I’m sourcing fireclay for mixing our mortar mix and wondering about mesh size. I can find fireclay in 20 and 35 mesh. What should I be looking for?
Either mesh size will work just fine 
When purchasing fire clay, make sure it is like powder…
Thanks. That helps, but one of these mesh sizes but be a finer powder, or does mesh size refer to something else?
Hi Eric,
“Mesh size” refers to the screen that is used to grade clays by particle size. A 20 mesh fireclay means that the particles were sifted through a screen that has 20 openings per inch. The result is that the largest particle in the bag had to fit through that opening.
It works just like sandpaper, and for the same reason: a higher number means a finer particle.
Since screens can be stacked “on top” of each other (not actually how it works in practice, but that’s the concept), manufacturers will run clays through successive screens. Starting with, say, a “1” screen, and working through the series until they get to the 300s which are great for ceramics, they grade the clay with a goal of packaging same-size particles in a bag. Clay that big generally goes back into the mill to be broken up further, because it’s not really usable for most applications.
For the purpose of mixing mortar, you want a relatively coarse powder, and either 20 or 35 mesh is just fine. By the time you get to ceramics, it all looks like powder, but there’s powder for the body of a piece and there’s powder that’s better for a glaze. You’re not worried about that here. You just don’t want it coming out of the bag in chunks; you want a coarse powder.
Here’s a link to an article about mesh size. It’s focused on ceramics (which is why all the ceramics references above!), but you’ll get a little more detail.
Bottom line, 20 or 35 are both suitable for the heat-resistant mortar mix.
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