We are always happy to answer customers' questions about these raw materials; we know that chemical knowledge in the ceramic realm is daunting in its scope, and there is a LOT of information out there. Because of this, we get a lot of phone calls! Within all of those phone calls, though, there are a handful of questions that keep popping up, and this blog is a great place to begin addressing our most frequent inquiries. Have an inquiry yourself? Leave a comment, and I'll be happy to answer!
Inches. Centimeters. Microns.
While all of these terms are units of measure that can be particularly useful to the ceramic enthusiast, only one -- mesh size -- comes across as somewhat ambiguous, as experience in raw materials consultation has taught us. Here, I explain what mesh size is -- and, more importantly, how you can start to consider it when you have option for purchasing the same material in different mesh sizes for your clay and glaze fabrication.
Mesh size is sort of like the thread count of the pottery world -- and beyond. This is a useful macro-level measurement in the fields of geology, soil science, archaeology, and more. That's right -- this is a standardized unit of measure that is utilized in many earth-science based fields, so you can push your glasses a little further up your nose and give a nod to your newfound earth-y science-y cohorts.
Taking a cue from one of those sciences, let's look at the classic geological soil size chart. This chart explains the difference in relative sizes between three categorizations of sediments -- sand, silt, and clay. Sand is the largest particle out of the three; silt is the middle child; and finally, tiny, little clay particles are the smallest.
So the graphic above lays out the technical size range of each of these important particles -- but if you're out in 'the real world' -- and by 'real world', I simply mean a world in which you don't have immediate access to, say, a microscope -- how can you tell what size particles you are dealing with? How do you separate sand from silt from clay if you are doing a project that involves mining and levigating your own clay? On my end, I've worked extensively as a field archaeologist, and I've often been called upon to do trenchside soil analysis, which means I have pretty basic equipment to work with to determine the relative particle-size makeup of any given deposit.
So what do I use?
Oh man, do I love shaking some dirt in a stack of mesh screens. Basically, in the graphic above, uppermost mesh size is your largest mesh size, with the largest-sized openings through which particles can pass. Anything larger than that stays in the tray, and when you look over at it, you know roughly how much dirt didn't pass through the 10-mesh -- meaning its particle size is larger than that. This continues down the line, with increasingly finer particles distributing through increasingly finer mesh sizes. The thing is, though, as scale of measure is not entirely intuitive. For instance, what does '10 mesh' ACTUALLY mean, size-wise, for the newbie?
Perhaps if you've purchased raw materials before, you've noticed certain chemicals come with a number attached -- say, 'Silica 325' or 'SMS 200' (SMS is the brand name for Stone Mountain Silica). Those end-numbers generally refer to a mesh size -- and this is important -- the larger the number, the smaller the particle! So Silica 325 has passed through a 325-mesh screen, which consists of particles that are smaller than 0.044 mm, while a (larger-sized) 200-mesh screen passes particles smaller than 0.074 mm. Those numbers are so tiny that both of these types of silica appear exactly the same if you place them side-by-side with the naked eye; both will present as extremely fine white powder. However, although these differences are literally quite slight in most applications, understanding relative mesh sizes, and how different sizes of materials might affect your clays and glazes differently is an important corner of the physicality of ceramics.
And with that, I'm really starting to veer into ranty, too-technical territory -- so, to sum it all up, I've compiled a neat little list of mesh size references that you are likely to come across while shopping for raw ceramic materials, as well as the typical application of each mesh size.
20 mesh -- These larger-sized particles -- just under 1 mm -- are considered coarse for the ceramics world. You will likely see this size in association with coarse grogs. This mesh size is too large to be attached to any fine powdered pulverized materials. To give you an idea as to what 20-mesh feels like, playground sand is a pretty good visual. This is generally the roughest stuff you can (commercially) add to your claybody, so if you are new to clay formulation and want something smooth for the wheel, I'd recommend a smaller mesh size (but larger mesh-size number). 20-mesh grog is a great additive for large-scale sculptural works -- it will give your claybody a great, rough skeleton, and often contributes to a beautiful surface texture, as well, when used in large enough quantities.
35 (or 40) mesh -- Still on the 'larger' size as far as ceramic-industry particles are concerned, 35-mesh is often seen in association with medium grog. If you are looking for a gritty (but not TOO gritty) additive for your throwing claybody -- one that isn't so rough that it will chop up your hands -- I'd recommend this. This mesh size, as a grog, will also serve sculptural claybodies well.
80 mesh -- Perhaps you've seen this as a suggested mesh size to use when mixing glazes. 'Sieve twice through 80-mesh (or higher)" is typically seen on dry glaze mixes. That size is usually sufficient for breaking down the clumped-together materials in a glaze, but for recipes containing commercial colorants such as Mason Stains, I usually recommend using a smaller sieve, such as a 100-mesh. When I worked as a studio tech, my 80-mesh test sieve was one of my go-to items. It fits right in the top of a deli container and is the perfect size for small test batches of, say, 200 grams.
200 mesh -- This is where clay additives and glaze additives start to overlap, in general. This is a mesh size that results in a fine powder -- baby-powder consistency, really --
200-mesh is also the standard mesh size of fine grog. I mention this because people frequently order fine grog -- which pretty much looks and feels like a very fine powder -- when they actually want the toothier, larger medium grog. Fine grog is just that -- fiiiiiiiiine. And, for certain applications, it still largely works the same way. This pulverized refractory material lends strength and durability to your pieces, but given its powdered form, it's not the best choice for, say, large-scaled sculpture or wheelthrowing. It does add great strength to fine claybodies without detracting from detailed-working qualities, though.
325 mesh -- This is the smallest/highest mesh size that you will see on ceramic products, for the most part. There are some 400-mesh products on the market, but 325 -- that's pret-ty small.
So, if mesh sizes like '200' and '325' are so small -- small enough that by handling or looking at them, people cannot tell the difference -- what's the main difference between the two? How, specifically, can we observe them actually behaving differently as glaze or clay additives? There are a few different thoughts on this. In terms of adding, say, silica to a claybody, having a variety of different particle sizes in the super-tiny range can actually cut back a bit on shrinkage. So, if you're working with a high-shrinkage claybody -- say, a porcelain you are mixing yourself -- and you're having trouble with glaze fit or any other number of issues that can arise from a clay moving and shrinking a lot, sometimes mixing in a silica content of varied grain size -- such as 200 and 325-mesh powders -- can help that a bit. As far as glaze formulation is concerned, smaller particles stay in suspension longer, so most glaze-mixers prefer Siilica 325 over the (slightly) larger, (slightly) heavier SMS 200.
These explanations, of course, are the greatly-abbreviated versions, and the staff at The Ceramic Shop always encourages further research (for ourselves, as well!). Hopefully this will provide you with a good starting point for understanding the correlation between mesh size, particle size, and practical application. Questions? Ask away in comments!