2 CRITERIA

The following criteria may be considered when choosing a compensation method for stone. The ideal compensation method:

• should be reversible;
• should not require removal of original material for its application;
• is inert;
• will not introduce soluble salts, highly alkaline or acidic materials, or mechanical stresses to the substrate;
• has less strength than the original stone;
• meets health and safety standards such as building safety codes;
• is cost effective;
• meets aesthetic requirements; and
• has desirable working properties.

While this generic list applies to any stone compensation, those exposed to outdoors are subject to additional criteria, such as:

• UV stability;
• durability on exposure to cyclic relative humidity, temperature, precipitation, and freeze-thaw cycling;
• ability to set (harden) in the treatment environment; and
• ability to emulate the same physical properties as the stone substrate. These properties include: appearance, water (liquid and vapor) absorption and exchange (as controlled by porosity and permeability), coefficients of thermal and hygric expansion, compressive and tensile strength, and modulus of elasticity.

An outdoor fill must possess similar physical properties to the original stone in order to allow equivalent exchange of water across the stone-compensation interface and to react to the environment in a compatible manner. If a fill is less porous than the stone, water and soluble salts can accumulate around it in the stone, leading to damage. Further damage can result along this interface if the stone and the fill have different dimensional responses to moisture and temperature. If the fill is harder than the original stone, the stone will be preferentially eroded. Since an exact match to the substrate is difficult to achieve, the fill should be somewhat more porous, more permeable, and slightly weaker than the original stone. Such a fill is “sacrificial,” attracting moisture and salts and thus causing disintegration of the fill instead of the stone. This condition is especially important when the stone is already extensively degraded. Discussion of these properties may be found in several sources (Peroni et al. 1981; Weber and Zinsmeister 1988; de Masy 1992).

It should be noted that in architectural contexts, certain specifications based on governmental legislation and other established standards must also be obeyed in conservation treatments. These include local building codes, historic preservation guidelines, and standards from organizations including the British Standards Institution and the American Society for Testing and Materials (ASTM 1980).

The above list is probably incomplete, and it is presented only as an ideal. For the purposes of this article, it may be used as a gauge by which to judge the acceptability of treatments in particular contexts.