Breakin' the Blooms : Compaction and cutting

Breakin' the Blooms
Compaction and Cutting - August 5, 2022
with Neil Peterson

For some time now, my primary smelting partner, Neil Peterson, has been coming up for one day sessions at the forge, converting smaller pieces of raw blooms down into working bars. This has allowed Neil to develop skills in forge welding and general hammer technique, at least as applied to the specific methods involved in working with bloomery iron. Bloom iron, especially at the 'raw' stage, always includes heavy slag inclusions and often pronounced diagonal internal fracturing, making this material quite unlike modern industrial steels.
For most of the experimental work at Wareham, the concentration has been on the problems posed by various historic furnace types, specifically those based on the archaeology of the Viking Age. There has always been consideration of creation of actual iron blooms, following the assertion by Lee Sauder : 'If you don't get any iron - you can not be doing what they did.' That being said - the DARC team has purposefully limited individual ore amounts to the range of 25 - 30 kg, with the expectation that the blooms produced will be in the range of 3 - 5 kg. We have understood that making these smaller blooms most certainly proves the viability of an test effort, and also means the masses are not so large as to create other handling problems at the bloom to bar phase.

That being understood, there have been a number of occasions when larger blooms have been the result, with a second group of those in the 8 kg range. The second stage 'bloom to bar' work has been undertaken using a two stage heating process, a pre-heat soaking in a two burner propane forge which as been shown to create internal temperatures into the range of 1040 C. The pieces are then shifted into my main coal forge, which has a bottom air blast, which has a significantly higher internal temperature (well over the burning temperature of iron, into the range of 1950 C +) (2).


Propane forge - maximum internal height at 10 cm (view to 'east)	Coal forge - maximum effective size about 10 cm diameter (view to 'south')

In propane - 1770 gm of bloom from #16	In coal - 625 gm of bloom from #30

In effect this combination limits the size of a piece of bloom that can be effectively heated to the required 'welding heat' (about + 1300 C) (3), to roughly 2 kg / about 'double fist' size. Up to now, Neil and I have been working through previously cut bloom pieces, with over 90 smelts behind us now, a pretty big assortment (4).

I have two larger three burner propane 'architectural' forges in the workshop. The larger, in terms of internal volume, was built by the Michigan Artist Blacksmith Assn, sold as a fund raiser. This has individually adjustable passive air burners, into a chamber I fitted with a vertical opening front door. The internal size is 21 x 21 (the door) by effectively 37.5 deep. During the later part of this exercise, the internal temperature was measured at 1160 C. As this is still well below effective forge welding temperature, it was understood that what would be possible was only *mechanical* compaction.
Because of the long soak time to allow heat to fully penetrate into the large masses, two blooms were heated together, the rear most still coming to working temperature, while the one at the front being removed for the working sequences.


Three burner propane forge	Working set up, forge to right, Neil using the hydraulic press	First pair of blooms at initial heating.

Of the ten available larger blooms, four were selected for that day's effort (in order of working) :

Smelt	Weight
P3-B / Sept. 2021	7.58 kg
Wind & Weather / Oct. 2021	8.18 kg
Slag Pit 3 / June 2018 (not seen)	6.0 kg
CanIRON Prep A / Nov. 2010	8.27 kg

Note the corrected weights against those indicated from the original production measurements.

Image by Neil Peterson

Detailed images of the individual blooms at the starting point (images by Neil Peterson) :

P3-B


	The image above shows what is most likely the original top surface of the bloom as it was positioned inside the furnace.

Wind & Weather


		The image above shows what is the original top surface of the bloom as it was positioned inside the furnace.

Slag Pit 3

	The image to left shows what is the original top surface of the bloom as it was positioned inside the furnace, the tuyere on the left side.

CanIRON Prep A

	The image to left shows what is the original top surface of the bloom as it was positioned inside the furnace.

The forming was undertaken through use of the 30 ton (actually 3000 psi) hydraulic press, using flat plate dies for compression, Typically there would be a number of cycles, attempting to re-shape the irregular blooms into a roughly flat sized 'brick' shape. One of the aspects of this press (mainly due to the long shaft and guide structure) is that it exerts a bit more pressure on one side (towards the frame) than the other, resulting in a slightly distorted compression. Several heats were required for each bloom. The final steps were using a cutting blade to render into smaller pieces.

Illustrating a working sequence
Images by Neil Peterson


start of the 3rd set of compressions	end of compression 3A	into compression 3B

making the first cut	flipping over to make matching cut	cutting down from one edge to complete

completed cut	to roughly 1 - 2 kg (CanIRON Prep A)	Showing the press in use

Final Results

	P3-B (L-R) A = 2.92 kg B = 1.56 kg C = 1.98 kg larger gromps = 0.39 kg start = 7.58 kg result = 6.46 kg loss = 1.12 kg / 14.7 %
	Wind & Weather (L-R / top-bottom) A = 2.39 kg B = 0.74 kg C = 1.25 kg D = 1.0 kg E = 0.56 F = 0.80 larger gromps = 0.25 kg start = 8.18 kg result = 6.99 kg loss = 1.19 kg / 14.5 %
	Magnetic fragments collected 0.52 kg
	Slag Pit 3 (L-R / top-bottom) A = 1.52 kg B = 1.4 kg C = 2.42 kg larger gromps = 0.31 kg start = 6.0 kg result = 5.34 kg loss = 0.66 kg / 11 %
	CanIRON Prep A (L-R / top-bottom) A = 2.24 kg B = 1.67 kg C = 1.36 kg D = 1.03 kg larger gromps = 0.42 kg start = 8.27 kg result = 6.72 kg loss = 1.55 kg / 18.7 %
	Magnetic fragments collected 1.31 kg

- Individual sections remain quite irregular, and were seen to have diagonal flaws remaining (lack of forge welding).
- Gromps at least 'half walnut' size were measured, as these could be recovered by welding to a small plate.

At the end of each pair of worked blooms, the area below the press was swept with a magnet to recover small fragments. Bloom input to output losses to pieces considered to be large enough allow forge welding were 4.28 kg lost. This against 1.83 kg recovered magnetically, so 42.7 % of that material gathered. Although too small to weld, these fragments can be added to a future smelt as ore enrichment.

Losses from bloom to workable pieces at this stage averge 14.5 %. It is expected there will be significant additional losses as the pieces are subjected to the complete forge welding process to convert each into solid working bars.

Notes:

1) 'Seeing and the Camera' blog posting, May 16, 2020 : https://warehamforgeblog.blogspot.com/2020/05/seeing-and-camera.html

2) Proved more difficult to cite than expected, one source : https://toolsowner.com/blacksmith-forge-temperature

3) There can be a lot of variation of the temperature an individual blacksmith desires for forge welding (skill / technique / alloy). For a discussion on forge / hammer welding, see blog posting, 'Forge Welding', Nov. 19, 2018 : https://warehamforgeblog.blogspot.com/2018/11/forge-welding.html

4) The full documentation, covering experimental work since 2001, can be found on the web site : http://www.warehamforge.ca/ironsmelting/index.html