STONE RESTORATION part 2

STONE RESTORATION PRACTICE IN PALESTINIAN TERRITORIES:

A CASE STUDY FROM JERUSALEM

part 2

DETERIORATIVE FACTORS OF STONES IN JERUSALEM

“Jerusalem stone” is an exceptional name known worldwide and is rated very high for its unique qualities, including both strength and durability ^[6]. The various techniques of local stone finishing reveal an intimate knowledge of the material, acquired by generations of stonemasons. The stone finishes range from rock-faced (with or without tooled margin) to pointed, crandalled and brush-hammered styles. The types of masonry, i.e. stone size and shape, also vary with the importance of the work, from polygonal dry-rubble masonry to finely cut ashlar work ^[7]. However, within a particular building, or even a neighborhood, the basic type of masonry construction will usually not change in style or in the quality of materials. It can differ, however, in its external surface finishing and in its jointing. The color of the local masonry follows the limited palette of nature, ranging from yellowish stone, with hues of red and brown, to all possible shades of gray limestone available in the Jerusalem region.

Most of the residential structures in Jerusalem’s Old City were built in Ottoman times (1516-1917), but there are still extant some important buildings dating back to the earlier Mamluk and Ayyubid eras ^[8]. The limestone used in most of these buildings was obtained from quarries situated next to villages located around Jerusalem, places such as Beit Hanina, Deir Yassin, A’nata and Hizma. The laboratory tests conducted at Birzeit University on stone samples from the area of Jerusalem show that this stone is a rock limestone ingredient that has an average compressive strength of 15.8 Mpa, an average density of about 2.08g/cm³ and an average absorption ratio around 7%. (Tables 1 & 2 provide more details about the density and absorption ratio of stones selected from different buildings inside the Old City of Jerusalem and from A’nata, the Jordan Valley, Birzeit, Bethlehem and Hebron areas).

The earlier restoration activities in the Palestinian territories concentrated on renovation of the plastering and/or the pointing between stones, doing necessary fillings where needed and replacing missing or corroded stones in the building façades. This process took no regard of the physical, chemical and mechanical nature of the existing stone material, and because of this the results were often very modest and impermanent. The past and present treatments of deteriorated sections of stonework involve two basic steps:

-          Extraction of the damaged parts, either totally or in part, and replacing them with other parts having similar dimensions, texture and color but without perceiving their geological characteristics. This manner of treatment depends, at its best, on the craftsman’s accumulated experience and his knowledge in classifying stones. At worst, it follows a random selection of materials, without paying attention to the stones’ origin and to what extent they correspond with the existing ones.

-          The use of handmade mixtures (mortars, fillings, sealants, etc.), all prepared without sufficient knowledge or definition of the specific qualities either for the existing stones under restoration or the new stones to be used in this restoration.

According to Khamaisi ^[9], 29.5% of the residential buildings in the Old City are currently in need of rehabilitation, and 11.4% of the residents require spatial expansion. Indeed, he explains that just 18.1% of current residential structures are actually suitable for living, which means that about 80% of all Old City residences require major rehabilitation, urgent maintenance or total revitalization! Unfortunately, the same general state of disrepair exists among public and religious buildings.

Our observations were conducted on selected public buildings with different functions situated within the Old City of Jerusalem: the Al-Ashrafeyyah School (Mamluk period), the Armenian Museum (Ottoman Period), the Spafford Children’s Center (mixed Ottoman and British Mandate periods), and Bab al-Silsila (Mamluk period). The observations of the stones visible on the exteriors of these structures show that these four buildings suffer from several causes of weathering and decay of stone masonry, the chief ones being salt crystallization, fracturing, crust and incrustation. Several different studies: Honeyborne ^[10], Zagari ^[11], Camuffo et al. ^[12], Camuffo et al. ^[13], Bonazza et al. ^[14], Rothert et al. ^[15], Smith et al. ^[16] explain that each of these phenomena has a recognizably different effect on the stone material. Some lead to a loss of substance of the stone while others lead to a disfigurement or disruption of the stone masonry, but not necessarily accompanied by any loss of substance.  The site visits and observations in the Old City of Jerusalem discovered evidence of the following causes mentioned, as follows:

Salt Crystallization: This was observed in all four of the buildings inspected. They all are exposed to humidity, which leaks through the stone walls due to the phenomenon of capillarity. The key factor is the differential composition (when considered in cross-section) of the traditional walls in the city: a rubble core between two layers of stone forming the wall faces, internal and external. Consequently, the salty crystals appear on the outer surface of the stone wall, forming a crystallized mineral layer. This is the crucial process creating damage to the main external surfaces of the buildings in the Old City of Jerusalem (Fig 2).   Fracturing: This damage appears in two of the buildings, due to repeated natural or man-made destructions and continuous population growth, i.e. the increasing pressure of ever more residents and the “life load” they are producing. In addition, improper construction techniques, specifically the thickness of the mortar filling and the laying of the building stones without regard to each stone’s geological composition (i.e. its pattern of stratified layers) has aggravated this kind of damage (Fig 3).

Crust and incrustation: appears in three of the buildings, apparently due to the absence of laws forbidding or at least limiting the movement of vehicles within the Old City, in view of the polluting emissions they spew forth. In addition, certain establishments such as bakeries serve to escalate such damage. All of this is in addition to the other environmental impacts on stone typical to a large city like Jerusalem (Fig 4).

It is already possible to conclude here that, among all the factors explored above, there is one common denominator: the crucial impact of humidity on the present status of the stones of Old Jerusalem. Therefore, the laboratory experiments in this study have concentrated on conducting an analysis of porosity, expressed as density and as water absorption ratio, for three groups of materials: stone samples taken from existing stones in use in the Old City; stone samples taken from the quarries supplying most of the stones used in the present restoration activities in Jerusalem; and finally materials mixed from various substances and used for restoration purposes in this city.