Finishing of Surfaces

After the sawing, the following step of processing is the final finishing of plates and other parts, through levigation, burnishing and lustering, or crushing and flaming. The levigation or rough-hew represents the reducing of plates, by creating plain, flat leveled and parallel surfaces. Burnishing produces the fine paring of the plate and the closing of the mineral grains, creating a smooth, opaque and more impermeable surface, than a natural face of the same stone. The luster is applied to brighten the surface of the plates, and is produced by polishing of the stone constituent crystal faces.

The levigation, burnishing and luster are made by abrasive grinding-wheel, based upon silicon and diamond carbide, in different granular metric system (thicker for the finer levigation; thinner and thinner for burnishing and final luster). The grinding-wheel, clasped to rotating headstocks, circulate on the surface of the plate, making a constant use of water flow for elimination of remainders and refrigeration of the face in treatment.

The results of burnishing and luster are defined after brightness, closing and shining of plates, and brightness may be checked through visual accuracy, or by using some device ( glossmeter). The brightness measurers had been developed for homogeneous metallic surfaces, however being very used in the field of ornamental rocks. The index of demandable brightness by the consumers must be equal or superior to 70 points, as measured on the device scale. The greater heterogeneity in the aesthetic features (movements) of a rock, the greater number of necessary measures for a representative average.

Mafic mineral concentrations (particularly rough biotite) and sulfates, produce more burnishing problems on plates and more remarkable changes on the products there applied. Nodules (mules), small dikes and veins (strings), especially in homogeneous rocks, leads to aesthetic standard problems and losses in the sawing of plates. Such features are expected in natural rocky materials, and it should be observed when its intensity leads to undesirable aesthetic effect as much as eventual physical and mechanical problems.

The most used equipment for burnishing of rocks are the manual polishing machines (gantry saw) (1 headstock), bridge polishing machine (1 to 2 headstocks) and polishing machine multiheadstocks (5 to 20 headstocks). The manual polishing machines (handle, veneer saw) are outdated, bringing about the low efficiency and great variation in quality of the former production. As for the bridge polishing machines, the movement of the headstocks is less random, allowing to achieve higher productivity and quality on finishings. The lines of more modern and efficient polishing machines are totally multiheaded and automatic, making it possible for the plate to be processed from 10 to 15 cm of thickness and 2 meters width, as well as dismissing previous operations (levigation) and posterior ones (luster) like in other machines.

Specific and usually automatic/semi-automatic equipment are used for crushing, flaming, sandblasting, milling, sawing, balling, scarfing, curvilinear cuts, perforations, etc. Such equipment is useful to obtain isolated parts, not necessarily standardized, normally requested at the marble shops.

The crushing and flaming techniques produce in some materials, it has already been cited, a more interesting aesthetic effect than burnishing ones. Flaming procedures are not advisable on plates with less than 3 cm of thickness, unless water application is done on the opposite face to the acetylene flame. The crackle of minerals during flaming and the impact of the metallic tips during crushing procedures, stimulates micro fractures that facilitate the infiltration of pollutants, and speed up the physical and chemical attack to the surface of the plate.

The general trend of technological evolution for processing and finishing of the ornamental rocks (dimension stones) is translated by the automation of the whole equipment line (looms, block chiseling machines, cut-plates, polishing machines etc.) and for a better specification of the consumption materials (blades, grit, abrasives etc.), towards the reduction of operations timing and cost, as well as an improvement of quality for the entire production.