FreeShip- Zircon, Zirconium Silicate, Opacifier - (Prompt rebate on orders with 3 or more FreeShip items!)

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This is the Milled Zircon (distinct from the zircopax) listing. We believe the particle size is larger than our Zircopax listing. Either one has the same chemical properties as the other but may be different enough to take a look at the two if you want to start using zircon in your glazes.
We found a bag of "milled zircon" we didn't know we had. The zircopax and zircon are identical in every way except the particle size, which sometimes can make a difference in firing characteristics. There may be differences in how it suspends in the liquid glaze, how it flows when applied and how it fires in a glaze.
We've ascertained a potentially large difference in particle size between the two. First we did an online search and last we actually looked at the particle size under a microscope.

Zircopax:
Digital Fire lists zircopax to have about a 1.3 micron particle size and superpax (zircopax plus?) a 0.74 micron particle size. Those sizes are incredibly small. I had a hard time finding the equivalent mesh size of one miron. I only found one source that calculates the equivalent as being 12,000 mesh.

Milled Zircon:
Let's compare the Zircopax to the often listed "milled zircon"s offered by ceramic suppliers. "Milled" is a relative thing. Put something in a ball mill and run it for 3 hours and then run some for 48 hours. There's going to be a difference in mesh size between those but you can call both sizes "milled".
I checked out a manufacturer of milled zircon and got their take on the mesh size. Since they're in business to sell the stuff, probably their size categories will be fairly common for ceramic suppliers and users. Here's what the manufacturer says:
fine grade is -400 mesh, coarse grade is -325, and extra-coarse is -200 mesh. You might find the middle-of-the-road -325 to be the most often sold.

We can assume there's some inexactitude, maybe a lot in that size information. But what we can probably safely say that zircopax has way finer particle sizes than milled zircon.
So, what does the microscope reveal? The most striking difference is the particle size range. Milled zircon has a broad range of particle sizes. The 2nd difference is that the zircopax does have finer particles and a tighter particle size grading. The overall average size of Zircopax is smaller than milled zircon, but not as much as the above online search would lead you to believe. The very finest of the milled zircon particles are about as small as the average smaller particles of the Zircopax.

So, all it boils down to is:
The Zircopax is a very very fine zircon.
The Milled Zircon is a very fine zircon.