Bob Murrell 

M3 Technologies

Photos by Bob Murrell

As I am writing this month’s column, people are getting ready for a Fourth of July holiday, and most businesses in my area of the country (East Tennessee) are open, albeit not 100 percent yet (phase II). I can go to my favorite restaurants and eat inside, with social distancing observed. I saw a commercial on television last night advertising that Las Vegas is open for business. Even though my corporate office in Cohasset (Boston area), MA is open, we still have limited operations with restricted personnel numbers allowed. We are, however, very busy shipping to areas all over the country. This is a good thing. 

I have written about this topic before, but in this article I would like to focus on some of the many properties of stone —especially those that are pertinent to us as stone restoration and maintenance professionals. In this article, we will revisit hardness and related properties.

Most of us already know that some stones are harder than others. But what exactly does that mean? We know about the Mohs Hardness Scale where talc is a one (relatively soft) and diamond is a ten. Marble is between three-four in hardness, and granite is about a seven on the scale. So how does this affect us in the business of restoration?

Well, we know that according to Mohs, it takes a harder material to scratch a softer material. Just makes sense, right? We normally use diamond in many grinding, honing, and even some polishing situations, so that takes care of the harder part. But as we have discussed many times, a diamond abrasive is comprised of not only diamond particles, of some size, but also a matrix which is used to suspend the diamond particles. The matrix used is determined by the hardness of the material to be ground or honed. In other words – and to make it simple – the harder the material to be ground or honed, the softer the matrix of the abrasive needs to be, relatively speaking that is.   

So for instance, a granite diamond disc would necessarily have a softer matrix than a marble diamond disc. This is to ensure that fresh diamond particles are exposed at a rate so as to provide a good and consistent cutting speed and still extend the life of the abrasive disc. So if we used marble diamond discs (a harder matrix) on a granite material, the matrix may not keep fresh diamond particle exposed and dull, or cease to cut effectively. Conversely, if we used a granite diamond disc (softer matrix) on a marble material, the matrix may wear too quickly and the disc may wear out prematurely. This is the way it goes, even though the diamond particles themselves are the same in both disc series.

There are other issues regarding hardness to consider, too. Let’s say you have a soft limestone or sandstone. Should cut very quickly, right? Well yes, it does, but the abrasivity of these softer stones will also wear out the matrix on most diamond discs very quickly. I recently sent some flexible metal (nickel bronze matrix) diamond discs to a job involving Tennessee Crab Orchard sandstone (fairly dense but very abrasive). These diamonds wore at twice the rate of a marble or similar-type stone. So always keep in mind that if you are going to grind or hone a soft stone like a limestone or sandstone, you will have very high diamond disc wear rates, and therefore a higher diamond cost. Of course, the stone should cut faster and maybe you can make up some of the losses in labor savings, but don’t count on it.     

Above, left: Problem: A dingy, dirty and scratched urethane coating on Tennessee Crab Orchard stone. After testing, it was decided that mechanical removal of the coating was preferred, so as to also flatten the floor. Above, top right: The restoration team tried a 220g Typhoon resin diamond disc to help smooth the floor after running coarse 40g, 70g, and 120g metals. Above, bottom right: The Typhoon resin did not hold up on the abrasive Crab Orchard sandstone. After barely 200 sq. ft. of use, this worn disc shows the discs were completely used up. Premium Flexible discs were then used with much better results, including less wear and tear on the discs to achieve the desired results.

Above, left: Problem: A dingy, dirty and scratched urethane coating on Tennessee Crab Orchard stone. After testing, it was decided that mechanical removal of the coating was preferred, so as to also flatten the floor.


Above, top right: The restoration team tried a 220g Typhoon resin diamond disc to help smooth the floor after running coarse 40g, 70g, and 120g metals.


Above, bottom right: The Typhoon resin did not hold up on the abrasive Crab Orchard sandstone. After barely 200 sq. ft. of use, this worn disc shows the discs were completely used up. Premium Flexible discs were then used with much better results, including less wear and tear on the discs to achieve the desired results.

Just for fun information, there is the Rock and Soil Abrasivity Index (RSAI) which is used to measure the wear mass of a target tool material while it is pressed against the crushed rock or soil medium, in a relative motion under high contact stresses based on a defined test protocol for reliability and repeatability. In simpler terms, I can tell you it means that a soft stone is typically more abrasive than a harder stone, so I recommend you use stone specific diamond abrasives. Let me repeat: harder matrix diamonds will perform and last much better on softer abrasive materials.

Let’s go back to the Mohs Hardness Scale for a moment. We know that hardened tool steel (like a knife blade) is about a 6.5. Therefore, it will definitely scratch a three to four hardness material like marble, limestone, onyx, serpentine, and other such stones. Yet what do we see? We are seeing honed, leathered, and polished marble and French Limestone routinely being used for kitchen countertops. 

What does this mean? It means, as restoration and maintenance professionals, you should expect more business restoring these types of kitchen surfaces. And, because we know that owners will likely continue to use marble (and other inappropriate stones)  as a cutting surface,  you’ll also have periodic, repeat business. 

Above, left: A 13-inch floor machine with a 20-inch drive plate was used to get under cabinet overhangs. ELF Flexible Electroplated Metal-Bbond Diamond Discs were used to remove urethane and flatten the floor. Above, right: the natural beauty of this Crab Orchard foyer has been restored after urethane and lippage removal.

Above, left: A 13-inch floor machine with a 20-inch drive plate was used to get under cabinet overhangs. ELF Flexible Electroplated Metal-Bbond Diamond Discs were used to remove urethane and flatten the floor.


Above, right: the natural beauty of this Crab Orchard foyer has been restored after urethane and lippage removal.

If you have your business in an area where there are plentiful amounts of silica (sand), you will also have a higher frequency of re-polishing marble foyers and lobbies. Silica sand is about a six to seven on the Mohs scale. Keeping floors free of this sand will help extend the life of polished marble floors. So work on protocols to help trap the sand and soil to keep it from being tracked in and across the marble flooring. These protocols should include implementation of walk-off mats, dust mopping or vacuuming frequently, and damp mopping or auto-scrubbing routinely.

If your diamonds are wearing prematurely, you may have a softer and more abrasive stone than you think. Also, make sure your project doesn’t have sanded grout lines as this can cause problems with unexpected scratching when scrubbing, honing, and polishing floors.  

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

I pray you all stay safe and healthy.


Bob Murrell has worked in the natural stone industry for over 40 years and helped develop some of the main products and processes which revolutionized the restoration industry.