The techniques most widely used in the current state of the art to cut "marbles" envisage the use of multi-diamond blade gangsaws, while granite cutting is almost universally performed with multi-blade gangsaws with abrasive slurry, where the abrasive element is made up of jagged high carbon steel shot of suitable granulometry. The two machines are conceptually similar by are substantially different in terms of construction, since granite cutting is a much more complex operation that requires accurate control of many parameters (density, viscosity, granulometry of the metal shot in the slurry, blade tensioning and wear status, etc.). The numerical values that express these parameters differ between cutting centres since there are many variables that can modify them significantly. It is equally evident that granite gangsaws achieve lower productivity, despite using larger and more powerful machines with more blades; this is the outcome of the greater difficulty involved in cutting granites and consequently slower downcuts.

The term "marbles" is used with a commercial and not a scientific meaning to identify all materials, including silicates, whose constituent minerals are no harder than 4-5 in the Mohs scale; the term "granites" is used to identify all the materials whose constituent minerals are harder than 6-7 on the Mohs scale.

At this stage, a question comes immediaetly to mind: "Would it be possible, and if so, also competitive, to cut "marbles" using multi-blade gangsaws and abrasive slurry?".

This question was posed by one of our main clients who had to cut a huge quantity of marble; modern granite gangsaws not completely engaged in work were available and we were asked for indications about this possibility. There was the further problem that the vast majority of these marbles was light in colour, so a system had to be found which excluding the possibility of stain formation caused by the presence of residues of carbon steel shot, that may oxidise over time.

The solution to this problem, in our opinion, lies in a new type of stainless steel shot with a martensitic structure. This new metal shot made it possible to use standard granite gangsaws for marble cutting, with the only necessity of ensuring a complete change of the abrasive slurry between different production operations. Initial laboratory experiments immediately highlighted that this was the right approach. At this stage, a diagram (see figure 1) is useful; it shows some values of the difficulty of cutting marbles, obtained using the Pometon rapid dynamic evaluation method of cutting performance, so far only used with granites. It is immediately evident that the application of this method involves consumption of metal shot for cutting marbles about 10 times less than that necessary for a granite of average cutting difficulty.

Production in cutting centres with this new stainless steel metal shot has been carried out for some months (currently, the new method is being used in several leading Italian cutting centres) and has confirmed the indications obtained through laboratory tests; consumption of metal shot, as already mentioned, is about one-tenth of that normally required for granites, while downcuts are about twice as fast as those attainable with "softer" granites (in some, simple modifications of the gangsaws were needed to increase downcut speed on occasion limited to values normally attainable with granites), and thus a significant increase in the productivity of the gangsaw.

Compared with the very low consumption of abrasive material, blades and lime, and with the downcuts measured in the production runs so far performed, it can be stated that this method is economically competitive when compared with diamond-blade gangsaws.

This new possibility also substantially expands the flexibility of use of gangsaws, that in relation to market requirements of the needs of the cutting centre, can be rapidly converted for cutting both granites and marbles. It can also be mentioned that the new metal shot, thanks to its stainless processing and thus resistance to chemical attack, also avoids possible formation of rust stains on the slabs.

The parameters that must be kept under control during marble cutting are the same as those that must be controlled during granite cutting. However, the reference values are substantially different from those customarily used in cutting granites. The Pometon Technical Service can provide the operative modes for the use of this new application, which obviously should be calibrated from time to time depending on the type of material to be cut, of the type of gangsaw and cutting centre practice.

Given the fine results achieved, this new type of stainless steel metal shot has been patented in terms of chemical-physical analysis of the product and the relative brand-name: Graninox.