Talc Talk

Hey, it's Tuesday, and what could usher in the new-ish week better than an expose on one of our popular glaze-room chemicals, TALC?

We do not sell talc in rock form. SORRY.

I decided to write about talc because during our recent back-to-school crush of business, we received one question pretty consistently throughout the busy school ordering season. One of our most popular clays for classrooms is Standard's low-fire White 105 (and its groggy version, 105G). Customers who have not used the clay before are sometimes surprised to see that in its wet form, it's grey -- NOT white, as it appears when it gets fired. Now, early on in any ceramics training, you sort of learn that unfired colors are often quite different from fired colors, but clays tend to be a little more close on that front than, say, glazes. 

Standard's low-fire white 105. 

It fires white, we promise!

Well, the grey color that you see in unfired 'white' clays is due to the addition of talc, or magnesium silicate, to the claybody. Talc can appear either grey or white, depending on where it's mined -- plenty is mined here in the states, where grey talc is quite common (although white talc, as seen in the image below, is also mined here). That color is a result of trapped carbon, much of which burns off in firing.

Left, Texas talc; right, Montana talc. Image courtesy digitalfire.com

Talc is an interesting material that can perform a variety of functions within clays and glazes, particularly when used for different things at different firings. That's a pretty vague description of a material, for sure, but it does serve a very specific function in low-fire claybodies: it acts as a flux, which means it aids in the melting of other materials in the claybody, leading to a more vitrified final product. This is, of course, an important factor for low-fire ceramics -- after all, you don't want to have a permanently-porous body for low-fire functional wares, right? Adding talc to a claybody, then, can force it to vitrify a bit more than it would without the additive.

But what is talc, really, and how did it come to be a ceramics staple? 

You've probably heard of soapstone -- heck, maybe you even have a knick-knack or two that has been carved out of the stuff. Soapstone's more technical name is steatite, and it can be found in many, many places throughout the world -- as evidenced by those aforementioned knick-knacks being fairly ubiquitous, right?

The Ceramic Shop is based in Philly, though, and this corner of the world has a very interesting relationship with steatite and ceramics. I'm not even talking about an historic relationship here -- more like PREHISTORIC. Yes, steatite artifacts and steatite-tempered ceramics are a very important part of the prehistoric archaeological record in the NE United States -- and that presence speaks to a more widespread technology that, in a mind-blowing turn, we still use today.

Pennsylvania and the surrounding areas were home to indigenous groups that go so far back, archaeologists can't even identify them as belonging to any specific Native American groups -- but needless to say, they were indeed ancestors of more easily-identifiable groups we see in this area at a later date. And on that note, when I say these groups went far back, I mean like 8,000 or 9,000 years ago. People living in this area, at that time, are generally referred to as Paleo-Indians. Their technology, which we see in the archaeological record, was a lot of worked stone and some ceramics here and there. Of course they worked in many other materials, but given the moist, seasonal, acidic conditions of the northeast, many organic remains (such as wood and leather) have simply decayed over time. Not so for our archaeological inorganics, however -- that's stone and fired clay.

Beginning in what archaeologists call the 'Transitional Period' - about 3900 years ago - steatite was used to make stone bowls by these people, and those stone bowls had a few things going for them. In the first place, soapstone is very soft -- the softest, easiest-to-carve stone there is, really - so working it was much easier than trying to make a bowl out of, say, magnetite (or some other very hard stone). In the second place, soapstone bowls had this wonderful quality of not just cracking and failing immediately when they were placed in direct heat, such as an open fire. This made the bowls wonderful vessels for cooking.

This is a pretty typical, well-preserved steatite, or soapstone, bowl. 

It could take the heat. Ohhh, it could take the heat.

At this time -- thousands of years ago -- you have to keep in mind that ceramic technology in this part of the world was fairly limited. Anything that was formed and fired was earthenware, with a low tolerance for everything. This pottery cracked and broke very easily due to the difficulty that ancient potters had in getting their open-air pit-firings as hot as they 'should' be for the clay they were using. You know how pottery in bisque form seems more fragile than fully-vitrified wares? At any rate, somewhere along the road these two materials came together -- pulverized talc, when added to a claybody, imparted some if its heat-handling properties to these ancient vessels while also acting as a powerful flux, resulting in more vitrified -- and heartier wares! Archaeologists aren't exactly sure of the where and when this took place, but you can almost picture it: thousands of years ago, and area of craft production is set up. Some people are making pottery, some might be carving, that kind of thing. Some of that super-soft soapstone dust, which might just be a byproduct of carving a soapstone bowl, winds up in someone's clay, either accidentally or intentionally -- local ancients were known to add all kinds of tempers to their clay. At any rate, maybe after firing, that soapstone-tempered bowl stood out as being slightly more durable, and the addition of that material was thus introduced into the ceramic technology.

So, bringing this little history lesson full-circle, it's amazing how ancient technology can still really be seen informing contemporary products today! Interested in more information on archaeology of Pennsylvania? Check out the great blog This Week in Pennsylvania Archaeology. 

We just mesh well.

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. 

...mesh size?...

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?

MESHES.

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!

In the Philadelphia area (and beyond!), The Ceramic Shop has really filled a niche of supplying raw ceramic materials to studio potters, university classrooms, and independent artists alike. We pretty much sell it all, in dry form, by the pound. So whether you are just learning how to make and test your own glazes, or you're a seasoned pro, we have a VERY stocked warehouse that has pretty much everything you need.