Bog iron in Kurpie
How much iron really remains?
A model batch of 25 kg of bog iron ore goes into the bloomery. Three bands show it as ore, bloom and bar, on one scale. The dashed, faded section is the mass that did not make it to the next stage.
Change the model assumptions · What is model and what is source
Model · sample data, not a smelt measurement
Tracked component: Iron (Fe). 3.5 kg of the 8.4 kg of iron in the ore remains in the bar.
Without JavaScript you see the reference values (scenario C) with the iron trace, and the table of three scenarios below.
-
1 · Bog iron ore, raw25.0 kg
Iron in the ore: 8.4 kg, or 33.6% of the charge mass. The rest is water, rock and oxygen bound in the ore.
-
2 · Bloom from the furnace, raw6.7 kg
The bloom weighs 6.7 kg, but only 4.4 kg of it is metal (65.0% of the bloom mass). A further 0.6 kg of Fe is held in the oxides of trapped slag. 3.5 kg of Fe stayed in the hearth slag.
-
3 · Bar after forging3.5 kg
Bar: 3.5 kg, or 41.7% of the iron from the ore. Forging removed 0.9 kg of Fe as hammer scale and 0.6 kg of Fe in squeezed-out slag.
Common scale for all bands and scenarios: full width = 25 kg of ore charge. The thick black line on the bloom and bar bands marks the boundary of what passes on to that stage; it does not depend on the selected component.
- Fe that ends up in the bar
- Fe lost as hammer scale
- Unreduced Fe, in the slag
- Rock and the rest of the dry ore, in the slag
- Water
Theory versus result
The textbook answer to the opening question is: all the iron in the ore. The model result is lower by two different losses, and each has its own dial here. The third group of settings changes the “theory” itself, that is, how much iron enters the furnace at all. The batch mass is fixed (25 kg); all masses in this model scale in proportion to the ore mass.
The sliders require JavaScript. Below is a table of three scenarios with results calculated for the model values.
The gap in numbers (kg Fe)
8.40Fe in the ore (theory) 4.03unreduced, in the slag 0.87hammer scale 3.50Fe in the bar
-
Ore25.0 kg
-
Bloom6.7 kg
-
Bar3.5 kg
The same three bands, the same 25 kg scale and the same selected trace as at the top of the page.
- Degree of reductionFe reduced to metal ÷ Fe in the ore
- 52.0%
- Bloom yield (mass)mass of the whole bloom ÷ mass of the ore
- 26.9%
- Metal share of the bloommetallic Fe ÷ bloom mass, the rest is slag
- 65.0%
- Bar yield (mass)mass of the bar ÷ mass of the ore
- 14.0%
- Ore Fe in the barFe in the bar ÷ Fe in the ore
- 41.7%
| Quantity | A | B | C | Current |
|---|---|---|---|---|
| Fe in the ore | 5.55 | 11.28 | 8.40 | 8.40 |
| Raw bloom | 3.53 | 9.78 | 6.72 | 6.72 |
| Bar | 1.36 | 6.23 | 3.50 | 3.50 |
| Ore Fe in the bar | 24.5% | 55.2% | 41.7% | 41.7% |
A: moisture 26%, Fe 30%, reduction 35%, slag in bloom 45%, hammer scale 30%. B: 6%, 48%, 65%, 25%, 15%. C: 16%, 40%, 52%, 35%, 20%. These are model scenarios from version V2, not descriptions of documented smelts.
| Component | kg | Where it ends up |
|---|---|---|
| Fe in the bar | 3.50 | forged bar |
| Fe in hammer scale | 0.87 | smithing waste (oxidised iron) |
| Unreduced Fe in slag trapped in the bloom | 0.57 | squeezed out during forging |
| Rock and the rest in slag trapped in the bloom | 1.78 | squeezed out during forging |
| Unreduced Fe in the hearth slag | 3.46 | slag solidifying in the hearth |
| Rock and the rest in the hearth slag | 10.82 | slag solidifying in the hearth |
| Water | 4.00 | steam from the upper part of the furnace |
| Total | 25.00 | = mass of the raw ore |
Values are rounded to 0.01 kg by the largest-remainder method so that the total always comes to exactly 25.00 kg. Figures with two decimal places are for checking the total; they do not imply measurement accuracy.
Where in the furnace it happens
Diagram of a single shaft-and-hearth bloomery. Choose a zone, and the same component you are tracking in the bands will light up in the drawing. The drawing is not to scale and measures nothing.
Original diagram, not to scale. The dashed line is ground level. The numbers correspond to the zone buttons.Numbers 1–4 correspond to the zone descriptions alongside.
1 · Upper shaft: drying the charge
Upper section of the furnace shaft
Bog iron ore tipped in with charcoal from the top of the furnace contained moisture. The rising gases evaporated the water and heated the charge as it sank deeper into the furnace.
Active trace: Iron (Fe). Iron compounds in the lumps of ore are dried and preheated.
In the calculation: 4.0 kg of water (16.0% of the ore mass) is lost at this stage.
Roasting the ore before charging could remove some of the moisture and make the lumps easier to break up.
Without JavaScript all four zones are described below, starting from the top of the furnace. The descriptions come from V2 and need checking by a specialist.
- 1 · Upper shaft: drying the chargeBog iron ore tipped in with charcoal from the top of the furnace contained moisture. The rising gases evaporated the water and heated the charge as it sank deeper into the furnace. Roasting the ore before charging could remove some of the moisture and make the lumps easier to break up.
- 2 · Mid-shaft: gas reductionCharcoal provided heat and made it possible for reducing gases to form. Reduction removes oxygen from iron compounds in the solid phase. The tightness of the furnace walls mattered, so that the reducing gases did not burn off prematurely.
- 3 · Above the tuyère: solid metal, liquid slagThe blast of air from the bellows keeps the charcoal glowing. The reduced iron sinters into a porous sponge in the solid state, while the slag components liquefy and run downwards. The bloomery did not melt pure iron into liquid metal.
- 4 · Hearth: slag at the bottom, bloom removedLiquid slag ran to the bottom of the hearth and solidified into a slag block. Above it the porous bloom settled, that is, iron with slag inclusions. The bloom was taken out and forged on an anvil; the method of removal needs checking in the literature.
Three materials, not three progress icons
Ore, bloom and bar are different bodies with different mass and composition. The patterns on the bands denote components; this page contains no photographs of samples, because there is no resource with a documented right of use. A comparative photograph of bog iron ore (from the Netherlands, not from Kurpie) is only a link in the sources.
| Material | Properties and form | Role in the process |
|---|---|---|
| Bog iron ore | Brown layers and lumps; besides iron compounds it contains clayey material, silt and organic remains. | Raw material from meadows and bogs. It needed drying or roasting before being charged into the furnace. |
| Raw bloom | A porous, spongy mass of reduced iron with slag inclusions in its cavities. | Direct product of the furnace. It was not a finished tool and required further smithing. |
| Bloomery slag | A product of the metallurgical process: a dark, glassy sinter of mineral components and oxides. | It collected in the hearth or was squeezed out of the bloom. It is not the same as the metallic bloom and it contains iron. |
| Iron bar | Consolidated, solid metal after repeated hot forge-welding. | A smith's semi-finished product, cleaned of part of the slag inclusions. The model treats the bar as pure metal. |
Krzysztof Braun [2], citing research by Elżbieta Kowalczyk-Heyman, describes a grant of Ruda Skroda to the ore miner Jan of Przasnysz. To be checked in the source work.
Atlas Historyczny Polski [Historical Atlas of Poland], cited by Braun [2], records ironworks on the Omulew, among them at Brodowe Łąki and Olszewka. To be checked in the source work.
The mass-band model describes the general logic of a bloomery, not the technology of a particular Kurpie ironworks. Modern experimental smelts [3] come from outside Kurpie, and a single experiment does not reproduce all workshops.
Sources, limits and changes relative to V2
Sources for further reading
- [1]Lasy Państwowe: Ruda darniowa [State Forests: bog iron ore] — environment of formation and composition of the raw material.
- [2]Krzysztof Braun, „Kurpie. Antropologiczno-historyczne studium kształtowania się tożsamości grupowej", UW, 2023, pp. 34–37 [Kurpie: an anthropological and historical study of the formation of group identity] — early ores and ironworks of northern Mazovia.
- [3]Muzeum Starożytnego Hutnictwa Świętokrzyskiego: wytopy doświadczalne 2023 [Museum of Ancient Metallurgy in the Świętokrzyskie region: experimental smelts 2023] — bloom, slag and study of the process; technological context from outside Kurpie.
- [4]Wikimedia Commons: Hands-on-bog-iron.jpg — comparative photograph of bog iron ore (the Netherlands, not Kurpie); author and licence as per the file page (V2 cited Rp22, CC BY 3.0). Link only, not embedded.
The links were carried over from V2. They were not opened when V3 was built, so their content and availability need checking.
Limits of the model
- This is a calculation of mass shares on sample data, not a full chemical balance of the furnace and not a measurement of a particular smelt.
- The entire non-iron part of the dry ore, including oxygen bound to iron, goes to the slag in the calculation. In reality oxygen from the ore escapes as gas, so the slag mass is overstated here. Conversely, the oxygen added to hammer scale is not counted.
- Charcoal and gases are not part of the charge mass. We do not give their masses: that is missing data, not zero.
- Degree of reduction, bloom yield and smithing purification are three different quantities; the sliders set them as assumptions, not measurement results.
- The page gives no coordinates or locations of sites, so as not to facilitate illegal exploration.
- The furnace zone texts come from V2 and need checking by an archaeometallurgy specialist.
What changed relative to V2
- Ore mass: the slider was replaced by a fixed batch of 25 kg; all masses in the model are linear with respect to it, so a slider would only rescale the chart.
- Charcoal ratio: removed, because in V2 it affected no ore-mass result. The fuel is marked as outside the calculation.
- Saving in the browser, copying and printing the balance: removed as peripheral; restoring the values remains.
- Five cascade bars: replaced by three bands of a single batch with cut lines.
- Photograph from an external server: removed from rendering; the link remains.
- Kept: the five remaining parameters and their calculation (with slag limited to the available mass), scenarios A, B and C, four furnace zones with traces of Fe, slag, gases and water, and the table of materials.