Cracks make up another key component to constructing a realistic artificial rock formation. When expertly administered, they add a powerful sense of motion and degradation.
Natural-appearing cracks must be created in appropriate groupings and contain the correct depth and width.
Installers also should avoid the commonly held misperception that cracking occurs randomly in nature. Actually, there is a strong sense of order in the formation of cracks and most varieties of rock undergo three distinct types: rift, grain and hard grain.
A rift is a crack that causes a rock to split.
The grain consists of surface cracks that run in the same direction, mostly horizontally. They compose the majority of cracks.
A hard grain crack runs diagonally against the grain. This type of fissure doesn’t happen often.
Installers should choose a direction in which to run the grain and, in the case of GFRC panels, set adjacent castings so the cracks proceed in the same direction.
Grain cracks usually are horizontal but also can be placed at a slant. Either way, the angle must be accurately communicated to the installation crew. To accomplish this, some designers drive two parallel 2-by-4 planks into the ground, then nail another 2-by-4 across them at the desired angle. Painting the crossbar a bright color will make it stand out.
Cracks should never intersect in X or T patterns. Though it’s occasionally seen in nature, humans generally can’t imitate the effect, so veterans recommend avoiding it. In addition, it’s best to be judicious when creating curved cracks, as they also rarely occur in nature. If designing such a crack, keep in mind that it should be composed of many small straight lines rather than a single, sweeping curve.
In general, the realism of cracks is affected by their depth. As with formations, the cleavage should be deep enough so that the bottom of the gap is invisible. Specific depths depend on width: The wider the crack, the more depth is visible. A hairline crack only needs a fraction of an inch of depth, while a larger rift should travel inward for a couple of inches to conceal the bottom.
Proper carving technique also can help cracks appear as if their depth is infinite. Holding tools at a slant, and carving the rift at an angle makes it impossible to see directly to the bottom.
Cracks should be produced in a variety of widths throughout a formation, since nature does show more randomness in this regard. While fissures often run in the same direction, they don’t tend to have the identical width.
It’s helpful to employ a variety of tools to create different-sized impressions. Brushes can add texture, while trowels and harder tools produce varying sized cracks. If the impulse is to rely on one brush, the installer probably should select three.
When sizing the cracks, installers also should consider the distance between the rock and the viewer. If a formation is within 6 feet, create hairline cracks in combination with thicker rifts to allow for more detail in closer viewing. Conversely, when seen from farther away, even wider cracks appear narrow, and hairline ones become invisible.
Finally, rock installers must resist the urge to carve too many cracks because it can create a cartoon-like effect. Professionals can follow two general rules: When you think you’ve carved enough, you’ve already carved too much; and when in doubt, leave it out.
Contractors should develop guidelines to help staff members know when to stop carving.