Wind & Weathering :

air delivery & long term erosion

Darrell Markewitz, with contributions by Neil Peterson


‘ A new furnace was constructed on a clean sand pad, set to one side of the main smelting area. Photographic and video recordings were made of the extraction sequence. The remains of the furnace and the resulting debris field will be exposed to weather, and the aging documented as the features erode. The intent is to continue these observations over the next ten years. ‘

    Starting in 2007, a number of furnaces have been placed around the base of the tree just visible in the extreme left of Figure 3-1. These have been allowed to decompose with time, described in ‘Evidence of Absence’  (June 2021). This has provided some observations of the effects of natural weathering on various clay shafts themselves.
    The intent of the experiment # 90 was to keep an entire working smelter area, with clay furnace remaining above the slag bowl produced, plus the associated debris field created from the extraction.

    Although not detailed during the description of furnace construction, a fresh working area was established, slightly separated from the normal working area. A frame was laid out using standard 2 x 4 lumber, creating an 8  x 8 foot square (240 x 240 cm). These were placed over top several layers of flattened cardboard, this done to at least limit the growth and penetration of the under laying grass and weeds.  With the boards laid on edge, the frame was filled with partially screened locally dug sand, bringing the depth of the sand pad to about 8.5 cm. The furnace was placed roughly centred side to side (east to west), and with 2/3 of the available space towards the front (north). This placement would allow more complete retention of the extraction debris. The tuyere and air system was placed on the east side. Most typically additions of ore and charcoal were carried out with the worker at the west to south west area.


Figure 3-1 : Setting of the experiment within the larger smelting area, looking to east.

The remains of the sod construction for Phase 3 at centre.
Earlier furnaces have been built under the main structure seen to left rear.

    Referring back to the extraction videos (seen in Part One) shows how the individual pieces of broken furnace were deposited, with the accumulated tap slag raked to one side. It should be remembered that through a combination of less than ideal building techniques, aggravated by the in furnace compaction process, there was considerably more damage to the body of the furnace.
    During this extraction, the bulk of the entire slag bowl mass was also pulled clear, rather than the more typical breaking free out of the bowl of just the bloom itself.

Figure 3-2 : Overall - after extraction, view towards east.
Figure 3-3 : Overall - after extraction, view towards south

Figure 3-4 : Detail -
remaining debris from extraction
Figure 3-5 : Detail -
body of the broken furnace
Figure 3-6 : Detail -
internal erosion around the tuyere

    Minimal changes were made to the overall working area at the completion of the smelt :


Image Mosaic A : Entire working area, images from October 31, 2021

The final change from the post extraction condition was to carefully remove the copper tuyere, and the exterior support for the air system. The heavy wooden block (used to aid levering out the bloom) was also removed.

Figure 3-7 : Imprint of the air system when removed Figure 3-8 : Detail of interior, with tuyere removed
(compare to Figure 3-6)

Short Term : Early Winter, End 2021

In previous decades, weather in Wareham typically has been temperatures starting to bracket freezing by early November, with the first snow that remains as ground cover by the middle of that month. Normally by end end of December, show has accumulated to depths in the range of 30 cm (or more) and day time temperatures remaining below freezing. (total winter snow fall has historically been into the range of over 300 cm)
Climate Change has continued to significantly impact weather conditions, effecting both temperatures and precipitation over November and December 2021. Temperatures were significantly warmer than in past years, and there was also more precipitation than normal. The combination, with more rain and temperatures oscillating between nights below freezing and daytime just above, created conditions of excessive nighttime frost expansion alternating with daytime water penetration. The overall result was considerably more weather created damage to the remaining structures.

Figure 3-9 : December 13, 2021 -
snow at this point was barely enough to cover the ground
Figure 3-10 : January 3, 2022 -
snow at this point was to about 10 cm.

The most significant damage is seen with the portion of the furnace wall on the tuyere side.

This accelerated rate of erosion, at least as compared to the other long term components under way, may not prove important. The exercise here is intended to shed light on the remains of bloomery furnaces on an archaeological perspective.  As stated in the introduction, the intent here is to provide at least 10 years of detailed observations, as an ongoing process.    

Unless otherwise indicated :
All text and photographs © Darrell Markewitz, the Wareham Forge.