The Chemistry of Pigeon Post
by Adam Quinan
Pigeon Post is in my mind in many ways Dick Callum's book. Here the Professor really demonstrates the uses of his scientific and engineering bent. My particular interest is in chemistry, though he also demonstrates his electrical and mechanical engineering skills in setting up the bell for the pigeons to ring. I have perhaps stretched the strict definition of chemistry to include geology and ore processing but as I am fond of asking "What in the world isn't chemistry?".
Arthur Ransome had had some training as a scientist at Yorkshire College in Leeds, forerunner to today's Leeds University (where I took my degree in Chemical Engineering in 1976). I expect that by the time he wrote Pigeon Post, he was a bit rusty. However, his descriptions of some of the chemical reactions are obviously based on some real experience and I suspect Oscar Gnosspelius, to whom the book is dedicated, probably demonstrated all the important experiments to him.
An important caution: chemistry experiments can be very hazardous and shouldn't be performed except under well controlled and supervised conditions and preferably in a well equipped laboratory. Reading about Dick making up aqua regia and pouring it on to his unknown powder in Captain Flint's study makes me quiver! Things were certainly different back then when it came to chemical safety!
The first clue to the geology of High Topps is found when Squashy Hat has painted the rocks. where gosson (or gossan) is to be found. Ransome describes it as looking like "rust and ashes, brown, reddish and black". Gosson is basically rust, or as chemists put it "hydrated iron oxides". It is formed by the action of the weather on exposed ore. The water from rain and snow and the oxygen in the air react with the ore and dissolve out some of the elements leaving behind an insoluble mass of iron oxides. Prospectors know that when they find gosson there is usually a good body of rich ore close at hand underground. Squashy Hat was on the right line, he just didn't find the seam first.
The ore in Pigeon Post is identified by Captain Flint as soon as he sees a sample. In Chapter 33 when he finds Dick in his study on his return to Beckfoot, he says "But this looks like perfectly good copper pyrites". Copper pyrites is a mineral compound of iron and copper sulphide (CuFeS2) also known as chalcopyrite. It is not a pure metal and is softer and more brittle than either gold or copper metal. This is one of the richest forms of copper ore, so no wonder Captain Flint was so pleased. Together with the better known iron pyrites (Fe S2), this ore is often known as "Fool's gold" because of its gold metallic colour. Iron pyrites ore is often found in the same area as real gold so only a fool would ignore it! In Pigeon Post, the parent rock in which the ore is found is quartz. Quartz is chemically the same as ordinary beach sand, silicon dioxide or SiO2 The main difference is that sand has been broken up into grains. Gold is often found in quartz seams so the prospectors seemed to be on the right track.
The one area where I think that perhaps Arthur Ransome had to make things easier than they would have been in real life was in the panning of the "gold". The extremely hard work of mining and crushing the ore is exactly right. That would have taken the hours of backbreaking work described in the book. However, they were lucky to be able to pan the "gold" so easily. Gold metal has a density of about 19.3 and the quartz of the main rock has a density of about 2.65. It is not too difficult to separate materials of such different densities by panning. However, copper pyrites has a fairly low density of about 4.25. While this is a little bit more than the quartz, it is not much denser and a lot of luck and probably a lot more water would have been needed to obtain even the small samples that the SADMC managed to extract. This is another clue which they miss. Real gold metal is so dense that the powder they extracted would have felt much heavier than the equivalent amount of pyrites.
The blowpipe was intended to melt the gold dust by concentrating the heat of the candle and later the spirit lamp on the samples of gold dust. When Dick tried it, his gold appeared to melt but when it cooled down, it became a black blob. Copper pyrites decompose when heated. What Dick saw was the material melting and then reacting with oxygen in the air to form a black copper/iron oxide. Simplified, the chemical equation might look like this: CuFeS1 + 3 O1 = CuO + FeO + 2 SO1 (gas)
Exactly which iron and copper oxides would be formed is hard to tell. Gold might have melted but it would have resolidified as a gold pellet.
Needing a hot fire to melt their gold, the children turned to charcoal. Not only were they familiar with the process of making charcoal, it is a good fuel to build a hot fire. Charcoal burning is an ancient art and charcoal has been used for fuel, drawing on paper (or even cave walls) from earliest times. It is still used in modern industry (or perhaps in a tap water filter in your house) to remove impurities from liquids. Making charcoal the way the Billies and the Swallows, Amazons and Ds did is time consuming but doesn't require a lot of technology. Charcoal is wood which has had all the volatile material baked away but the main wood is not burned. This is done with a slow fire with very little oxygen. The body of the wood doesn't burn but the fire is hot enough to boil out all the sap and other volatile material which does burn enough to bake the wood to charcoal.
The blast furnace was intended to melt the gold metal so that they could cast it as an ingot. In practice it would have been very difficult to get a hot enough fire to melt gold, however, the SADMC didn't have that problem, instead their "gold" disappeared when the pyrites decomposed. They would only have the black oxide powder that would easily be lost when the crucible broke.
Aqua regia, as Captain Flint says, will dissolve almost anything. Gold, chemically, is a very inert metal. It doesn't react with oxygen or atmospheric pollutants which is why it stays nice and bright and shiny and doesn't rust like iron or tarnish like silver. However, it will dissolve in that nasty mixture of concentrated nitric and hydrochloric acids, aqua regia. (Latin for Royal Water, perhaps because of its effect as "King" of solvents). When Dick adds aqua regia to his test-tube of golden dust, he gets quite an exothermic reaction with a gas evolved. He ends up with a clear yellowish liquid and some dull sediment. The sediment is mostly solid sulphur (and other impurities from the ore). The copper pyrites reacts with the nitric acid in the mixture.
CuFeS2 + 4 HNO3 = Cu(NO3)2 + Fe(NO3)2 + 2 H1(gas) + 2 S(solid)
Until I looked it up in a text book, I wasn't sure why the liquid in the test-tube was yellow. The yellow colour is due to the copper ions forming a complex with the chloride ions in the solution. A complex ion is one where one ion (copper in this case) forms bonds with another species so that the whole complex behaves as one ion. Any ionic material dissolved in water is in effect a complex of that ion with water molecules. These bonds are not as strong as true chemical bonds but they can allow for ions to stay in solution when otherwise they might deposit out. Complexes often form coloured solutions and the yellow colour of this solution indicates that there was a strong excess of chloride ions. This is what you would expect using concentrated hydrochloric acid. A weaker solution would have produced a greenish colour as some of the copper would have had water associated with it instead of chloride. Copper solutions in water are often a bluish colour.
The reaction that is described when Captain Flint adds ammonia to the test-tube, so that the colour changes from yellow to brilliant blue is the replacement of the chloride ions in the copper complex with ammonium ions. Ammonium ions form a stronger complex than the chloride ions and the very bright blue colour is a sure sign of copper.
As we expect from Arthur Ransome, all his technical details are as correct as he can make them while keeping the story moving along. Pigeon Post would have come to a sad end if the prospectors had not been able to pan their "gold", so I believe that we can forgive him a slight exaggeration on this point. The rest of the description of the chemistry and ore processing is completely correct; but how many people would have known if it hadn't been? That is the mark of his integrity as a writer.
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