Granite used for carving not only needs to meet the strength requirements of general building stones, but also has extremely high requirements for homogeneity, fineness, and long-term weather resistance. The evaluation should be conducted comprehensively from two aspects: laboratory physical index testing and on-site investigation of the quarry.
One. Core physical indicators: density and water absorption rate
These two indicators directly determine the durability, weathering resistance, and ability to maintain carving details of granite.
1.Density (apparent density, g/cm ³)
High quality standard: The density of high-quality granite used for carving should be ≥ 2.56 g/cm ³, with an ideal range between 2.60-2.80 g/cm ³.
Evaluation significance:
Fine texture: High density means that the grains inside the rock are tightly bound and there are few pores. It is not easy to produce chipping or burrs during carving and can achieve sharp edges and fine lines.
High strength: High density corresponds to higher compressive and flexural strength, ensuring the structural safety of large carved works.
Hand feel and weight: High density stone has a more "jade like" texture after polishing, and the weight is more sufficient for the same volume, giving people a sense of heaviness and stability.
Low density risk: If the density is below 2.56 g/cm ³, it indicates that the rock may have many micro cracks or weathering, and is prone to fracture during carving, resulting in poor polishing effect.
2.Water absorption rate (%, mass percentage)
High quality standard: The water absorption rate of top-level carved granite should be ≤ 0.4%, preferably ≤ 0.2%.
Evaluation significance:
Frost resistance: In outdoor environments (especially in cold regions), low water absorption is the key to frost resistance. After water seeps into the micropores, it freezes and expands, causing the stone to crack and peel off. The lower the water absorption rate, the longer the lifespan of the artwork.
Anti-pollution: Low water absorption can effectively prevent rainwater, oil stains, and pigments from penetrating into the interior of the stone, keeping the carved surface clean and color stable.
Microbial inhibition: Wet environments are prone to the growth of mosses and lichens. Stone with high water absorption rate will accelerate biological erosion and damage the carved texture.
High water absorption risk: Granite with a water absorption rate>0.6% is generally not recommended for fine carving or long-term outdoor display.
Simple on-site testing (reference): Drop a few drops of water on the surface of a dry block. If the water droplet is hemispherical and there are no obvious signs of infiltration within 30 minutes, it indicates a high density and low water absorption rate; If water droplets quickly spread out and infiltrate, leaving wet marks, the quality is poor.
https://www.gsdmemorials.com/about-us
Two. Quarry selection and on-site assessment of blocks
In addition to laboratory data, it is crucial to directly assess the quality of the blocks in the quarry.
1.Geological occurrence and mining site selection
Priority selection: Choose granite bodies formed by slow cooling and deep intrusion of magma (such as biotite granite and diorite granite). This type of rock mass has sufficient crystallization and uniform mineral distribution.
Avoidance area: Avoid areas where rock mass edges are prone to rapid cooling and numerous fractures, structural fracture zones, and veins (such as pegmatite veins and fine-grained veins) are frequently interspersed.
Mining and stripping method: Priority should be given to using mechanical sawing or static expansion agents to extract blocks. Avoid blasting and mining blocks, as blasting shock waves can easily generate invisible microcracks inside.
2. Inspection of block appearance and structure
Uniformity of color and pattern: Carving requires a consistent overall color tone. Observing all sides of the block, there should be no sudden changes in color spots, color lines, or clustered abnormal mineral aggregates.
Mineral particle size: observed by touching or magnifying glass. High quality carved stone should have a fine-grained to medium grained structure (with main mineral particle sizes ranging from 0.2mm to 2mm). When the particle size is too coarse (such as>5mm), there is a large difference in the hardness of different minerals in the carved details, and the surface is uneven after polishing (commonly known as "rough surface").
Crack inspection: This is a fatal defect. check
Visible crack: a penetrating crack visible to the naked eye.
Hidden cracks: Tap different parts of the block and listen for sound. A clear, loud, and lingering metallic sound (steel sound) indicates good integrity; Make a dull, hoarse, and short sound, indicating hidden cracks or weathered looseness inside.
Weathering layer: Check for yellow, loose "stone skin" (semi weathered layer) on the surface of the block. All carving blocks must be made of fresh rock, with all weathered skin removed.
3.Mineral composition assessment
Ideal composition: Quartz content of 25% -35%, feldspar (mainly potassium feldspar and plagioclase) content of 50% -60%, and dark minerals (biotite and amphibole) content of<15%.
Adverse factors:
Excessive dark minerals, especially biotite, can cause the stone to peel off easily and have poor polishing properties.
Sulfide minerals such as pyrite and magnetite exist and can oxidize and rust (yellow rust spots) in the air, seriously damaging the appearance.
https://www.gsdmemorials.com/about-us
Conclusion.
The granite used for carving must be carefully selected. When we pick up a pcs of stone before carving, we will polish the 6 sides to make sure there is no defect in the stone. We only supply premium quality material to our customers. For more info, please kindly see below.
https://www.gsdmemorials.com/about-us
