Masonry Magazine June 1997 Page. 35
Fabrication of
Standard Granite Products
Natural granite is manufactured into wide variety of products. Monumental and memorial applications are probably the most commonly recognized forms, followed by building claddings, pavements, and landscape features. Granite products are used extensively in interior wall, floor, and countertop applications; both in commercial and residential construction. In addition to memorial and construction uses, there are several common industrial applications. Granite is used to make "surface plates" which are used as a precision measuring datum or for precision machine mounts and inertia plates. Industrial applications where chemical resistance is required often use granite products, as for the lining of sulfuric or hydrochloric acid tanks used in the "pickling" of steel, or for the construction of bromine extraction towers. The discussion of fabrication in this article is limited to building application products, which is the most standardized of the different product lines.
Slabbing
After the raw block has been inspected, graded, and transported to the fabrication site, the first process in fabrication is the "slabbing" of the block.
While there are generally standard slab thicknesses in the industry, available thicknesses and tolerances may vary slightly from one producer to another. Typically, 10, 20, 30, 40, 50, 80, 100, 150, and 200 mm nominal slab thicknesses are produced by most fabricators. Of these thicknesses, 30 mm is the most commonly used thickness for curtainwall cladding.
Slabs are cut from the blocks by three methods; a reciprocating blade using steel shot as an abrasive media, a twisted wire carrying a silicon carbide slurry as an abrasive media, or a 3.5 meter diameter circular saw fitted with diamond abrasive segments.
Reciprocating saws, commonly called "gang saws" or "gang shot saws", are the most popular method of sawing slabs worldwide. A gang of up to 140 blades is fixed at constant increments to achieve the desired slab thickness. A mixture of steel shot, hydrated lime, and water is fed to each blade. The steel shot serves as the abrasive, while the water and lime hold the shot in suspension. The entire gang of blades, which weighs in excess of 30 tons with its fly wheel and eccentric arm, reciprocates together as one unit. Most of the modern gang saws oscillate in a pendulum motion instead of a straight line back-and-forth action. Energy usage is dramatically reduced by this design, as the reversal of momentum is provided by the pendulum motion and the energy consumption is limited to that which is required to overcome friction. The reciprocating saws provide astounding accuracy. Slab thickness can stay within tolerances of ~1.5 or even + 1.0 mm across the 2.5 to 3.0 m length of the slab.
Face Finish
There are a wide variety of standard and proprietary face finishes available for natural granite. The most popular finishes for building granite panels are the thermal, honed, and polished finishes.
A thermal finish (also called flamed), is a rough textured finish produced by a rapid heat application. A propane flame is passed over the face of the granite slab, and the rapid heat gain produces stresses in the stone that overcome the mechanical bond between adjacent minerals. The outer crystals flake off leaving a textured finish, sometimes with a sparkle effect. The degree of coarseness varies due to the size of the grains in the stone. In North America where we enjoy relatively low propane costs, the thermal finish is the most economical to produce, and is widely used in pavement applications due to its frictional properties as well as its economy. The ADA (Americans with Disabilities Act Bulletin #4, 1990) suggests a static coefficient of friction of 0.60 or greater for level walking surfaces. Most thermal finish granite will demonstrate a static coefficient of friction (ASTM C 1028-89) of 0.70 or greater, comfortably complying with the recommendation.
A honed finish is a smooth finish, free of visible scratches, but not visibly glossy or reflective. A honed finish is produced on an automated line with a series of rotating heads holding abrasive bricks which are held against the face of the stone with hydraulic pressure. The abrasive varies from course to fine as the slab progresses through the machine. Honed finishes are used where refined appearance is desired, but without the distractions of the reflections occurring in polished surfaces.
A polished finish is a high gloss, highly reflective surface. By polishing the granite, the different minerals comprising the stone will demonstrate distinctive colors and many will be identifiable with the naked eye. This finish is produced with the same machinery as the honed surface, but with two additional heads. These heads use an aluminum oxide abrasive compound to produce the final polish of the granite.
Panel Sizing
Once the slabs are cut to thickness and a finish has been applied to one surface, the next step in the fabrication sequence is to cut the slabs to specified dimensions.
The slabs undergo another grading process, where subtle color ranges are sorted so that the color variation can be minimized for a particular project. In some materials, color can vary so rapidly that opposite ends of the same slab may be destined to two separate projects. The slabs are then manually marked with a rough "lay-out" of what finished panels are to cut from them. All panels on a project are assigned a unique number. It is at