Stone extracted from the quarry in block form is processed at the stone processing plant. At the plant, parts of various purposes are made from the blocks. According to Russian regulatory documents, products made from natural stone are classified by types. Each type of product has its own technical requirements. Currently, in Russia, technical requirements apply to the following types of products:
We have examined the technical requirements for natural stone when used in cladding and urban architecture. Now let's consider how to ensure the required quality of natural stone products, so that when purchasing stone, one does not end up with poor, low-quality material. The supply of natural stone to construction sites that does not meet technical requirements can lead to the complete failure of the entire facade improvement project and architectural complex. It is not uncommon for the unsuitable stone to be completely replaced with another, quality stone, which is associated with significant financial costs.
🔍Everything starts with a block extraction quarry. Quality control of the products arriving at the construction site must also begin here. The fact is that the natural stone mass, as a geological object, can be heterogeneous in properties. And not only in terms of color and pattern but also regarding its technical properties. This is related both to the conditions of formation of the geological mass and to its subsequent changes. For example, at one of the granite block extraction quarries, up to 4 varieties of stone are simultaneously extracted from one face (Fig. 1), and all of this is sold under one brand name.
🔍Fig. 1. Granite quarry. In the same face, the main extraction is conducted from the yellow stone (yellow arrows), gray stone (gray outlines), and light pink granite (orange outlines). Other varieties are also extracted incidentally: pinkish-gray, gray-pink, grayish-yellow, etc.
Therefore, a conscientious supplier usually classifies the blocks by the quality of the stone and its color characteristics while still at the quarry, even before purchasing the stone. Blocks made of the highest quality stone, which typically have the prefix "extra" added to their commercial name, are much more expensive than blocks made of second, third, and lower grade stones. Naturally, the supplier does not inform the buyer that the stone extracted from the quarry is of somewhat inferior quality; instead, such stone is given double names. You can determine which grade the stone you are interested in belongs to by the price range from the same deposit. Worse still, the supplier often accompanies a batch of lower quality stone with a technical passport based on laboratory tests of high-quality stone. Therefore, laboratory testing of a sample taken from the supplied batch is considered absolutely necessary to ensure the quality of the stone cladding.
Quality control of blocks usually begins with determining the conformity of the decorative parameters of the stone blocks to the stone of the reference sample. As our experience has shown, for comparison, it is necessary to use the split edge of the reference sample and conduct a comparison with the sample chipped from the block. The size of such samples should be within 15-20 cm in diameter. The samples are moistened and pressed against each other. In such conditions, it is possible to determine the slightest differences in the mineral composition and structure of the stone (Fig. 2). If necessary, magnifying glasses with magnifications of 5x, 10x, and more are used. Comparing samples with different textures, for example, polished and cut, is ineffective. Possible variations in the stone's color and pattern should be taken into account. For this purpose, it is necessary to inform the consumer about possible variations and produce several reference samples that reflect these variations.
🔍Fig. 2. Control of decorative and mineralogical-petrographic properties of the stone blocks by comparing the reference sample (on the right) with the chipped piece from the block (on the left)
Buy stone from the quarry correctly
If you are going to buy natural Jurassic limestone in blocks, it is recommended to buy directly from the quarry by first contacting a company with an established reputation. In this case, you do not overpay for the brand but actually acquire the material specifically for the task you set for the supplier. Jurassic stone blocks, like Jurassic limestone blocks, can contain many more hidden defects than granite because it is a sedimentary rock. A block of Jurassic marble with a defect (Fig. 3.)
Companies processing natural stone in Bavaria that have their own quarries employ staff who have been working directly in stone extraction for decades and can very likely distinguish suitable blocks from low-grade rock. Therefore, stone is differentiated only for interior cladding and marble for facades. Products made from natural Jurassic stone for facades are manufactured only from verified blocks of the appropriate quality level.
🔍Fig. 3. A block of Jurassic limestone with an interlayer crack, limiting the possibilities for the production of three-dimensional products.
As can be seen from the picture above, the Jura Beige block (Jura Beige limestone) at a height of about 120 cm has a usable height of a maximum of 80 cm, as the interlayer crack prevents the entire volume of the stone from being used. Typically, when communicating with the quarry, a knowledgeable buyer provides drawings of future products and, together with the supplier, selects blocks (Fig. 4) according to the set production task.
🔍Fig. 4. Jura Beige blocks (Jura Beige limestone) of good quality selected for a specific task from the block list of the quarry company with specified dimensions and photographed at the quarry.
It is more difficult to control the technical parameters of stone blocks, as it is impossible to cut out 35 cubes from each block and send them for laboratory testing. Moreover, there is no need for that. In geological exploration, it is considered that parts of a geological body with the same mineral composition, structure, and texture will also have the same technical (physical and chemical) properties. Since blocks are usually sold in batches extracted from one location in the quarry, it is sufficient to determine the similarity of their geological structure features, and this will serve as the basis to consider their technical properties as identical (or very similar).
To increase the reliability of block control, instrumental methods can also be used. Thus, in quality control of blocks, in addition to visual geological analysis, the speed of ultrasonic pulse (USI) propagation through the rock and its radioactive properties are sometimes used as indicators. Average speeds of ultrasonic pulse passage were determined for each block. Blocks with a USI speed 20% lower than that of the standard were rejected as not meeting physical and mechanical property requirements. The value of the additional radiation background of the block stones is determined using a dosimeter. This value should not exceed 34 µR/hour (0.34 µSv/hour). Only in this case is the stone suitable for use as raw material in the production of facing products. The obtained values of the additional radiation background of the blocks served as their rejection criterion (Fig. 5).
🔍Fig. 5. Instruments for non-destructive instrumental quality control of blocks at the quarry and stone parts at the factory
According to our data, up to 12% of granite blocks (by volume) extracted from the same ledge may not meet the standard for technical properties. For blocks made from other types of natural stones (especially sedimentary origin), such figures can be significantly higher than those mentioned.
Quality control of blocks is only half, and indeed the lesser part, of the problems related to improving the quality of natural stone products. We will discuss the other problems in the next article.