Durable and readily available, stone is a basic material used in art and architecture. Stone sculptural objects date to “goddesses” of fertility, carved over 40,000 years ago. Basic traditional stone types include sedimentary sandstone, limestone and travertine; metamorphic marble, slate and schist; as well as igneous basalt and granite. A wide variation within each stone type exists, effecting mineral composition, grain and crystal size, pore structure, color and texture. These variations in turn affect stone compressive and tensile strengths, chemical resistance, water absorption rates, thermal expansion coefficients, and salt and frost resistance.
Atmospheric pollutants, thermal cycles, freeze-thaw cycles, fatigue, inappropriate design, misapplication of materials and external stress lead to a host of detrimental conditions – soiling, grain loss, disaggregation, spalling, delamination, cracking, breaks and loss – each typically more endemic to basic type.
A carefully designed conservation treatment appropriately addresses the specific stone properties, the existing object condition, the environmental setting, the desired structural needs, and material compatibility. Testing, monitoring and logical sequencing of each treatment phase ensures appropriate material selection and application procedure. Allowing adequate time in the treatment schedule for material cure times is crucial, particularly when stone consolidation is required.