The Coal Collection
The Department of Geology has over a thousand specimens of coal in its collections from all over the world.
Approximately 800 specimens are of Welsh coal, many fortunately collected in the early 1900's, when coal was being mined from deep underground collieries. The majority of these localities are now completely inaccessible, and the museum's collection is irreplaceable. Many of the specimens in the collection have been used for research.
The coal rich areas of Wales are all in the Upper Carboniferous rocks a time spanning approx. 315-300 millions of years ago. Coal is an organic sedimentary rock, with over 33% organic content. Areas that have coal seams were once covered in forests that grew and died in cyclic sequences as water levels rose and fell. Plant material was compressed by the weight of sediment above to eventually form coal. It is thought that around 10m of plant material forms just 1m of coal.
Thin sections of coal magnified x 6.3 showing how the specimens are made up of organic layers.
A common sequence through the Carboniferous Coal Measures area is :
Types of Coal
Different types of coal can be formed depending on the original plant matter, and the history of the deposit - was it buried deeply, was it heated, or how long was it buried for?
In Wales different types of coal are historically associated with different regions: House coal in South Wales and North Wales, steam coal in Glamorgan, anthracite in the western part of the South Wales coalfield and cannel coal in North Wales. This indicates that each area has had a slightly different geological history.
These types of coal are all different and can be classified in many different ways.
Industrial users of coal use a ranking system based primarily on the on the amount of carbon (C) and volatile gasses such as hydrogen in the specimen. The table below, based on South Wales coals, shows how the amount of carbon increases from peat to anthracite. The amount of volatile gasses generally decreases down the list, such that anthracite, the best quality coal, has only 3-8% volatile gasses compared to house coal with 22-40% .
Peat 53 %C
Lignite 61 %C
Sub Bituminous 75 %C
Bituminous/House Coal 84-91 %C
Steam Coal 91-93 %C
Anthracite > 93 %C
Cannel coal , and other coals such as boghead coal, and torbanite, are part of a group called 'sapropelic coals' Instead of being formed from land plants they are formed from sapropel - slime and sludge layers of organic matter (mostly algal), that form in stagnant oxygenless environments on the floors of lakes and seas.
Scientific study of coal has shown that the remains of the original vegetable matter can be seen using high powered microscopic techniques. These can be divided up into different categories by their properties. Some of these names appear in the database, particularly with older specimens.
An early division was into three types: bright coal, dull coal and mineral charcoal (or mother of coal). Another system first devised by Dr M.C. Stopes in 1919 was into four types: fusain, durain, clarain, and vitrain. This coincided with the classification put forward by Dr R. Thiessen of the United States Bureau of Mines where coal is divided into anthraxylon - bright layers of plant material and attrius - darker debris material.
Today coal classification is based on the organic material seen using different optical microscopic techniques. There are three main groups: liptinite(or exinite), vitrinite and inertinite with many further divisions. e.g. in the liptinite group: alginite - tiny, fluffy cloud-like bodies, probably originally algae. Coals can be composed of just one group, or, more commonly, contain two or three groups and are named according to a standard scheme. For example: liptite is 95% liptinite coal, clarite is a combination of vitrinite and exinite, and vitrite is 95% vitrinite.
Magnified fragment of coal in reflective light showing layers of organic matter. (NMW 2001.9G.R216b Mag. x20)
Seam Names and Stratigraphy
In the past every colliery tended to have its own scheme for naming the coal seams it was extracting. Some collieries simply numbered the seams downwards whilst others used more descriptive terms: e.g. the 'Peacock seam'; the 'Big seam'; the 'Four Foot seam'; and of course the 'Bute seam' after the Marquis of Bute who controlled most of the South Wales coal fields.
The largest seams have now been correlated from colliery to colliery by the British Geological Survey and the National Coal Board, and a list of standard names has been developed. However, the nature of the original deposition means that across an area the seams can split or combine, or be completely absent, and there are often smaller un-named seams in the sequences. In this database the most up to date geological memoirs of the area have been used to give the official standard seam names where possible. The original name is recorded in the notes.
The stratigraphical chart shows a very generalised view of the coal fields in different parts of Wales:
Today coal is difficult to find in exposures in Wales as it easily weathers, and also as so much of it has been worked and removed. We are always interested in new exposures, particularly of named seams to add to the collection.
This database at present contains all of the specimens in the museum's Welsh coal collection. It does not at present contain related material from the coal measure sequences such as seat earth and fireclay, nor does it contain fossil material or minerals. It is hoped to expand the web site to eventually include these items (nearly 8,000 specimens), but in the meantime they can be accessed by contacting the museum directly.
The next addition to the database will be coal from the rest of the world.