Tuesday, October 21, 2014

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. 

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