True Grit

Goal

 Understand the basic numbering schemes for grits, and know which grit to use when.

Overview

Mirror makers grind mirrors much the same way that carpenters shape and sand a piece of wood:  they use rougher (larger) grits first to get the basic shape, and then a series of finer grits to make the surface increasingly smooth and glossy. Each finer grade removes the pits and scratches from the previous grade of abrasive.  

Just as sandpaper is available in various grades from "extra coarse" to "extra fine", ATM's grits also are graded by size.  While there are dozens of sizes available, there's certainly no need to use all of them.  What we're after is a basic series of grits, from large to small, such that each size will remove the grinding marks from the previous size in a reasonable amount of time.  Usually, a series of five to seven grit sizes will be most efficient on most types of glass.

Modern grits are graded primarily in one of two numbering schemes:

For example "#80 carbo" has a grain size of about 1/80th of an inch in diameter.  Or, said another way, if you were to lay out 80 grains of #80 carbo side by side, the line would be 1" long.  (The would take excellent eyesight and some really fancy tweezers, however.) #320 carbo grains are obviously about 1/320th of an inch in diameter, or about 4x smaller than #80.  

So, as the mesh size numbers increase, the size of the grit decreases, so you start with the smaller numbers and work upwards. With micron sizes, it's the opposite:  you start large and go small. 

The following table shows the approximate equivalents for mesh and micron sizes.   It may be a bit confusing at first, but there's really not too much room for mistakes.  When you say you're working with 220, everybody knows you mean 220 mesh, and not 220 micron, which would be some big stuff, indeed! 

A few good general purpose sequences for hand working of glass are:

As you can see, there is a smooth decrease in size without large jumps or gaps.  When working by machine, some use fewer grades, such as 80 and 220 mesh, followed by 30 and 15 and 9 micron.  There is no "magic" sequence, just a series of tradeoffs between grinding time and number of steps.  

There are two types of abrasives used in grinding a mirror: Silicon carbide (SiC) and aluminum oxide (Al₂0₃).  While these may sound exotic, it's the same material that's used in common sandpaper.  Both are very safe to handle as they are non-reactive.  Grit is just very hard, finely graded rock.

Silicon carbide (or "carbo") is much harder than glass, and almost as hard as diamond.  The usual form of supply is as rough grains, which look like tiny boulders under a microscope.  It's used in the early stages when the goal is to remove glass quickly.

Aluminum oxide is a bit softer, but still much harder than glass.  It works slower than carbo, but leaves a smoother surface, hence it's use in fine grinding and final smoothing stages before polishing.  While it is available in rough grains like carbo, the much preferred type of abrasive for smoothing is shaped like a disk. The disk (or platelet) shaped grains cut wide and shallow, making it easier to remove with the next finer grade of abrasive.  The most common supplier of disk-shaped grits here in the US is Micro Abrasives Corporation's Microgrit, which is universally praised as an excellent finishing abrasive.