Digging Up Raw Clay!

[Tone9Solar]

Hi all,

 

I live in South America, Suriname. Unlike in the States, there is no possibility to buy clay here. In fact I cannot buy any clay or clay accessories anywhere. There is a small sculpting community and the only clay we can get is that dug up from old mines. It comes to us extremely raw after which we must prepare it for work which is a long process that takes up to a month. Our clay chamber is first filled with large clay deposits which are mixed with all sorts of rocks, stones etc. We are talking large quantities here! Imagine a room of 5m by 6m filled with large rocky chunks of clay. We than take a portion we can handle a put it in a large vat filled with water. We then wait for the clay to dissolve and separate from all the other materials e.g. rocks and stones. After a week or two we are ready for the next step. The separated raw clay is placed in yet another tray in order to dry. At this point the clay is far from being clean mind you. It it still filled with millions of tiny stones. Once it has dried enough we pack it in manageable packages we can take home and individually clean further before we start using it. There are always stones left in the clay, as it is virtually impossible to completely clean it by hand, so once you have began working on a project you continually run into stones. Smooth surfaces and sharp straight line for instance are therefore very difficult to maintain as you always run into a stone that ruins it. All this aside it is good quality clay, the problem is we are still forced to process it by hand rather than machinery. I see all this beautiful clay used by everyone visiting the site and i truly envy you!I often read someone wrote how important it is to choose the right clay and I laugh to myself. Hehe.....I wish I had that choice!Anyway....the reason I am posting is the following. I live next to the river and we are currently building a house. While we were building a dam, we stumbled upon a large deposit of clay. This clay however is not the orange clay I am used to, it is entirely gray almost black. It is almost completely smooth with no stones whatsoever. Of course, as you can imagine after my long story, I am ecstatic!This saves me so much time in cleaning!However, as I do not really know that much about types of clay etc., I do not know if it is really good clay and if I can use it for my sculpting or what I should do with it before I try to do so. I've never come across such clean clay so I am a bit baffled and honestly can't believe my luck. It's just too good to be true!I would really appreciate it if tips and advice on what I should or could do, and advice on what sort of clay I am dealing with here. I it's any help, I can always post some more pics as well. I hope you can help!

 

Ira

[Chris Campbell]

After reading about how hard you work on the other clay, I am as HAPPY as only another potter can be to hear that you found this clay deposit.

 

I think other more technical minds will have scientific ways for you to proceed,...I hope you are looking forward to the adventure of learning how to use it.

I would try to make a few simple forms and fire them to see what happens ... start at bisque temperatures and move up from there.

We have a potter in our area who only uses clays he finds on construction sites, fields and roadsides. Sometimes they are wonderful and sometimes not.

Please let us know how it progresses.

[ron]

I have gotten some clay from a river here in Mis.,its very black and think it came from the sediment a the bottom of the river because they are pumping it up for the sand,anyway I've processed it and rolled out a slab about a quarter of an inch to see how much it would shrink. It didn't shrink much but cracked big time,what should I had to it? It's very plastic as far as rolling around my finger.

[Seasoned Warrior]

I dig my own clay for my pottery. I like to test the clay initially by hand to see if it is plastic enough to use and I do that by taking a piece of the clay between my thumb and forefinger and squeeze it to see if it extrudes well and how far you can extrude it. The next step is to take and purify a piece of the new found clay and make it into a pyrometric cone-like shape. I then put it into a boat with pyrometric cones and set it so I can observe it through the peep hole of a kiln as I fire other wares. I watch to see when the new clay cone falls along with the pyrometric cones. The vitrification temperature is at the point where the new clay cone falls along with a known pyrometric cone. Most native clays I find can be readily modified to increase the vitrification temperature by the addition of other known clays, my favorite is the fire-clay that is readily found in hardware stores where they sell brick. Through the addition of fire-clay and subsequent firing you can adjusts the vitrification temperature. Just as an aside it is a lot easier to break up the clay you use in a ball mill rather than waiting for it to saturate with water. A ball mill can be as simple as a 55 gallon steel drum rotated with a motor and filled half way with clay and river rock. The result after tumbling is a fine powder which can be sieved, the rock removed and the clay can be readily mixed to the consistency you want. Remember aboriginal potters dug their own clays and made quite fine pottery with them.

 

Regards,

Charles

[Tone9Solar]

Well, so far I have cleaned the clay I found. At the moment it is settling in a bucket. I'll come back to it in a few days. Because I couldn't wait, I took a piece of clay and cleaned it by hand before leaving the rest to settle. I've mixed it with some fine sand and made a few things to see how it would react. So far it isn't cracking but I think it does have a tad too much sand. I do not have my own kiln yet, but I do have access to one so I won't be able to test fire it yet. Perhaps in a few weeks. Also, I do not have a chance to test it with cones. I hand my pieces over to someone to be fired and only get them back once they're done. (Which is why I cant wait to get my own kiln!) I am extremely curious what will happen with my piece once it's been fired. I am not entirely satisfied with the viscosity at the moment but I strongly suspect this is due to too much sand. As I have mentioned before, I cannot buy any clay or clay additives here so adding fire-clay would mean me grinding bricks myself by hand which is something I rather not do. I've added coal to my clay once and it was a very time consuming process to get it grinded. The work turned out wonderful b.t.w., as light as a feather! So for now I have to wait to fire my piece and for the rest of the clay to settle. Once that is done I have to be careful about the amount of sand I add. Anyone know the relation of sand to clay by any chance?Here are a few development pics!

[Seasoned Warrior]

I don't know what kind of bricks you have but usually grinding bricks won't get you clay but grog unless the bricks have not been fired in which case its pretty easy to break them up. Why do you add sand? I find that grog is a much better additive than sand for specific purposes and I like a heavily grogged clay for work that is exposed to the weather, also grog can make the clay a bit more forgiving in firing although it makes a somewhat poor throwing clay. Most commercial clays are a mixture of various clays and additives. There are an excellent series of books put out by one of the development councils in the UK but I am not sure if its the Rural Development Commission, I have a series of their books and the one on Clay is outstanding including references to the chemical composition of most commercial clays available. I am away from my studio for the next couple of weeks but I can for you when I get back.

[Marcia Selsor]

I have processed local clays and worked with clays all over the world. One interesting experience was in Uzbekistan where we had a very plastic lowfire clay. There the potters add cat tail (the plant) fuzz. This was worked into the clay and acted like what we want when adding nylon fiber. It held handles together which would have cracked without the addition of the cat tail fuzz.

 

Other things people have added to clays is sand, as you have already done. If you have problems with this, ex. if it disintegrates after the bisque, try soaking it in water within 24 hours of the firing.

 

People have added things like coffee ground to open up the clay body.

Have your friend fire this clay to bisque temperature first. When you high fire it, make sure your test i in something in case it melts.

 

Once you figure out its firing range , have fun!

Marcia

 

 

[MMB]

So I have been digging a wine cellar amongst rebuilding a studio to start my ceramics up again....and well after 2 feet into my north Georgia ground it starts to become nothing but clay and decaying organics. It has a lot of buildable attributes but still lacks. Ive blended it up dry and sifted it through and yet I still come across rocks. I havent purified it totally but was curious what could be done to natural clay to help. I made a lil pattie to try during my next 06 firing to see how it does.

[Ben]

If you can find it, buy a copy of "Pioneer Pottery"

There is a seller on potterbarter that has one for sale right now or you can try Abe books website.

 

First test to do uses a glass container so you can see the layers of materials in the clay sample.

Mix a sample with water and let it settle in the glass container. Tall and narrow containers are better.

Note the different bands of materials after they have settled. This will tell you the fractions of clay to silt to sand to stones in a given clay deposit.

 

 

Easiest way to process raw clay without equipment is by using water.

First, remove all the water from the clay. Dry it out completely.

Next smash it up. Small as golf balls should do but smaller is OK if you have means to do so.

Mix with at least 2 times its own weight in water. let it sit overnight. Mix well.

If you have some window screen pour the slip through the screen to get out the big junk. If balls of clay are left on the screen, dry them then return them to the first mixing container with the next batch.

Let settle for 2 hours, pour off water.

Make a bottomless frame large enough to hold your batch of slip. You can also use this frame with the window screen as a bottom. You want the water from the slip to be able to get out the bottom. I have agravel driveway that I use so my frame is 4x4 lumber and has no bottom. Line the frame with cloth and pour in the slip. The thinner the layer of slip the faster it will dewater. 2 inches thick is about right for me in my climate.

Remove it from the frame when it is stiff but a little sticky.

 

More about testing later.

Ben

[Ben]

You are gonna need a notebook.

 

 

test plasticity:

 

roll out a coil of clay about the size of your little finger. Wrap it around your finger. Note how well it survives, how much or how little it cracks.

 

 

Test shrinkage and absorption.

 

 

Make test bars about 6â€x1â€x1/2†thick (150mm x 25mm x 13mm)

 

Mark 5†or 100mm on them. This will be used to test shrinkage.

 

Don’t let these warp as they dry. I make them by putting clay down on cloth or paper then cutting a slab to thickness, cutting bars out and discarding every other bar. This works well if the clay is a little stiff. Then put a board on top of them until they can be turned on edge without bending them too much. Space them out or it will effect how they dry and they may warp.

 

Measure the length between the marks and calculate drying shrinkage.

 

When completely dry weigh each bar to the nearest gram.

 

 

Fire these bars supported on each end so that you can judge if they have slumped in the firing.

 

When they cool enough to handle weigh each again (the difference before and after firing will tell you loss on ignition) then place in boiling water for 1 to 2 hours.

 

Weigh again. The difference in post firing weight and post boiling weight will tell you the absorption rate.

 

 

Measure and calculate bisque fired shrinkage.

 

Repeat at hotter temps until the bar slumps or you reach the limit of your kiln. Fire over a disposable surface like a slab of a known quality clay that will fire to your top temp.

 

What the results may tell you...

 

high drying shrinkage and plasticity will make for a clay that can be problematic for attachments like handles and may tend to warp a lot in drying.

 

Low shrinkage and plasticity will make forming more difficult.

 

Color change in firing can indicate iron content which may limit the firing range.

 

 

Low absorption indicates that a clay has reached its maturation temp and will start to slump if fired much hotter. High absorption means the opposite.

 

 

 

 

 

Have fun!

[Marcia Selsor]

I use to dig clay in my primitive pottery class in Montana. I would have the students pick out stones as digging, slake it , then I would make slurry in 5 gallon buckets with my little machine made with a 1/4 hoer power motor that sits on a channel iron support in a stand that fits a 5 gallon bucket. I cut a slit in the lid and drill a hole at the appropriate spot for the shaft with propeller like blades that are low in the 5 gallon bucket. The we screen the clay through a fiberglass window screen on a frame and pour it into plaster vats to dry. We burnished these pieces and fired them in a pit with cow dung piled on top.

This stuff was lowlier. High fire clay could be found near the city dump of Lewistown.

Marcia