3. PERFORMANCE REQUIREMENTS FOR CONSOLIDANTS

Performance requirements for stone consolidants have gradually emerged as understanding of the factors controlling their performance has improved. These requirements can be divided into two major categories. The first category, which we will term primary requirements, consist of invariable performance requirements that consolidants must fulfill regardless of the specific application. In the remaining category are secondary performance requirements. These are requirements imposed on a consolidant by a specific application. The distinction between the two categories will become clearer as they are discussed.

3.1 Primary Performance Requirements

Primary performance requirements for stone consolidants are applicable to essentially all stone consolidants regardless of the specific use. These requirements are based on the premise that the main functions of such materials are to restore the cohesion, physical properties, and appearance of a deteriorated stone to near its original condition. Considerations are given in the requirements to the necessary levels of performance of consolidants for consolidation, durability of the consolidated stone, depth of penetration, effect on stone porosity and moisture transfer, computability of consolidant with stone, and effect on appearance. These performance considerations could become the basis for stone consolidant specifications.

3.1.1 Consolidating Value

The most important function of a stone consolidant is to re-establish cohesion of the particles of deteriorated stone [1-4]. Methods which have been used to obtain a qualitative assessment of the consolidating value of stone consolidants include the measurement of the tensile strength of treated unweathered stone [25, 53], measurement of surface hardness of treated weathered stone [54,55], and the abrasion resistance of treated unweathered stone [56]. A more quantitative test needs to be developed, which incorporates the testing of standard deteriorated stone. A standard deteriorated stone may merely be finely crushed stone, which is then treated with consolidants and its tensile strength measured. The tensile strength of the treated crushed stone could be compared to the strength of the original stone.

Only one recommendation was found in this review for what constitutes an acceptable performance level for consolidation value. Gauri has recommended [54] that the compressive strength of a treated weathered stone should be at least 10 percent above that of the untreated and unweathered stone. However, increasing the strength of weathered stone substantially above that of unweathered stone may accelerate the decay of the unweathered stone [68], unless the complete structure is treated with the consolidant.

3.1.2 Durability of Consolidated Stone

The consolidated stone should generally be as durable as the unweathered stone. If the consolidated stone is substantially less durable than unweathered stone, it may be advantageous to replace the deteriorated stone with new stone. In addition, treated stone should weather (in terms of appearance) at nearly the same rate as the original stone, to retain, as closely as possible, the authentic appearance of the stone. Torraca [l] has suggested that it is not justified to demand that a consolidant should protect a stone forever from any environment.

Durability of a consolidated stone is dependent on several factors, including the durability of the consolidant, compatibility of the consolidant with the weathered stone, properties of the consolidated stone, and the environment. The complex interaction among these factors makes the development of appropriate accelerated durability tests difficult. A methodology is given in ASTM E-632 [69] which can be useful in the design and development of rational accelerated durability tests for consolidants.

3.1.3 Depth of Penetration

Past experience with stone consolidants has shown that their ability to penetrate weathered stone is one of the main factors controlling their performance [3, 16, 25, 57-60]. Superficially-penetrating consolidants tend to fill the pores of stone surface layers, thereby reducing the permeability. This may result in the accumulation of moisture and salts behind the treated layers [58, 61]. Furthermore, interfacial delamination often occurs because of a marked difference in the thermal properties of the treated and untreated stone [10, 11, 58]. Price has suggested [58] that a good consolidant should be able to penetrate a weathered porous stone to a depth of at least 25 mm. This should result in a gradual transition in the thermal and mechanical properties from the exterior treated surface to the inner layer of untreated stone. A slightly different requirement has been given by Torraca [1]. He proposed that a stone consolidant should penetrate a weathered stone to a depth that all incoherent material is solidified and attached to the sound core of the stone.

Properties of a stone consolidant which affects its ability to penetrate a specific stone at a given temperature include its viscosity [57, 59, 61], surface tension [59], the rate at which gel or precipitate is formed [16], method and conditions of application [2, 63], and rate of evaporation of any solvent [66]. Tammes and Vos [67] developed the following relation for the horizontal transport of liquid through porous materials;

              1/2
          s     1/2       1/2
    X =  -----    t      = At
          2N

where X is the displacement of the liquid front, s (sigma) is the surface tension of the solution, r is the average capillary radius, N is the viscosity of the liquid, and t is time. A is a measure of the permeability of the porous material to a particular liquid. Munnikendam [59] showed that this equation can be used to estimate the penetration ability of a consolidant into a specific stone.

This survey revealed that a need exists for the development of a standard test to measure the penetration of consolidants into stone. This standard test should specify the testing temperature and relative humidity, condition and size of the stone specimen, methods for applying and curing consolidants, and techniques for measuring the depth of penetration.

3.1.4 Stone Porosity

As previously mentioned, the porosity and pore size distribution of a stone can have a major effect on its durability. For example, the resistance of a given type of stone to frost damage and to salt damage decreases as the proportion of fine pores increase [11, 33]. Therefore, a stone consolidant which reduces the size of large pores but does not close them may be harmful. For example, Dukes [2] found that silicone ester consolidants decreased the frost resistance of portland stone used as gravestones by increasing the number of small pores.

A method which can be used to determine the effects of a stone consolidant on pore size distribution was described by Clifton et al. [70]. In their work, the pore size distributions of both impregnated and unimpregnated hardened cement pastes were measured using a mercury porosimeter. Both pressurization and depressurization studies were performed. Pressurization results give an indication of the total open porosity and pore size distribution. Information on the shape and continuity of pores is obtained by depressurization work. Pressurization studies on stone have been performed by Biscontin and Pavan [71] and by Alessandrini et al. [72].

3.1.5 Moisture Transfer

Many stone preservatives and stone consolidants have performed poorly because they form a surface film which impedes liquid water migration through the treated stone, but allows water vapor to pass. This can lead to a situation where water evaporates behind the treated stone leaving deposits of salts [59, 61]. Because water vapor can pass in and out of the stone, the deposited salts may rehydrate and possibly be converted into different crystal forms. Further, larger crystals may grow through the dissolution of small crystals and reprecipitation on larger crystals. These processes may result in the disruption of the microstructure of the stone and breakdown of cohesion between stone particles [14]. Further, impeding the passage of water vapor may increase the susceptibility of a stone to frost damage and to thermal shock.

If a material produces a film which prevents the passage of both water and water vapor, large amounts of moisture could accumulate in a structure. In addition to decreasing the resistance of the stone to frost damage, excess moisture in a structure could cause the rotting of wood, corrosion of metals, and degradation of plaster and roofing materials.

Munnikendam has suggested [73] that a consolidant should be hydrophilic to allow moisture to pass through the treated stone. In addition, he suggested that the water vapor transmission of treated stone should not be decreased by more than 30 percent compared to untreated stone.

The water absorption and water vapor permeability of stone, untreated and treated, can be measured by following the procedures of ASTM C97 [74] and ASTM C355 [75], respectively.

3.1.6 Compatibility of Consolidant with Stone

Experiences with stone consolidants have demonstrated [16, 61] that they should be compatible with stone to form a durable composite. Specific compatibility requirements should include the following:

1. Cured consolidants should have thermal-dimensional properties similar to those of sound stone [16, 73]. Otherwise, delamination of the consolidated stone from the untreated stone could occur [76], especially if the stone is subjected to thermal shock. In addition, the cured consolidant should not become brittle [16, 76, 77].
2. The consolidant should not severely disrupt the microstructure of the stone [16]. For example, if the crystals formed from the precipitation of an inorganic consolidant exhibit crystal growth sufficient tensile stresses may be produced to cause the development of microcracks, and ultimately macrocracks in the matrix. Further, Marsh [50] pointed out that replacing a constituent of stone with another of a larger molecular volume can cause dilation stresses leading to cracking. The effects of a few inorganic consolidants, e.g., hydrofluoric acid, depend on their reacting with the constituents of the stone to form insoluble products. Many conservators [5, 16, 76, 78] are opposed to the use of such materials because the reaction products usually fill the voids and pores, thereby sealing the surfaces.
3. Consolidants should not form by-products which can be harmful to the stone. Many of the inorganic consolidants are precipitated as a result of the reaction between two dissolved salts [1, 16, 50]. In addition to the precipitate, at least one soluble salt is formed, much of which is deposited in the stone as the water evaporates. These soluble salts can damage the stone through recrystallization processes and/or produce unsightly efflorescence on the treated stone's surface.

3.1.7 Effect on Appearance

Ideally the application of a stone consolidant would not cause any change in the appearance of a stone. Most inorganic consolidants, however, produce a white deposit within the voids and pores of a stone, the color of which may not match the color of the stone. While organic consolidants usually form transparent polymers when cured, they can change the reflective properties of a stone [79]. Further, the optical properties of organic polymers may gradually change because of their degradation by photochemical processes, oxidation by oxygen and ozone, and attack by air pollutants.

The permissible extent of change in appearance accompanying the use of a consolidant is probably best dealt with by the conservator responsible for the preservation of a structure. This is because the extent of the change in appearance is partially controlled by the interactions between a consolidant and a specific stone, and also by the environment. Furthermore, the location of the deteriorated stone within a structure, the extent of stone deterioration, the nature of the change in appearance, and the importance of a structure should be considered in deciding what constitutes a permissible change in appearance.

In laboratory evaluations of the performance of stone consolidants, the change in appearance of a standard stone specimen should be quantitatively determined. The preparation of test specimens, exposure condition and evaluation methods should be standardized. Then the results of such a standard test could form the basis for selecting promising materials for specific applications. Gauri et al. [54] developed a laboratory performance test for organic consolidants in which the absorbency of a polymer at 254 mu was compared to that of bisphenol-A epoxy resin.

Several methods can be used in the laboratory and in the field to measure the change in appearance of stone caused by consolidating materials. In a laboratory evaluation of preservatives, Sleater [12] instrumentally compared the color and gloss of treated and untreated stone specimens following the methods given in ASTM D 2244 [80] and ASTM D 523 [81]. He also recommended that a visual estimation of color differences be made. The Munsell color system [82] can be used in both the laboratory and field to determine color changes. Winkler [83] has developed a promising method to measure rapidly the reflectance of stone using a photographic light meter and reflex camera.

3.2 Secondary Performance Requirements

Secondary performance requirements are those requirements which may be imposed in addition to the primary performance requirements because of specific preservation problems encountered at certain structures. For example, the leading cause of stone deterioration in England is salt crystallization [24-25]. Therefore, in England the capacity of stone consolidants to encapsulate salts or to otherwise mitigate the effects of salts is an important consideration in their selection. Arnold and Price [84] have observed that certain consolidants facilitate the extraction of salts from stone, which could be more important than their ability to immobilize salts.

Other attributes which may be required of stone consolidants in certain situations include the ability to prevent further microbiological growth, to greatly increase the resistance of stone to abrasion by foot traffic, and to rebond large stone fragments.