A lot has happened since my last Materials Monday post. i had intended it to be a regular monthly column, but then well, a lot happened…as we start to return to work and familiar activities, I thought it was time to get this started back up…..
Today we’re talking about structural aggregates….Grog, Kyanite, Mullite, Mulcoa, Molochite, Sand, Shot and maybe we’ll even talk paper pulp. There are a lot of “things” that people add to their clay to achieve some miraculous property ranging from plasticity to strength, lowering shrinkage, reducing dunting, improving glaze fit and so on. It is interesting to watch how trends evolve as word spreads about something people are adding to their clay body with great success in a certain application. Lately, the magical ingredients are kyanite, mullite and mulcoa. But before we get to those, let’s back up and talk about GROG.
What is grog, anyway? Basically, it’s some sort of fired clay that can be added to a clay body to increase green strength, reduce shrinkage (because it is already fired and therefore has done its shrinking) and provide better and more even overall drying. The most common these days is flint grog, which is a calcined fireclay. (Calcined, in case you don’t know, means that the ingredient has been taken up to around 1200ºF, just hot enough to drive off any chemically bound waters and has been through quartz inversion. Quartz inversion means the item has undergone a chemical change and can no longer be slaked down in water.) Brick grogs used to be readily available; it was literally pulverized brick, and it was great stuff, but it is very hard to find these days. For a short amount of time, Pyrofrac Grog was a thing, and it was great because it added some pyrophyllite to the body as well, but that, too, has sadly “gone the way of the proverbial dodo” as my mom likes to say.
In addition to decreasing shrinkage, grog will also leave the fired body much more open, which can be ideal if you are making, say flower pots or containers (which you might know i have been doing a lot of lately) because if you over-water your soil, the open clay body will help with drainage. Grog is often favorable also in applications that include significant thermal shock (such as Raku). Clay bodies usually will use a 30 to 35 mesh grog. (*heavy sculptural bodies may use 10 mesh grog as well, but it is very rarely, if ever, found in a throwing body) The Fireclay that is in most stoneware bodies is also usually 30 mesh, so often the grog is indestinguishable from the fireclay. FREQUENTLY people will refer to the grog in a clay body hurting their hands or impacting their carving or similar struggles, when in fact, it is the hawthorn fireclay that is the culprit. Potters who need a finer clay body will often get a custom blend that uses a 40 or 50 mesh hawthorn. There are other options as well, but that’s a whole other article. I would be remiss if i didn’t also share that almost any clay body problem you might experience has about a 70% chance of being caused by impurities in the grog. Both flint grog and its pre-calcining partner, fireclay are notorious for this. To some degree, all veins of clay have varying degrees of undesireable material in them, we are, afterall, talking about something dug out of the ground…the ground is not necessarily the cleanest thing on the planet. The further you get from primary clay (Kaolin) the dirtier it is likely to be, and although all of these materials are processed in some way for the purpose of removing the junk, that process is not flawless.
Both fireclay-based grog and brick grogs are generally dark in color, so if you are working in porcelain, or even a very light stoneware or earthenware, you might not want dark specks. Molochite is perfect for this purpose because it is pulverized fired china clay. Molochite. Please note that autocorrect frequently thinks i am trying to write malachite (totally different…..that’s a green copper mineral…i’m not sure you can get much more opposite than that!), which is frustrating. Molochite, on the other hand is NOT frustrating. Because it is a low-iron Kaolin to begin with and is then calcined, it does not suffer from the same problems as regular grogs do. The mechanical stability of the molochite structure also provides resistance to thermal shock and a very low thermal expansion. It’s available in many mesh sizes, but the most common are 30, 50X80 (meaning approximately 90% of the particles are between those mesh sizes) 120 and 200. So even if you are doing really fine carving, the “medium” mesh will be undetectable (other than in the strength it provides). So why wouldn’t you just always use molochite instead of grog? My answer is a question back to you…how much do you want to pay for your clay? 30 mesh molochite is about $1.50/# at the full bag price, while 30 mesh grog is about 37¢/#.
Some popular clay bodies are often made with grog or with sand (BMix, for instance comes in cone 5 OR cone 10, both as “original recipe”, or with grog (I like to call that extra crispy) and with sand (i call that one rotisserie…🤷🏼♀️). So why would you want sand instead of grog? ..and what is “shot”?
Sand doesn’t refer to the stuff you find on a playground or a beach, it is a silica sand. “Shot” is also a silica-based product. The two are often used as interchangeable terms as they provide the same chemical and aggregate benefit in clay. The difference is microscopic….literally. If you look at sand under a microscope, you will see that the particles are all irregularly shaped and rather random. the size, also, varies slightly. Shot under a microscope is all absolutely uniform dodecahedrons or maybe they’re 30 sided, but i don’t know the term for that. My point is that they are all exactly the same size and essentially they are tiny round balls. So back to the question about why would you want sand/shot over grog? It goes back to what i said in the first sentence of this paragraph. It’s straight silica. Silica in a clay body can aid in glaze fit and reduce crazing in fired ware, HOWEVER, it should be noted that too much silica content in a clay can cause dunting. Silica has a high thermal expansion, which means that as it goes through quartz inversion (both during firing on the way UP and during the cooling on the way DOWN). The other thing is that shot and sand are cheap and a good amount can be used as a “filler” in a clay body (up to about 30%). If it is a high temperature clay body, there is almost no noticeable effect because the hotter temperature and longer firing time is going to melt that silica more completely, and it can drop your clay price by 5-10¢ per pound. Another benefit of silica content in your clay body is that if there is also limestone (dolomite) present in the body, the two can combine to create wollastonite which adds considerable strength. (Note: based on the longer firing time necessary/ideal if you are using shot/sand, does the “rotisserie” reference make sense now?).
The downside of silica content, though, is that dunting problem I mentioned, caused by the thermal expansion. Enter the magic of mullite/mulcoa/kyanite. All of these materials are getting a lot of attention right now, but what are they? Let’s start with Kyanite. What is it? This crystalline structure is a naturally occurring source of Alumina and Silicate (AL2O3•SiO2). It is mined primarily in Virginia. At 63%, it is an inexpensive source of alumina, but perhaps more importantly, Kyanite expands at its decomposition temperature, thus countering a good amount of firing shrinkage of various clays. The amount of expansion is dependent on the particle size; it ranges from 25% for 35 mesh material to about 3% for 325 mesh. Calcining Kyanite changes the the crystal shape from a lath-like crystal to a lenticular crystal. This composition and shape mimic the rarely naturally occuring mineral Mullite. (The only known deposit of this is the Isle of Mull off the coast of England, hence the name Mullite). Kyanite Mining corporation calls this product Virginia Mullite. It is purported to be 80% mullite, 11% amorphous silica 7% quartz, less than 1% cristobalite. Chemically, it is still high in Alumina (56-61%) but the calcining process neutralizes the thermal expansion benefit of Kyanite. Mulcoa, sometimes known as mulcoa grog is also a version of mullite. This is a product of Imerys Corp rather than Kyanite Mining Corporation. They have several types; Mulcoa 47 is the one commonly used in the ceramics industry. This version is 65% mullite, 20% glass and 15% cristobalite. Mesh size also plays a huge part in this. The higher mesh size (smaller) particles will offer better glaze fit and a stronger body, but during a long firing process (think wood firing), they will be more likely to form cristobalite, which is going to result in cooling cracks in the ware.
With all that in mind, here at Bracker’s, we have given deep consideration of what to keep in stock. We will be ordering from Kyanite Mining Corporation once a year, right around this time and our typical order will be a full skid (3500#) of Kyanite 100mesh, and a mixed pallet containing 1000# of the 35mesh kyanite, 1000# of 48 mesh mullite, 1000# of 100 mesh mullite and 500# of 200 mesh mullite. We think that should cover all of the needs people might have. Mulcoa, for those that specifically want that product, will be available by special order, expect a 12-16 week lead time.
Paper pulp is the last of our list of common strengtheners that can be added to clay. The long fibers paper help to provide structural integerity to your work in the raw state, and since the paper will then burn out in the firing, the resulting work is also quite a bit lighter. Paper clay will also re-wet more easily than it’s non-paper counterpart, which can be beneficial when adding attachments to work. Paper clay is also good to mend cracks in pieces, even ones not originally made with paper clay. the downside is that the paper CAN (by which i really mean WILL) break down as the clay as it ages, and when it does that. it stinks. truly. It’s also harder to throw with, so it’s typically used by handbuilders. We just made a fresh batch of our Domestic Porcelain Paperclay (also known as Linda’s Paper Clay, for Linda Ganstrom, professor at Fort Hays State University who uses it for her figurative sculpture works as well as with her students for their classwork in figurative sculpture). We also have a Terra Blanc Paperclay for low-fire folks! Check them out
If you’re still reading at this point, i applaud you for making it through. Be sure to leave questions or comments if you have any, and watch for the next materials monday post coming hopefully in July!