Architectural Stone: properties and techniques Harvard Graduate School of Design

Stone classified according to its formation Igneous: originates in volcanic process; hard and crystalline; (granite, basalt) Sedimentary: formed of bonded mineral sediments; generally softer and less stable than igneous rock; (sandstone, limestone) Metamorphic: igneous or sedimentary rock whose molecular structure has changed over long time under pressure + heat; (slate, marble, gneiss)

Stone characteristics Hardness Strength Density/unit weight Porosity Color Pattern/markings

Finishes/ surface treatments -- (from roughest to smoothest) 1. split face 2. thermal/flame-treated (granite only) 3. hammered 4. sandblasted 5. honed 6. water-polished 7. mirror-polished Surface treatments affect: texture color and legibility of grain porosity and resistance to water/dirt sense of thickness/depth

Finishes/ surface treatments split face -- irregular, “natural” texture created by forcing stone to shear along internally weak plane; most common in slate and sandstone. thermal or flame-treated (granite only) -- heat of torch causes crystals near the surface to expand and flake off, exposing a rough texture of interlocking crystals. hammered -- tiny indentations hammered into the stone surface (variety of scales possible) sandblasted -- smaller indentations caused by sand granules hitting the stone surface at high speed hammering and blasting processes tend to * lighten color * de-emphasize grain or irregularities, and * increase surface area and thus porosity.

Finishes/ surface treatments honed -- minimal polishing to produce a smooth, matte surface; medium color water-polished -- moderately polished, smooth, deep color, semi- reflective surface mirror-polished -- highest degree of polish, deepest color, reflective surface, most impervious to water; too slippery for most floor surfaces

Stone construction techniques Structural masonry rubble or cut stone/ ashlar; uniform through depth of wall. Balanced wall construction load-bearing stone facing on masonry wall; examples are mainly historical. Veneer stone: “wet” construction -- veneer stone bonded with mortar to structure; rarely used for exterior walls; “dry” construction -- most common today; stone attached to structure by metal clips and angles.