(Lies, Damn Lies - and Statistics)
(originally posted 1/11/21, with corrections 1/19/2021)
Be careful about what some people are stating, when it comes to promoting, rather than reporting, their work with experimental iron smelting.
This sample of iron ore was reported as containing 89% iron!
Do you see the impossibility here?
(Do you remember your high school chemistry?)
Naturally occurring bog iron ores (what I call ‘primary bog iron’) are mainly composed of FeO(OH).
The atomic weight of iron is 56, of oxygen 8, hydrogen 1. So FeO(OH), combined, has a total weight of 89. Of which only 56 is the iron. The maximum amount of iron possible in pure Fe0(OH) is 63%. (1)
Many of the iron ores used by current experimenters contain the other forms of iron oxide, being Fe2O3 (red) or Fe3O4 (black). Here the maximum possible contribution of the iron by weight is slightly higher, at 70 (2) or 72 % (3)
Obviously, any naturally occurring bog iron ore is going to have other components. One of the major ones is typically silica - SiO2. Again, atomic weight for silicon is 28, so here the silicon makes up 47% of that component. (4) A typical primary bog ore may vary considerably in silca content (usually the other major component), in the range of anything from 5 - 25 % of the total weight.
The amount of silica available is also extremely important in a functioning bloomery furnace. Silica is the major component of the slag, which combined with some of the iron, creates the slag bowl, which both contains and protects the developing metallic bloom. Another major source of this needed silica is from melted furnace walls, so the composition of the furnace plays a role (type of clay, use of brick or stone). Most naturally occurring ores are more likely to contain too much silica (so also less iron). Although some industrially prepared ores (especially hematite blasting grit) may require addition of some silica by way of sand, there is usually no reason to add any kind of 'fluxing' agent into a bloomery furnace. (5)
A last important consideration with a natural ore will be the organic and water content. Both of these do burn off (the Loss On Ignition measurement). This will not show up on many lab analysis methods. It becomes important when actually calculating raw ore to final bloom yields however. Pre-roasting ore now becomes a factor.
My team here, which suffer from not having any naturally occurring ore in our region, has worked with more different types (and iron concentrations) of ore than most other groups. In my own experience (which is considerable), you need at least 45 - 50% iron content in the ore to expect any successful production from a bloomery furnace. Even a ‘good’ natural ore is typically in the range of 65 - 70 % Fe2O3 - which places it in that same 45 - 50 % iron content range.
If the sample reported above was 89% iron oxide FeO(OH)- it most certainly would represent a reasonable quality ore. This would be 56% iron content however. For comparison, the samples analyzed from the 2001 excavations by Dr. Birgitta Wallace at L’Anse aux Meadows (the Norse site in Vinland) showed iron in the 58 - 68% range . (6)
I’d be curious to see some actual reporting of the iron bloom yield figures from any smelt attempt using this (clearly) mis-represented ore, as illustrated above. Waxing poetic about ‘produced excellent iron’ is meaningless. Ore to bloom yield, density, potential carbon content - those are what current serious researchers and experimenters offer up as description of their results.
I'd certainly suggest anyone distorting basic science should not be trusted about any other claims made...
1) https://www.webqc.org/molecular-weight-of-FeOOH.html
2) https://www.convertunits.com/molarmass/Fe2O3
3) https://www.convertunits.com/molarmass/Fe3O4
4) https://www.convertunits.com/molarmass/Silicon+Dioxide
5) This piece was sparked by a recent 'first time' question on that same 'Iron Smelters of the World' discussion group. Bloomery furnaces almost never require addition of additional materials as 'flux'. In the production of liquid cast iron, the mechanism within the furnace is quite different. Here it is quite important to ensure the already high carbon iron (which has a lower 'burning' point that solid bloom iron) is protected from the air blast inside the furnace. Historically, powdered limestone was added for just this purpose.
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