Before we get started, I should talk a little about the different types of sandpaper that there are available to you, the consumer.
When we talk about sandpapers we are really talking about a wide assortment of backers with different abrasives attached with a variety of glues or bonding agents. It's a lot of variables! Really, the mind kind of boggles at the wide assortment these combinations bring about.
For backers we have, of course, paper. But besides paper we utilize some sorts of cloth, PET film, fibre, or rubber. The glues are generally hide glue, but for waterproof sandpapers they will use a resin bonded onto a waterproof backer. The actual abrasive (the sand part of sandpaper) can be glass, garnet, aluminum oxide, or even diamond.
Grit is, of course, how rough the sandpaper is. The finer you want, the higher the number. Sixty grit is very coarse, and 10,000 is super fine.
Now that we're all on the same page, let's talk about garnet sandpaper.
Step 1: What Are Garnets, Anyway?
Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
Basically, they are naturally occurring shiny crystals which have about the same level of hardness as quartz. I looked everywhere but I couldn't find any specific details about whether the garnets used in sandpaper are naturally occurring or whether they are manufactured (as in grown). It surprises me that this detail seems impossible to find on the internet.
Either way, these garnets are crushed and sorted to a specific size using meshes. These crushed garnets are then bonded to their backer and are used by everyone for sanding wood and metal alike. That is, they used to be.
Along came aluminum oxide.
Step 2: Welcome, Aluminum Oxide!
It was our buddy, Karl Joseph Bayer, who was an Austrian chemist that developed the process in 1888 that could cheaply obtain aluminum oxide from bauxite.
Wait, wait, what is Bauxite, you say?
Bauxite is an ore that contains a large amount of aluminum hydroxide. The Hall-Héroult and Bayer methods are still in use today producing the majority of the world's aluminum.
Ok, so what does this have to do with sandpaper? The abrasive in modern sandpaper is made from it!
Annual world production of aluminium oxide from Bauxite in 2015 was approximately 115 million tonnes, over ninety percent of which is used in the manufacture of aluminum metal. That other ten percent is what we're talking about.
If you're interested in a video on how aluminum is made click here.
Step 3: So Why Is Aluminum Oxide Better Than Garnet?
The advantages of aluminum oxide over garnet is two-fold: Cost (by weight, Aluminum is the 3rd most abundant element in the earth's crust having 8.13 wt.%) and Fragility.
It's really this second feature that is of interest to you and me, since the industries of scale have made both garnet and aluminum oxide incredibly cheap.
The garnet granules fracture more easily than aluminum oxide. On the one hand this is good because it exposes fresh cutting surfaces, but it also leaves tiny shards of garnet behind to be swept into the grain and to ruin sanding with finer grits. Since garnet paper tends to be the cheaper stuff, the glue that binds it to the paper is weaker than that used in better quality aluminum oxide paper, so whole garnet granules drop off to further mess up your finish.
So, when reaching for sandpaper in your local hardware store (I know, I know, we all buy online these days), keep this in mind.
Step 4: What Other Types of Sandpapers Are There?
I see you really want to learn about sandpapers and found the topic as interesting as I did! There are a bunch of other types of sandpapers, as I alluded to in the intro.
Materials like alumina-zirconia, which is super hard and used in machining operations, and silicon carbide, which is often used in wet-sand applications, broaden the use-cases for these types of abrasive tools.
I found this article detailing the process of making sandpapers particularly interesting.