Planning for a Rudimentary Furnace

by Jamie Hall on March 1, 2010

The aim is to build a small outdoor furnace, constructed primarily from unfired clay, and fueled bycharcoal. The furnace needs to be able to achieve melting temperatures for sterling silver, so in the region of 900C. Failing that, the furnace needs to achieve about 650C for the purposes of annealing. Eventually, I’ll try to replicate specific designs of Viking and middle age furnaces, but for now, I want to work with the basic principles.


At a later date, my intention is to try different fuels. I’ve already discussed this here.


I’ve started making enquiries about buying clay – if anyone has any advice please post a comment. Because of the kind of fires that I’ll be working with, it looks like earthenware crucibles are the most likely items for me to produce – they are fairly porous, and need lower temperatures than other clays to produce – I’m not intending on achieving temperatures high enough to make other kinds of pottery, and the methods I’ll be using will likely be limited to crude pit-firing. I’m hoping that I can use the same clay for both tasks.

Initial Furnace Plan

The initial site will be a disused flower bed in my garden – I can pack the soil into a slope, and build into it. The pit will be lined with unfired clay, or perhaps a layer of sawdust and then clay – I have a feeling that soil might prove a poor insulator. It might be possible to face the firepit with clay, leaving a hole on top, (to rest the crucible in), and a hole at the bottom front for the tuyere pipe to enter. It may be necessary to provide a lid of some sort to reduce heat-loss through the top.


From what I’ve read, the basic structural requirements are a refractory (or heat-proof) material, an insulating material surrounding that, and a protected area for the tuyere pipe to blow air into the fuel. Apparently, tuyere pipes can be damaged by extreme heat, and can be blocked by dirt, fuel, metals and residues, so the pipe needs protecting, while still injecting air as close to the centre of the furnace as possible. This might be achieved by placing several rocks or clay lumps around the end of the pipe, to stop objects falling directly into it. I’m intending to use a pipe that angles down into the furnace, ideally driven by bellows.


Beyond the basic materials to create heat, I’ll also need a few other items, which I’ll discuss in detail elsewhere.

  • Crucible
  • Bellows
  • Tongs to lift crucible
  • Cuttlebone moulds
  • Gloves, goggles, apron, leather-soled shoes

I’ll prepare a number of hardened pieces of sterling silver to test for annealing temperature, and some cuboids to test for melting (cuboids are easy to measure, and will visibly change shape if they melt).

{ 10 comments }

Jerry Fowler March 1, 2010 at 19:24

Besides gloves and goggles please try to find some welding leathers to wear, splatters of molten metal tend to hurt a bit as they burn through clothing. Wear only clothing made from natural fibers, no synthetics here. Wear leather soled shoes, rubber soles melt when you step on molten metal that may spill from broken crucibles. Cover shoe laces with duct tape to keep small splatters out of them. Be safe!

Jamie Hall March 1, 2010 at 20:42

Updated.

Michael Johnson March 2, 2010 at 04:29

As I look through books of ancient jewelry, I’m not so sure that casting was a preferred method. And, I don’t see any evidence that cuttlefish was used. Have you researched this? Cuttlefish smells terrible, and makes that signature pattern on everything.

But, I hate to sound negative. Your furnace plan looks great.
Soil should insulate just fine, but keep in mind it will kill the soil around where you put it. It will be years before anything grows there again.

Jamie Hall March 2, 2010 at 08:29

I’ll bear that in mind about the soil – do you know if it’s just a matter of killing the biomass in soil, or is it chemical damage that will contaminate other soil that’s mixed into it.

As for cuttlefish – you are right about the pattern of cuttlefish – I’ve not seen it anywhere. My reasons for mentioning it are it availability around coastal Europe through history, and it’s availability to me down the local market. My aim is move onto using clay and soapstone moulds, but cuttlefish is something I’m confident enough with, and gives me a point of comparison for my early efforts to melt and pour the metal. If cuttlefish was used, then I expect it was only for making bullion products – if that was the case, then there would be little record of it.

Your comment about preferences to casting versus other methods surprised me. Could you expand on it?

Tim Blades March 2, 2010 at 09:49

Hi Jamie, I did a cameo years ago for Time Team and was lent two pairs of bellows from a friend who had done this. I quizzed him about the exact shape of the furnace and he was very sketchy about the details , he mostly said it didn’t matter! He was right, I made a shallow bowl with old clay I got from a potter friend, not even dried it out. Barbeque charcoal crushed and sieved to pea size ( you can use the dust left over to mix with clay for moulds) , a bit of copper pipe to push the air to a hole in the bottom of the bowl which was about 10″ across an d4″ high and a crucible and stand fired from heavily grogged clay.
It took about ten minutes to melt two ounces of silver, it went faster when I stopped blowing so hard, slow and steady does it . You can maintain a steady airflow with two bellows. A lid on the crucible helped. I needed someone else to pour, you can’t get much superheat, the unburnt charcoal acts as a good insulator.
See if you can find a copy of ‘The Work of Angels ,Susan Youngs’ it has pictures of old crucibles in it. I think the shallow ones are for metal tongs and the tall ones you can lift out with a hazel stick bent into a ‘U’ shape , on a tall crucible the top is not that hot.

Just try it , it is much easier than you would expect!

P eter Bond March 2, 2010 at 10:17

Jamie – you might find it interesting to research some of the Ashanti casting methods; they use a mix of clay, sand and fresh horse manure (i.e. not actually rotted down for the roses) for casting.

I’ve made a few impromptu charcoal forges for demos using Thermalite blocks to hold everything in (with a Japanese-style bellows driving them); a friend of mine used a pair of hollowed-out logs with simple flap valves as the bellows, while his hearth was genuinely nothing more than a hole in the ground – soil is a pretty good insulator so long as it doesn’t have lots of nicely flammable material in it (so don’t do it in peat).

Oh, and Tim’s spot on about the slow & steady (of course!); there’s no advantage to rushing it. I could hit welding heat with ease using my bellows, but all it took was pumping a little harder, not faster .

Michael Johnson March 2, 2010 at 16:48

Jamie, my reference to the Viking (or Nordic) preferences for fabricating and forming verses casting are just from observation in photographs. I am no expert. But the fibulas, pulled wire chains, and stamped designs that I’ve seen just seem more easily fabricated and formed, similar to the methods of Native American metalsmiths. But, as I said, I am no expert. But, if I were going to make these things, I would take the easiest route, which would be fabricating, IMO.

But, all in all, it may just be the examples of jewelery that you may emulating. I don’t claim to have seen “all” examples of these artifacts, so it may just be that casting is correct for what you are wanting to make.

Don’t let me deter you. :o) I’m just the Devil’s advocate, LOL.

I am not sure exactly why the grass won’t grow afterward. I just know that there is a permanent dead spot where my first forge and foundry was. It has been six years, and nothing grows there, and the tree that was close by is starting to not look so good. I’ve mulched it and turned it with a tiller, but I can’t even get weeds to grow.

buck March 2, 2010 at 21:19

Your forge design may be overkill as far as athenticity. A hole in the ground is fine,it’s the air flow that brings up the temp. People forge iron in no more than a hole and some sort of air source. As far as killing the ground, water passing thur charcoal turns to lye,thur coal turns to sulfuric acid. Easily acessable clay is kitty liter,oil dry, mix with water and let sit . Try litter clay mixed with sawdust,chopped straw ect for melting pots. The sawdust,straw,horse manuer,burns out leaveing insulating air pockets. Be careful low tech is still dangerous.

Calvin March 6, 2010 at 18:28

When I was a child, by brother made furnace in the back yard. It was very similar to your design except it was lined with red brick and the air supply was the exhaust end of a shop vacuum. It was also fueled by charcoal.

You may be able to get higher temperatures then you expect with this. We turned the edges of the red bricks to glass. I don’t know what temperature is required to do this, but I would guess that it is more than 900C.

Grimbold September 15, 2013 at 19:46

Look at http://web.comhem.se/vikingbronze/casting.htm or anything else by Anders Soderberg for hearths.

Early forges, melting hearths etc. were rather identical, with evidence at some sites workers did non-ferrous casting and ironsmithing on the same hearth in both Anglo-Saxon and Viking eras. Two+ inches of river-dug clay without too many inclusions works fine for a liner, and either a drilled soapstone or cast ceramic tuyere ‘blowing stone’ protected the end of the tuyere pipe, which can be cast ceramic (Bronze Age and up) or iron (Viking-Medieval).

Casting, fabrication and especially heavy reworking cold of cast pieces are all period.

Book from the 1300’s mentions ‘Cuttlefish beloved of the workers in silver’ or some such, don’t remember the book at the moment and they might be referring to it’s use as an abrasive.. Lost wax casting in tempered clay, 2-part molds in tempered clay, and stone molds in soapstone, limestone (ingots only) and slate (Britain) were used at various times before 1200, loam (sand) casting was used in bell-founding around 1350, maybe before that. After chasing and filing/grinding/Scouring Rush use it’s difficult to firmly eliminate any particular casting process, above is from finds plus written evidence (bell founding).

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