Mohs Scale Explained…Sort of
In the world of dry media abrasive blasting, sandblasting to the general public, there are a quite large list of blast media options. Each option presents a different set of properties that make it suitable, or unsuitable for a given job.
Setting aside wet blasting (mixing the media with water in a tank, or at the nozzle head) and dry ice blasting (generates no dust and is not abrasive), abrasive blasting that I am discussing here is indexed according to its hardness by the Mohs Scale.
The Mohs Scale of mineral harness was created in 1812 by a German geologist Friedrich Mohs. I know this because I am able to use Wikipedia, but until I wrote this post, I had not a clue who this person was, now that I know he was a geologist from more than 200 years ago, I have no opinion on whether this makes any difference. Keep in mind there are other scales and test, Rockwell scale and Brinell scale being two examples.
Regardless of Mr. Mohs and his work of the early 1800’s, the scale is simply described as a grading of minerals based upon how they scratch other minerals, ranked from 1 to 10. Diamonds are the example of a 10, talcum powder is a 1. In blasting I can give you a sample of the most common 5-7 media options we use, and their range of Mohs Hardness rankings.
- Silica Sand – Sandblasting Sand. We don’t ever use this, it generates dust, which is extremely hazardous, and for that reason alone we don’t use it. However, it is a 6 or 7 on the scale.
- Aluminum Oxide – Mohs hardness 9. This is a very hard media, highly recyclable and does a good job on rust and steel.
- Crushed glass – Mohs hardness 7, maybe a 6. Glass is inexpensive and usually single use.
- Garnet – Mohs 7. This is our go to media for general blast booth blasting. Sturdy, recycles fairly well, minimal dust compared to glass.
- Corn Cob – Mohs 4.5. Good outdoor blasting option, absorbent and surprisingly hard.
- Walnut Shells – Mohs hardness 3.5. Less hard than corn cob, but travels really long way on ricochet, and with the square angles can really cut into jobs.
- Baking Soda – Mohs 2.5. Food grade, as are walnut shells, good for blasting car panels, molds and areas where abrasive needs to be minimal with little heat transfer. Dusty as all get out, however.
The list can go on and on for more options: Glass bead (5), Black Beauty (coal slag) (6.5), Steel Shot (8), Silica Carbide (9.5) and a host of plastic types (3-5).
As a final thought on the individual media choices, the really hard options, say aluminum oxide will put a really good profile on steel, it’s a 9 on the scale, whereas baking soda won’t put any profile on the same piece of steel (2.5). Then of course, you have the size of the particles you are using, air pressure, air volume and other variables that will affect the final surface.
When we are quoting a job, we are looking at a range of options for what our customer wants. Do they want a profile (etching in the steel for example), or no profile? Removing paint or rust? Both rust and paint? Mobile blasting or in house? Are they going to paint it afterwards? Is it aluminum and they want it to look uniform? The options are a numerous, and that’s where experience comes in.
Your takeaway here is that the Mohs Scale is a guide to the hardness of your abrasive blasting media choice. Understanding the range of substances and their hardness may make picking what you want easier and provide you with a better finished surface.