Wood Tar Distillers
The older method of making charcoal (without recovering the byproducts) was to pile the wood up, cover it in turf and set it alight, the last chap doing it this way in the Forest of Dean, and probably the country, was Edward Roberts in the 1950's. By the early 19th century works were being set up to use 'destructive distillation' to separate out the byproducts and still recover the charcoal. There is a complete description of this process, and numerous useful photographs of the installations, in the book 'The Severn & Wye Railway Vol. 2, published by Wild Swan publications. This should be essential reading for anyone considering adding such a works to a layout, it is also a good read.
During the 1800's the people making charcoal began using industrial pyrolysis methods, putting the wood into long iron retorts to recover both the charcoal and also the various by-products that were driven off in the retort.
Small 'Chemical works' or 'Acid Works' shown in forested areas on Ordnance Survey maps are probably one of these wood distillation plants. These took in stuff called 'cordwood' (basically odd bits of branches and thin tree trunks, see also 'Wagon Loads & Materials Handling - Timber') and heated it in large tubular iron retorts, sealed from contact with the air. This drove off the oils in the wood which were then condensed to produce 'Pyroligneous Acid' and residual tar.
The 'acid' contained mostly acetic acid but also a range of useful chemicals such as naphtha, turpentine, acetone and methyl alcohol shipped out in drums (or very occasionally in tanks), acetate of lime (a powder) in bags and of course charcoal (shipped in bags). Not all works processed the acid on site, some shipped it for further refining to another, larger, works.
In areas where pine trees were grown these establishments produced turpentine, a complex oily resin which is distilled to produce a range of useful materials including 'rosin' (used to coat violin bow strings), 'thinners' for paint and as a source of synthetic camphor oil (natural camphor oil is produced almost exclusively in Taiwan by steam distillation of the camphor tree wood itself). The products of pine trees, turpentine, pine oil, tall oil and especially rosin and the residual tar or 'pitch' are sometimes referred to as 'naval stores' as they were used for repairing wooden ships.
Where hard woods were abundant (birch, alder, oak and beech) the main product was usually acetic acid, shipped out in drums. The acetic acid was used to manufacture sodium acetate, a useful mordant for the cloth dyeing industry (see also Lineside Industries - Dyes and Inks). Acetic acid produced from wood distillation was cheaper than that produced from malt vinegar. The residual tar was shipped out in barrels or in bulk using tank wagons, other outgoing cargo would include sacks and casks.
Modelling a wood distillers
The sign at the entrance might include mention of 'wood distillers', 'chemical works', 'acetic acid', 'charcoal' and 'blacking millers' although 'chemical works' (where the trees were mainly pine) and 'acid works' (where hardwoods were abundant) seem to have been the most common.
The characteristic features of the distillation works were the long retort in which the wood was 'cooked', the associated cooling chamber, at least one tall chimney and some large open topped tanks. There was a lot of wood lying about and most works had a saw (often water powered) to cut up the timber for the cradles.
A 'cord' of wood is a measure of volume, being 128 cubic feet. The wood was stacked at the factory about 4 feet wide by 4 feet high, hence every 8 feet was a cord, in later years the wood was often purchased by weight, so there would be a weighbridge at the road entrance to the yard.
The pictures I have seen of the Scottish works all show substantial buildings of conventional 'industrial' appearance. The pictures I have seen of the works in the Forest of Dean showed a much more ramshackle scene with timber buildings mixed in amongst more substantial brick buildings.
The Pola (or Model Power) 'pickle works' makes a good basis for a Forest of Dean wood distillation works, it has a timber clad central building with large tanks under cover at each end and a moderate sized chimney. To serve as a wood distillers plant just paint everything in various shades of very dark grey (representing weathered black).
Pola Pickle Factory kit
You will need a retort, and its associated cooling chamber and even in N these are fairly large, nothing less than about three inches long for each of them. One simple option would be a length of strip wood about 3/4 inch square (18mm) with a central curved top (plastic card formed round a broom handle and dipped in hot water, or a section cut from an old Venetian blind). Wrap the whole thing in 'sooty brick' brick paper and add black and slightly rust stained 'iron doors' to each end from post card, the door hinges can be added from paper. The narrow gauge track running through the retort and cooling chamber can be represented with Z gauge track, but that looks a bit heavy for this job and benefits from being well 'bedded-in' to the ground surface. Home made 3mm gauge track from plastic strip is probably a better option but tedious to make.
One point to note is that these works were often built on uneven ground, and they seldom bothered to level the site when building the plant. The buildings would all be on the level, but they might all be at slightly different heights.
Incoming cargo would include cordwood (lots of small pieces of wood, as described in the 'Wagon loads and materials handling' section under 'timber'). The works would receive lime in roofed wagons, sulphuric acid and caustic soda, both often shipped in spherical glass carboys (see also Appendix One - Packaging Materials & Containers for a description of carboys). Sulphuric acid can also be shipped in iron tanks (see also 'Lineside Industries - Chemicals, salt and plastics industries') and caustic soda can be shipped as a liquid in tanks or as a white pelleted material in small metal drums holding 10 lbs.
Description of the process
At the works the cord wood was loaded into more or less cylindrical metal cradles about 6 foot in diameter and 10 foot long running on railway tracks. These cradles were wheeled into the 'retort', typically a rectangular iron box 7 or 8 foot square or a fire-brick structure with a flattish curved roof of about the same size with large doors at either end and long enough to hold several cradles. At least one 'acid works' in Scotland had the retort in what looked like a long garage with a pitched roof of corrugated iron, so you have considerable leeway on how much work you need do in that department.
The retort doors were closed and hot gasses from a coal furnace admitted. As there was no air (or rather oxygen) in the retort the wood 'cooked' without being allowed to burn, over two thirds of its weight being given off as gasses which were extracted via large copper pipes from the top of the retort. After about a day the cradles were pulled from the other end of the retort and placed in a similar structure which was cooled with water, the end product being charcoal (about a ton of this being produced from 5 tons of the cordwood). A fresh cradle of wood was immediately run-in to the retort, the two ends alternating to allow near continuous operation.
The gas meanwhile had been passed through a condenser which dropped out most of the heavier tar (which was drained into a large tank), and the 'pyroligneous liquor' (which contains a range of liquid materials). This mix of hot liquids was dropped into large storage tanks where the tar settled to the bottom and the naphtha/acid mix floated on top.
The naphtha/acid mix was passed to storage tanks (iron vats, perhaps 10 foot across and 8 foot high, usually under a simple 'carport' type cover of corrugated sheeting on a simple wooden framework). These tanks were heavily stained black (although I am not sure if this was from the process or some kind of protective coating), and there was a raised walkway provided for inspection purposes. As far as I am aware they were open topped.
The liquor was often then fed into a series of stills called 'separators' where the acids, soluble oils and water were removed.
The still hot gasses and vapours from the separator were passed to further condensers all set up indoors, produced grey acetate of lime powder, and the naphtha (mostly methyl alcohol). The gasses which did not condense in this process (mostly carbon dioxide, carbon monoxide and methane) were used to supplement coal in the furnace used to heat the main retort.
Up to the 1940s acetic acid was commonly recovered from hard woods by neutralising the acidic liquor (by adding lime, to produce a solution of calcium acetate) and evaporating the resulting liquid to separate the salt from the water. The resulting calcium acetate was then mixed with sulphuric acid to liberate the acetic acid from the salt. This is why 'acid works' was often included in the name of the works. By the later 1930s better ways of extracting the acid had been found and few if any works used this process after World War Two.
One of the more valuable products in hard wood areas was methanol, made by the destructive distillation of the calcium acetate, however by 1940 alternative methods for producing methanol had taken over (mainly by producing acetic acid from acetylene or ethyl alcohol). By the time war broke out in 1939 less than 10 percent of the countries supply of methanol was from wood distillation plants and the increased demand (for explosives and plastics) was met by expanding the synthetic methanol industry.
The charcoal itself was valuable, although replaced by coke for iron and steel making it is still used today in metallurgical processes in which the low content of ash, sulfur, and phosphorus is important. Wood charcoal is slightly different from coke (technically coke is a type of charcoal) and the slight differences in its properties mean it continues to have some specific industrial applications. A lot of the residual tar or pitch from wood distillation works was apparently exported in barrels for painting round the base of rubber trees in the Far East.
As noted above there were quite a number of smaller works which did not process the liquor on-site but shipped it out for further refining. These smaller works would comprise the retorts (invariably with a tall chimney) and a few ancillary buildings including (often) the home of the owner or manager of the plant. The retort might be made of brick at a smaller works (at the larger establishments they were typically an iron box). The vapours driven off were passed via copper pipes in which the vapour condensed and ran down into large wooden barrels. Once full these were left to settle out, the tar sinking to the bottom, the 'acid' floating to the top. The acid and tar were then decanted into fresh barrels for shipment. Such a small works would only just qualify for a railway connection.
In forested areas 'works' were often rather large establishments, with large buildings several stories high to contain the various settling tanks involved in the process. There is a complete description of this process and numerous useful photographs of the installations in the book 'The Severn & Wye Railway Vol. 2, published by Wild Swan publications.
Access to the railway was a major advantage for a larger works, some works were arranged to either side of the local branch line, these would have a level crossing (an 'occupation crossing') or perhaps a small bridge to allow works traffic to cross the line. These large works would process the liquids recovered to obtain a range of chemicals.
British 'Acid Works' were still operating certainly into the later 1970s and although some specialist markets remain viable (for example wood methanol can be used as a denaturant for ethyl alcohol) I am not aware of any wood based chemical firms still operating in the UK since 1980.
Since the later 1960s there has been an increasing demand for charcoal and new methods of producing it have been developed. Some firms use waste sawdust and shavings from other industries and a lot of charcoal is now sold for use in bar-b-que fires as 'briquettes' (formed lumps of powdered charcoal mixed with starch and a little water). The byproducts which used to be so important are no longer valuable enough to justify a works but they can still (in many cases) be sold on (methanol, acetic acid and wood oils).
Having said which chemicals and materials recovered from wood today include; Acetic acid, acetone, cellophane, cellulose, acetate, charcoal, dyestuffs, ethyl alcohol, explosives, lacquers, methanol, molasses, oils, photographic films, pitch, plastics, Rayon, sugars, synthetic sponges, tannins, tar, turpentine, vanillin, yeast, etc.
More modern products include ablative carbon composite (made of wood reduced to long filaments of carbon) capable of withstanding temperatures of up to about 6,000 degrees Centigrade (5,000 degrees F) and carbon fibre composite or structural graphite composite, which is stronger and lighter than metal.
Activated Charcoal
It had been known for hundreds of years that charcoal could deodorise and remove discoloration, charcoal made from bones was in use to de-colour sugar from about 1815 but an improved material called 'activated charcoal' was invented in about 1900. Activated charcoal is made by heating the charcoal in a stream of steam or carbon dioxide, which results in partial oxidisation of the surface of the charcoal. Activated charcoal is many times more efficient at removing contaminants but it was the early 20th Century before scientists worked out why. In essence the charcoal is very porous, meaning it has a large surface area. Because it is made of carbon a lot of things stick to the surface (including oxygen) and whilst held in close proximity some of these materials undergo chemical changes. In gas masks the poison gas combines with the oxygen on the surface of the charcoal and the resulting compound remains stuck in place. In industry charcoal was used up to the 1930s to extract petroleum oils from natural gas, but then better ways were found to do this. Activated charcoal remains important to recover volatile solvents in various industries such as the manufacture of artificial leather. Town drinking water is treated with chlorine to kill bacteria, but this reacts with traces of phenols in the water and produces an unpleasant smell, so activated charcoal is used to recover the smelly chlorophenols at the local 'water works' (the charcoal is mixed in as a powder and absorbs them,it is then filtered out by running the water through gravel beds).
Some example coal tar distilleries
The Forest of Dean was a major area for this activity and prior to World War One the Government built a large plant there (they had to buy the equipment from Germany). This one plant alone went through 12000 tons of wood a year, producing 384 tons of acetate of lime, 270 tons of wood tar, 1380 tons of charcoal and 90 tons of wood spirit. These works were rail-served and the works remained in production until 1971.
There were works in most forested areas, in Scotland the one chemical company operated several plants from about 1913, using the products to make mordants for the dyestuffs industry. This firm was known variously as 'Turnbull & Ramsay, manufacturing chemists' (1825) 'Turnbull & Co., manufacturing chemists and vinegar makers' (1831), 'Turnbull & Co., manufacturing chemists' (1889), 'Turnbull & Co., wood distillers and blacking millers' (1933) and in 1942-3 the company was 'Turnbull, Stuart & Co. (Camlachie) Ltd.
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