TOOLING-INDUCED ALTERATION OF DIMENSION STONE: COMPARISON OF TRADITIONAL TOOLING AND COMPUTER-CONTROLLED METHODS

The exposed surfaces of dimension-stone elements used on facades of historic structures have traditionally been fabricated using hand tooling techniques. Experience has shown that weathering of ornate stone features typically progresses at a faster rate than for similar planar stone exposed on vertical surfaces, and after 50 to 100 years replacement of these ornate elements is commonly required. In addition to the type of stone and its location on a building facade, this weathering rate is thought to also be related to surface alteration associated with initial tooling, in combination with shape of the tooled surfaces as it relates to water run-off.

Both traditional hand-tooling methods and computer numerical controlled (CNC) methods, are now available to fabricate many ornate dimension stone elements. Hand-tooling methods include removal of stone fabric with sharp steel tools struck with a hammer in combination with abrasive grinding. CNC fabric removal is typically done using diamond-coated grinding tools that remove precise amounts of material by micro-abrasion. The current study compares identical ornamental dimension stone features prepared using traditional and CNC methods. The studies describe evaluation of the type and extent of surface alteration associated with these various tooling methods. Laboratory techniques developed to evaluate tooling-induced surface alteration are described, including a modification of a standard plastic vacuum desiccator capable of simultaneous treatment of multiple samples.

Comparison of surface alteration by different tooling techniques was conducted for metamorphic and sedimentary rock types commonly used as ornamental dimension-stone elements. Orientation of the prepared surfaces with textural features of different stone types was considered. Based on distribution of tooling-induced alteration for various dimension stone types, a system for selection of optimum tooling methods/stone combinations was developed.