How to Process Clay

How to Process Clay

So you found some natural clay, great, but now what? How do you process clay into a usuable material suitable for building pottery or sculpting? It is actually not all that hard to process natural clay, primitive people have been doing it without the help or any specialized equipment for millennia and so can you! All you need to get started is some wild, natural or native clay (all words used for the same thing), a little time and last and surprisingly, sand! So let’s get started.

There are two primary ways to process clay, wet and dry, the steps to those processes are listed out below.

Dry

  1. Dry the clay thoroughly
  2. Grind the clay into powder
  3. Add about 20% sand
  4. Add water and knead until plastic

Wet

  1. Soak the clay
  2. Mix it up into a slurry
  3. Pour through a screen
  4. Let stand for several hours
  5. Pour water off the top
  6. Pour liquid clay into a pillowcase to dry
  7. Add 20% sand and knead until mixed well

Note: This article deals only with processing natural clay, if you want to learn how to find clay check out my article How to Find Clay linked here.

Pros and Cons 

Both processes will result in a good usable clay body when completed given that the natural clay is of reasonably good quality to begin with. Here are some of the factors to weigh in deciding how to process your clay.

Labor

  • The hard part of the dry process method is grinding the clay.
  • The hard part of the wet process is all the kneading required to mix the sand in thouroughly. 

Time

  • The wet process takes more time, using it will result in good usable clay in about a week. 
  • The dry process is fast, using it can give you good usable clay on the same day if your clay is already dry. 

Materials

  • The wet process takes more equipment, a bucket, a screen and a pillowcase. 
  • The dry proccess only requires a way to grind the clay up but even two rocks will work for that. 

Efficiency

  • If the clay you have found has a lot of junk in it like rocks and sand or sticks, then the wet process will do a better job at removing those.
  • The dry process will only remove the larger chunks and attempt to grind up the smaller bits of non-clay materials into temper (more on that later).

Purpose of Temper

You may be surprised to learn that I add 20% sand to my clay. This is done to promote more even drying, clay shrinks when it dries so uneven drying puts stress on a pot as some areas shrink faster than others. Putting your damp pot in the direct sun or wind to dry will cause it to crack for the same reasons. Pure clay is dense and does not let water move through the clay body, adding course material to clay opens it up and allows moisture to move out of the clay more easily and therefore promotes even drying.

cracks in natural clay
Clay naturally cracks when dry, adding temper will prevent this.

This grit or “non-plastic material” added to a clay body is called temper, in most modern clay the temper is fired clay that is ground super-fine called grog. With a natural clay body such as we are working with sand is the most easily available grit but primititve people have used all sorts of materials for temper some of these are;

  • Sand
  • Volvanic ash
  • Ground pot sherds
  • Ground rock
  • Ground sea shells
  • Diotomatious earth

I tell you to use 20% because that is a good average for the natural clays I use but the amount of temper needed will vary based on the clay and the amount of grit it contains naturally. You may need to experiment, if your pots keep breaking when they dry, add more, if cracking pottery is not a problem you can try using less.

Dry Process Clay

Dry the clay thoroughly

Grinding clay which has even a small amount of moisture is nearly impossible because it will want to gum up on the grinding surface. So completely dry clay is essential to starting the dry process for wild clay processing.

In most cases spreading it out on a table or other surface in a place where it will not get rain or dew or other atmospheric moisture will do the trick. But if you live in an exceptionally damp climate or are in a hurry you can speed it along by putting it on a cookie sheet and placing it in an oven on low heat. The oven cannot get hot enough to turn your clay into ceramics even at the highest temperature, so there are no worries with heating it this way.

Grind the clay into powder

clay being ground in a mill
Grinding clay with my corn grinder.

The clay needs to be ground up, not into a fine powder like baby powder, it only needs to be as fine as course sand. You can use a hammer or stone on a patio or driveway, or could even crush it between two rocks. I use a corn mill to grind my clay and it even pulverizes small rocks in the mix. You can see the grinder I use at this link. While grinding the clay you should throw out large stones, sticks and roots and allow smaller of such to be ground up with the clay.

Add about 20% sand

Since the clay is dry powder at this point it is easy to measure out with any small container. I just use a cup and measure out 4 scoops of the powdered clay and add one scoop of sand that I have screened to remove larger rocks and sticks. This is a 4 to 1 ratio which makes for a 20% temper mix.

Mix the clay-sand mixture thoroughly. I usually just pour this dry mix back and forth between two buckets until it is consistent with no spots of pure clay or sand left. Alternatively you could use your hands for a small batch.

Add water and knead until plastic

Add water a little at a time so you don’t accidentally get it too wet. It can go over the threshold from too wet to too dry surprisingly fast. Add a little water knead, add a little water, knead, and continue like this until you reach the desired plastic but not overly sticky or “gooey” consistency.

Freshly prepared clay
A freshly prepared block of clay ready for forming or aging.

If you accidentally get it too wet, you can add a little bit of the dry clay/sand mixture to bring it back the other direction.

Wet Process Clay

Soak the clay

In order to wet process wild clay it will need to be soaked until all the lumps have dissolved. Some clays tend to resists soaking and may need to be ground up first. Drying before soaking can sometimes allow the clay to soak up water better. Using a paint mixer drill attachment can help to break up lumps and get it soaked through more quickly. Here is a link to one such product on Amazon.

Mix it up into a slurry

The soaked clay should be thin like paint or pancake batter, not thick like tar or bread dough. Depending on how much clay you started with it may be advantageous to divide the clay between two buckets so you can add enough water to achieve the desired thin consistency. Add the water and mix it up good so the clay is suspended in the water, you can use your hands for this or a paint mixer drill attachment will work, the link is above in the previous section of this article.

Pour through a screen

clay being screened in buckets
Screen ready on left and clay ready to pour on right.

Right after you stir up the clay mixture, while the small particles are still suspended and swirling around, pour the clay through a screen to separate out sand, rocks, sticks and roots. You can use a window screen for this, other options are screens specially made to fit on a 5 gallon bucket and paint strainer bags which can give you a finer grained screen. Here is the link to the bucket screens on Amazon, and here is the link for the mesh paint strainers.

Let stand for several hours

Next you will begin the process of separating the clay from most of the water. How long it takes the clay to settle varies greatly depending on the qualities of the clay, some clay will settle out in just an hour while others will take days. One clay that I collect from a dry lake bed will never settle, it will stay suspended in the water forever until the water evaporates away, for that clay I just avoid wet processing it.

You are waiting for a layer of clear water to develop on top of your bucket of clay/water slurry. Sometimes it is crystal clear and other times it will retain a bit of murkiness, but if you can see through it, then it is probably safe to discard.

Pour water off the top

Now slowly pour off the water on top, when you get down to the layer of clay, you will see a distinct difference in the thickness, heaviness and color. Stop just before you start pouring off clay. You will probably want to do this standing and pouring off of water several times over a period of hours or days until you reach a state where little or no water is accumulating on top anymore.

Pour liquid clay into a pillowcase to dry

a pillowcase full of liquid clay
A pillowcase full of liquid clay drying in the sun.

Now that much of the water has been removed you will be left with a thick, clay slip in the bottom of your bucket. It may not settle much at this point but it’s still a long way from being able to form into a pot.

Get an old pillowcase that you don’t care about ruining, I like to check yard sales or thrift stores for these. Look for one that is good cotton, if the material is too thin it may cause clay to run through the fabric.

Now pour the clay from your bucket into the pillowcase, wrap or tie off the end and leave to sit outside to dry out. How fast it dries will depend a great deal on your weather, the drier the atmosphere, the faster the clay will dry. It will help to turn the pillowcase over every so often so that the clay on top doesn’t become too dry while that on the bottom remains too wet.

Add 20% sand and knead until mixed well

clay and temper ready to be mixed together
Damp clay ready to have 20% sand kneaded in.

When your clay is plastic enough to knead, take it out of the pillowcase. Some parts of the clay mass will be drier than others so some kneading will be necessary to form a consistent mass. Any parts that have dried too much to be kneaded will need to be put aside to be reprocessed later.

Measure out a pile of screened sand equal to about 20% of your clay mass. I usually just eyeball it, a ratio of 1 part sand to 4 parts clay is what you are shooting for.

Now knead the sand into your clay until it is fully and consistently mixed through the lump, this is that hard labor part of the wet process method. You do not need to worry about “wedging” your clay, which is nothing more than a useless tradition. Air bubbles do not cause pottery breakage, moisture does, so go ahead and wedge if it makes you happy but just a good old fashioned kneading such as one would do to make bread will be sufficient.

Aging Clay

Now the the clay is finished it can be used right away, although aging the clay for a few days or weeks can improve the quality. Wrapped up in plastic or sealed in an airtight container the clay can remain usable indefinitely and the longer it ages the more plastic and workable it may become.

Learn More

There is much more that could be said on the subject of how to process clay. I have put together a detailed video based class on how to find, test and process wild clay, you can learn more about this class by following this link. There are videos that show how to find and process clay on my YouTube channel that you can access at this link.

I've been making primitive pottery since I was a teenager in the 80's. My work focuses on reproducing the beautiful and complex polychrome pottery styles made in the fourteenth century. I have taught workshops and lectured at venues all over the Southwest. While traditional potters are notoriously closed about sharing information, I believe that "when information is generally useful, redistributing it makes humanity wealthier no matter who is distributing and no matter who is receiving." Because of my feelings about sharing information I created AncientPottery.how to help others who want to learn.

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