Introduction

 

After block cutting operations, slabs are usually subjected to abrasive processes such as levelling and calibrating (to ensure perfect planarity and dimensions within pre-defined tolerances) followed by final polishing to achieve the best possible "mirror-finish". Since materials often have pores, tiny gaps and micro-fractures (that may even be formed during the previous high-power abrasive processes), it was once common practice to perform a final processing stage with metallic lead. The very mild metal was forced to penetrate into and fill such imperfections and small pores; this process produced final surfaces of excellent quality with a fine and highly reflective mirror-finish. Health regulations related to the toxic nature of lead have seen a ban on its use in this kind of process and thus the need to develop other methods capable of ensuring acceptable-optimal mirror-finishing of slabs. The problem is particularly evident for granite slabs which (as a primary igneous material) often present a great many microimperfections. Moreover, the increasingly widespread use of diamond abrasives to improve productivity of polishing plant causes extensive micro-fractures that in many materials generate unacceptable effects (a typical example is red granites). Even some marbles (green materials, for instance) may present large, dull areas where the colour seems rather faded and shine is rather low.

 

Processing

 

The problem of poor polishing as described above has been tackled in various ways and with different technologies, with applications that vary from case to case, since the response of materials to various processes is not always identical. In any case, the most widely accepted solution is to use, after the final mechanical polishing stage (with abrasives), a chemical polishing agent, normally applied with specific and rather simple apparatus known as "waxing machines". Waxing machines are made up of: a) a system supplying the polishing product (normally a liquid) that is poured by gravity on to the slab or by means of a simple, small cock, or a somewhat more sophisticated system, such as a special dosing pump, b) motor-driven, oscillating workheads (normally four): the first spreads the polishing product evenly over the entire slab as it moves through the system, while the others "work in" and eliminate excess using discs in fabric, felt or polymer materials (e.g. polyurethane). The slab finished in this way is removed from the system, lined with a protective film of polyethylene and appropriately stored. Since storage periods may often be rather lengthy or involve sites exposed to humidity or agents that may be even partially aggressive, the polishing process should also ensure water-repellent characteristics or other forms of protection for the slabs.

 

Certain countries achieve a "shine" for tiles in certain materials (Cream Marfil, for example) by means of felt pads and sawdust, as can be seen in the photograph page illustrating the processing cycle at a big company in Novelda (Spain).

 

Polishing materials

 

• Nature of the carrier solvent

 

The nature and mechanism of action of the various products used to enhance the surface characteristics of the slabbing are normally kept jealously secret by their relative manufacturers; even the formulas are changed constantly in terms of the base components and the carrier solvent. Polishing formulas may be based on organic or water solvents. Water-based products are less aggressive in terms of both the environment and the personnel involved in supervising the machinery used for such processes. However, there is no doubt that water involves problems associated with its higher surface tension (and thus lower wetting and penetration into the stone to be treated), its lower evaporation characteristics, etc. While the use of special and specific additives has substantially overcome these problems, that initially seemed almost insurmountable, their complete resolution is perhaps still some way off. The use of organic solvents seems to ensure better results in terms of wetting, penetration into the stone and evaporation, etc. but their utilisation may create long-term problems for workers and the environment. This is worsened when applications are performed indoors (in industrial processing premises) or when suitable air recirculation systems are not available, although currently almost all producers prefer using solvents with low toxicity and low environmental impact. In some cases, systems have been installed to ensure air circulation or suction systems with relative filters that capture the vapours emitted by the solvents.

 

• Nature of the polishing product

 

In general terms, the polishing products currently used can be divided into two main families: a) wax-based polishing agents, b) polymerbased polishing agents. It is also often possible to mix these two types of material and appropriate blends can enhance effects.

 

Wax-based polishing agents are effectively waxes that are applied to the slabs; their fundamentally plastic nature means that they fill micro-imperfections and after final polishing ensure a particularly shiny and mirror-like finish. It should be borne in mind that the use of waxes (especially beeswax) to enhance the "shine" of marbles (floorings and even statues) is a very ancient technique already known to the Greeks and Romans. Naturally, the overall result depends on the type of wax used and its intrinsic characteristics. Waxes with a low melting point tend to be oily and thus may make the surface "pasty" and afterwards create problems with dust, dirt, etc., while waxes with a higher melting point are more difficult to work to a shine and thus may create problems of a different type. The wax in any case must ensure good protection for the material against water and its effect must be as durable as possible (although, given the nature of waxes, this will not be particularly long-lasting…). Good results are achieved with micro-crystalline waxes, although more highly-prized natural waxes (Carnauba wax, for example) may ensure even better results but at much higher cost. Suitable blends of various waxes may often be used to ensure greater versatility and facility in terms of application, shine, waterrepellent features and enhancement of certain colour shades in the stone (especially for green stones, as mentioned above). Waxes are well suited to applications as water-based products or in organic solvents but in any case the ultimate effect is often associated with the action of the final finishing felt pads of the waxing system that must always ensure a good polish for the product applied. Unsuitable, old or otherwise deteriorated felt pads may create serious problems in terms of the final shine of treated slabs, even if the product used itself is entirely valid. Many formulations envisage the use of blends of waxes and styrene-acrylic resins that help achieve better results and more durable bonding with the stone.

 

Polymer-based polishing agents are a more recent development thanks to progress in the chemicals industry; their composition is changing continually with the discovery of new products and new characteristics. Their effect is based on the deposition of a very thin film of polymer material on the surface to be treated in order to ensure a shiny, mirror-like finish. Structurally, the polymers utilised may be thermo-plastic or thermo-hardening types: the former tend to generate very thin films through coalescence, while the latter (normally applied as blends of oligomers) also tend to form subsequent reticulation processes that ensure better bonding with the stone and longer duration of the treatment. Some polymers also have a marked impregnation effect (that is, they penetrate into the stone matrix) that gives rise to a bond that is difficult to eliminate (especially if thermo-hardening materials are used). They often also create a more or less intense "wet effect" that helps disguise any imperfections (micro-fractures or spider cracks) caused by the action of abrasive tools that, as mentioned above, may substantially damage the polished surface. This effect is held in high regard when applied in the processing of certain granites and can be attributed to variations in the index of refraction of the treated material and the formation of areas with a slightly darker appearance that enhances chromatic contrasts in certain coloured materials (with particularly appreciable effects for red and green materials, etc.). The most widely used polymers are fundamentally siliconic and acrylic, and related blends. The average molecular weight of the polymer or the pre-polymer is very important in terms of good penetration into the pores of the stone to be treated in order to ensure good bonding. The chemical structure of the monomer (or monomers, if co-polymers are used) is also extremely important, since it determines the final features (brightness, water repellence, elimination of spider cracks, partial "wet effect", etc.) of the product used. In particular, a functional polymer containing polar groups will ensure high adherence and stability on the stone. The tenacity and durability of the treatment depend on the stability of the polymer (chemical inertia, stability in relation to photooxidation deterioration, etc.), the kind of bonding with the support, the eventual presence of reticulation processes, penetration into the pores of the stone, etc. In general, it is rather unlikely that a single polymer material will simultaneously embrace all these characteristics, although siliconic materials provide very satisfactory performances, especially for granites. More often, appropriate blends of polymers or other substances are used that combine to achieve the desired characteristics. A typical example is the use of acrylic-siliconic resins offering high synergic action that often achieve particularly brilliant surface characteristics. The situation regarding white materials is more delicate, since they require special formulations that do not affect the peculiar colour shades of these materials with darkening effects. In particular, siliconic polishing agents are not recommended for the treatment of white marbles since they may give rise to undesirable darkening effects or shading along the veins; better results seems attainable using wax-based polishing products.

 

Since new and different stone materials continually appear on the market just like new polymer and wax-based materials for treatment, continual up-dating and research work is essential. Moreover, it must be borne in mind that new topics and/or problems also emerge: the use of food-grade materials and products free of pollution or deterioration problems, etc.

 

A solution to the problem of complete surface refinement of slabs and subsequent enhancement of their shine has also been attempted by applying a surface stratum of resin (normally epoxy) that is then polished. The process (resin finishing), however, is very costly in terms of both the raw materials used and the plant (systems for drying the slabs and applying/seasoning the resin, etc. are needed). Moreover, special epoxy resins should be used that are very resistant to light (free of structural chromophores that cause yellowing following prolonged exposure to light) to avoid altering the colour of the surface. Other collateral problems may also arise since the resin, if exposed for long periods in unfavourable climatic conditions, may be subject to deterioration and undesirable secondary reactions. As a result of a number of negative experiences, certain markets (especially North America) often do not want slabs treated with this technology that, in effect, may alter the features of the entire surface.

 

Conclusions

 

Research into technologies that enhance the shine of slabs and, at the same time, also ensure a protective effect (or enhance chromatic contrast in coloured stone) seems particularly important for the valorisation of many stone materials. Developments in chemicals and the continual proposal of new materials with always different characteristics require constant and in-depth investigations in order to adapt and up-date the various formulations.