Frederick M. Hueston, PhD  

Stone Care Consultant 

Over the past few years I have received numerous calls from stone restoration contractors as well as fabricators who are having trouble polishing certain marble types.        

The major complaint I receive is they can’t produce that deep factory polish – what some contractors refer to as a “ten foot shine.” These same contractors tell me they have tried less powder, more powder and all different types of marble polish, all to no avail. Some even complain of orange peeling.

In order to properly polish marble and most calcium-based stone, you must understand how stone achieves its shine. The following article will discuss polishing marble, as well as tell you some of the techniques and materials you will need to achieve a factory polish.

Why Does Stone Shine?

When stone becomes dull and scratched, it has obviously lost its factory shine and luster. At this time the stone needs to be refinished and polished to restore the shine it had originally. Why does stone shine, and how can a lost shine be recovered?

The deep shine we see on polished stone is achieved by rubbing the stone with a series of abrasive materials. The process is very similar to sanding a piece of wood. The stone is rubbed with a coarse abrasive grit, followed by finer and finer grits until the stone becomes smooth.  

The scratches left behind from one grit are removed by the next, creating finer and finer scratches. The process continues until the scratches are microscopic. 

The shine on the stone is achieved by abrading the surface to the point at which it becomes extremely smooth and starts to develop some reflectivity. The shine on the stone is thus, a product of optics. 

In a rough or scratched piece of stone, when light is reflected from the stone, the light rays become scattered, producing a dull, flat appearance to our eyes.  In a very smooth stone, the light is reflected on the surface and the light rays return in a parallel pattern, producing a deep reflective appearance. 

This same optical property can be observed on a pond. When the wind is blowing and the surface of the pond is wavy, it becomes difficult to see a reflection; when the air is still and the pond is calm, a deep reflection can be observed. So, in order to achieve a deep shine on stone, all that really needs to be done is to smooth it until it shines.

Achieving a Polished Surface

Now that we understand why stone shines, we can discuss the techniques used to achieve the polish. A shine can be placed on stone with three basic techniques:

1. Coatings

2. Crystallization

3. Polishing 

Let’s take a close look at each in detail.

Coatings

Coatings are commonly waxes, acrylics, floor finishes, urethanes, etc., that are placed on top of the stone surface. Coatings are designed to provide a sacrificial layer that will take the abuse that stone receives. They have been used by the janitorial industry for many years on tile and other surfaces, and are used on stone the same way.  

It is my belief that coatings on stone surfaces should be avoided except in circumstances where slip-resistance is an issue. In this case, a coating may be necessary. Coatings are also used on furniture-top surfaces and do not receive the type of abuse experienced on a floor, and are therefore acceptable. 

Why are coatings used? The manufacturers of coating materials are well-established. There’s a huge market and demand for janitorial coatings. For this reason alone, these companies will continue to promote the coating of stone. Janitorial trades are very familiar with application of coating, so it is natural and convenient for them to continue using them. 

Coatings are generally economical, easy to apply and will provide stain and slip resistance, and will add luster or shine to the floor surface. On the other hand, coatings are much softer than stone, and for this reason will scratch, scuff and mar very easily. This will require frequent buffing, burnishing and/or reapplication.  

They can also build up, causing an unnatural plastic appearance. Poor quality coatings will yellow and can cause permanent staining to some stone types. The necessity for stripping makes coatings an undesirable choice for stone for several reasons. The chemical strippers are very high alkaline materials, which can cause spalling. 

The stripping pads used are very abrasive and will often scratch the stone surface. Some brands of strippers will attack the plastic matrix in agglomerate stones, eating away at the polyester binders.

The most important reason coating should be avoided is they may block the breathing capability of the stone. Moisture can become trapped below the surface and may lead to spalling.

If a coating is necessary, make sure that the coating is designed for stone surfaces, that it is applied properly, and most importantly that it is monitored for potential problems.

Crystallization

Another way to achieve a shine on stone surfaces is with a process known as crystallization. The term crystallization has entered the language of the marble polishing field to describe a process used to maintain a shine on marble surfaces.  

Crystallization can also be called “vitrification” or recrystallization.” The procedure has been used in the United States since the 1970s, and has generated some controversy among the experts. Before we discuss the pros and cons of this process, we need to take a look at what it actually entails.

The recrystallization process consists of spraying a fluid onto the marble floor and buffing it in with steel wool under a standard buffing machine. The steel wool generates heat through abrasion and the chemical reacts with the marble, producing a new compound on the surface of the stone.

Chemically, the crystallization process works as follows. Almost all crystallization chemicals contain three main ingredients:

1. Acid

2. Fluorosilicate compounds

3. Waxes

How do these components react with a stone surface? First, the crystallization process can only react with calcium-based stones such as marble and limestones. Although the process can work on non-calcium-based stones, such as granite, the reactions are entirely different.  

What happens when an acid is placed on marble? The acid attacks the calcium carbonate, leaving an etch or dull mark on the stone surface. The reaction is as follows:

CaCO3 + HAc -----> Ca++ Ac + H2O + CO2

CaCo3- calcium carbonate

HAc- acetic acid

Ca++Ac- calcium acetate           

H2O-water

CO2-carbon dioxide gas

When the crystallization fluid is sprayed on the marble surface, the acid attacks the calcium carbonate as outlined in the above reaction.  The fluorosilicate compound then attaches itself to the calcium ion, forming a new compound called calcium fluorosilicate.

Note: The fluorosilicate compounds found in crystallization fluids consists of three types: aluminum fluorosilicate, magnesium fluorosilicate and zinc fluorosilicate. One or more may be found in crystallization fluids depending on the brand.

The overall reaction that takes place during the crystallization process is as follows:

Reaction 1

CaCo3 + HAc-----> Ca++AC + H20 + CO2

The calcium carbonate molecule is broken or attacked.

Reaction 2

MgSiF6+ H20 + Ca++  --------> CaSiF6

Magnesium fluorosilicate reacts with the calcium ion and forms calcium fluorosilicate.

Reaction 3

ZnSiF6+ H2O + Ca++ ------> CaSiF6

Zinc fluorosilicate reacts with the calcium ion and forms calcium fluorosilicate.

OK, enough with the chemical reactions. What does this all mean?  Simply put, the crystallization process works by breaking down the marble surface and forming a new compound on the surface of the marble.  

In order for this reaction to take place, heat must be generated. This is the purpose for using steel wool.

Why the controversy? Proponents of the process claim the new compound formed protects the surface of the stone, adds shine, and may even harden the stone, increasing its wear.  Opponents of the process claim that the new compound that is formed blocks the stone’s ability to “breathe,” traps moisture and causes the stone to rot.

The crystallization process must be performed by trained craftsman who are familiar with the workings and techniques of this process. Excessive moisture in the stone can hamper the crystallization reaction and can cause problems. For further information on crystallization, see my blog, or my youtube video at the following link: http://youtu.be/SdMI7lGN5OU.

Powder Polishing

Powder polishing uses superfine abrasives and oxalic acid to achieve the deep shine on marble surfaces. Granite powders use only abrasives. As described earlier in this article, the stone is smoothed so that we achieve a super flat surface resulting in a glassy appearance. 

The oxalic acid is used to accelerate the polishing  process by reacting with the calcium carbonate. The combination of the abrasive and the attack of the oxalic acid result in a polished surface. The use of oxalic acid as an accelerator allows the craftsman to polish most marble at a 400-600 hone. 

Granite, since it does not react with acids, cannot be polished using this technique. For granite, you need to smooth the stone using finer and finer abrasives. The final polish is achieved using superfine abrasive powders and special felt wheels or polishing discs.

Powder Polishing Abrasives

What type of abrasives are marble and granite polishing powders made of? Most marble polishing powders use aluminum oxide as the abrasive, oxalic acid as the accelerator and a combination of other ingredients such as shellac, sulfur, salt, etc., which add color, lubrication, and so on, to the process.  A “hot” powder is one which contains over 50% oxalic acid.  

It is possible to blend your own polishing powders by mixing aluminum oxide and oxalic acid. I don’t recommend blending your own, unless you are experienced at doing so.

Caution: Oxalic acid, which is used to speed the polishing process, can burn marble. Burned marble has a characteristic dimpled appearance; the stone will have a molten, plastic shine. This burned appearance is commonly called “orange peel,” for reasons that are obvious to anyone who sees it. If you orange-peel the floor, you will have to re-hone the floor to remove it. 

 

Fred Hueston will continue this marble polishing article next month.