Which Abrasives Are Best to Use For Coldworking Your Glass
How Do Coldworking Abrasives Work?
If you've tried coldworking your glass at any point, then you know that there is a wide array of different abrasive materials that you can use for grinding your glass. So which abrasive is the right one to use for which process?
It can be very confusing trying to find the right processes. Knowing how each abrasive works will go a long way towards understanding the best time and situation to use each abrasive. Let's break down the main abrasives used for coldworking and see how they each perform and what they do to your glass surfaces
How Hard Is It?
One of the first things we need to know is how hard is the abrasive and how hard it the material you're trying to abrade. To do this, we're going to use something called the Mohs Scale, which is a measurement of how hard a substance is from 1 (the softest) to 10 (the hardest).
If you're working with glass, then it sits around 5–5.5 on the Mohs Scale. Anything softer than this will not scratch or abrade the glass, anything harder will scratch or abrade your glass surface.
Our four main abrasives when coldworking glass are: diamond, silicon carbide (carborundum), aluminum oxide, and pumice. Diamond is a 10 on the Mohs scale, so it is one of the hardest substances on the planet. Silicon carbide and Aluminum oxide are both a 9 on the Mohs scale, so they are slightly less hard than diamond. Pumice is a 6 on the Mohs scale so it's only slightly harder than the glass itself.
This already lets us know several things about each abrasive. Diamonds will remove material faster as it is the harder abrasive. Silicon carbide and aluminum oxide will also remove material from the glass as they are only slightly less hard than the diamond. Pumice, being so close in hardness to the glass itself, will remove very little from the glass surface when used as an abrasive.
How Friable Is It?
"What does friable mean?", you may ask? Friable is how brittle an abrasive is, or how likely it is to break under pressure. This will help determine how quickly an abrasive breaks down when you use it compared to other abrasives which will help in determining the longevity of that abrasive.
Diamonds are not very friable at all. They are extremely strong and it takes a lot of pressure to break them. They are the longest lasting abrasive you can use for grinding your glass. Silicon carbide and aluminum oxide are both highly friable abrasives, so they will break down quickly when you use them. Their lifespans are very short compared with diamond. Pumice is the most friable of the abrasives so it will break down quickly as you use it.
How friable an abrasive is, combined with the hardness helps determine which abrasive tool to use for which process. Whether you need faster removal and longer lasting or less removal with a shorter life. But are these the only things to consider?
Sharpness Will Determine Lifespan
Along with how hard an abrasive is and how friable and abrasive is; how sharp an abrasive is will also determine its lifespan. The sharper an abrasive is, the shorter its lifespan will be. The duller the abrasive, the longer the lifespan will be
Diamonds come in many different degrees of sharpness. Most diamond tools are a mixture of these types so you have very sharp diamonds that do the initial grinding with a tool, but the break down quickly and then you run on diamonds that are slightly duller, but will last longer. Silicon carbide is a very sharp abrasive, so it will remove material quickly from a surface, but it will break down faster as well. Aluminum oxide is a very dull abrasive, so it removes smaller amounts of glass, but will last longer. Pumice is also a very dull abrasive so it removes a minimal amount of glass.
What Have We Learned?
When we combine all these characteristics of the different abrasives, it can give you a good insight into how each abrasive will interact with your glass
Diamonds are one of the hardest materials on the planet, combined with low friability they will give you a long life as an abrasive, coupled with higher removal rate. This also means, that at finer grit levels, diamonds will still give you an aggressive finish on the surface of the glass.
Silicon carbide and aluminum oxide are similar in hardness, and friability, but they differ when it comes to how sharp they are. Silicon carbide, being a sharper abrasive, will work better for removal of glass from your surfaces but also perform well at finer grit levels due to how quickly it will break down from very sharp to quite dull. Aluminum oxide, on the other hand, is a dull abrasive so it will struggle to give any heavy removal of glass from your surfaces. This is why you often see it as a pre-polish abrasive that will leave a very fine surface on your glass in finer grits. This also gives it a longer lifespan as a pre-polish abrasive against silicon carbide and, used in a sandblaster, a better choice for surface blasting, while silicon carbide will excel at sand carving.
Pumice, being so close in hardness to the glass, along with being friable and duller is an excellent choice for pre-polishing surfaces where you want the minimal amount of damage to the glass surface to occur coupled with the finest surface appearance you can achieve. Almost exclusively used as a loose abrasive, it's a great choice for surfaces that have uneven surface texture or areas where you wish to maintain as much detail as possible but still give a more uniform pre-polish to the finish.