Bob Murrell 

M3 Technologies

Typical Grit Sizes
How scratch reduction works: polishing in several steps with a higher grit will remove the previous layer and add a new, finer  scratch pattern.

Above: How scratch reduction works: polishing in several steps with a higher grit will remove the previous layer and add a new, finer  scratch pattern.

This polishing line at a slab processing plant is applying a uniform “factory finish” to slabs, prior to distribution.

Above: This polishing line at a slab processing plant is applying a uniform “factory finish” to slabs, prior to distribution.

Surface Roughness
A radial arm polisher is an important machine in many fabrication shops. Here, an old reliable Sawing Systems machine restores a slab back to a factory polish.

Above: A radial arm polisher is an important machine in many fabrication shops. Here, an old reliable Sawing Systems machine restores a slab back to a factory polish.

Photos by Bob Murrell

Grinding, honing, sanding, and even polishing are all based on the same concept. Simply put, smoothing and stock removal are accomplished by imparting big scratches into a surface, followed by successively smaller and smaller scratches until the desired surface condition is achieved. This is the mechanics of finishing stone.

The evolution from raw quarry stone to polished slab starts at a processing plant where large blocks of stone are delivered. Here, the blocks are reduced to either slabs or billets (for tiles). This is accomplished with either a gang saw or a wire saw. Wire saws are mostly used for exotic materials, which tend to be more fragile, but not always.

These large processing saws leave the face of the stone with a coarse or rough surface which must be smoothed. The slabs or tiles are then moved to either an automated polishing line, or a manual radial arm type machine, where they are laid horizontally for finishing.

Coarse grit sintered metal-bond diamonds are normally used to begin the initial smoothing (grinding) process. These are typically followed with either resin-bonded diamonds or synthetic cement bricks, which usually contain either silicon carbide and/or aluminum oxide as an abrasive polishing agent.

What happens next, on a mechanical level, is that the surface stock is removed, and the coarser the abrasive, the faster the stock removal. Of course, every fabricator who has polished a countertop knows that the hardness of the bond or matrix of the abrasive also plays a role regarding the speed of the stock removal.  

During each abrasive series, a particular pattern of scratches is imparted into the surface. I have heard many contractors describe each successive grit level as removing the previous level. This is exactly what happens. However, I prefer to describe it as each step replacing the previous scratch pattern with a finer scratch pattern. Defining it this way should help you to understand the importance of making sure each step is consistent. 

Consistency is the key to success when grinding, honing, and polishing. Make no mistake about it – if you do not have a consistent scratch pattern, you have merely just made random scratch patterns in the surface, which will stick out like a sore thumb in the end. And remember, in polishing, you can never make up at the end for what you did not do correctly in the beginning. 

So if you don’t make each successive scratch pattern consistent and you end up with some deep 60-grit scratches remaining after you have polished, you will have to go all the way back down to a 120-grit to remove those missed 60-grit scratches. This is a big waste of time for certain. Unfortunately, it is only after a surface is polished when these random deep scratches tend to become the most noticeable. This is especially true of darker materials. 

Of course, stone processing plants or fabrication shops have a totally different set of work parameters than do the restoration and maintenance professional. The plants can use copious amounts of water, have specific abrasives which have been determined to work best on their particular material (with respect to the actual particle type and the bond), and of course, have no grout lines or edge concerns to interfere with the process. Everyone who works with stone grinds, hones, and polishes, but how we get there can be a totally different course of action, although they are conceptually related.

You can measure the profile scratch of most any surface using a profilometer. There are mechanical profilometers and optical profilometers – which can be very expensive. Portable profilometers can be purchased for around $1,000 and are certainly accurate enough for the measurement of stone surfaces. Many years ago, using a profilometer we determined that average foot traffic scratch patterns were between a 200-grit (about 90 microns) and 400-grit pattern (about 40 microns). It is important to know what scratch pattern is developed by foot traffic and normal wear so that an accurate diagnosis and prescription for polish restoration and maintenance may be appropriately prescribed.  

Looking at a grit/scratch diagram, you can see how the progressive scale works. Each successive grit used replaces the previous grit scratch pattern, if correctly and consistently applied. If the surface is polished to a very high grit, say above 3,500, foot traffic will necessarily tend to migrate the scratch pattern back down to between 200-grit and 400-grit in the affected areas. Our job as restoration and maintenance professionals is to restore and keep the surface consistently at the same level of polish.

In most cases, you can easily restore the polish to marble and similar stone using Majestic 5X Polishing Powder, XXX Shine Polishing Powder, or Marble Polishing Compound. In commercial applications, this should be accomplished quarterly or as needed. 

If the surface becomes too degraded, meaning closer to the 200-grit scratch pattern, some light honing with a 400-grit and possibly even an 800-grit may be necessary before polishing to ensure a good monolithic (mirror-like) flat surface.  

On surfaces that are not calcium-based like granite or similar stone, you will not be able to use the faster effect of an acidic polishing powder. Materials like granite can only be polished with the mechanical action of finer and finer grit scratch patterns imparted to the surface.  So when a traffic wear pattern becomes evident on granite, it is typically too late to merely use Majestic Granite Polish to restore the problem. Some honing will necessarily be required, typically starting with an 800-grit and possibly even down to a 400-grit, depending on the severity of the wear. Wherever the process is determined to be started, it will need to be successively finished to at least to a 3,000- or 3,500-grit before polishing with the Granite Polish, Majestic Crystallizer, and steel wool pads. Crystallizer is recommended because it tends to fill small surface pits between the crystals in granite, which also helps enhance the color. This is a still called a complete mechanical polish, despite the use of Crystallizer.

So, if you have ever wondered about the mechanics of grinding, honing, and polishing stone, hopefully you now you have a better understanding. Just take a look at most any 12 x 12 polished stone tile. Look at the back side, then the polished side. What is the difference? You should now know that the difference is simply the scratch pattern on either surface. The back is a big scratch pattern (sawn finish) which diffuses light, and the polished side has a very small tight scratch pattern that allows the surface to reflect light back to the eye. This is pretty much the crux of the finished stone industry. 

As always, I recommend trying a test area to confirm the results and the procedure prior to starting a stone restoration or maintenance project. Also, the best way to help ensure success is by partnering with a good distributor who knows the business. They can help with technical support, product purchase decisions, logistics, and other pertinent project information.

Bob Murrell has worked in the natural stone industry for over 40 years and is well known for his expertise in natural stone, tile and decorative concrete restoration and maintenance. He helped develop some of the main products and processes which revolutionized the industry, and is currently the Director of Operations for M3 Technologies.