I have had a heck of a time over the years struggling with sand and grog. Without knowing it, problems I’ve wrestled with for years such as “bumpy” clay and glazes, and ware cracking, have originated from trouble with these two clay components.
Sand is what? Tiny granules of silica, ideally. Geologically, rocks of various kinds weather down, eroded by abrasion, until small grains of the very hardest component--silica--are all that remain. Everything else the rock is made of weathers down faster and blows away or is washed away. Deposits of silica sand are mined for construction-grade use, and are sold by the bag. Beach sand has shell parts, salt, feldspar, mica, iron, and organic debris included--which can cause unwanted chemical complications. Impure beach sand should NEVER be used in making clay, even though it is a lot cheaper than silica sand.
Grog is what? Clay that has been fired once in the kiln, then crushed into fine particles. Grog can come from low-fired clay--such as bricks (sweepings from brick factory floors is occasionally bagged and sold cheaply as grog)--or higher-fired clay. Grog can be ground and sieved carefully and sold by size of particle. Grog can be red (as in brick grog), tan, or white.
What does sand or grog do to clay? Either sand or grog are added to clay to separate the platelike clay particles from each other. Mechanically, the rounded and rough clay or grog particles keep the flat and smooth plates of clay from sticking together. The result is twofold: a clay body with sand or grog added will dry out faster than a clay body without sand or grog, because water can exit more easily (think: pure sand in the garden drains water very fast while pure clay retains water a long time. Mixing the two gives you something in between). Secondly, adding sand or grog makes the clay body more able to “stand up:” higher pot walls can be thrown without collapse, therefore larger pieces can be created. Sculpting clay has a great amount of sand and coarse grog added. The reason sand and grog allow greater strength in clay walls escapes me, but thus it is.
What’s the difference between using sand and using grog?
Anyone reading this who'd like to fill me in, please add a comment. I am still not sure, beyond my understanding that grog absorbs water and sand does not. But I've had a bad experience with sand, and so now I prefer grog (fine gog). Here's the story:
I used to make up my own clay bodies from scratch. Without the addition of either sand or grog, my large pieces would inevitably warp or crack. I tried adding whatever I could get cheaply, but I would get new problems, without understanding that my choices came with prices. Sand (and large-size grog) abrades my hands and wears my fingernails to the quick. Though grog is more expensive, it is available in SMALLER particle size than sand does, and it still does the job of “opening” a clay body. Now I know that I can avoid the price of raw hands from the abrasion of sand, by adding very fine grog instead: I get the “open” clay, and increased pot wall height, without the abrasion.
I decided to buy clay ready-made from a supplier. It's not that expensive (clay is, after all, "dirt cheap"). Years ago I decided this was altogether a better solution for me than mixing my own. Some potters get very excited about digging their own clay, and preparing it; but for me it was too much exhausting work, and I finally gave in to buying it ready-made.
However, I encountered another hellish problem for years, and only recently found out that bagged clay was not infallible. Through two competitors giving me information, I heard that the maker of my favorite clay body was adding beach sand instead of more expensive silica sand to their proprietary mix. Not good! I would have smooth, very smooth bisque ware, but then discover after the final firing that my clay was now mysteriously bumpy. These rough bumps were quite noticeable on the unglazed undersides of my pots, but also made my should-have-been-smooth glazes intolerably bumpy as well. After looking at the clay with a 10x loupe magnifier I discovered bubbly whitish “zits” to be the culprit--and it was explained to me that these were feldspar blemishes caused by impurities in the sand.
When I switched manufacturers, the zit problem disappeared; my naked clay is now smooth and the glazes are smooth as they should be. I must assume that impure beach sand indeed was being added to that commercial clay body. But finally! I've got clay that stays porcelain-smooth through cone 6, yet has enough "tooth" due to the addition of fine grog to hold up in nice, thin walls rather than collapse.
When I want to make the occasional larger piece, I've got a supply of fine white grog in my studio--I just throw a handful on the wedging table and wedge it in to that chunk of clay I'll be using to make the larger piece. The glaze still fits, everything's still smooth, no cracking or warping. Bingo!