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Step 7How this 'replica' differs from other attempts
Presuming it is a battery, the reproductions I have seen always assumed the following: an acidic electrolyte (lemon juice, red wine vinegar, etc.) with an iron|copper electrode pair contained in a porous unglazed jar. Because the jars and electrodes are present as artifacts, we can assume most aspects of previous reconstructions are accurate, but the evidence for an acid electrolyte or uncoated iron electrode is weaker.
7 mm diameter by 75 mm length iron rod with an Fe3O4 coating instead of uncoated iron
The assumption of an uncoated Iron electrode is perhaps not correct. The artifacts (as I understand from written accounts and a few low resolution photos) show a mixture of black and red colored iron - which I assume is Fe3O4 ('black rust') and Fe2O3 ('red rust'), two of the more common oxidation states for iron. I am far from an expert on Iron Oxides, but from what I understand, Fe3O4 does not typically form over Fe2O3 unless placed in boiling water. As an example, I have not seen black rust (Fe3O4) form on an old car covered with red rust (Fe2O3), but that does not mean it is not possible, I've never buried an iron artifact for 2000 years either. However, Fe2O3 (red rust) can work its way into a layer of Fe3O4. Since photos of the artifacts I have seen show a mixture of Fe2O3 and Fe3O4 on the artifact, in my mind's eye, it implies there was originally a layer of Fe3O4 that was later infiltrated by Fe2O3.
So, I presume the electrodes were Fe3O4 and not plain iron - it certainly was not Fe2O3, since this does not produce a viable cell. Additionally, as shown in Step 6, Fe3O4 produces a better electrode than plain iron, and if this was a rechargeable battery being used for extended periods, the Fe3O4 coating would provide some protection from the formation of Fe2O3. However, the strongest piece of evidence is that iron of this time and shape would have been forged under a high temperature that exceeded the point of decalescence for low carbon iron 425 F-525 F. Beyond the decalescence point, iron will readily form a coating of Fe3O4. So, by choosing an iron rod without Fe3O4, previous replica creators would need to assume that the Fe3O4 was purposefully removed from the iron rod - but we know that most forged objects of this time retained their Fe3O4 coating.
Why:
Plain iron would likely oxidize to Fe2O3 (red rust) and this oxidation state does not work well as an electrode. As Fe3O4, the iron is actually protected from the elements and produces a better cell than plain iron, and significantly better than Fe2O3.
How:
As discussed in step 2, iron is easily converted to the Fe3O4 oxidation state - likely through the forging process, but a piece of already rusted iron can also be boiled in water for a few minutes, or iron can simply be immersed in seawater, all methods work fine, and would have been accessible to civilizations 2000 years ago.
A non acid electrolyte and thus a rechargeable cell
The copper electrode is completely oxidized during discharge in the acid chemistry, making for quite a mess (see photo). If an acid electrolyte, the artifacts should show substantial corrosion on the electrodes and in the jar, but none of the archaeological evidence or published accounts I have seen indicated this level of telltale corrosion. The BBC article listed in step 10 mentions corrosion and a test indicating acid, but does not provide any further detail. The corrosion shown in the artifact photographs, actually closely resembles the corrosion patterns I've seen after removing the copper from an alkaline electrolyte (see photo).
If it was an acid chemistry, cider vinegar seems more likely than the lemon juice or red wine vinegar used by some investigators - my understanding is that red wine vinegar was not used in earlier times. Perhaps mashed grapes were used instead, as seen in Arne Eggebrecht's replica demonstration, this would provide tartaric acid, but since acetic acid and citric acid were both known to the Parthians, these are equally possible too. However, an alkaline electrolyte seems just as likely to me, and as I've explained, creates a very interesting battery.
The available construction details also seem to support the possibility of this being an alkaline chemistry. Most investigators, including myself, have wondered why the copper electrode is sealed in the clay jar, since completing a circuit to extract the current is much more difficult - this is puzzling since concealment of the copper electrode does not appear to be necessary with an acid electrolyte. However, with an alkaline electrolyte, the copper electrode is quickly oxidized even when partially exposed to air. Since the artifacts found indicate that the copper was sealed in the vessel with only the iron electrode exposed - this construction detail seems to support the argument for an alkaline electrolyte, not an acid electrolyte, since exposed portions of the copper electrode are less prone to oxidation with an acid electrolyte (see photo).
Why:
An acid chemistry means the electrodes are only good for one use. An alkaline electrolyte allows for a rechargeable cell. The cell chemistry is also much less robust with an acid electrolyte, and provides a cell with only 1/5 the capacity of a single charge cycle of the alkaline electrolyte rechargeable cell.
So, maybe I've convinced you this is a rechargeable cell, but how was it charged? Perhaps an earth battery - stick two conductors (Copper, Iron, Carbon, etc. all work fine) into the Earth and you will get around a 1.2 V potential with a few mA of current. I charged my replicas using voltage/current that simulates an earth battery.
How:
KOH, an alkaline urine, or perhaps another alkaline substance, are all possible, but my experiments only used KOH. A diet rich in citrus fruits, legumes, and vegetables is known to raise pH and produce urine that is more alkaline - this would have been close to the diet of this time. I've been eating a lot of citrus and legumes lately, so maybe I will retest my urine pH and give it another go. Soap production is claimed to have been present during this time, so it is possible there was familiarity with alkaline chemistries. In my wife's DIY soap book, common lore states that soap was discovered when ash from fire pits drained with the fat from animal renderings into the nearby stream where people washed clothing and other items, and it was noticed clothes were easier to clean - whether this account is true or not, I do not know. However, mixing ash with water is not rocket science. Even today, most KOH is commercially produced by taking the ash form burned wood, soaking in water, and then evaporating the water off to leave KOH crystals. It might make sense to have a pit of water near your night fire, if the ash was raked into the water you should get an alkaline solution that would work well as an electrolyte. Since the Copper sheet was not exposed, the jars may have been placed into a reservoir of this solution (e.g. a hole dug in the Earth filled with urine or ash water) with the iron rod connected to another conductor in the Earth, now you should have an Earth Battery charging station with a ready source of electrolyte replenishment. I'm stretching a little here, but this explanation still seems more plausible than an acid electrolyte. After playing with several replica permutations, I'll reemphasize the electrodes are protected in an alkaline electrolyte, and are oxidized with an acidic electrolyte - furthermore, that the artifacts indicate the copper was isolated in the container, only really matters with an alkaline electrolyte.
7 mm diameter by 75 mm length iron rod with an Fe3O4 coating instead of uncoated iron
The assumption of an uncoated Iron electrode is perhaps not correct. The artifacts (as I understand from written accounts and a few low resolution photos) show a mixture of black and red colored iron - which I assume is Fe3O4 ('black rust') and Fe2O3 ('red rust'), two of the more common oxidation states for iron. I am far from an expert on Iron Oxides, but from what I understand, Fe3O4 does not typically form over Fe2O3 unless placed in boiling water. As an example, I have not seen black rust (Fe3O4) form on an old car covered with red rust (Fe2O3), but that does not mean it is not possible, I've never buried an iron artifact for 2000 years either. However, Fe2O3 (red rust) can work its way into a layer of Fe3O4. Since photos of the artifacts I have seen show a mixture of Fe2O3 and Fe3O4 on the artifact, in my mind's eye, it implies there was originally a layer of Fe3O4 that was later infiltrated by Fe2O3.
So, I presume the electrodes were Fe3O4 and not plain iron - it certainly was not Fe2O3, since this does not produce a viable cell. Additionally, as shown in Step 6, Fe3O4 produces a better electrode than plain iron, and if this was a rechargeable battery being used for extended periods, the Fe3O4 coating would provide some protection from the formation of Fe2O3. However, the strongest piece of evidence is that iron of this time and shape would have been forged under a high temperature that exceeded the point of decalescence for low carbon iron 425 F-525 F. Beyond the decalescence point, iron will readily form a coating of Fe3O4. So, by choosing an iron rod without Fe3O4, previous replica creators would need to assume that the Fe3O4 was purposefully removed from the iron rod - but we know that most forged objects of this time retained their Fe3O4 coating.
Why:
Plain iron would likely oxidize to Fe2O3 (red rust) and this oxidation state does not work well as an electrode. As Fe3O4, the iron is actually protected from the elements and produces a better cell than plain iron, and significantly better than Fe2O3.
How:
As discussed in step 2, iron is easily converted to the Fe3O4 oxidation state - likely through the forging process, but a piece of already rusted iron can also be boiled in water for a few minutes, or iron can simply be immersed in seawater, all methods work fine, and would have been accessible to civilizations 2000 years ago.
A non acid electrolyte and thus a rechargeable cell
The copper electrode is completely oxidized during discharge in the acid chemistry, making for quite a mess (see photo). If an acid electrolyte, the artifacts should show substantial corrosion on the electrodes and in the jar, but none of the archaeological evidence or published accounts I have seen indicated this level of telltale corrosion. The BBC article listed in step 10 mentions corrosion and a test indicating acid, but does not provide any further detail. The corrosion shown in the artifact photographs, actually closely resembles the corrosion patterns I've seen after removing the copper from an alkaline electrolyte (see photo).
If it was an acid chemistry, cider vinegar seems more likely than the lemon juice or red wine vinegar used by some investigators - my understanding is that red wine vinegar was not used in earlier times. Perhaps mashed grapes were used instead, as seen in Arne Eggebrecht's replica demonstration, this would provide tartaric acid, but since acetic acid and citric acid were both known to the Parthians, these are equally possible too. However, an alkaline electrolyte seems just as likely to me, and as I've explained, creates a very interesting battery.
The available construction details also seem to support the possibility of this being an alkaline chemistry. Most investigators, including myself, have wondered why the copper electrode is sealed in the clay jar, since completing a circuit to extract the current is much more difficult - this is puzzling since concealment of the copper electrode does not appear to be necessary with an acid electrolyte. However, with an alkaline electrolyte, the copper electrode is quickly oxidized even when partially exposed to air. Since the artifacts found indicate that the copper was sealed in the vessel with only the iron electrode exposed - this construction detail seems to support the argument for an alkaline electrolyte, not an acid electrolyte, since exposed portions of the copper electrode are less prone to oxidation with an acid electrolyte (see photo).
Why:
An acid chemistry means the electrodes are only good for one use. An alkaline electrolyte allows for a rechargeable cell. The cell chemistry is also much less robust with an acid electrolyte, and provides a cell with only 1/5 the capacity of a single charge cycle of the alkaline electrolyte rechargeable cell.
So, maybe I've convinced you this is a rechargeable cell, but how was it charged? Perhaps an earth battery - stick two conductors (Copper, Iron, Carbon, etc. all work fine) into the Earth and you will get around a 1.2 V potential with a few mA of current. I charged my replicas using voltage/current that simulates an earth battery.
How:
KOH, an alkaline urine, or perhaps another alkaline substance, are all possible, but my experiments only used KOH. A diet rich in citrus fruits, legumes, and vegetables is known to raise pH and produce urine that is more alkaline - this would have been close to the diet of this time. I've been eating a lot of citrus and legumes lately, so maybe I will retest my urine pH and give it another go. Soap production is claimed to have been present during this time, so it is possible there was familiarity with alkaline chemistries. In my wife's DIY soap book, common lore states that soap was discovered when ash from fire pits drained with the fat from animal renderings into the nearby stream where people washed clothing and other items, and it was noticed clothes were easier to clean - whether this account is true or not, I do not know. However, mixing ash with water is not rocket science. Even today, most KOH is commercially produced by taking the ash form burned wood, soaking in water, and then evaporating the water off to leave KOH crystals. It might make sense to have a pit of water near your night fire, if the ash was raked into the water you should get an alkaline solution that would work well as an electrolyte. Since the Copper sheet was not exposed, the jars may have been placed into a reservoir of this solution (e.g. a hole dug in the Earth filled with urine or ash water) with the iron rod connected to another conductor in the Earth, now you should have an Earth Battery charging station with a ready source of electrolyte replenishment. I'm stretching a little here, but this explanation still seems more plausible than an acid electrolyte. After playing with several replica permutations, I'll reemphasize the electrodes are protected in an alkaline electrolyte, and are oxidized with an acidic electrolyte - furthermore, that the artifacts indicate the copper was isolated in the container, only really matters with an alkaline electrolyte.
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