Clay can be more or less plastic and workable. Plasticity is the property of clay that allows it to turn from dry dust into a workable body when mixed with water. A workable and plastic clay will allow you to create any shape without ruptures, and maintain this shape in the drying and firing process.
As a rule of thumb, if you have the doubt that the samples you are collecting will form workable clay bodies, you can do a simple test: wet and roll it in a sausage-like shape and bend it sharply. If the material bends without breaking, you most likely have a very plastic and workable clay at hand. If around the bending point you can see lots of cracks or tearing points, you can be in front of a ‘short’ clay, that lacks plasticity.
Clay’s workability improves with multiple drying and hydration cycles.
You will want to completely dry the clay you have found in a non-humid environment, ideally at the sun
You can then use a hammer to break your clay blocks into smaller chunks. A rolling pin can then be used to crush down the smaller bits into fine powder.
You will then want to fill a container with water, and slake down the clay in the water. This process will allow the dry clay to re-absorb moisture at a molecular level.
If you repeat this process a few times, you will have a much more plastic material. This can make the difference between a piece successfully crafted and fired in the oven with a piece that will crack and break.
Clay might have high levels of impurities once collected (such as rocks, roots, etc…). Some of these particles will float to the surface during the hydration processes, but most likely few bits will also be incorporated in the body of the clay.
Now that the clay has matured, you want to sieve it and refine it, to make sure that only the finer, and more homogeneous particles will make-up your workable clay. You can play with the grain of the sieve and run some tests to see which one give you the most interesting clay consistency. I suggest to start with a quite loose sieve of 500 microns, but you can go down to 250, 150, 125 etc… The finer the mesh, the fewer inclusions will be present in your final clay sample.
In a container (bucket or basin) dissolve the dry clay powder in some water
Rest the selected sieve on top of a container. I usually rest the sieve on a big bucket on 2 strips of wood, and place the sieve on top of the strips, at the centre of the container.
Pour the wer mixture on the sieve, and use a brush to help pushing the material through the sieve
Collect the sieved clay in the container. You will also want to make sure that your container has a far larger circumference than your sieve so that all sieved material will be collected in the container.
Some clays are quite difficult to get through the sieve, so pouring more water to further dilute the mix in the sieve will help. The more water you add, the bigger container you will need.
Let the sieved clay particles sediment at the bottom of the container. Depending on the type of material, it will take few hours to a day to obtain the separation.
Remove as much water as possible from the water layer
Pour the muddy mix on a dry plaster slab. Plaster is well known for its ability to suck out water from the clay mix.
Once the layer of clay in contact with the slab will have solidified a bit (but still maintaining some moisture), you will want to peel the mix off the slab and turn it around so that the moist top layer reaches the same consistency.
Remove the clay from the slab, make it into a ball and knead it as you would do with bread. This will help clay to reach a uniform consistency.
If you want to preserve clay in this form, place it in a plastic bag, making sure it is not in contact with air.
At this point, you really want to make something… I know… But you also want to first understand how your clay shrinks during drying and firing. This is because when clay starts to dry, water evaporates. Clay particles are drawn closer together, resulting in shrinkage.
When you buy clay from an industrial company, you will know its shrinkage percentage. It will be written on the package. Different clay bodies shrink at different rates. Very stable clay bodies will have a 4% shrinkage, but with home-made clays I have observed up to a 20% shrinkage.
To test shrinkage:
Take some of your clay body and form it in a rectangle of roughly 22cm x 3cm, with even thickness (5 to 7 mm will do)
Place a ruler on the rectangle and with a modelling tool, make a notch on the clay for each of the cm on the ruler (cover 20 cm).
Let your sample dry. While drying your sample will start shrinking, meaning that the notches that are now 1 cm apart from each other, will come closer. You will probably also understand if your clay is still ‘short’ if cracks will start appearing on the borders.
Fire the sample. To be more accurate, you could make a range of samples with the same material and fire them at different temperatures and cooling times. You will be able to observe different colouration and behaviours based on the kiln’s temperature
Measure the distance between the first and the last notch, that originally measured 20 cm. You can compare the new length and find out the shrinkage %. For example, if your 20 cm have reduced to 18cm, you will have a clay with 10% shrinkage.
The more clay samples you collect, the more you will realise how they differ in colour both when wet and after firing. You can catalogue each sample based on colour and shrinkage and you can start mixing different types of clays, using different percentages of each, to obtain in-between shades as well as levelling the shrinkage. For example, by mixing 50% of a clay with 20% shrinkage with 50% of a clay with 4% shrinkage, you will obtain a much more stable material.
You can also mix more ‘greasy’ clays with drier ones, to make them more plastic.
To make your clay more stable you can also add fine chamottes.
To make it lighter and porous, you can add organic material that will burn at the high temperatures of the oven. For example, try adding saw dust or hair or foliage.
When you design and make your object out of clay, you will need to keep into account the risks of the drying process. Thinner or more readily exposed parts will tend to dry first. This will produce stress in the clay, that sometimes will show up as crack or warpage, and other times will show up as breakage in the kiln.
To make sure that drying happens evenly:
Make sure that the thickness of your objects is as uniform throughout the piece as possible
Slow down the drying process by covering your piece in plastic film. Inside the film, a micro-moisture environment will be created. Moisture evaporating from the faster-drying parts of the clay will redistribute and will help the whole object drying at the same rate. This is particularly helpful when you had pieces that you want to maintain flat.
However, errors are beautiful, so you can play with the drying process to obtain interesting warpage.