Harrow and Hillingdon Geological Society

Previous Meetings

To the Mines of the Pharaohs

Andrew Shortland

Centre for Archaeological and Forensic Analysis,
Cranfield University.

The three substances with which the speaker was concerned were:

1. Alum, Pickeringite and other magnesium sulphates which are used in dyeing, cosmetics, tanning and aftershave.
2. Turquoise CuAl6 (PO4)4 (OH)8 .5H2O used primarily for jewellery and other adornments.
3. Natron used in mummification, putrefying and washing. Many of the uses remain the same today as they have for thousands of years. Natron is also used as a flux in glass making.

The cultivated area of Egypt is associated with the River Nile and the width of fertile land varies from 100 yards in some areas to between 5 and 6 miles in others.

The alums are obtained at two oases at Kharga and Dakhla approximately 150 miles from the Nile. Turquoise is mined from the Serabit el Khadim mines in Sinai and natron is extracted from the Wadi Natrum sited approximately halfway between Cairo and Alexandria. There are two roads from Cairo to the oases, one essentially following the Nile while the second is a desert track but free from checkpoints and military vehicles unlike the “Nile” road. The area crossed was a white desert of limestone terrain covered in mushroom-shaped formations. The party crossed the edge of the sand desert in three land rovers and camped in tents. Their biggest concern was to check for the absence of rodent tracks, snakes and scorpions. Kharga and Dakhla are about 150 miles from the nearest water and hence there is very little life there, not even mosquitoes. The trips were designed to coincide with the long Oxford summer holiday and in August daytime temperatures are 110^oF and 75^oF at night.

The scarp slopes in the region indicate the presence of artesian wells but any water now is very deep and can only be obtained by powerful pumps. The alum forms in veins in the silts and clays and the bands are often yellow in colour as this is an iron-rich area. Much of this alum is used for tanning. There are also deposits of gypsum and these are differentiated by “licking” the sample as the alum has an acidic taste not unlike lemon. The mine also contained pieces of ancient pottery. At Kharga is a Roman fort and settlement area dating to early Roman times. There are masses of ancient alum mines which have been scraped and contain a huge amount of human spoil. Some of the alum or MgAlSO₄ is pink in colour due to contamination with manganese.

Turquoise is used in jewellery but is extremely important in Egyptian iconography, as is lapis lazuli. Both minerals are blue or blue/green in colour and are a symbol of rebirth, possibly in association with the fertility of the Nile. CuAl₆ (PO₄)₄(OH)₈.5H₂O is associated with Sinai although other sources came from Cyprus, Greece and Sardinia. Between 1500-100BC, the Egyptians built a temple to Hathor, the Goddess of childbirth and the lady of turquoise. It was constructed on top of the mine, and this temple was an exception as it was not near the Nile. The mines date from 3000 BC to the present and are set in the side of the hill. The turquoise appears pink when it is extracted from the ground but it is only semi-stable and turns blue when polished.

Natron is obtained from a series of lakes in an area along the desert edge on the road from Alexandria to Cairo. Sodium carbonate is formed at approximately 60 ft below sea level. The lakes, which are 1.5-2 miles wide, are fullest in March/April from winter rain. The pH is 9-10 and the water is full of salts which precipitate in summer forming pink and white crusts, although the deposits never dry completely. Natron does not produce much sodium carbonate but forms deposits in polygonal ridges. Another lake is mostly halite with some sodium sulphate, but all the lakes are very impure. During winter, water enters the lakes from run off but mostly from springs. The less soluble salts are at the edge and bottom and the most soluble are at the top and centre, however there is no concentric pattern of salts, it is much more complex. The springs tend to dissolve the old crust and the water becomes so concentrated that the minerals precipitate at both the top and bottom of the water. Eventually, the crust becomes solid and continues to form leaving a very complex structure.

Glass and the deposits of the Wadi Natum.

Glass was originally produced from sand. The temperature required for quartz to melt was 1710ºC but the Romans added 25% sodium carbonate and reduced this temperature to 1250ºC. By using natron it reduces the temperature to 1100ºC. Glass samples found in tombs was very impure. Natron levels fell to a minimum in the 6th century and plant ash was used as an alternative flux in glass making during this period.

Why did the natron levels fall? There are several possible explanations, such as trade variation or war but mostly likely was the change in the lakes. A wetter climate put more water in the lakes, reduced salt production and being cooler reduced evaporation. Oxygen isotopes from the Dark Age Cold Period (DACP) showed that it was much cooler between the 3rd and 4th centuries. There was also the 536 AD event, from which tree-ring evidence showed vast amounts of dust in the atmosphere possibly as the result of a comet or volcanic eruption. The effect was experienced for 20-30 years. The dust veil event lead to famines, bread shortages and the Justinian plague of 541 AD started in Egypt and killed 25% of the population of Egypt. In Constantinople 300,000 people died together with 40% of the population of the Mediterranean basin by 600 AD.

The speaker, who was a graduate from Oxford, had his first degree in geology and his masters in archaeology giving us a unique approach to both the geology and historical background of Egypt. It was a well-illustrated and extremely interesting talk which provoked a lot of questions.

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