Mind the BLAST

Correcting Reported Air Volumes

    One of the critical elements in any bloomery iron smelt is the amount of air provided to the furnace. Air, or more specifically the oxygen component, supports the burning of charcoal, so the production of both heat and reactive gases. In simplest terms:

O2 (air) + C (charcoal) = CO (heat) 

CO + Fe2O3 (ore) = CO2 and Fe (bloom)

    The mechanics of the air blast itself is a lot more complicated than just the chemical equation. Air volume effects the size of the burning zone, delivered pressure how far the burn zone penetrates into the furnace, and entry angle of the blast determines how the burning zone is positioned within the diameter of the furnace. ( see ‘About the Air’, blog post May 23, 2013 )


    Most working teams actually don’t concern themselves with recording air volumes, as a much more practical measurement is how air blast effects charcoal burn rates. Over the course of a working smelt, knowing just how long it takes to consume a fixed measure of charcoal is one of the best indications of what is happening inside the furnace. Over past experiments, recording the addition times of fixed measures of charcoal has been a standard method of indicating shifting burn rates within each experiment. This instead of mathematically generating an average (total time against total charcoal consumed), a method commonly used by other teams.
    Directly related to this, most other teams have one standard air production method, either repeated use of the same electric blower or human powered bellows. With any bellows equipment, obviously the greatest variation in delivered volume and pressure, minute by minute over the course of the long smelt effort, will
be through the differing activities of individual (changing!) bellows operators.

The long series of experimental smelts recorded here have seen a large number of quite different air equipment used, even when considering those used on multiple tests :

Vacuum cleaner blowers - two different
Norse blacksmith bellows - two different
Norse ‘smelting’ bellows - three different
‘Compression’ blower - the commonly used standard (seen below)

Partially because of this wide variation in air equipment, there was an attempt made fairly early in our working series to at least roughly measure air volumes produced.

Current wind speed gauge, about life sized

 

    It has been mentioned numerous times, those working without Institutional support often can only provide the simplest of measuring instruments. I had investigated possible methods of measuring air volumes back in 2007, and what proved feasible was the purchase of a simple wind speed anemometer, a type used by sailors and wind surfers. The precision of this unit was low, and in retrospect, the durability of construction is likely to have further effected even that level of accuracy over time. With the gauge set in line, measurements would be recorded as a rough estimated average while observing for several seconds, as Kilometers per Hour. The  KpH number combined with the pass through area (2.5 cm diameter) and a calculation made to convert to Litres per Minute. This was never considered more that accurate to within 25 LpM at best. Additionally, from the vary start it was recognized that any measurements from these methods could only be considered roughly approximate.

High capacity electric blower - US Navy surplus

Set up of wind speed and pressure gauges, April 2008 (smelt 32 - D14). 

    Along with a 'box' fitting to hold the wind gauge, an additional fitting can be added to allow inclusion of a simple pressure gauge. This set up was used infrequently between separate smelts, plus only recorded irregularly during an individual experiment.
    Air volume from the most commonly used industrial blower was controlled using a sliding plate styled ‘blast gate’. This had been marked with a set of lines roughly calibrated to 100 LpM amounts, determined as indicated above. Out of concern of the wear being caused by the high output speed of this blower, most frequently these marks would be used to record air volumes, not repeated use of the wind speed gauge (which was only left installed for the measurements).

    Given the duplication of method, it should be considered that the air volume numbers recorded over those smelts using these systems should at best be considered ‘relative’, as a high level of variation in precision was certainly to be expected. One major reason for continuing with these methods, despite the known lack of accuracy, was to be able to make some estimation of the functional air delivery from the various human powered bellows equipments used. It was quickly observed that any theoretical volumes as calculated via dimensional measurements, were quite different than the actual working volumes produced. There was also considerable variations between individual operators, and even the same operator over even a short working cycle. (Further information on this can be found as ‘Air Delivery Test’ - October 2007 )

    Neil Peterson had acquired a new, and significantly more accurate, air volume meter - the HHF1000 by Omega, which was available for the smelt on September 4, 2021. This instrument uses a small sensor that is positioned through a 6 mm diameter hole into a pipe section, with both direct digital read out and even the ability to wirelessly transmit data to a near-by computer. It makes readings down to every 1/4 second, with an expected accuracy of 1.5 %. Needless to say the addition of this significantly higher quality instrument can be expected to seriously improve accuracy of measurements into the future.

Over this experiment, four separate measurements were made using the new instrument. This was done roughly each time air flow was changed, that done using the simple marks on the sliding plate of the blast gate. Changes were made because of variations in burn rate or other observations of how the furnace was performing. The meter measurements were taken inside a 40 mm pipe section, the rough multiplier from meter reading to LpM is X 75. Note how there are clear gaps in the recorded measurements :

Time	Event	Plate	Meter	Calculated
10:11	fill rough charcoal	700
10:15	air increase	(900)
11:19	full graded charcoal		11.5	865
11:23	air reduced	700	(no record)	550
12:00	air increased	800	8.8	660
12:44	air increased	900	9.6	720

    What is significant to remember is that the plate marks are ‘hole size’ only (so a maximum amount available). There can be considerable changes in flow into the furnace as conditions inside change (charcoal size, any ‘draw’ effect as stack ignites, formation and position of slag bowl, amount of ore contained in the upper stack - all come to mind at the least). The meter is measuring the actual air flow at the time point indicated, so differences at similar plate marks should be expected. This is clearly seen even with this limited set, in the differences between the two measurements with the plate setting at 900 LpM, early with the furnace full of rough ‘out of the bag’ sized pieces compared to later in the smelt, with smaller gaps between graded charcoal, plus the addition of about 3 kg + of combined ore / slag pieces within the stack.
    Clearly this limited set of measurements is at best a ‘proof of concept’, definitely not enough data to draw conclusions of the operating changes within the air / furnace system over the duration of a complete smelt!

    Comparing the plate marks back to the volumes calculated from the meter readings shows an error factor of roughly plus times 1.25 (how much larger the plate mark is than the meter calculation). Although there will be some shifting of potential accuracy between the earliest wind speed gauge readings and the more recent (due to wear in the vane shaft), certainly this set suggests all the earlier numbers will be 20 - 25% higher than what was actually being involved at those times.


    Obviously, an important set of observations can be gathered by leaving the new instrument in place over an entire smelt series (or at least taking measurements over a short set of time points overall). Although the pressure gauge now on hand is both an analog and far less precision unit, recording variations in delivery pressure will also provide some insights.
    One important set of measurements employing this new gauge will be to undertake at least some of the air delivery from the next smelt using the current V3 Norse styled smelting bellows. At best, the current estimates for air delivery using this air system are based on measurements made using the earlier wind speed gauge as described above.

30 That Never Sold - 'Wind Widgets'

 The last couple of years I exhibited at Summerfolk in Owen Sound, I mounted a series of work based on the Four Elements : Earth / Air / Fire / (Water).

I never completed the last element, work based on water. This largely because of repeated trips to Scotland and Europe after 2014, to participate in bloomery iron research projects, all over that same part of later August. 

For the 2013 year, the theme at Summerfolk was AIR.

In an attempt to have a body of lower priced objects, that I thought suitable and of interest to a Folk Festival audience, I had created several styles of individually made 'Wind Widget' outdoor spinners. These were all made of durable metal, unlike the slightly cheaper plastic spinners becoming available about the same time. There were two broad groupings, either made of solid copper or brass, plus those made of aluminum or stainless steel. Some of the curved aluminum ones were spray painted  with merging bright colours.

The more expensive (considerably!) material copper and brass were priced $20 (taxes included). The aluminum and stainless were priced at $16 (taxes in)

I don't think I sold one...

I still have over 30 of these. I personally think they are good objects. The spirals and mobius strip types are roughly 8 - 10 inches in diameter, the strip versions about 18 inches long. With the light weight against the curving surfaces, they all are quite mobile to the wind. They certainly are extremely durable - I've had some of the first prototypes for these hanging outside at Wareham, year round, for a decade.

And no - I've never been able to 'Understand the Marketplace'.

36 seen at Wareham

Walking through Wareham
the Yard Art Tour

So here's the thing.
I've now been in Wareham over 30 years - also the length of time as the Wareham Forge.
Over that long a time, you tend to accumulate a lot of pieces.
Some of these are concept tests and samples that lead to major commissions.
Some of these are good pieces, that for one reason or another just never attracted a buyer.
Some of these were intended as 'show' pieces, which simply got marred after being repeatedly being hauled and exhibited.
Some of these are 'just because' pieces, using novel techniques or conceptual designs that cried out to be created.

Many are often what any artist considers some of their best work.

For an overview of what is mounted up around the yard :
Go on to the Yard Art Tour

 

For those who are wondering why contributions have been thin of late?
I've experienced a medical, an am pretty much limited to 'one hand hunt and peck' on the keyboard right now.

Setting up for Phase 3 - B

 Those who have been following the recent work have seen a number of commentaries related to the ongoing series of bloomery iron smelts based on elements from the excavations at Hals, Iceland, by Kevin P. Smith. 

The basic undertaking for the phase three experimental series was build a potential Hals type turf / sod furnace, containing a relatively thin clay liner, using a mixture simulating (as best possible) the material found close by that location. The full build was done in mid June, with the first complete firing cycle on June 20. The intent was to subject the furnace to multiple firings, including one after a winter exposure to freeze and thaw.

After the first use of the furnace, there was pretty much the normal expected damage to the front section of the furnace, with erosion of the walls around the tuyere, and breakage at the extraction arch.

Furnace - just at end of the smelt

20 cm turtle - and damage

What was not expected was the invasion of the warm furnace by a large snapping turtle, which resulted in a large section of the front wall section breaking clear. Weather at Wareham has been unusually wet over later June, July and now into August. (This normally a period where there is little or no rainfall = Climate Change!). Although the top of the furnace was fitted with a metal cover, there has been considerable damage to the 'baked mud' outer sections of the furnace walls, plus considerable slumping to the original grass sod surrounding structure. 

Field drawing of the furnace - August 7, 2021
Note combination of metric and US Imperial units (single marked tape!)

Typically, the next scheduled smelt attempt would take place over Canadian Thanksgiving, this being October 10. I have some concern over additional damage to the existing furnace structure, so have decided to undertake the required repairs to the lower front of the furnace, including breaking clear the internal slag bowl which was left in place after the first smelt.

Interior - just after smelt (tuyere still in place - at top)

Roughly same view, slag bowl broken clear

 The slag bowl from the June smelt did nicely resemble those exposed at Hals, a distinctive C shape with a cupped cross section, the front edge broken away where the bloom had been pulled free during the front extraction. The remaining slag broke free of the interior wall surface quite easily, using a chisel tipped bar from above. There was little actual damage to the wall structure from this process. 

Condition of furnace - August 7

As seen in the images and drawings above, a considerable part of the front wall surface had been broken away. 

The original line of the sods can be distinguished as the place where the upper exterior wall changes from smooth to a wrinkled texture. That top area, about 10 cm wide, despite the cover, had been flaking away in the rains. The slumping of the sod structure is obvious here as well. The heat from the smelt had largely destroyed the binding root structure, leaving basically baked earth. Again the rains were slowly washing this earth down and away. 

The original build had constructed a full cylinder of clay, with the stones supporting the arch and tuyere placed to the outside. As a result, there was no exposure of the stones to heat, other than the short time of extraction. The archaeology at Hals indicates fire marked and slag adhering stones. For this reason, in the repair, a set of small flat stones were used to block in the eventual extraction arch, which will expose the inner surfaces to the heat of the lower furnace.

Interior after repair. Dark grey is the new clay added

Lower section, replaced lintel stone with small flat stones filling extraction arch

In the initial build, it proved quite easy to keep the wall thickness to 4.5 cm overall. This simply was not possible when adding new clay to repair sections considered badly eroded, and those locations where the walls needed to be replaced entirely. new material had to be added by reaching up inside through the extraction arch, most often working from touch alone. There are certain to be places where the fresh clay will be much thicker. Additional clay had to be added around the edges of the small flat stones used to seal the extraction arch, as these where chosen from a random pile of available (gniess) pieces. The same granite lintel stone was used as in the first build, only this time it's inner surface will be exposed to the full heat at tuyere point. Once again, this slab serves to support the upper layers of grass sod (total of three).

Finished repair build, tuyere yet to be installed

An extra row of grass sod was placed around the furnace liner, building the supporting structure back to approximately the same level as at the start of the first smelt ( about 65 cm above hard base). Extra dirt was placed in a ring around the top of the sods, with the hope of reducing any fire damage during the next firing. One additional aspect seen in this image is that the grass composing the sods laid in mid June has continued to grow, and least around the edges of the sod cone. 

For this repair, a total of five prepared balls of the clay / sand / manure mix were required, each about the size of a large grapefruit. (This material left over from the initial construction of the clay liner.)

Depending on team availability and other ongoing work here at the Wareham Forge, the hope is to undertake the next smelt in this series some point over the next two weeks...