Making the CUTS - (Blooms to chunks)

This post is a (kind of) continuation of two earlier ones:
'More on Angle Grinders'
'Making Spears - Ore to Bloom to Object'
A sub text on both was the problem of how to cut up large bloom masses into 'working' blocks.

Lee Sauder has (wisely) repeatedly stated 'A bloom is never as hot as it will be just as you extract it'. Lee obviously has a huge more experience than I do in not only the bloomery iron process, but also both 'bloom to bar' and 'bar to object'. 

Well, ... As I have mentioned a number of times in pieces here, I get limited at the point of extraction by a couple of major elements here at Wareham :
1) By the time I get through the preparation, full smelt sequence, the actual extraction itself? I'm pretty much exhausted!
2) I rarely have any 'skilled' hands for actual blacksmithing related tasks. Although the DARC supporting team have undertaken a certain amount of hammer work at previous smelts, none are trained past the 'introduction' level.
3) Honestly, I have been concentrating on the mysteries of *historical* iron smelting themselves. I have not built the specialized equipment (mainly oversized forges) required for secondary heating of the blooms.
4) Although I * do * have some modern powered equipment (75 lb 'Robertson' air hammer and self built hydraulic press) this is all installed in the actual forge workshop. This at the front of the property, a good 150 feet from the smelting area at the rear of the lot.
This is all aggravated by the size limits on both my main coal, and currently available propane, forges. Anything much larger than about 6 x 6 x 4 inches (15 x 15 x 10 cm) I just can not effectively heat to forging temperatures (1)

So - How do you CUT a full bloom when it is cold?

The only realistic way I have found is to use a 'zip' metal cutting disk on an angle grinder. (2)
There are any number of basic problems with this method :
1) How to secure the irregular bloom.
2) Relatively shallow depth of cut (depending on the unit)
3) Maintaining control over the long duration of the cut
4) Cost of multiple disks - which quickly abrade beyond useful size. (3)

Here are the results of this week's adventures in bloom cutting

Two blooms were cut into smaller pieces. Both the result of group training / public demonstration smelts. The original promise was to divide the bloom between the active participants.

SCA 50 Year - June 2016

Starting Bloom = 4.836 kg

First Cut = 2.841 / 1.829 kg

Piece to right (above) sectioned again = .838 / .876 kg

Larger piece (to left above) sectioned = 1.594 / 1.136 kg (a)

(a) You can see that I was able to complete the cut on the larger 'half' after I had heated the partially scored block.

Starting Bloom = 4.836 kg

Pieces Total = 4.444 kg

Total Loss = .392 kg / 8% over 4 complete cuts

CAMELOT - September 2018

Starting bloom = 5.26 kg (b)

First cutting = 2.82 / 2.29 kg (c)

Finished cuts = 1.14 / 1.04 + .641 / 1.136 / .756 kg

(b) You can see the surface is at least scored from a quick attempt at hot cutting the bloom at extraction. The other shallow marks are from one of the (failed) University Lab attempts mentioned below (2)

(c) You can see the deeper cut lines, from the use of the monster 7 " x 13 lb grinder (which can provide up to a 1 3/4 inch deep cut)

Starting Bloom = 5.26 kg

Pieces Total = 4.713 kg

Total Loss = .547 kg / 10% over 5 complete cuts

You can see that in all instances, it was not possible to make a depth of cut entirely through the bloom mass. A scoring cut was made on all four sides, to the best depth possible with the grinders on hand. Then the mass was placed on the hydraulic press - cold. The cutting blade was able to force open the line. After several compressions (alternating sides force was applied), the central area effectively shattered.
One nice result is that this process reviled the internal crystals - both in terms of size / structure and colour. This also provides a 'relative' clue to potential carbon content (at least in the interior). The larger the crystals and 'whiter' the colour, the higher the carbon content. (4)

One other advantage to using a grinding cut is that the density of the interior is easy to view. When cutting the bloom hot, there is a combination of both compacting and slicing as the blade pushes down through the bloom. Freely admitting that of course the end purpose is usually to compact the bloom into a solid block - the 'bloom to bar' phase.


The next part of what all resulted from a simple question will be a look at just that 'bloom to bar' - in terms of offering up some data on overall yields...


(1) Loose plans afoot to build a standardized, larger sized, 'heating base' - using two or three individual propane burners installed into a shallow circular metal containment. A number of different purpose designed top shells can then use the same heat source : larger forge / bronze casting / raku ceramics.
Event looser plans are to convert an antique cast 'oval bowl' forge into a side blast unit, specifically for working blooms. 

(2) Aspects of the selection, effectiveness and use of various angle grinders was detailed in the 'More on Angle Grinders' posting.

In two earlier experiments, the finished blooms were handed over to University Engineering Departments. In both cases, after several attempts (damaging expensive cutting equipment) these folks returned the uncut blooms - with only shallow scoring on the surfaces.

As returned from a University Lab - reported two blades destroyed.

A much earlier attempt to use a 'hobbyist' quality (10 inch x 1/2 HP) fixed cut-off disk saw ended up melting the windings on the electric motor!
Use of a band saw is also not successful. The slag inclusions scattered through any bloom mass quickly tear the teeth off most standard alloy blades (Carbide tooth blade might prove more durable, but are not available for the smaller sized band saw I have.)

(3) The two bloom cutting processes detailed here consumed roughly $40 in disks. Typically a 5 inch diameter disk will only last about 5 minutes before it has been worn away past its effective cutting diameter. This results in a cut across one of the four sides to best depth.
A high quality diamond grit wheel might prove more durable or effective. Cost does intrude however.

(4) At least that is my understanding, from examining some pieces of bloom from the Japanese Tatara process. (Happy to stand corrected here!)

Late Addition:

I had this comment come in (via Facebook) from Gotz Breitenbucher, a member of the wider Experimental Iron Smelting community (based in Norway)

Götz Breitenbücher I agree about the coal -propane difference, but not about the charcoal being colder.I have never measured it, but in a well built furnace, using my viking age bellows ( medium size, about 60cm long) a bloom seems to heat about as fast as in a coal forge, somewhat more evenly, because the fire is less concentrated . In the beginning (like, 20 years ago) i had problems with the heat , but it depends a lot on a good bellow man, and the construction of the furnace. I found also, that using wet charcoal, sprinkling more water on the coal in the fire increases the temperature quite some . In the last oroshigane smelt i smelted a 300 by 40 by 40 bar of bloom in less then 30 min (don`t have the exact time) The temp MIGHT be lower, hard to tell, but it sure does not make much of a difference, in my opinion. No need to say that i found the experiment very interesting, and will spend some more time reading it !