover were around 110-120 degrees. The image shows the clamp at first light on 1st May.

Clamp 1: 12 hours

By 20 hours most of the steam had gone, and it was apparent that the clamp was burning very slowly. The hottest zone, near the summit, gave temperatures of 480 degrees just below the cover. Additional smoke holes were opened about half way up the sides.

Clamp 1: 38 hours

By 40 hours the hot area was at 630 degrees. The slow rate of burn meant that after two days of 24 hour cover, it was decided that a continuous watch during the night was unnecessary.

Clamp1: 45 hours

At almost 6 days into the burn the clamp is showing signs of subsiding on one side. Much of the smoke escaping has a distinctly brownish tinge, and substantial condensates are building up around the vents.

Clamp1: 140 hours

After a full week of burning at least half of the clamp shows a high degree of subsidence.

Clamp1: 168 hours

After almost 10 days of burning the subsidence of the clamp has increased significantly. Ian has taken the decision to shut the clamp down and in 3-4 days it should be cool enough for opening.

Clamp1: 236 hours

15 days after lighting, the clamp flared up again, but has been covered with additional turf and damp sawdust.

19 days after lighting, the clamp is no longer smoking, but is still extremely hot to touch.

27 days after lighting, the clamp was finally opened. The SE corner was slightly burned and the NW had some unburned material, but the majority of the clamp yielded good charcoal. Around 180kg was collected and the first batch is to be used on Monday in the iron smelting.

180kg of charcoal from clamp1

31st May 1999, smelt 16 employed the first batch of our charcoal. Some 86kg of oak charcoal from clamp1 provided the fuel for the smelt. (see also the smelting campaign diary)