How surface luminescence dating (SLD) works – SLD dating of several famous Egyptian monuments

In this post we present selected parts of the very interesting paper titled “Surface luminescence dating of some Egyptian monuments“, by Ioannis Liritzis and Asimina Vafiadou (2015).

“Physical methods for the determination of age of stone structures (monuments, altars, temples, monoliths, buildings, cairns, field walls, mortars etc.) almost always use material associated with the construction period, that may contain 14C datable material rather than material directly from the fabric of the construction. However, in many cases, appropriate organic debris is either not available, or the association with the archaeology is insecure. The direct dating of stone surfaces has been an ongoing subject of research since its first application, until today, and it is coined surface luminescence dating (SLD).

Map of Egypt from GoogleEarth. The sites are pined in the map

“Though archaeological dating in Egypt relies on written sources, there are instances where the Dynastic chronologies do not satisfy the construction age of some monumental structures. Here, we have applied SLD to a selection of six Egyptian monuments for revisiting their dating in the light of current opinions”

The surface luminescence dating (SLD) works as follows: during the process of the preparation of stone blocks (cutting and carving, or sculpturing) and prior to the setting one upon the other (or construction of a building), the solar radiation (UV and optical spectrum) bleaches the stored geological luminescence in the carved stone surface, down to a depth determined by the depth of penetration of light in that material. This exposure – just minutes required for quartz and feldspar bearing rocks used here i.e. granite and sandstone – erases the luminescence to a zero or a near zero residual value. On construction, shielding of the surface occurs and re-accumulation of (archaeo-) luminescence is initiated due to irradiation from ambient radioactivity and continues till excavation and measurement.

In fact, the decay of natural radioactivity viz. uranium, thorium, potassium and rubidium along with cosmic rays, provide as a first approximation a constant irradiation field. The minerals in stonewall are therefore irradiated at a constant rate, and hence, acquire latent luminescence at a constant rate. The latent luminescence is released upon exposure to light, setting the signal to zero or near zero, whence the trapping process begins anew. Events which zero the pre existing geo- or archeo-luminescence are intentional (construction) or accidental (seismic events, destruction, that follows sediment cover) exposure to daylight which provides sufficiently energetic photons to induce zeroing.

In the laboratory, the same process is mimicked. The trapped electrons population can be measured by stimulating the crystal by heat (mostly up to ∼ 400◦C) or visible [mostly blue or green diodes or infrared (IR)] light. These stimulations lead to release of charges some of which eventually recombine with opposite charges and emit luminescence in either or all of ultraviolet (UV) and visible spectrum. The intensity of this light is proportional to the number of charges recombining and this in turn is proportional to trapped charges. This fact is exploited to convert light units to dose units.

The intensity of the emitted light is proportional to the concentration of trapped charges/electrons and hence, to the radiation dose. The relationship is proportional. The latent luminescence signal increases till a saturation of trapped charges occurs.

Complete eviction of electrons from traps of crystalline minerals is desirable although a residual unbleachable (residual) luminescence component often remains. Quartz and feldspars in monolayer are bleached within minutes of sun exposure, but in rocks, it needs dozen of minutes to zero the signal due to overlying layers. This is because sunlight penetrates the upper nano to micron scale depths easier, resulting in fast total bleaching, and goes further too attenuated according to Beer–Lambert’s law and other scattering phenomena more complicated, implying random transport of photons in matter through pathways and cascade effects. The direct optical transition to the conduction band (photoionization) gives rise to the near-exponential dependence of bleaching efficiency on photon energy. Theoretical calculations and experimental tests define the penetration depth of solar radiation that comply with experimental data on various rock types (marble, granite) i.e. a complete absorption at around 4–5 mm. However, reservations are made for the penetration as exposure time indicates slow bleaching at greater depths. For calcitic rocks, the bleaching is much slower in the order of hours to several dozens of hours, where a residual luminescence level is reached. The latter serves as the initial level upon which radiation growth builds up.

Laskaris and Liritzis have produced a generalized approach for the bleaching of luminescence signal as a function of depth for every surface rock (marble, marble schist, granite), promoting the functional behavior of cumulative logarithmic or normal distribution type of error function and attributing to the variable coefficients a physical meaning. The construction of a particular equation unique for each material exposed to sunlight versus depth and exposure time has been tested on various rock types and data sets inhering variable errors, that at the end, offers a new way to surface luminescence dating and authenticity.”

The temple of the Sphinx

“The dated samples derive from the following sites:

1. Abydos (Seti A’ Temple and Osirion) (RHO-109, Seti I; RHO-110,Seti II; RHO-111 Seti IV; RHO-136, OS3; RHO-137, OS5; RHO-138,OS6; RHO-139, OS7);

2. Giza: Valley Temple (RHO-98, VT1; RHO-103, VT6; RHO-105, VT8;RHO-106, VT9a; RHO-107, VT9b);

3. Giza: Sphinx Temple (RHO-55, ST1; RHO-56, ST2; RHO-57, ST3;RHO-58, ST4; RHO-59, ST5);

4. Giza: Osirion Shaft: (RHO-53, OT1; RHO-54, OT2);

5. Giza: Menkaure’s Pyramid: (RHO-119, MYK);

6. Fayum: Qasr-El Saqa (RHO-129, QAS1; RHO-130, QAS2; RHO-131,QAS3);

7. Kings Valley: Khasekhemui Tomb (Kings Valley) (RHO-132, KH1c;RHO-133, KH2c; RHO-134, KH1)”

The Osirion temple

“The choice of applying luminescence dating to several constructions on the Giza plateau is the intriguing nature and probable (re-)use of them that may mislead construction age. However, current archaeological opinion is that they were built under the auspices of the Fourth dynasty pharaohs Khafre, Khufu and Menkaure. This has been firmly established through the historical record and subsequent discoveries of cartouches at the site. However, the discoveries of cartouches and funerary evidence from earlier dynasties, clearly suggests that parts of the site may have been re-used, and it is a reasonable assumption that some structures were already present at Giza when the large-scale works of the fourth dynasty began.

Today, the traditional theory prevails, that Giza was built as a funerary complex for the 4th Dynasty pharaohs. However, the lack of contemporary human funerary remains from any Egyptian pyramid and the obvious astronomical and geometric nature of the site, that prove their orientation was not by chance but inhere knowledge and star configuration patterns at the period of construction) imply that the “pyramids as tombs” theory is no longer sufficient and a broader determination of age, function and re-use of both Pyramids and Giza is required. The Old Kingdom monuments are a mystery and conventional dates have been questioned and critically discussed.”

“The Sphinx Temple seems to be dedicated to the Great Sphinx, but we know very little about this, because there is no textural evidence (…) The luminescence ages concur with the swayed opinion of a 3rd millennium BC age with an indication of an early 3rd mill. BC and a possible later reuse (intrusion?) during the 13th century BC.”

The Valley Temple is next to the Sphinx (…) The buildings of the Valley Temple and the Dead Temple were repaired and modified later by priests operating in the temple during the 5th and 6th dynasty, according to Reisner. Therefore, later finds and repairs may have actually occurred that mislead their construction ages. A verification of these assumptions has been acquired by SLD method. Luminescence ages gave an early 4th mill BC (3000 ± 420BC) and a New Kingdom one (1050 ± 540 BC, 19th Dynasty).”

“Along the Chephren Causeway lies the Osiris shaft around 35 m below Giza plateau that consists of three carved underground levels with chambers (…) The obtained SLD ages of two sarcophaguses pinpoints to a 4th dynasty age in contradiction to a 16th or early 7th century BC, according to archaeological findings inside. It is still puzzling the difference that may explainan early construction subjected to later reuse activities.”

“The third group pyramid of Giza located in the southwestern corner of the area attributed to Mykerinus (Menkaure) by Herodotus and Diodorus Siculus. It was constructed of limestone and granite. The first sixteen courses of the exterior were made of granite. The upper portion was cased in the normal manner with Tura limestone. The lower part made of granite has smoothed facings, and sampling was made here. The pyramid’s date of construction is unknown, because Menkaure’s reign has not been accurately defined, but it was probably completed in the 26th century BC. The luminescence age inheres a large error that falls within the early 3rd millennium BC.”

Khasekhemwy tomb

“At Abydos, Khasekem the last king of the 2nd Dynasty changed his name to Khasekhemwy (“the appearance of two powers”) apparently after the outcome of political struggle for succession. His tomb at Abydos is a significant departure from the square tomb, in a long and irregular pit, divided into forty warehouses.The obtained luminescence age (3100 ± 660 BC) of the rectangular limestone tomb gave a date in agreement to epigraphic and historical evidence (2nd Dynasty, 2890–2686 BC).”

“The Osirion at the back of the temple of Seti I (1294–1279 BC) is at a lower level and in direct contact with temple. It is a cenotaph and designed to give the impression of an underground mountain or island surrounded by water channels. While there is disagreement as to its true age, despite the fact that it is situated at a lower depth than the structures nearby, that it features a very different architectural approach, and that it is frequently flooded with water which would have made carving it impossible had the water level been the same at the time of construction, Peter Brand says it “can be dated confidently to Seti’s reign”. The luminescence ages for Seti I Temple gave 1550 ± 200 a concurrent age to archaeological opinion, but one of 3rd mill BC on a sandstone cast doubts. The former is confirmation of textural evidence carved on the sandstone.

Regarding Osirion, of the two dates one on sandstone with a large error falls within Seti I reign (1300 ± 500 BC) and the other on granite and low error (1980 ± 160 BC) indicates an earlier by some hundred of years age, and comes from the older part of the temple. The latter implies a somehow earlier construction age of part of Osirion. However, one has to bear in mind that it cannot have been later than about 1800 BC because no building of this kind took place in Egypt between 1800 and 1500 BC due to social collapse. Therefore, the builder had to be from the 12th Dynasty of the Middle Kingdom. There were 12 pharaohs in that time, and the accepted dates of this dynasty were 1976–1793 BC. Egyptian governance and construction did not then recommence until about 1500 BC, i.e. at the New Kingdom. It is known that Seti I carved the sandstone and his reigning dates were 1290–1278 BC.”

“The Temple at Qasr-el-Sagha is a small temple and without inscriptions, 8 km north of the lake Birket Qarum, the front end of an horizontal plateau about 34 m above sea level in the northwest of the Fayum (…) Nothing is known about the pharaoh who built the temple or the god to whom it was dedicated. Sherds found near the temple covering a period from the Ancient Kingdom until the Ptolemaic, Roman and Islamic times. Arnold argues that the building dates to the Middle Kingdom. Our dating result does not solve definitely the problem but contributes to that. It indicates an earlier construction re-used probably during much later Ptolemaic times.”

Temple at Qasr-el-Sagha

“Concerning the luminescence measurements, OSL and TL measurements made on particular rock types, that is, granites and sandstone, which comprise mainly of quartz and feldspar, and limestone with or without traces of quartz, has proved the potential use of these materials for dating in archaeology (provided that ancient monuments were made of these). This result is based on the well-known quartz and feldspar solar bleaching of sedimentary deposits. The most rapid bleaching of the optically stimulated electron traps is observed for sandstone, followed by granite, while for the thermal stimulated electron traps the faster bleaching is for granite followed by sandstone. The granite with quartz, feldspar and biotite (e.g. in Mykerinus) bleaches slower than granite with its two-grain phases, mainly feldspar with little quartz and biotite (e.g. in Osirion) (…)

The criteria applied for dating purposes, that is, the solar bleaching and the radiation dose growth (either in additive or regeneration mode), are both well satisfied with thermal and optical stimulated luminescence, for three rock types, while the calcites studied follow the known behavior of TL bleaching verifying earlier studies (…)

The potential dating of ancient monuments made by carved granites, sandstones or limestones, by TL and OSL methods is reconfirmed. The solar set zero-clock of TL in granites, sandstones and the residual for limestones offers new applications to luminescence of dating ancient megalithic buildings made of these materials.”

Research-Selection for NovoScriptorium: Philaretus Homerides

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