There really is no mystery to this question - oil lamps were the main source of illumination of work in tombs and detailing on the inside of dim temples (Clarke and Engelback, Ancient Egyptian Construction And Architecture, 1930, p. 201). These lamps are sometimes depicted on tomb reliefs and often take the form of an open receptacle with a base that can be grasped with the hand (see figure 5.8 in Arnold, Building In Egypt, 1991, page 218 for example). Dieter Arnold mentions distribution lists of linen wicks for work in Biban el-Muluk, and Kent Weeks has found receipts on ostraca for wicks among the rubble of KV5 (The Lost Tomb,1998).
Some lamps were little more than open dishes, floating a wick, with a pinched rim from which to pour the oil. As such lamps provide meager illumination, many must have been needed to have been effective. Olive oil is fairly clean burning, and the addition of salt may have help keep the soot at a minimum. It may also have been that the lamps were covered in some way to minimize smoke.
In some cases, torches may have been used. Examples have been found at Dier el-Medina. They are made of various woods and are about 10 to 15 inches in length and their tops were wrapped in resin-soaked linen. It has been suggested, however, that the torches may have been used in ritual processions in later times.
The use of mirrors is sometimes suggested, but there is no evidence that this method was in fact used. This method would seem impractical because the light beams must be constantly tracked with the relative motion of the sun, and also the path of the beam must be at all times kept clear of obstruction, the main source of which would likely be the workman himself.
Some have proposed the use of electric storage batteries as a possible source of illumination. Though I personally know of no examples found in Egypt, in 1936 a battery-like artifact was unearthed at Khuyut Rabbou'a near Baghdad. Found in the 2,000 year-old Parthian layer, the yellow clay jar, later dubbed the "Baghdad Battery," had a copper cylinder stuck into it, held in place by asphalt. The vase was about 6" high and the cylindrical tube (with a closed bottom made of sheet copper) had a diameter of 1" and a height of 3.5". Inside the cylinder was an oxidized iron. Similar examples were found in Seleucia (which had papyrus relics inside them) and Ctesiphon (which contained rolled bronze sheets).
Because of the similarity of the form of the relic to the modern dry battery (though the materials were different -- modern batteries employ a zinc cup and carbon rod), it has been speculated that the arrangement of components in the jars must have served as a kind of 'galvanic' function -- as a battery. Others argue that the objects may have been containers for blessings or incantations, which would have been written on organic material placed inside.
In order for a battery to be useful, it must generate a reasonable flow of electrons for a reasonable length of time. Models of the above "battery" have been tested using as an electrolyte a solution of copper sulfate. Such devices did work but only for a short time. More recently, the use of benzoquinone (which occurs naturally in beetles and centipedes) as an electrolyte has been tested with successful but short-lived results (the use of naturally occurring organic acids and sour fruit juices were found to be too weak). Because the copper element was sealed, the oxygen rapidly depleted and the current quickly decreased to negligible levels. Strong mineral acids (unknown at the time) would be required for the two metals to generate a useful current.
Experiments on a faulty model (with no bottom on the copper cylinder) have been more successful because oxygen diffused from the outside feeds the reaction and supports a continuous operation. For this reason, the "Baghdad Battery" is seen by some to be a faulty deviation form the other "batteries" found, which instead of a tightly close copper cylinder contained loosely rolled bronze sheets sealed only at the top and bottom. Thus, rather than just the cylinder, the entire vase would be filled with the electrolyte and, due to the porous walls of the vase, a constant supply of oxygen could diffuse into it.
Attendant to the question of whether the vases were used as power sources is the question of exactly what electrical apparatuses or processes they were meant to support. The most popular speculation is that they were used for electrotherapy or electroplating. Unfortunately, there is no archaeological or historical evidence to corroborate this.
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