Laser surface treatment of dimension stones

Granite that has been surface treated with high-power diode laserS has a finish that resembles the rustic look of the stonemason

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José Fernández could be called a visionary in his field. He is the CEO of Digafer, S.A. (Porriño, Spain), a machine manufacturer that has been involved in the field of processing stones for more than 30 years. Fernández has seen the evolution of stone processing from the early years when almost all the work was done manually and most of the products were big blocks shipped to Italy for manufacturing. Now he is thinking about a revolution in this industry—the introduction of the laser as a manufacturing tool for the processing of dimension stones. To better understand the application in which the laser will be involved, it is interesting to have a brief look at this stone-processing field.

According to the American Society for Testing and Materials (ASTM) and the U.S. Bureau of Mines the term dimension stone refers to those stones that are finished to specific dimensions and shapes and are used for building, paving, decorative or other purposes. Almost every variety of rock can be used as dimension stone, including sandstone, marble, limestone, granite, slate and more. However, this discussion concentrates on the granite family.

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Figure 1a. Granite plate cut by disc-saw.
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Figure 1b. Granite plate cut by gang sawing.
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Figure 1c. Granite plate surface after laser treatment.
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Granite is one of the best materials for beauty, durability and ease of maintenance in all types of construction. Its use dates to the origin of civilization, and buildings made of this stone still survive from ancient times. The aesthetic appeal of granite is shown by the many attempts to simulate it in synthetic materials. To the petrologist, granite is a visibly crystalline rock with interlocking texture, composed essentially of quartz, feldspars and mica. It is a member of the family of rocks that includes all those with the granitic texture and composed essentially of quartz and feldspar. Granites are generally harder rocks than marble or limestone and therefore are more difficult to machine. From a commercial point of view the term "granite" is used to refer to all kinds of intrusive igneous rock such as diorite and gabbro to metamorphic rocks of similar structure such as gneiss and to the real geological granites totaling more than 100 varieties. Commercial granites are commonly named for the quarry or location where they are found, with names modified by adjectives giving color, texture or some exotic description, such as "Balmoral Green" (Australia), "Baltic Brown" (Finland), "White Topaz" (Brazil) or "Rosa Porriño" (Spain).

The stone industry is in good health in countries such as Italy, Spain or Brazil. Even in the U.S. the situation is optimistic. Although many of the world markets are expecting some degree of decline, the U.S. stone industry appears to be a notable exception, according to a market study published by the trade magazine Stone World (January 2002). According to this study a total of 68.4 percent of companies saw increased business last year and 41.3 percent of all stone fabricators saw their business increase by a factor of more than 20 percent.

From the first manipulations in the quarry, the granite follows different steps up to the production of the plates or tiles ready to be commercialized. These steps comprise the extraction of the big blocks from the quarries, the squaring of the blocks, sawing the blocks by gang-saws or by big disc-saws to obtain slabs, surface treatment of the slabs and cutting to the final sizes. In the last few years the use of granite for cladding the façades of single-family houses and cottages and also the increment of the use of granite for paving and flooring expanded the demand for granite tiles having a rustic aspect. Moreover, architects are obtaining excellent results combining the polished and rustic finished granite tiles in big buildings.

The usual way to produce this rough surface finish on the granite slabs is the bush hammering technique, which produces large amounts of powder and generates a high noise level (>120 dB). And, severe mechanical stresses introduced in the slabs could cause their breakage at the end. Another problem is the wear of the tools used in this technique. After a few days of continuous production the machines must be stopped to change the tools, which can be sharpened and reused although they must be replaced after some cycles. As Fernández says, "you cannot imagine the huge amount of noise and powder that this [a three-head bush hammering] machine produces if you have never been, just for a moment, in front of it." Thus, there is a need for new techniques that will produce this roughened surface on the granite slabs while reducing the noise and the powder production and increasing the productivity of the overall process.

Knowing this set of problems, the group of the Applied Physics Department at the University of Vigo (Spain), led by Prof. Mariano Pérez-Amor, conducted a series of experiments to evaluate the possibilities for industrial lasers surface treating these dimension stones. The results were exciting for the scientists, but Fernández had another view. "They come with samples of granite showing a surface that was a mixture of glazy particles and craters, something perhaps very exiting for an artist but surely not for a building contractor. Anyhow, there was something interesting in this work, something really new." After a long wait for funding, the regional government (Xunta de Galicia [grant: INFRA 99-43]), the Spanish Ministry of Science and Technology and the European Union granted the project (FEDER/CICYT, 1FD97-2395).

The high-power laser diode (HPLD) was selected from various types of high-power lasers because it has a high efficiency (40 percent), the use of gases as lasing medium is not necessary and it does not need excitation lamps. These new laser systems have a lifetime of more than 10,000 hours, which makes sense for industrial applications. And the new HPLD is portable, compact and robust, characteristics that are required to effectively introduce this new technique to the stone manufacturing industries.

Figure 1 shows the surface finish of three plates made of the same type of granite (Rosa Porriño). Figure 1a shows the surface of a granite plate as cut by a disc-saw and before the laser treatment. Figure 1b shows the surface finish of a similar plate after being cut by gang sawing and also before the laser treatment. In Figure 1c the typical aspect of the surface after the laser treatment can be observed. The surface finish is similar when starting from a disc-saw cut surface and from a gang-saw cut one. Note that the HPLD gives a more rustic aspect to the surface of the granite plates than the bare sawing.

To evaluate the observations quantitatively, the profile of the untreated and laser-treated surfaces and the roughness of those surfaces have been measured. The profiles of the samples corroborate the observation made on the optical images of the surfaces of the granite plates. The Ra values confirm that the laser treatment gives a roughened finish to the granite plates that is closer to the rustic aspect than the surface finish obtained after the sawing.

Stonemasons used to work the stone, beating its surface with hammers and chisels, so that chips of the rock were extracted mechanically in each hit. In our case, the energy from the HPLD is absorbed by the rock causing the breakage of stone chips due to the relaxation of the thermal stresses induced on the superficial layers of the granite.

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Figure 2a. Surface produced by bush-hammering machine.
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Figure 2b. Surface produced by the HPLD.
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By tuning the processing parameters, the surface finish can be made to resemble that produced by the bush-hammering method. Figure 2 shows the surface aspect of two tiles made of a white-greyish colored granite called "Blanco Alba" from Galicia in Spain. As can be seen in these two images, the aspect of the surface given by the HPLD is similar to that produced by the bush-hammering machine.

The success of the application of the HPDL has convinced Fernández to further develop the system. Digafer, S.A. in union with the Applied Physics Department at the University of Vigo has already registered a patent (P200101590), and major stone manufacturers in the area such as Euro-Roca, S.A. have expressed their willingness to buy laser-based surface treatment machines to be installed in their factories. Today the stone industry is dominated by diamond-coated tools. Diamonds are very expensive, but the producing companies envisioned the good business that this market represented and moderated their earnings at the beginning to be able to increase their sales. The laser manufacturers should realize that this market is very big and, if they want to enter it, they need to lower their prices, because the laser is a large capital investment, and without lower prices they will not be able to compete with other techniques. Fernández admits, "We are exploring other alternative technologies such as sand blasting or water jet blasting. Of course, they have drawbacks but none compared to the high capital investment of the laser heads."

Acknowledgements

The authors wish to thank M. Rodríguez-Lage, R. Soto, A.F. Doval, M. Boutinguiza, F. Lusquiños and F. Quintero, for their continuous collaboration.


The authors are associated with Dpto. Física Aplicada, Universidad de Vigo, Lagoas-Marcosende 9, 36280 Vigo, Spain.

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