Q: Someone told me they were carving rotten granite. What is that?
A: The term granite brings to mind concepts of hardness, permanence, and intractability of stone outcrops. But boulders can be weathered to a crumbly material, often brown stained that may be easily carved even without carbide or diamond tools.
“Rotten granite” is an informal term used rather accurately to describe this partially decomposed granite.
All rocks at or near the earth’s surface are subject to weathering. Changes may be brought about by physical, chemical, and biological processes. The end product is sediment and soil, although from a geological perspective it’s not the end, but simply a step in a great cycle toward sedimentary rock and beyond. Rotten granite is part way there.
By geologic definition, granite and granitic rocks contain visible grains of quartz, feldspar, and various iron-bearing minerals. These grains all formed from molten material, cooling and crystallizing into a tight, interlocking, three-dimensional puzzle. Subjected to rainwater, groundwater, organic acids, and biological activity the quartz changes very little, but the feldspar begins to alter to clay minerals. The iron-bearing minerals convert to brown, yellow, and red iron oxides: what artists know as ochre. The loosening of the puzzle pieces would hardly be noticeable in a human lifespan.
Weathering moves inward from the rock surface or outward through cracks and fractures, any feature that allows water to contact the rock. With an appropriate set of fractures, granite weathers much as an ice cube melts, from a cubic form to a rounded form—a boulder. A partially weathered boulder may still be coherent enough to carve. It won’t take a polish (it may not even tolerate grinding and sanding) but broad forms with rustic finishes could be carved. However, rotten granite may not be uniformly rotten, a core or other areas of unaltered material may remain. But this offers an opportunity to contrast stained, altered, and unaltered surfaces with only a minimal investment of carving time. Of course there is also the risk of the stone’s simply falling apart with the first (or last) hammer blow.
As weathering progresses beyond any coherence, granite masses may disintegrate to sandy, granular debris called “grus” (from German for grit or gravel). This decomposed granite is used commercially for landscaping, road surfacing, and chicken grit.
The Stone Column
Bill Laprade of Seattle is a stone sculptor and geologist. As a NWSSA Board Officer for four years, Bill helped shape the policies for many Association procedures and Symposia. This is the third publication of the Stone Column series, having begun in January, 1993, with soapstone. This series continues to provide both veteran and neophytecarvers with information and useful insight into their treasured medium.
Waxy, multi-colored alabaster has been the choice of stone for artists and artisans for millennia. It not only serves as the source of beautiful sculptures, but historically it has provided utilitarian objects such as jars and casks. It was prized by the Assyrians and the Egyptians for its beauty. One of its most handsome and unique characteristics is its ability to pass light; imagine, a stone that can transmit light through it. While we sculptors use it for carving, alabaster’s cousin, gypsum, is around us most everywhere we go and is handy on all our workshop shelves.
Read more ...