Saturday, 6 July 2013

Hearth Steel

There has been a lot of discussion on the forums about using small hearths to melt low carbon iron and steel and carburise (adding of carbon) it to produce a hardenable carbon steel.

After discussing some ideas with my friend, Dan Prendergast, we decided to have some experiments trying to produce some of these “raw” steels, at his workshop- Didbrook Forge in Gloucestershire.

Whilst the produce of one of these small furnaces appears very much like a true “bloom” from a bloomery furnace, the method is dissimilar because in a melting hearth, you do not start with ore and thus the whole process is much simpler.

Didbrook forge is a historic forge in the heart of Gloucestershire, which has worked continuously as a forge for over a century with parts of the house dating to the  17th century and beyond. It was a true pleasure sharing the workspace with Dan for a few days and being in an old workshop again, for me, was very reminiscent of my father’s old forge, “The Wharf” which we left some 7 or 8 years ago and has now subsequently been demolished.  I grew up knowing the old workshop and I have always felt comfortable in such spaces.

I first met Dan when working at Hampton Court Palace in London and discovered we had shared interests in Bladesmithing, Ironwork, history and tradition. We had crossed paths before without meeting as I had callously bought a powerhammer that Dan had been keenly watching for while…

But back to our melting.

We had three attempts, each time altering the furnace a little but each time using similar ingredients of mild steel bar and a small quantity of cast iron, perhaps ten percent on each melt, which we believed would likely add bursts of high carbon to the mix.

For fuel, were supplied with some locally produced charcoal from old coppice woodland, this charcoal was very good and clean to work with, chopping well with a pleasant clink.  Charcoal I have used previously had left me so filthy with the soot that I was hesitant to handle the stuff too much, but this batch barely left a mark on the skin when handled.

It was either Mark Green or Dan O’Connor  that I read had done a great deal of research into the variability of charcoal and I remember that one of the traits of good forge/furnace fuel was this cleanliness.

The ‘Coal was roughly split and chopped and half-heartedly sieved to separate the very fine finings and powder from the proper sized pieces before the hearths were lit.

The first “furnace” was built quickly with house bricks and a rich natural clay from Dan’s garden, using the water cooled tuyere from one of the forges. The furnace was approx. 10 inches square, which with hindsight was certainly much too large as we used much more charcoal than we needed to- That said, it did work.


We were both surprised to find a “bloom” in the basin, swaddled with ceramic slag, weighing about 1200g. This first bloom was allowed to cool before we cut it up to investigate and process it. We found it to be a high carbon material so we decided to forge the pieces into strips, weld and fold and weld again until we felt it was suitably consistent.
These processes were undertaken with striking sledge and hand hammer as Dan’s very beautiful, 19th century, French made power hammer is not quite up and running yet, so it was porridge power all the way.


 We then divided the material and each forged a knife.  

Dan had in his collection an excellent little book called “Knives and Scabbards” which featured artefacts taken from excavations in London dating to the medieval period.  One blade form took my fancy and I decided to produce a blade in that style as I felt the Hearth steel’s material aesthetic qualities would suit it well.

The hearth steel was forge welded in coke onto a wrought iron back and tang to maximise the size of the blade, but also to best display the limited amount of the decorative steel we had available.   

The knives were both quenched in warm water and took great hardness and did not crack. Dan proceeded to polish his knife and etched it, revealing the wonderful character but also a distinct hardening line. My knife has not progressed this far but I am optimistic for the vibrancy that will be displayed in the steel once it is finished. The knife I am copying had a small brass bolster which I will replicate. The knife will have horn or antler scales, fitted with hand-made brass or copper tubular rivets. I will of course share its progress here when I get round to it. We were both surprised and pleased at how well the metal performed and we decided to proceed with a second burn.

This time we forwent the brick furnace, instead opting for an all clay and straw structure with a much reduced diameter and thus improved fuel consumption.


The second burn was excellent, taking under an hour before we decided the tuyere had become obstructed with the bloom and we stopped feeding it.

This time we forged the bloom right away after breaking the furnace open. Working the bloom quickly with striking sledge and hand hammer- despite being hot, the bloom was noticeably tough and resisted our force, yet it displayed no “red-short” brittleness and held together very well with little to no pieces coming off.


Once we began squaring the stock it became clear that there was a large split running neatly through the piece, dividing it nearly in half. We decided to separate these two halves and proceeded to work them separately as Dan’s forge has two large coke hearths.

I was struck with how solid the steel felt under the hammer and we were both able to forge the material into barstock without folding the bloom at all.

Spark testing again suggested some good high carbon content

Dan did a quick polish and etch and it showed a lot of clear activity.


These pieces were divided equally into seven parts before being dispersed as much as possible in a stack to ensure optimum carbon distribution, these were welded and folded an number of times and left as stock for Dan to approach at a later time.

The third furnaces followed more closely to the second burn, only with more stout walls to help prevent collapse. The hearth bloom we extracted from this furnace weighed 1400g and sparks with good carbon on each of the points tested.


I took this raw bloom home to be forged at a later date as I have a power hammer to help with the processing.

This last burn took just over two hours from digging the earth out of the yard, mixing the clay, building a furnace, lighting and doing a burn and extracting the piece. I think we were both impressed at the speed of the turnaround.

This was a fun experiment that produced some great steel, I’m sure we’ll do it again, perhaps with a smelt as well….
Thanks Dan, Vivian, Peanut and Betsy, it was a real pleasure.

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