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Help with historical stone mystery - slag, or something else?
  
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MikeC




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PostPosted: Jun 25, 2021 09:46    Post subject: Help with historical stone mystery - slag, or something else?  

Hi there

I was wondering if someone could help me with a historical mystery? I'm doing some research and ran into a query relating to rock identification, and thought I'd turn to the experts!

A large volume of something resembling pumice stone was washed up onto a beach in Scotland in the 1860s. A sample of the stone was sent for chemical analysis to determine its origin, as it was suspected to be volcanic. The analysis concluded that the composition of the stone was consistent with it being a product of a volcanic eruption AND with it being iron works slag, i.e. implying that both of those materials have identical composition, and that there's no way of determining for certain which it is.

This seems strange to me. I know nothing of geology, but it sounds odd that a man-made waste product (iron works slag) would have identical composition to naturally occurring pumice stone. I know that the term 'scoria' is used both to refer to a certain volcanic stone and to waste from iron production, but I didn't think the 2 were actually compositionally identical?

Can anyone here shed any light on this? Does the analysis make sense?

Here's the original report from the 1860s:

"Chemical analysis has proved the mysterious material which you have forwarded to me to be essentially a silicate of lime, containing, in addition, oxide of iron, alumina, magnesia, sulphuric acid, hydro-chloric acid, and phosphoric acid; of the latter only traces. These are exactly the consituents which are met with in the products of volcanic eruptions, but they are also the constituents of the slags which are found in every smelting furnace of our iron works. Chemical analysis, then, is unable to decide the origin of the material in question."

Any insight would be greatly appreciated. Thanks.
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Pete Richards
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PostPosted: Jun 25, 2021 10:38    Post subject: Re: Help with historical stone mystery - slag, or something else?  

This is an interesting question. I suppose the overlap in chemical composition could be considerable between the two sources, depending on the nature of the iron ore. I think the analyses use archaic reporting units that reflect the analyses done to measure the various elements. For example, sulfuric acid and hydrochloric acid almost certainly were not present in the material; rather there were other compounds containing sulfur and chlorine.

Here's another way to look at the question. This large mass washed up on the beach. That implies that it floated or, if not, was nearly of the same density as water. Especially so if there was no local source of an appropriate natural material, which seems likely. This is consistent with some very frothy volcanic rocks, but I wonder if it is consistent with slags. I've seen slags with numerous bubbles in them, but none that would float.

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SteveB




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PostPosted: Jun 25, 2021 11:10    Post subject: Re: Help with historical stone mystery - slag, or something else?  

Chemical analysis in 1860 differs from chemical analysis in 2020s. So it may have seemed identical enough at the time with the tools and techniques they used. But today “identical” could be easily confirmed or dismissed.

Both volcanos and smelters are high heat environments and there are various similarities of the byproducts. Both produce a large number of gases, some produced with reactions of the air and the extreme heat source, some further from minerals and chemicals trapped in the heated material that are released. So waste material like pumice, scoria and slag often contain contain bubbles that get trapped in the molten waste as it cools. Likewise molten silicas create glasslike materials too in both instances. With smelters the point of smelting as opposed to kilns or fuirnaces is to contentrate as pure and element as possible, typically a metal like gold. The molten form gets mixed with other powdered minerals which dont react chemically with the element being purified, they trap everything else unwanted to form the slag, different densities mean the pure molten material and the slag form different layers which are mechanically separated. So in both smelters and volcanos I would the same chemical elements are burnt off to gases if the are present and denser elements settle to the bottom of the molten source and the “cream rising to the top”, that is the foamy slag rises to the top. They would both contain a similiar subset of elements so chemical analysis back then would produce similar results which to the accuracy tolerances of 1860 would seem identical but today there would be distinctively empirical differences. Plus we have an array of electronic (spectroscopic) methods to get high precision and databases of other analyses to compare to not only give accurate results but possibly give geographic source locations.

There is also the possibility of the human factor. Given a lump of something that could be pumice (fairly worthless and common) or slag (certainly worthless and common). How dedicated an anlysis would most people perform ? Maybe it wasnt considered important enough to warrant more time or costs in further analysis? Have you seen the analyis report itself? Do you know the credentials of the “expert”. Pumice is simple to identify by even a child, Its not like a valuable land claim (oil, gold etc exploration/exploitation) would have been at stake. The pumice could have fallen off a ship, its been used in construction, cosmetics and powdered as an abrasive for polishing, the 1800s was a time for the “gentleman scientists” where it was fashionable to have microscope and telescopes and go on expeditions all to climb the social nobility ladder and win favour with royalty. So again Who did the analysis and was only a cursory examination done for the time?

All up I dont see any mystery here. Two similar possibilities at a time when experts had much less extensive knowledge than today. Even today its a political result to say “A is this but there are some uncertainties that could mean its this other thing or another thing.” Its a way of covering your butt that mangers and politicians use so if in future someone proves identity 100% they can say “yes thats what I wrote I thought it was” or if its proved 100% to be something else entirely they can also say “see I was right, It seemed like one thing but I also wrote it could be a number of other things”. Its a social engineering technique used probably forever to prove an answer that can be interpreted both one way and the opposite.

Interesting to think about . The truth is unlikely to be determined. Unless the original sample pieces can be examined as well as the original report and a historical biography of the expert. They may not have had the knowledge at the time to make an accurate determination. Perhaps there was a monetary or status gain involved by someone. What you’ve read was it the source of the person making the determination? Was it word for word accurate? Was it paraphrased? Each stage the information is passed on it can change plus word meaning over time so how you are reading it could be clouded by current understanding. A library should have old Oxford English Dictionaries so you can see the exact historical and modern definitions of key word in the analysis report, have you read the full report or just the generalised conclusion.

Many parts for you to follow if you want to understand the truth. Dont forget not too long ago to be Happy was to be Gay, but today has a vastly different meaning to most people. Botany has been undergoing revisions as plants would be named where they get “discovered” but more recently its been found the same species of many plants and trees have been given different names, much difficult back then to quickly look up something you think is new. Also the brontosaurus is another example. I dont. Know if pumice was a specific rock or meant a class of volcanic rocks or something else back then, which is why the exact wording needs to be examined to comprehend what was being said 160yrs ago.
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MikeC




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PostPosted: Jun 25, 2021 12:02    Post subject: Re: Help with historical stone mystery - slag, or something else?  

Pete and SteveB, thanks so much for your responses!

I realise now I should have given more specifics about the case. To be honest, I wasn't sure how much interest there'd be, so didn't want to throw too much information at people to start with. But I'm happy to delve into more detail!

First off, I should clarify that the pumice stone that washed up was a shoal of many thousands of small pieces, rather than one large mass. They were found over a stretch of coastline some 10 miles in extent.

The arrival of the pumice stone coincided with a fall of black sulphurous rain over the area - a very bizarre happening! This is what led to the interest in the pumice stones, and why they were analysed - it was part of an effort to establish the root cause of the incident. Some argued it was volcanic in origin, others that it related to local pollution, hence the relevance in getting to the bottom of the source of the stone.

The analysis was carried out by Professor A.W. Hofman of the Royal College of Chemistry. All that survives of the analysis is a summary given in a letter he wrote, part of which I quoted in my original post. Below reproduced is the entire relevant section of the letter:


"Chemical analysis has proved the mysterious material which you have forwarded to me to be essentially a silicate of lime, containing, in addition, oxide of iron, alumina, magnesia, sulphuric acid, hydro-chloric acid, and phosphoric acid; of the latter only traces. These are exactly the consituents which are met with in the products of volcanic eruptions, but they are also the constituents of the slags which are found in every smelting furnace of our iron works. Chemical analysis, then, is unable to decide the origin of the material in question.

Nor does the physical state of aggregation appear to throw additional light upon the subject. The scum of iron slags very frequently presents exactly the same appearance as the samples which you have sent me. Not having any specimens of such light slags at my disposal, I applied to some friends of mine, Messrs Levick & Co., Blaina Iron Works, Monmouthshire, who kindly transmitted to me a quantity of the scum produced in enormous quantities at their iron works. I forward a sample, together with the material which you sent me, and on comparison, you will see that, with the exception of the colour, which, in the case of the iron scum is somewhat lighter, the properties of the two substances agree in a most remarkable manner.

I refrain from examining the evidence contained in the letter of your correspondent. I feel utterly incompetent to enter into the merits of the case, and must limit myself to look at the question exclusively from the chemical point of view.

Had the specimen which you transmitted to me been placed into my hands without any further remark, and had I been asked to state what I thought the substance was, I would have said the frothy slag of an iron work."


For additional context: nothing like this pumice stone shoal had ever happened in the area before, though the same thing happened again a few months later, and (as far as I can ascertain) it never happened again after that. If it had been related to local pollution and iron smelting, you'd expect it to be a regular happening, thus I suspect some kind of freak volcanic occurrence is the more likely cause. Of course, we could be more confident in our conclusions if we had a more in-depth analysis of the stone!

SteveB, that's fascinating to read that modern electronic methods could potentially even identify geographic source locations. Are such tests difficult to get access to? I may be able to provide a sample, if it were possible to get it tested.

Pete, that's an interesting point about slag's likely inability to float. The closest iron works at that time was some 140 miles away from the point of arrival, so it seems unlikely common slag could have made it that far.
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Peter Lemkin




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PostPosted: Jun 25, 2021 12:38    Post subject: Re: Help with historical stone mystery - slag, or something else?  

note the exact date they were found and see if anywhere upwind [usual upwind direction] there had been any volcanic eruptions, such as Iceland.
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Pete Richards
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PostPosted: Jun 25, 2021 12:51    Post subject: Re: Help with historical stone mystery - slag, or something else?  

Peter Lemkin wrote:
note the exact date they were found and see if anywhere upwind [usual upwind direction] there had been any volcanic eruptions, such as Iceland.

A good idea. But I think (any true petrologists, please confirm) that really light (floatable) pumice is probably rhyolitic in nature, while mid-occean basalts such as at Iceland are likely basaltic. If this is true, it has implications for the likely chemistry as well.

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PostPosted: Jun 25, 2021 12:53    Post subject: Re: Help with historical stone mystery - slag, or something else?  

Peter Lemkin wrote:
note the exact date they were found and see if anywhere upwind [usual upwind direction] there had been any volcanic eruptions, such as Iceland.


Yes, thanks, this has already been done, and other investigate approaches too. It was really more the stone analysis details I was focusing on with this post.
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PostPosted: Jun 25, 2021 14:01    Post subject: Re: Help with historical stone mystery - slag, or something else?  

Keep in mind that great pumice "rafts", sometimes several square km in extent, have been washed up on shores in Japan, Fiji and elsewhere that came from undersea eruptions and did not correspond to any known volcanos on land.
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PostPosted: Jun 26, 2021 01:28    Post subject: Re: Help with historical stone mystery - slag, or something else?  

What is not said is where it washed up. That might give a clue as to its source, but might not! I have found pumice on the beaches of Fife, typically on the northern coast near Crail and round the corner in the Firth of Forth. I always wondered if it was natural or man-made, but never took it any further than 'a curiosity'. I suspect I discarded the material
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Peter Lemkin




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PostPosted: Jun 26, 2021 01:55    Post subject: Re: Help with historical stone mystery - slag, or something else?  

Agreed. Unless you get the actual details, such as date/month/year, location in Scotland, more details than that very old-fashioned chemical analysis, a piece that someone collected and it remains - then there is just too little scientific information to do anything with. It is just a folktale without more information. You give, for example a ten or more year span. Iceland and other places have volcanoes, but the month and year - if not the days are known. Yes, Iceland usually does not have volcanoes the produce much pumice-type stone, but I think on occassion they do...but I am not even going to research that with so little information. Or is could be something a local or not-so-local factory or sinking ship let into the ocean. Not enough information....... I see no mystery other than the lack of data. Here is one [there are other] lists of large volcanic explosions - the very large ones can sometimes send pumice-like ash far away by air or water. https://en.wikipedia.org/wiki/List_of_large_volcanic_eruptions_of_the_19th_century One volcano in Iceland exploded in 1860 with enourmous force and has been known to send pumice-like tephera as far as Scotland in the past - It is thought that Katla is the source of Vedde Ash,[9] more than 6 to 7 cubic kilometers (1.4 to 1.7 cu mi) of tephra dated to 10,600 years BP[1][10][11][12] found at a number of sites including Vedde in Norway, Denmark, Scotland and North Atlantic cores. Just click on Katla volcano to see this data.
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PostPosted: Jun 26, 2021 02:23    Post subject: Re: Help with historical stone mystery - slag, or something else?  

Given the direction of the Gulf Stream, I think pumice is more likely to reach Scotland from the Azores than from Iceland, and the Azores certainly have more silicic pumice-producing eruptions than Iceland does, although as you point out, tephra from Iceland is possible too.

The chemistry, and any microminerals in vugs, would distinguish an Azores source from an Icelandic source.
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PostPosted: Jun 26, 2021 03:14    Post subject: Re: Help with historical stone mystery - slag, or something else?  

That 1860 Iceland eruption was level 4 [5 is like Krakatoa]...so very large. Such an eruption sends tephra into the statosphere and it can rain down quite far away. You are correct about if moving on the sea via currents. If this was such a large event that so many took notice of, surely some museum or private person has still a sample. With modern techniques the exact volcano [if it was a volcano] can be named. Back in 1860 things were different and the internet was now well organized ;-)
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