Bonded abrasives used for granite lapping. Natural stones like granite and marble are capable of taking a high polish and find use as decorative stones. These are vastly for cladding purpose in the construction industry, as well as for monumental and memorial work. The basic difference between marble and granite is that marble forms due to the metamorphosis of lime. Therefore, marble is a much softer stone. On the other hand granite, which exists in nature as igneous rock and is quite hard.

BASIC PRINCIPLES

The basic principle of polishing any natural stone is to hone the surface of the stone. By progressively using finer mesh of abrasives, we can obtain the natural lustre of the stone. A polished piece of natural stone is capable of reflecting a major portion of the light falling on it. A surface, on which honing is not proper has irregularities, is not able to reflect a substantial portion of the incident light. Instead, it dissipates the light. As a result, the viewer finds that the surface lacks certain lustre.

Smoothing and polishing are the jobs that bring surfaces to a smooth and a polished state, respectively. The process of polishing is different on marble and granite. Marble is polished chemically with tools using oxalic acid and acetoacetyl in the final stage of the process. By deliberately reducing the amount of water in the final stage of the abrasive treatment, the material overheats and a patina forms on the surface due to the interaction of the aforesaid substances and the constituents of the material itself (mainly calcium carbonate). Bonded abrasives used for granite lapping are done mechanically.

ABRASIVES USED FOR GRANITE LAPPING

Reaction between oxalic acid and calcium carbonate generates calcium oxalate which plays the prime role in polishing. Polish of granite is mechanical. In fact, while the final stage uses felt pads + lead monoxide and stannic oxide. This is called the material’s closing and is done with progressively finer abrasives. While this is not the place to discuss the parameters to keep under control in the smoothing-polishing process, it should be stressed that, from the practical standpoint, both categories (marble and granite) require great care. Increased abrasion and consequently faster processing with great savings of time and labour. Abrasives with magnesium sulfate and barium chloride do possess health hazards, while handling it.

TRADITIONAL SYSTEM

The traditional system of polishing marble employed the use of wax to impart a gloss to the marble. Waxing does not bring out the natural lustre of the marble. It is an artificial coating applied to the surface of the marble. This gives temporary gloss to the marble. With use, wax wears out and the marble looks dull and lifeless. In some cases stone processors even use wax on granite at the final stage of polishing. This too results in a temporary polish. Apart from this, wax is slippery and can cause accidents.

In order to achieve a good and permanent polish on the surface of the marble or granite, it is therefore imperative to grind of the surface irregularities to such an extent that the surface reflects light rays falling on it to a large extent. A proper grinding and honing of the surface will result in a permanent polish since no artificial coating is in use to enhance the reflection of the light rays.

BONDED ABRASIVES

In most modem stone processing plants, abrasives used for granite lapping includes bonded abrasives for grinding and polishing natural stones. Earlier, the use of silicon carbide powders was common for grinding and polishing granite with the help of a rotating steel plate. Pressure applied by the steel plate on the free particles of silicon carbide resulted in the grinding action. Water acts as coolant. After the honing was completed with 400-mesh silicon carbide, a copper plate was used to lap the surface with 800 or 1000 mesh silicon carbide. In case these fine mesh powders were not available, coarser mesh of SIC was used until it was ground down to the finer mesh. The final polish was achieved by lapping the finely honed surface with the help of lead plate and tin oxide. This lapping action imparted a high gloss to the granite.

The above mentioned system had the following drawbacks:

• The process was comparatively very slow.
• Waste of abrasive without doing useful work.
• The soft lead forms a thin film (Similar to the wax) on the surface of the granite.
• If the polished granite used in the exterior facades of building or for memorial work, the polish would wear out with the effect of rain, dust and wind over a period of time.

Today, the use of mixture of abrasive grains and bonding materials is common all over the world. This is because they grind and polish the marble and granite at the same time. Also, it is very rapid and very cost effective. Binding Silicon Carbide powder in a magnesite bond makes the most common type of bonded abrasives. This is the most economical and efficient method of grinding and polishing of granite and marble.

Since granite is a hard rock, use of special grinding and polishing heads is common for rapidly grinding and polishing the slabs of granite. The principle of these heads is to increase the contact pressure between the abrasives and the stone, so that the abrasives can rapidly grind the stone. All the grinding heads utilise the principle of line contact between abrasive and stone, to ensure a faster grinding action. The most common type of grinding heads in use today are the oscillating heads (or the rocking heads) and satellite heads (or planetary heads).

OSCILLATING HEADS

The rocking heads consist of 2, 4 or 6 rotating abrasives used for granite lapping. These oscillate about their own axis while they are grinding the stone. As a result of the oscillating movement, the abrasives generate a line contact with the stone. This increases the specific pressure on the stone tremendously and results in high material removal.

SATELLITE HEADS

The planetary head on the other hand has a number of abrasives used for granite lapping. These rotates in their own axis as well as revolving around the central axis of the head. The abrasives incline at a small angle, so that they generate a conical shape, such that there is only line contact between the abrasives and the stone. Due to the line contact, there is high specific pressure plus high grinding velocity. This action is able to grind the stone very rapidly.

Both the above systems are efficient forms of grinding and polishing granite. However, the planetary head has a higher incidence of maintenance. This is due to the high rotating speeds of the abrasives and subsequent shocks on the bearings. The oscillating heads incur low maintenance cost and are also more efficient in the utilization of electric power.

For polishing soft stones like marble, it is necessary to have a complete surface to surface contact of the abrasive and marble. A line contact will apply too much pressure on the slab of marble and the grinding action will therefore be very severe.