Note on some “tricky fractures” in Volga Blue Granite

Abundant hairline features in kitchen benches manufactured from slabs of 20mm thick polished Volga Blue granite (selected by the client) have been interpreted by the client and his representatives as structural defects likely to lead to future problems. Additionally, there has been a suggestion that these hairline features could represent a potential health risk by harbouring pathogenic bacteria.

Volga Blue is the trade name for an exceptionally coarse-grained, plutonic rock quarried in the Ukraine. Crystal size is measured in centimeters rather than millimeters and it contains highly distinctive, very large feldspar crystals displaying a bluish opalescence. The textural rarity coupled with the schiller texture of the feldspar places this rock in the “upper class” price bracket. The rock appears to be fresh geologically with very little obvious weathering of the minerals. This mineralogical freshness is borne out by a high-pitched “ringing” sound when struck. Moreover, the quality of the sound is also an indication of its integrity. Any substantial discontinuities in the rock whether textural, mineralogical or structural, would result in a noticeably duller sound.

Quarrying of this granite is carried out by drilling vertically and horizontally. Primary blocks are extracted by a combination of expanding cement (vertical faces) and lifting the floor with an air explosive. Although this technique is widely used in granite quarrying and can be reasonably successful on certain rock types the technique can lead to problems with brittle granites. Inspection of many slabs of Volga Blue reveals the damage that has been done by both the expanding cement and the explosive. Much of the damage is directly attributable to the rock’s freshness and coarse grainsize (i.e. brittleness) and, unfortunately, the reduced recovery of unfractured blocks translates to elevated costs for the end product. After the blocks are cut and polished (usually in Italy) slab inspection generally separates the good product from one not suitable for further processing. Some operators will attempt resin fillings to hide structural blemishes but these are obvious to a ‘trained eye’.