MICHAEL FARADAY - TOUR IN WALES - 1819
THURSDAY JULY 29th
. . . . . continued from underground trip . . . . .
Mr. Irewick now took charge of us and showed us the work above ground. We went first to the kilns and in our way passed other mine workings belonging to the Mona Company. At the kilns the following process is carried into effect. The ore is raised from the mine and broken by the women as described, is placed in heaps about 35 feet long, 10 wide and 10 high. Larger pieces of ore are used for the outside which is something like rough brickwork but the ore is wheeled in anyhow into the interior. Four or five large holes are made in the mass below like ash pits and when the heap contains enough ore flues are built across and along the top, the large pieces of ore which are connected with another flue running two or three feet from the kiln on the ground and this being done the whole heap is covered with earth and clay so as to prevent the entrance and exit air or vapour except by the holes before-mentioned and the flues. A brick chamber is built a few feet from the kiln and connected with its flue at one end, the other having a small aperture. Some lighted coals are now thrown into the holes left at the bottom of the kilns and in the course of a day they heat and inflame the ore immediately about them and afterwards no further additional fuel is necessary but the combustion goes on with the ore itself one part roasting the other. This lasts five or six weeks and all the sulphur separated and sulphurous acid generated pass through the flues into the chamber and are there condensed. In this way very little vapour escapes and the process instead of being a general nuisance as at Swansea is a very magnificent and agreeable example of sublimation.
When the kiln goes out of itself and is cooled it is pulled down and the ore taken away in carts to the refineries near the port. Those parts which happen here and there to be only half burned being carefully selected and put into other kilns. The chamber is not disturbed for the first, second, even third kiln but after the sulphur of many kilns has been sublimed into it it is opened the brimstone taken out, washed from the acid which adheres to it and is fused and then it goes to market.
From hence we went to the precipitating pits. I have already said that the water which gathers in some of the workings is a very strong solution of sulphate of copper from its action on the sulphuret. This water is pumped up by a steam engine into large reservoirs and it is let down by sluices from there into small tanks placed side by side each about I2 feet long, 8 wide and 18 inches deep. Into these tanks is thrown old iron of all sorts, hoops, nails, saucepans, etc., and they frequently procure what they call iron from the iron works, but it is generally a mixture of slag and iron containing about half its weight of the latter. In this state the iron and water remain in contact for some time being turned now and then to expose fresh surfaces to their mutual action and then the water is drawn off and fresh let in. The waters are not thrown away after having been once over the iron but that which has been acted on in the highest tank is let down into a second where there is more iron and then again into a third, fourth and fifth in all of which there is iron until it is so poor as not to be worth working any longer. The result of this arrangement is the production of copper in these tanks occasioned by the play of affinities which takes place between the substances. The water contains sulphate of copper or blue vitriol to which iron is added and iron having a stronger attraction for oxygen and sulphuric acid than copper has, it takes both these substances from the blue vitriol uniting to them and forming a soluble salt and consequently the copper is thrown out and remains as a sediment in the tank. This sediment is never pure copper but always a mixture with the rust or oxide of iron a part of which comes from the dirty state of the iron when thrown in, and another part from the spontaneous decomposition of the salt of iron which is produced, for you must understand My Dear Girl that the combination first made by the Iron and Sulphuric acid is what is commonly called green vitriol or copperas. Now when the salt is dissolved and exposed to air it absorbs a portion of the oxygen of the air and the Iron becomes more oxidised. In this state as it is not so soluble in the acid as before and therefore a part is deposited as a red powder mixed with the copper rendering it impure, consequently the sediment is always copper mixed with oxide of iron and it is richer in copper from the first tank or the strong water and poorer when obtained from the last tank. It is found from experience that if the sediment yield less than 5 per cent of copper the expense of the iron is more than the worth of the copper obtained so that waters reduced until they yield the mixture of only 5 per cent copper are thrown away. In the first tanks the sediments are so rich in copper as to yield 80 or 90 per cent. These tanks are emptied of their sediments once a quarter. When the substance is dry it is taken down to the refineries and soon rendered fit for market. From 40 to 50 tons of copper are produced annually in this way.
When the water first runs from the tank it is of a fine red colour from the per-sulphate of iron it contains. The pools which receive it and the rivers it forms in passing to the harbour, look as if filled with blood. In the harbour it soon becomes diluted by the sea but the rocks to a great distance are stained by it.
We then walked on to the Parys mine. This is an immense excavation open to today on the other side of the same mountain. An extraordinary accumulation of ore was found in this place which, when worked, proved of immense value and brought in enormous incomes to the proprietor. It appears that 3 or 4 veins of copper here converge together and caused a single disposition of ore which has made the place so deservedly famous. At present the ore is not so abundant and the mine is worked by underground shafts and galleries like the others though still a little is done above.
In our way from hence to the assay we passed several groups of children who were engaged in searching the rubbish of ancient workings. Formerly the ore was not so perfectly produced as at present and much was thrown away with the slag. Now these heaps of refuse are eagerly sought for the better parts selected and sent to the refineries to be reduced.
At the Assay office we found the Assay master and his assistant busy in ascertaining the relative value of different specimens of ore slag metal etc., and according to his report are workmen paid and the calculation made, I saw nothing very particular there.
Now having viewed everything and spent 4 hours very pleasantly among the works we returned to the mine office pocketed our minerals, shouldered our bundles, bade adieu to our very kind friends Messrs. Irewick and Leaman and again set off on our journeyings. We endeavoured to find a nearer way from the Mine to Bangor Ferry than we had taken from the ferry to Amlwch and succeeded to a certain extent but the sea was rising over the sands in the bay of the coast and two or three times turned us a little aside. Our walk was much finer than yesterday and contained more coast scenery in it. We frequently had bays on our left. with the waves rolling into them and shipping in the distance and the day was neither so hot nor so misty.
The rock on the Parys mountains is slate. A few miles from its eastward end we came on to sandstone and breccia of white quartz pebbles and then on to a limestone full of organic remains eneryne and alcyine and shells.