SCA 50 Bloomery Smelt a la Wareham Forge

It was my great good fortune to attend the SCA 50 year celebration, and while I was there I spent a fair amount of time hanging out (and occasionally helping out) at the "Bloomery Iron Smelt" experiment that Darrell Markewitz ran.

It was the first time I'd ever actually been at the forge when the bloom was extracted, and I have to say I was pretty overwhelmed. I had to leave early Wednesday morning, so I'm not sure how the into-the-night reheat or next-morning compaction went, but I was glad to be there nonetheless.

Here's a short video showing some of what we did:

Hope you enjoy.

Upcoming events Spring/Summer 2015

It's been a busy six months, mostly working on chemistry rather than metallurgy, but metallurgy season is coming around, so here are some upcoming events you might want to check out:

Events in 2015

Early Iron IV
When: April 24-26
Where: Ashokan Center, Olivebridge NY, USA

Iron Smelting Course
When: May 24-26
Where:  Proton, CA

Prehistoric Metallurgy Class
When: July 3-6, Butser Ancient Farm
Where: Butser Ancient Farm, Chalton, UK

UMHA AOIS (BRONZE AGE)
When: Late July, Early August
Where: Skibereen Co Cork, IE

Assaying galena for silver

In which I create large quantities of a small molecule with unforgettable properties while simultaneously inventing a new Olympic Sport.

I was recently exchanging email with Fergus Milton (a most excellent Brit who teaches a class on copper smelting at Butser Ancient Farm) on the possibility of adding a unit on cupellation to the class. This jogged a memory and I recalled that I had purchased a quantity of galena some time ago. I hadn't had any particular drive to smelt lead, but now that I might get lead and silver out of the effort, I was moved to retrieve it.

The galena was in my Ore Storage Facility, known to the less appreciative as "that big dusty pile of rocks and bags of yet more rocks you keep behind the bar". After a time I had moved enough of the contents of the OSF to identify the correct unlabelled tan canvas sack which contained my galena. I towed it over to the scale and learned that I had almost exactly 7.5 kilos of the stuff.

Now we all know that some galena is argentiferous and some is not. Having been (to date) unsuccessful in coming up with a decent excuse to blow the cash for an X-Ray Fluorescence analyzer, I was seemingly left with two alternatives: Send the sample off to a lab to have it analyzed, or do a smelt & cupel, trusting that I'd get it all right the first time, and find the silver if there was any. Neither appealed.

Then I had a thought. (This happens more often than I like.)

Like any aspiring mineralogist, I have some hydrochloric acid for identifying metals in rocks. Lead chloride is soluble. Silver chloride is not. It should be a simple matter of dissolving the galena in hydrochloric acid and watching for an insoluble precipitate. Silver chloride has some well-defined characteristics, so it should be easy enough to confirm the silver and estimate the silver content of the original lead.

Being a Man Of Action, I grabbed the jug of acid and a handful of galena and headed to the kitchen. I tossed the galena in a glass pie pan and promptly poured some acid over it.

I can hear the chemists in the crowd wincing.

Now the chemistry I had concerned myself with was fairly straightforward:

   PbS + Ag₂S + 4HCl = PbCl₂(aq) + 2AgCl(s)

But this is not a balanced equation, since it doesn't account for the hydrogen or sulfur on the left hand side. A correct and balanced equation looks like this, with aqueous, solid, and gaseous products:

   PbS + Ag₂S + 4HCl = PbCl₂(aq) + 2AgCl(s) + 2H₂S(g)

Anyone remember hydrogen sulfide? The more enterprising among you may have produced it in stink bombs when you were kids. It has an eye-watering odor of rotting meat crossed with severe flatulence. It's also flammable and quite toxic, but I can personally assure you it's the odor that will speed your pace when clearing the area.

Flash forward in time five minutes. Every window in the house, as well as the front and back doors, are wedged fully open. Every fan in the kitchen, lavatories, and HVAC system is on "high". The pie pan has been transferred to the backyard.

Ever try to run through your living room while squinting and holding your breath? While carrying a pie pan full of corrosive liquid which is actively producing toxic gas? Getting the sliding glass door to the backyard open under these conditions yields a difficulty rating of 6.0. I believe I have a new Olympic Sport in the offing.

Happily hydrogen sulfide is a small and (mostly) hydrophobic molecule, so once it's cleared, it's cleared. It doesn't linger. At that point it's only a problem for the people downwind of you.

The adventure over, I was able to discern that my galena does in fact contain silver. Stay tuned for an account of smelting lead and cupellation of silver.

---

Postscript:

DANGER: Usually in metallurgy the worst thing that could happen to you is a disfiguring burn. This is different. Every one of the reactants and byproducts of this reaction are toxic to humans. Don't even start this unless you know how to handle and dispose of them safely.

For those qualified to do this properly, the stochiochemistry follows: Galena (lead sulfide) has a molecular weight of 239. Lead chloride has a molecular mass of 278 and a solubility of 10g/L in water. The acid I used is 31.45%, 20° Baume, or 8.5molar hydrochloric acid.

Scaling everything for 0.036 moles, we get the following values: 8.6g of lead sulfide + 4.3cc of 8.5M hydrochloric acid + 995.7cc of distilled water results in 10g of lead chloride dissolved in 1L of water. Excess acid and/or water will assure only the silver precipitates.

Not dead or dying

Despite the dearth of posts in 2014, this blog is not dead. I've just been involved with experiments involved in other disciplines. (See my paleotechnology and paleochemistry blogs via the links above for more info)

Bronze age bellows a la Brenda Craddock

Brenda Craddock taught an excellent unit on making primitive bellows on the second day of the Butser archaeometallurgy course, and this is an attempt to capture some of that excellent teaching.

The bellows pattern used was referred to locally as a "Roman Kite Bellows", although I've been unable to find that term used elsewhere. Brenda confirmed that "No one actually knows what bronze age bellows looked like.", but her design is intentionally "As simple as possible" and based on "later descriptions and depictions" of them.

The bill of materials is relatively short:

1. one large tanned goat skin
2. one hollow nozzle (we used a segment of bamboo)
3. two short, smooth, regular, handles (dowels work)
4. leather awl with thread
5. a few feet of rough twine
6. (optional) gelatin glue

The bellows pattern (figure 1 below) itself was designed to fit into an animal skin with maximum utility, with the narrow bit to the left terminating at the scruff of the neck, and the rounded section on the right straddling the tail and haunch. The rest is made as broad as the hide will allow. Caveat: I was unable to trace her pattern directly, so this is a projection from some images captured during the class.

Production

General notes

• The bellows are produced such that the handles have the rough/suede side of the hide on their outside, so that they seal suede-against-suede rather than smooth-to-smooth. This gives a better seal, which is essential.
• The bellows can be fashioned such that the seams are on either side, although in this class all the bellows we produced had the seams on the outside.

The foot

See figure 2a. The hide is bent slightly along the A-B line, then points C and E are brought together. The edges A-C and A-E are sewn together starting from point C/E and progressing toward A, so as to avoid any misalignment. Then points D and E are brought together, and sewn as above, from the point D/E toward B. Lastly, a small triangular gusset is sewn into the V formed by the edges CF and DG. The result should look something like figure 2b.

Fig 2a

Fig 2b

 

The snout

See figure 3a. The hide is curled around the A-B line, joining point C with E and D (roughly) with F. Sewing proceeds from point C/E toward point D/F, so that the joint at the handle tabs is clean and any discrepancies occur at the snout. Figure 3b shows the curl and the point from which to sew.

Fig 3a

Fig 3b

 

The handles

As we mentioned before, the hide is wound around the handles such that the rough side is on the outside to provide the best possible seal when used. Once the handle sleeves have been sewn, the handles are inserted most of the way into them, and the bottom of both sleeves are pinched together and sewn flat to prevent air from escaping from the joint. Figure 3b above shows the measuring for this process. Figure 4 shows the bellows at this stage.

Fig 4

 

The vent

The hollow vent for the bellows is placed in the "snout" of the hide we have sewn. It may be liberally coated with gelatin glue, as may the inside of the snout. Then a meter of twine is wound about the joint between the two. The twine is laid so that its midpoint is beneath the snout/vent joint (see Fig 5a), then wound from both sides. It is single-knotted at least every other winding to prevent slippage, then tied securely when the ends are reached. Fig 5b shows the completed tie. Note that there is considerable linear coverage along the joint.

Fig 5a

Fig 5b

 

Usage

Here is Ryan Watts, a classmate, using the constructed bellows:

Airflow

We measured the approximate air displacement of a single push of the bellows and arrived at a volume of 1.4 liters. With an average of 1.25 pushes per second over an hour long session, we get an airflow of 105L/min. Vigorous pumping increases the pushes per minute but usually decreases the volume per push somewhat. It would be extremely difficult for someone to sustain over 200L/min for an hour.

Conclusions

• The airflow is easily sufficient for artisinal production of gold, silver, lead, mercury, copper, and tin. It is doubtful whether a single bellows of this type would be sufficient to smelt iron.
• Given a good leather awl and all the materials required, a bellows of this type could probably be constructed by an experienced craftsman in less than two hours.
• The materials required could largely be taken from a single animal (Brain-tanned hide, gut/sinew thread and twine, bone nozzle)
• Considering the number of materials, parts, and processes required to make them, a bellows might be among the most complicated tools on a bronze age farmstead.

Upcoming events

May 24-26: Ontario, Canada
Darrell Markewitz, author of Hammered Out Bits, is teaching an iron smelting class, where I hope to smelt my first iron ore.

May 31-Jun 3: Hampshire, UK
Fergus Milton at Butser Ancient Farm is teaching a four day archaeometallurgy class covering copper and bronze smelting. I hope to walk away with a cast axe head.

Jun 14-16: London, UK
The Historical Metallurgy Society is having their 50th anniversary conference.  I have just paid my membership and registration fee so that I may attend.

I also plan to visit Malta and Morocco, so if anyone knows of any events related to early mining or metallurgy in or around those places, please let me know!

Success, sort of.

A second three hour high-heat smelting reduced the contents of both crucibles completely. The copper came out as a single large button. The tin was different. There was one small button (5g) and a three layer "cake" of slag... a light grey very granular layer was the bulk of it. Atop that was a dark grey layer and atop that were little balls of tin, like a froth, embedded in the borax.

In researching the tin, I find that tin forms a significant amount of tin silicate slag, given the chance. Historically quicklime was added to displace the tin from the silicon. I will try that next time. 40% of the mass of the cassiterite in quicklime is the proportion recommended.