Blackton   Mill, The Process. 

Blackton Lead Smelting Mill 1820

Blackton mill was constructed in the 1820's. Chief agent Rober Stagg of the London Lead Mining Company commissioned the mill to serve the local ore fields to smelt Galena ore into Lead and Silver that would be presented to the market as billet. Robert had a good eye for profit and an understanding of progression, the mill was developed and used as an experimental base for the improved smelting and refinement of Galena - the mill was considered to be at the top of the London Lead Mining Companies technological advancement for the time.

The mill was situated on a terrace along side Blackton beck (slightly North East of Blackton, Eggleston). It featured a set of twin flues that ran almost 1790ft up the hill side before combining into a single large flue. The larger flue continued for a further 837ft to reach the head which terminated at the 152ft tall chimney to take the fumes into the atmosphere beyond. It was known that Lead smelting fumes (mainly sulphur dioxide) had a serious detrimental effect to the health of mine workers and their families if vented at village level as was done in years gone by. 

 

The mill was abandoned in 1904, soon after in 1905 the London Lead Mining Company was wound up. Very little remains of the mill with almost all of the dressed stone having been re-used in the construction of other houses and buildings around Eggleston.

 

This pile of stones is all that remains of the hexagonal Blackton Mill chimney.

The Chimney stood until 1930 when the Kidd brothers (of Blackton House) wanted to reuse the 2000 tons of dressed stone for other purposes. Tom Allinson, a retired Lead miner and resident of Eggleston was employed to demolish it, which he did with great effect with the use of explosives planted around the base of the structure.

 

One of the reservoirs that remain at Blackton Head that serve as a reminder of the industry that they one served.

Two reservoirs stand at Blackton Head and were constructed to feed the mill. The water was primarily used to power a water wheel which was located down at the mill site and would have driven a large set of bellows. A continuous supply of pressurised air was required during the smelting process and was used to feed the flames which would drive up the heat.

 

A secondary usage of the water was later discovered and put to use, where by it was found that if the lead rich smoke that was being expelled through the mill's chimneys were to be condensed, then it would stick to the inside surfaces of the flu and could be harvested from the stonework. This unfortunate job was generally given to young boys whom would have to crawl up the flues whilst knocking any deposits from the walls by scraping, then flushing with water. The dust deposits could then be collected and re-smelted.  

 

Image 1: Remaining flu wall with collapsed second flu built parallel to the first.

Image 2: Lead deposits still exist on the wall of the flu.

It was discovered that the construction of a horizontal flue (the type that we see here at Blackton Mill) was preferable to that of a large vertical chimney. The longer and damper (or wetter) the flu could be, the more natural draw and the greater the condensation effect on the smelting gasses - this of course would help the recovery of the lead from the fumes and also aid an increased air flow to maintain a high smelting temperature needed at the hearths.

 

Remains of condenser timbering within the existing flu wall.

Having spent some time looking at this remaining section of the horizontal flu, one can't help but notice the amount of timber that is present within the stonework. I conclude that this is likely the remains of a condensing chamber that was put to use in the final years of the mill. In effect, the London Lead Mining Company knew that the lead ore was becoming expensive to mine, and the returns were rapidly diminishing on the market - mainly due to imported lead billets from abroad. It was important to ensure that every ounce of sellable lead was retrieved from the expensive process of smelting the ore.

Condenser chambers were compartments constructed inside the chimney (or flu) that were designed to rapidly cool the fumes so that it would deposit any lead dust onto the chamber walls. This was achieved in many various ways, but at Blackton the use of timbering ledges and obstructions were put in place in the horizontal flu near to the base of the vertical chimney. The idea being that the timbers placed throughout the condenser area were to be kept wet via a means of packing the external surface of the stonework in moss and grass. This in turn would wick water from the moorland inwards via the internal timbering which would create a moist humid atmosphere within the flu. This would massively aid thermal conduction reducing the temperature of the flu gasses as they moved through the many moist obstructions within the structure.

The Smelting Process

Mining for Galena, the Lead Ore. (image taken in LLMC mine in near to local area)

Galena (lead ore) was found with rich deposits all over the Pennines as well as many areas throughout much of the UK. The galena was naturally formed millions of years ago whilst the earth was in its infancy and is generally found in vertical columns called 'lodes'. The lodes have horizontal and vertical 'veins' of ore that typically form alongside quartz and other minerals, and it is these veins that the miners would follow in search of finding a lode. Once (or if) a lode or vein was found, the miners would create a 'level' (a horizontal passage) or a 'stope' which usually means that the miners had to mine upwards, downwards and horizontally to follow the vein or lode. It was usual to build a series of timber levels inserted into the rock in order to gain height, this was called 'stoping'. (examples of images I've taken, click here). The galena would be mined out and brought to the surface for sorting by means of pure human power, horse/ponies or in later years by mechanical winches. 

Waterwheel driven sorting mill that crushes the galena to split it from the rock, and agitated flotation tubs. 

The galena ore would be roughly sorted and reduced to small pellets on the dressing floors at the mine site. This work was undertaken by women, children and older people that were not capable of underground work to separate the galena ore from the stone and other minerals. This was done in the beginning by means of basic hammering, but the process was later semi-automated with the introduction of water wheel powered mangles, stamping mills and flotation tubs. (as pictured above)

 

The sorted ore was brought to the mill by the 'Jaggers', which were men with long horse drawn trains of carts. The ore was collected from the mine site and transported often over many miles across moorland and difficult terrain. Jagger stops can be seen all over the Pennines with small dwellings and stone encirclements where the horses could be kept overnight - though few remain intact in the modern day, some remains are clear to see scattered across the moor with several visible on the Stanhope to Eggleston road.

Map from 1892 showing the Blackton Mill layout, the flu's can be seen exiting to the moor from the smelting furnace buildings. 

Blackton Mill was constructed as a 'test bench' for introducing new smelting processes that would be put in place to the other mill sites over time. The mill employed a new gravity feed system where the ore was brought first to the top of the mill site.

As the ore entered the mill, large chunks of galena would have to worked into small pea sized fragments or even into a dust. This would serve to aid the release of the lead from any pockets of other materials - be it any other common mineral such as quartz or fluorspar that would commonly be found with the galena, or to release any pockets found in small fragments of any remaining rock. The ore would pass through the set of mangles and stamping hammers to break the ore down, these machines were powered by water wheel (hence the need for the large water reservoirs and to be constructed near a beck).

 

The pre-processed ore would be thoroughly washed and sorted, the larger pea gravel sized galena chunks would pass to the roasting furnace, with the water and dust being transferred to the newly invented process of froth flotation.

The Ore Processing Plant at Blackton Mill showing the flotation cells.

The large cylinders in the image of Blackton mill (above) show a very similar process to that which is currently in use in the modern day. The milled galena slurry (water and galena dust) would be fed into the middle-upper area of the vertical cylindrical chamber. Pressurised air was driven through a central column within the cylinder to cause a violent froth (or bubbles) to form which would rise to the surface - the effect was increased by the addition of adding lime to the galena slurry. The froth would gather at the top of the cylinder and be drawn off by wipers and fed onto a drying lagoon where the water would be encouraged to evaporate to leave behind a crusty lead concentrate. 

Twin bellows powered by a basic cam system from the waterwheel shaft.

Once the ore was concentrated, it would be passed to the roasting hearths which would heat the ore from between 700 degrees to 1400 degrees celsius, more lime was added to the mix to act as a flux which would cause the galena to give up a lot of the impurities. These generally consisted of sulphur dioxide, arsenic and lead oxide which were drawn up the flu and into the condenser chambers. The molten lead would be tapped and sent downhill via a series of chutes, as it solidified it would be broken up back to smaller pieces before being passed to the smelting furnaces.

 

Blackton mill was at the forefront of 'modern' technology, so new refractory or cupola furnaces were constructed which were far more fuel efficient than the older conventional direct fuel smelters. The cupola furnace differs in that one large hearth was used which would be fuelled by coal (or peat in other mills), air was injected under the fire grate to increase the burning temperature and the hot air that resulted was directed over a set of ore hearths positioned alongside the furnace. The pre-processed lead was shovelled onto the ore hearths and the hot air would smelt the metal at a temperature of 1100 to 1200 degrees celcius (generally slightly lower than that of the roasting furnace), again more lime would be added to remove any further impurities and would result in vast amounts of carbon monoxide to be produced, chemically reacting with the lead fumes to form carbon dioxide as it leaves the flu.

 

The lead would then pass to the refining furnaces which would maintain a temperature of 327 degrees celsius. The lead would remain molten and form a skum on the surface, this would contain almost all of the last of the impurities such as copper and other metallics which would be scraped from the surface and disposed of in dross heaps away from the mill site. (in modern times, these dross or spoil heaps were reprocessed to extract fluorspar, barytes and other minerals that had no known use at the time).

 

After refining, the almost pure lead concentrate would be poured into moulds to form ingots. It is these cast iron moulds that we find scattered throughout the village being used as water troughs.

 

This gravity system that was employed at Blackton reduced the man power needed to move ore around the site, it helped maintain the ore temperature from the previous process and improved smelting from ore to product. (which reduced labour and fuel costs). The resulting billet would be good quality and almost pure lead, this would then be sold at market to be reworked into sheets for roofing and tanking or rolled into pipework (most Victorian houses featured lead pipes). It was also used in foundry processes, glass making, paints, toys(!) and other lead containing products.

Remains of part of the ore hearth or furnace fire grate burried in the mill dross heap.

The Traditional Smelting Process

In early smelting mills; the hearths were fuelled by either timber, peat or rarely coal (due to the high cost and general distance from coal mines). Air was driven over the fuel to increase the burning temperature and was produced by driving a large set of bellow, again powered by the mill's water wheel. 

 

The ore was placed under the burning fuel, lead having a low melting point would immediately be released from the ore and would drip through the hearth into a crucible leaving much of the other mineral deposits behind. The crucible containing the molten lead was then tapped into a mould to form a lead billet. The billet would have been of less pure grade and would result in a lot of slag (or crusty deposits) to form when it was remelted to be moulded/formed etc into a usable object - this was of course undesirable and lead to the improved methods of refining the ore into a higher grade product.

 

In Blackton, coal was the main source of fuel that was used as the London Lead Mining Company had interests in the vast reserves of coal and mines at Copley, which is located in the next valley over the hill. The local collieries offered a perfect and convenient supply of coal fuel for the smelting processes at Blackton. 

 

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