(Image: Iwo Bulski, Pixabay)

Deterioration of the Stone of the Great Sphinx

ARCE Newsletter, no. 114 (Spring 1981), pp. 35- 47.

Reproduced by permission of the American Research Center in Egypt  (ARCE)

The following report on the deterioration of the stone of the Sphinx is a summary of a more detailed report now in the process of completion. This report is based upon my field survey of the Sphinx during March 30 – April 4, 1980, sponsored by the American Research Center in Egypt. The laboratory investigation of samples collected at the Sphinx was conducted in the Stone Conversation and Water Resources Laboratories of the University of Louisville.

The problem of stone decay in all portions of the Sphinx is intrinsically the same. For the sake of clarity of presentation, we have distinguished four components of the Sphinx stone: the bed-rock, the granite, the limestone blocks, and the mortar.

A. Bed-rock

A direct relationship has emerged from comparison of the degree of decay of the stone with its mineral composition. The degree of deterioration is readily apparent, as illustrated by Figure 1.

Figure 1 Front view of Sphinx
The compositions are expressed as percentage weight of the stone.

It is easily seen that the recessed rock units have undergone more deterioration than those which are projecting outwards. The composition of this limestone rock, as it is pertinent to the deterioration phenomenon, relates to its content of such water-soluble salts as halite (NaCl) and gypsum (CaS04.2H20) and to such clastic components as silt and clay minerals. The relationships that have emerged from these comparisons are:

1. The halite is present in larger quantities than gypsum in the more deteriorated rock units, while gypsum occurs in larger quantities than halite in relatively sound rocks, even though all rock units thus far investigated contain both halite and gypsum.

The soundness of rocks containing halite is affected adversely due to the ease with which halite, in the presence of liquid water or water-vapor alone, dissolves, recrystallizes and grows into larger crystals. As a result, large tensile stresses are generated, disrupting the rock. The source of moisture in the case of the Sphinx appears to be the subsurface water.

Gypsum, acting in the same fashion, but to a lesser degree than halite, is also a potential hazard. In certain instances, however, discussed in the section on “duricrusts,” the gypsum surface coatings, rather than being hazardous, have even protected the stone from decay.

2. All the rock units exposed at the Sphinx are extremely fine-grained limestones of the packed biomicrites type. They contain, in addition to the predominant calcite and above-described water soluble salts, some quantities of non-carbonate clastic fractions composed predominantly of clay minerals and very small amounts of silt.

Once again the sounder rocks contain smaller quantities of clay minerals (approx. 2.5 – 3.5%) as compared with less sound rocks which contain 5.5 – 8.5% clay minerals.

The clay minerals, due to an abundance of free valences at their surface, have an excessive ion-exchange capacity. As a result, chlorides and sulfates, etc., sequestered at the site of these minerals, generate more tensile stresses which fragment the rock.

3. The head region of the Sphinx is composed of limestone and is similar to the rest of the Sphinx. But these bands are not so thick and their content of NaCl, CaS04.2H20, and clay minerals is smaller. As a result, the head region has suffered much less deterioration, as seen from the 1925 photograph (Figure 2), taken prior to the restoration.

Figure 2 1925 photograph showing the south-east side of the Sphinx. This figure reveals a relatively less deteriorated head region as compared with the rest of the rock units of the Sphinx structure. Courtesy of the Center Vladimir Golénischeff.

The above considerations indicate that the occurrence of such salts as halite and gypsum is the main cause of the decay of stone at the Sphinx. The following examples further support this inference.

B. Stela

The Stela is made of granite which, in its original composition, is devoid of halite and gypsum. But the Stela is in contact with the bed-rock whence these water soluble salts have migrated into the Stela. The accompanying x-ray diffraction trace (Figure 3) clearly shows these salts; they are also visible to the naked eye as whitish encrustations or flakes that continually fall from the surface of the Stela.

Figure 3. X-ray diffraction trace of water soluble salts extracted from weathered chips exfoliated from the granite of the Stela

The condition of the Stela as seen in 1925 photograph (Figure 4) when compared with recent photograph (Figure 5) shows that the weathering of the granite has occurred to a greater degree at the portion of the Stela which is in contact with the bed-rock.

By corollary it may be inferred that the deterioration of the granite at the valley temple also is primarily caused by the same salts.

Figures 4 and 5. 1925-6 and, respectively, recent photographs of Stela to show that during the transitional period the weathering has occurred more on the south side (left) because the granite here is in contact with the bed-rock. Photograph 4 courtesy of the Center Vladimir Golénischeff.

C. Limestone Blocks

Limestone blocks of variable compositions were used in the construction and restoration of the stone veneer surrounding the lower portion of the Sphinx. From the point of view of weathering of stone, two varieties of such blocks may be distinguished: those with duricrust, and those which are without it.

1. Limestone blocks with duricrust. Duricrust is a natural surface coating (Figure 6), which protects the stone from weathering from within (by salts, etc.) and from abrasion by the sand laden winds. The duricrusts occur commonly on rock surfaces in the desert regions.

Figure 6 Showing side-by-side (at south flank of the Sphinx) some blocks with and others without duricrusts; the latter even though younger have suffered more weathering.

The evaporation at the stone surface during higher temperature results in the precipitation of salts which were dissolved from the body of the rock by underground water. Such coatings, termed duricrust, are highly impermeable and therefore, by inhibiting movement of water towards the surface, protect the rock from continual decay.

Commonly the duricrusts are made of iron and silicon oxides. The duricrusts observed at the surface of certain limestone blocks and surfaces of certain bed-rock have been found to consist primarily of gypsum (Figure 7), although we are continuing to determine trace elemental composition of these duricrusts.

Figure 7 X-ray diffraction trace of duricrust, showing that mainly is made of gypsum.
The limestone blocks with duricrusts are from a period earlier than the 1925-26 restoration. A detailed study of the composition of these blocks will yield information regarding selection of limestone for future restorations.

2. Limestone Blocks without duricrusts. Most such blocks, especially those used during and after 1925-26 restoration, have suffered extensive damage due to efflorescing salts. Figure 8 shows a stone block which was selected but not used in an apparently recent restoration. This block already reveals extensive peeling associated with efflorescing salts.

A detailed study of the composition of such blocks will yield information as to which type of blocks to avoid and the need to remove the potentially efflorescing salts from stone blocks selected for replacement at the stone veneer.
Figure 8 Unused block from a recent restoration, deteriorating due to efflorescing salts.

D. Mortar

One major source of the deleterious salts are certain mortars applied to bond the limestone blocks of the stone veneer. Such mortars have not only damaged the stone block in conjunction with which they were used but also, by penetrating behind the surface of certain well-preserved stones, they have literally exploded the stones (Figure 9).
Figure 9 Salts from the mortar have penetrated behind the duricrust causing its exfoliation.
Figure 10 shows the presence of large quantities of gypsum in the mortar employed during the December 1979 restoration. Obviously this mortar shall prove harmful to the stone. The use of such mortar to fill space between bed-rock and the veneer, as in the past, shall further aggravate the situation contributing to the accelerated decay of the stone blocks as well as of the bed-rock.
Figure 10 X-ray diffraction trace to show occurrence of gypsum in the new mortar


On the basis of the knowledge gained from the field survey and the laboratory investigation, it is my conclusion that the deterioration of the Sphinx is due to the presence of water soluble salts. Such salts were deposited in the limestone at the time of its formation and they have also been derived from mortars used in the construction of stone veneer at the lower level of the Sphinx.

These salts, by themselves, should be harmless provided they are not repeatedly dissolved and crystallized, for which moisture is essential. Therefore, the essential area of future study is the determination of the source of water and mechanisms of its movement through the pores of the stone. These studies will involve obtaining representative samples in a vertical profile from the top of the bed-rock down to the water table, and determination of such physical properties as porosity, pore-size distribution, and permeability of the samples. These investigations shall also form a basis for designing a preservative treatment of the Sphinx.

K. L. Gauri
Director, Stone Conservation Laboratory
Dept. of Geology
University of Louisville