Wednesday, April 25, 2007

Sand and Grog

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!

12 comments:

vin said...

One of my pieces, that took me months to build and refine, finally fell apart. The teacher had been telling me that the clay needed more grog in order to avoid problems with larger pieces but I loved how smooth this white clay was. I just kept going thinking that it was going to be ok and that I'd repair any crack that appeared, big mistake lol. I'm glad I came across your post, it's extremely helpful and informative. :)

barb said...

vin:

Glad I helped. Barb

Nathan said...

My understanding is also that silica sand can be substituted for grog.

I found a local building supply place that sells silica sand for about $40/ton, that sure beats $20/bag!

Check out quarries if you are having trouble finding silica sand in bulk.

Also even if you want it bagged, the quarry is always going to be waaaay cheaper than getting this type of material from a ceramic supply place.

I've been dabbling in clay architecture and after working with some highly engineered mixes, I have come back to just a basic mix of sifted local clay and silica sand.

barb said...

Nathan:

Silica sand (especially at that price) would indeed be a good substitute for grog, if you can get the size particle you want. It's usually highly cleaned, because cement people have worse problems than potters if the sand is adulterated with stuff like salt (a la beach sand) or feldspar.

Great idea, to check for alternate suppliers, yesssss!

Just note that grog will absorb water & sand will not--resulting in slightly different throwing (or handbuilding) properties.

Barb

Daniel Cullen said...

Hi There,
Just read your very helpfull post on sand versus grog,thanks.
I live by the beach in Irelands East coast.I have tons of lovely earthenware clay all about me which I just cannot resist using in spite of the white calcium or feldspar?I make large-ish sculptural pieces which I paint(glazes pop of course) I just love its sea smell and its real earthy feel,smooth as chocolate....
Anyhow I welcome the idea of using silica sand.Is this the same as childrens playsand?that would be a lovely coincidence,as I use this in my Art and play Therapy work.
Question;how much sand should I use?.I read somewhere 7% dry weight,.Any advice?
Daniel

Barbara Szabo said...

Daniel--

Silica sand is children's sandbox sand, or construction sand. It should say on the bag "100% silica sand." You may find there are different sizes of grain available to buy, or you may not. I would mix up some at 3% and some at 7% and test them before doing anything major. Testing, meaning: mark a six-inch flat piece before and after firing so you can tell shrinkage; weigh and then immerse the fired piece 24 hours, dry and reweigh to find out absorption %; and build something tallish with your 3% and your 7% silica bodies to see how much the silica helps against sagging, and what you're comfortable with.

Barb

sara barden said...

Thank you so much. You are very helpful.
Sara

Barbara Szabo said...

Thanks, Sara :o)

meleena said...

Which type of clay has the most grog earthenware, stoneware, or porcelain.

Barbara Szabo said...

Stoneware usually has the most grog, although different clay "bodies" in the stoneware category have differing amounts of grog. Laguna Clay Co. for example has some very smooth clay bodies and some very "toothy" bodies (with lots of grog). Earthenware (low-fire) Sculpture clays exist that are super groggy. Porcelain normally is very smooth.
Barbara

Anonymous said...

I have a pot that is beautifully thrown. I bought it from a potter who was not a professional at the time. She is a very skilled individual. I was amazed that she could throw such a large piece about 12 inches high and about 5-6 inches across at its greatest diameter. (vase type...with grog...) How she did it without lacerating her hands I will never know. I love it and tried to recall some pottery terms so I found you. I used to do a bit and wish I could have done more. Thanks for having the info I wanted!

Anonymous said...

Thanks for the info. I've only been throwing and sculpting about a year-and-a-half now. I've been having problems with some of my throwing platters. They always seem to crack around the outer rim before I build my wall that is only about 2 inches tall. I'm going to have to try grog in my mix. I was thinking about adding Raku in with my bmix to see if that would help solve my problem. Appreciate the good info and Hope to use your advice soon