MARBLE

 

Introduction

Marble is a natural product, which has a long tradition of being typically Italian. It is a recognized fact that Italy leads the world in this field. This leadership is not only a matter of economics and statistics; it is a result of constant, dedicated effort, not always immediately visible, combined with accurate, technological research, carried out with intelligence, experience, and creativity.

 

Marble, a term with many meanings

The term marble is applied to polishable, decorative rocks, a category which largely comprises those minerals with a hardness in the order of 3-4 Mohs (calcite, dolomite, serpentine). Included among the commercial marbles, therefore, apart from the true marbles, are numerous other polishable, calcareous breccias and the calcareous alabaster, the serpentines and ophicalcites.

 

Besides the commercial marbles mentioned before, other rock materials used ornamentally are illustrated. These include the commercial granites (a category compromising not just granites in the true, petrographic sense of the word, but also other magmatic rocks with granitoid or porphyritic structures and some metamorphic rocks of analogous composition such as gneiss and serizzo), travertines and stones.

 

Marble, because of its many wonderful characteristics, can be considered from many different points of view. No other material can match its wide range of shades, or the pattern of its veins. When it is used properly, its beauty is eternal, and neither men nor time have ever shown it to be false. 

 

Quarrying

Successful quarrying is the result of a thorough knowledge of the natural marble deposits, a rational stoping (excavation) method, use of suitable production techniques, and sound promotion and marketing of the products. Many particulars are connected to the deposit location including access, distance from technical facilities and processing centers, and water and power availability. All these data vary for each site and therefore must be carefully considered. 

 

The Products

The quarrying products that can be classified according to dimensions and uniformity of shape, as well as quality, are: square blocks, semi-squared blocks, shapeless blocks, and, in some cases, monoliths of different sizes. The designer must take special care in selecting materials according to use and type of installation. Even if a strict correlation is not evident between the physical-mechanical characteristics, resistance to stresses, ect. Apart from chromatic effects and constant color characteristics, exterior veneering will preferably make use of marbles characterized by low thermal expansion coefficients and low sensitivity to frost; floors, marbles with higher wear resistance and low porosity. For monumental buildings, marbles should be chosen that have suitable processing characteristics for each of the extremely wide range of uses artistic and architectural inspiration demands. 

 

Today, the main working cycles can be divided into the following groups: 

Working thick material (headstones, memorial tablets, columns, blocks) used in monuments and works of art. 

Sawing marble blocks into small-medium thickness (1.5-6cm), large-size slabs to be used in the manufacturing of the principal building products (with special processing according to job details). Sawing products having constant dimensions (or variable size only in standard production of large quantities) directly off the marble blocks; this method excludes the production of semi-finished articles. 

 

Processing thick material

This type of working consists mainly of products having a high added value attributed to the considerable amount of manual skill and the high-level craftsmanship generally required for this kind of processing. 

Special care must be given to the detailed plans by which the marble-worker must construe (and often redesign) the graphs, sketches and/or models that are generally prepared in a rough way by sculptors and architects.

The selection of the blocks must take into account the actual production potential of quarries, especially in the case of building elements having off-standard characteristics and dimensions.

When the products are to have static functions (columns, trabeations, lintels, quoins, etc.) careful inspection is required in order to find any small cracks and discontinuities, and also traces of residual humidity left on the faces during drying. Careful inspection is required to search out weathered ore (particularly in extrusive rocks); inclusion recemented during diagenesis and porous karst phenomena, more or less recemented, in carbonaceous sedimentary and metamorphic rocks. Taking the aspect of color fastness into account, it is important that the blocks be free from ores that atmospheric agents may oxidize or dissolve, thus causing pigmentation and staining which would disfigure the desired original chromatic effects. 

The primary cutting of rough blocks- in order to obtain a solid circumscribing the final form,

- Is generally carried out with the following equipment- cutting by wires and abrasive compounds 

1. –Cutting by single-blades (diamond edged) frame saws

Should a considerable amount of stone need to be removed to reach the final shape, removal will be aided (particularly when the rock is rather hard and abrasive) by additional side-by-side cuttings (generally obtained by milling –shaping machines); then the stone can be hand-chiseled to its final shape by means of pneumatic and electric rock-drills. 

Final processing methods vary according to the requirements of the job and no series of steps is universally accepted. However, a certain number of machines and tools generally used in the final stonecutting operation can be listed:

Standard or mechanical drills, milling machines and churns, pneumatic or electric bush-hammers and sanding disks; mechanical lathes, pantographs and special machine for the reproduction of ornaments and complex processing; pneumatic or centrifugal sandblasting machines. 

 

Large-size slabs

Through this type of processing, slabs are sawn into semi-finished products, rough slabs, which can be marketed without further processing. Rough slabs can be surfaced-finished and then converted into manufactured articles (mostly granites and other granitic igneous rocks), or they can be cut for manufacturing, and then surface-finished (in the case of carbonaceous sedimentary or metamorphic rocks). 

 

Rough Slabs

Blocks are generally sawn into rough slabs by means of reciprocating, multiblade frame-saws. The energy transmitted to the blocks by the blade movement is use it to cut the rock sections having a thickness not much greater than the blade width. The result is blade penetration into the block throughout its height, at a cutting speed called “cala” (downward speed). 

 

The “cala” depends, among other things on the pressure exerted by the blades on the block, and on the blade-block friction coefficient; it is also limited by the blade frame’s kinesmatic features, i.e., by the length go the past along which the blade is in contact with the blocks, and the relative speed. 

Today, blocks are cut by two types of frame-saws:

-First, where friction is produced by abrasive compound injected between blades and blocks (traditional saw); the blade frame has pendular movement.

-Second, where friction is produced by diamond covering the lower edge of the blade (diamond-edged saw); the blade frame has a straight-line motion.

Frame-saws using abrasive compounds are still widely employed in cutting granites and other igneous rocks of equal hardness. In this case the abrasive compound is made of water and cast iron and/ or steel grit delivered by a pumping set located under the frame and is sprinkled all over the block being sawn. A portion of the abrasive compound is constantly tapped from the delivery cycle and undergoes purification to remove the finer fraction of the contained solids, which is no longer abrasive. Special automated distributors supply water and new abrasive material, as well as antioxidizers in order to prevent slab from rusting.  

Softer rocks (generally carbonaceous sedimentary or metaphoric rocks) are now cut by frames equipped with diamond-edged blades, cooled by water sprinkling. 

These frames allow high downward speeds, thus reducing production costs thank to lower operational costs of the frame, and to reduce inventory of marble blocks because of the increased slab production rate. 

Models where the blade frame oscillates along a fixed horizontal plane while the block-carrying trolley moves upwards are in wider use. 

Frames with diamond-edged blades having vertical motion are in limited use, though they are considered high-yield equipment. Interesting experiments are being conducted in vertical motion frames equipped with special diamond-edged blades to saw the harder igneous rocks. 

 

Rough slabs and finished products

Large-size rough slabs cut by means of frame-saws are semi-finished articles to be transformed into a finished product. 

Rough slabs are cut to desired sizes and shapes (provided for by the detailed working drawings) by means of trimming machines equipped with diamond-edged disks. 

The trimming machine consists of a bridge-trolley running on fixed crosswise girders. Along the bridge runs a spindle carrying a disk which cuts the slabs (individually or in piles) placed on a special bed. 

This bed can rotate 360 degrees around a vertical axis. With the most advanced machines, it is possible to cut slaps to a precession with 0.5mm by programming cutting length (i.e., disk cutting programming cutting stroke), cutting depth (i.e. bed upstroke), and product width (i.e. side traverse of the bridge and disk). 

Trimming machines can be equipped with a double bed to obtain higher performance, and fix-shaft spindles can allow cutting by means of disks revolving round non-vertical planes (shaping machines).

The surface of the slabs and finished products can be treated by means of hand tools or fixed automatic machines. Here is a description of widely used surface treatments:

 

Honing and polishing

honing and polishing are done to obtain a smooth and flat surface by successively using grinding disks with different grains (coarser to finer). The most widespread equipment is of the discontinuous type, i.e. the machines are equipped with on or more spindles to which grinding disks with different grain sizes are successively fitted (coarser to finer), the working cycle is discontinued each time a disk is changed. Wheel-mounted machines are also used, but they can polish only laid floors.

Special models (where the toll has a rotation and revolution movement) are used with harder materials (granites and similar rocks).

The use of articulated-arm machines of the stationary type is now restricted to oddly shaped products or articles requiring special treatments. On the other hand, honing-polishing machines with fixed bed and automatic traverse and oscillation of the tool spindle are widely used. 

A remarkable higher yield is obtained with advanced multi-head machines (where the automatic pendular traverse movement is programmed) equipped with fixed beds of considerable length (about 40m). Of particular interest are those equipped with a large rotating disk mounting 8-10 spindles carrying an equal number of abrasive disks having rotation and revolution movements. 

Should large investments and use of large amounts of electrical power be justified by high hourly outputs, polishing machine trains (continuous type machines) can be used. The slabs or finished products are conveyed to special beds where they go through honing and polishing by means of spindles with abrasive tools, progressively fitted from coarse to fine, operating in the direction of material feed until final polishing is obtained. Materials requiring preliminary and, possibly, final filling make use of processing trains (including fully automatic ones) equipped with diamond disks for thickness correction and pre-honing, and drying and fill in areas, possibly alternated with initial and final polishing areas.

Equipment for shaping and polishing sides is also widely used. These machines are particularly suited for making step treads for chamfering or beveling veneering. 

 

Bush-hammering

This is one of the oldest surface treatments for stones, mainly used on products for exteriors. Rough bush-hammering is carried out with pointed chisels which produce rough, medium or fine scabbling according to the size of the cutting edge. Rough and medium scabbling is still done by hand; its cost is constantly increasing while its use is decreasing. 

The most widely used automatic bush-hammering machines are equipped with moving beds (roller type) and a pneumatic tool that runs transversally on a fixed bridge; or with fixed bed and a tool running crosswise on a mobile bridge which in turn runs on crosswise tracks. These machines are suitably soundproofed. Bush-hammering machines produce any kind of surface chiseling, from fine scabbling to medium,, fine and very fine benching, as well as classic medium-fine, San Marco and very fine bush-hammering. 

 

Scorching (flaming)

This is a surface treatment of granitoid igneous rocks in general and acid rocks in particular, that uses high-temperature flame obtained from oxygen and gaseous fuel. It gives the rock a rough and vitreous appearance with characteristic chromatic effects, and lends remarkable resistance to both chemical deterioration caused by atmospheric agents, and mechanical deterioration when used as treading surface. The slab surface is generally heated by a basic equipment identical to a bush-hammering machine, where a hammer is replaced by a special blowpipe (single or multiple) which blows out a flame that hits the slab surface at an angel of about 45 degrees. 

 

Other surface treatments

Other surface treatments are sanding (also used to engrave words and patterns), and surface etching (down to a 3-4mm depth), hot acid etching of the surface of carbonaceous rocks.