Simple introduction of how clay is formed – it is believed that is the most common and abundant raw material in the earth. It is an end product of the geologic weathering of the earth’s surface. In remote geological time our earth was a molten mass of material covered by a thin layer of earth crust. Gradually, the geology changed and the earth started to cool off. The molten mass on the upper layer solidified to a much thicker earth crust and rocks were formed called igneous rocks. As the earth cooled off the moisture in the atmosphere also become more dense, then torrential rain began and has lasted for millions of years. The rain water has a profound effect on the clay’s formation. Water seeps though cracks of rocks and it expands when it freezes, the rocks are broken into small pieces. These small rocks continue to undergo abrasion and grinding by water or wind borne weathering, and break into even smaller pieces then to fine particles. Water continues weathering these particles making them even finer and are carried away from where they are original formed until finally laid down in the estuaries or deltas as clay.
The Clay Project
The first project from the tutor is to understand the physical properties of unrefined clay. We have been asked to collect some local clay. It could be found in tidal estuaries, rivers banks, building sites, beaches or in our own back garden. It is rarely found on the surface where usually is sand or mud. The clay should be slightly moist and malleable. So we need to dig at least 40 cm deep below the surface. As the clay has not been processed, pebbles, roots, leaves, stones or sand may be found in the clay. These pictures were taken at a building site on Colchester Avenue, Penylan. A deep hole was dug and the clay is still moist. With the permission of the site’s builder I was able to get the clay easily. In these pictures you can see the clay is rusty brown colour so it should be contained iron.
Back at the workshop I crumbled the clay and discarded the large-sized rocks and pebbles then placed them on a bat. I found that to place a piece of newspaper underneath the clay not only helps the clay dry more quickly, due to it’s water absorption ability, but it also makes it easier to transfer the clay to a bucket later. After the clay is completely dried transfer it to a bucket and fill it with water. The water should be just enough to cover the clay. Allow the clay to soak in the water until it has turned soft, then pour off the excess water. At this stage the clay should be a thick paste-like texture but still have small stones and sand which need to be separated. I used #40 mesh sieve to sift the clay. a brush or rubber rib will help to push the clay through the sieve. If necessary other tools can be used for helping to push through the big lumps.
The sifted clay looks like slip. If it is too wet, coil some clay round the edge of a plaster bat to prevent it slopping out.
This is my clay and it was not too runny so I poured it onto a plaster bat directly without coiling the edges, then spread it out and smoothed the surface a bit. The reason for using a plaster bat is its water absorption ability and also the clay won’t stick to its surface. It took about 15-20 mins to become stiff. I would suggest checking it occasionally sometimes the slip can turn stiff fairly quickly. When the slip has stiff on one side, turn it over to allow the plaster bat to plays its part on the other side. As long as the clay has reached the right consistency, wedge and knead it then it is ready to use.
We can check the plasticity of the clay by rolling coils and bending them into tight curves to see if they crack easily. To check shrinkage percentage, rolled out six strips of clay into 12 cm long and scored a 10 cm line on each one. Marked down on each strip a firing temperature of 1000℃, 1100℃, 1200℃, 1260℃ oxidation and 1260℃ reduction and a strip marked as ‘raw material’ to check the clay shrinkage percentage as it will decrease in size after it has dried. An extra 100g of clay was measured out for checking it’s weight changed after drying.
After firing, the clay strips showed different colours. These colours are usually produced by iron oxide. Due to different firing temperature and the atmosphere in the kiln the clay body will produce various tones of colours, they could be from light pink, yellow, orange, red, brown, dark brown to black.
In a plastic clay the interstitial spaces between clay and other mineral particles are filled with water. After the first firing (biscuit) the spaces are left empty. In this stage the clay body is porous. To test the porosity percentage, first weigh every strip then put them in boiling water for an hour. When it is done, take the strips out and weigh again. This is the formula :-
dry weight – after boiling weight 100
_____________________________ X ____ = percentage of porosity
weight of dry 1
For example, the weight of my 1000℃ strip is 71g and after it has been boiled it is 81g, the difference is 10g. 10 multiplied by 100 = 1000 then 1000 divided by the weight of the dry strip is 71. The percentage of porosity of my 1000℃ strip is 14%.
Temp After firing After boiling porosity percentage
1000℃ 9cm 71g 81g 14%
1100℃ 8.8cm 73g 81g 11%
1200℃ 8.6cm 81g 84g 4%
The raw material after drying is 9.1cm
The 100g clay after drying has decreased to 80g , after firing has decreased to 74g
If clay contains lime it could cause the clay objects to break. When lime is fired it is altered from calcium carbonate to calcium oxide. Calcium oxide is an unstable oxide which takes on water or hydrates. This hydration will go on cause the lime swell and the swelling against the pressure of the clay objects during firing the piece will break. A simple lime test can reveal whether lime is present in the clay. Roll a small ball of clay and skewer it on a tooth pick or bamboo stick then submerge it into 50% solution of hydrochloric acid. If lime is contained in the clay it will effervescence. My clay ball has bubbling a lot so it contains a large amount of lime.
Glaze test by using my dug clay
All glazes are a mixture of my dug clay, wood ash, whiting and feldspar, the proportion of which are shown in the following picture. Only the the wood ash (A) has the best result out of the other two. It is smooth and has a clear distinction between colours. The other two only have a subtle change.