1987/1 p2

Epsom Well water

The medicinal properties of Epsom well water are usually regarded as being due to the presence of dissolved magnesium sulphate. This substance in the crystalline state (MgS04.7H20) is appropriately known as Epsom salts. One can purchase this chemical without restriction. Medicinally it is used as a purgative, but it has some other uses such as the treatment of soils which are poor in magnesium content. Epsom well water also contains iodine in the form of the iodide ion and quantities of calcium. Inorganic salts such as magnesium sulphate are highly ionised in solution and their concentrations are often expressed in terms of the ions.

The well is situated on the outcrop of the London Clay. This clay was formed as a mud on the floor of a sea which covered much of South East England about 40 million years ago. It is normally a dark bluish grey colour when freshly exposed. At depth it is extremely wet. This was shown recently in excavations carried out in Christchurch Road, Epsom. The colour is mainly due to finely divided pyrite, a mineral form of iron sulphide. In many places London Clay contains considerable quantities of calcium sulphate which crystallises as the mineral, selenite, a form of gypsum. Colourless crystals of selenite, for example, can be picked up on the beach below cliffs of London Clay along the North Kent coast.

Magnesium is the eighth most common element in the earth’s crust. It is always present in sea water, usually in the form of the chloride. Attempts have been made to extract it from sea water by electrolytic methods. It is, however, easier to obtain the magnesium from salts left in ancient dried up sea beds.

The actual magnesium ion is a very small particle indeed with a radius of 0.065 of a nanometre (which is 10-9 metre). It has a strong positive charge and can be attracted by clay particles, which, in the very fine state, carry a negative charge. It is thus possible for magnesium to be trapped in clays for release at a much later date. The sulphate part of the Epsom salts is almost certainly produced by oxidation processes of the sulphide in the pyrite. Since magnesium is very soluble in water it rarely forms crystals naturally, certainly not at Epsom. However, there are a few locations in Canada and the United States where natural crystals do occur and go under the name of Epsomite. The well-known salt locality, Stassfurt in East Germany, yields a mineral known as Kieserite (MgS04H20). In the past this was converted into Epsom salts.

Unlike magnesium sulphate, calcium sulphate is much less soluble in water. When it is present in the London Clay fossil shells are absent. It is thus reasonable to conclude that the calcium has come from the shells since their main constituent is calcium carbonate; it is possible that they are responsible for supplying a little of the magnesium needed for the Epsom salts.

I have not seen any record of magnesium sulphate having been extracted from the Epsom well water direct. It would be a tedious process to do so. One could, however, extract the magnesium and the sulphate parts separately and then join them together again. It would hardly be a viable process, since there are other much cheaper sources.

It should be mentioned that the Epsom well is not unique as magnesium sulphate is present in water elsewhere in the district.

Richard Butler