What Is Unfired Clay?
Unfired clay is called Greenware. Unfired clay contains a lot of moisture. It actually contains water in a sufficient amount so that the unfired clay is soft and malleable. It’s at this stage that the unfired clay can easily be formed into different shapes. The clay bodies are in a plastic like form due to the moisture content but when fired they become hardened bodies of clay.
The unfired clay plate in the photograph above was formed into it’s final shape and is now almost ready for the first firing in the kiln. The clay used above in the picture contained enough moisture so that it could be formed by hand. In addition, decorative patterns were embossed into the clay. Once fired the clay will no longer have sufficient moisture to be shaped and will be impossible to bend and form again.
The clay will sit and naturally dry out till it has about 15% moisture content. This state is called Leather-hard. At this stage the clay is still unfired and very delicate. It could easily break if handled the wrong way or dropped accidentally.
When the clay plate reaches a moisture content at or near 0% it is ready to be fired.
The clay plate above will be painted and then fired in a kiln up to about 2,000 degrees. If it survives and does not break or crack it will then take its final form as all, or nearly all moisture, will have been removed.
What Is Greenware Clay?
Greenware is unfired clay pottery. Greenware refers to a stage in the production process when the clay is mostly dry (leather-hard) but has not yet been fired in a kiln. Greenware can be in any of the different drying stages, including wet, damp, leather-hard, dry, and even bone dry. The main thing to remember is clay in this stage can still be shaped. More water can be added to unfired clay for reshaping.
Clays Used In UK Pottery
Did you know China clays were discovered in this country for the first time in the 1740s? China clays can be found at St Austell, on Dartmoor’s western flanks, and on the western and southern areas of Bodmin Moor in England. I find the quarrying method is rather unique. They employ the use of high-pressure hoses aimed at the clay pit walls. The fine clay is washed down in a slurry. The majority of contaminants are left behind.
Due to their low plasticity, China clays are frequently combined with additives such as ball clay and bentonite. China clays are, on average, faster to cast than sedimentary clays. They are prized for their whiteness, which is why they are used in bone china. Clays burned at 1300 degrees Celsius shrink by around 12%.
These clays are sedimentary deposits from the Tertiary or Mesozoic periods. They were dug out of the ground as bricks or balls at first. That’s how they acquired their moniker. Their geological ages, chemical compositions, and geographic locations are all quite different. The primary areas, however, are in the Newton Abbot district of South Devon in the United Kingdom.
It’s worth noting that the clay is extracted using both open cast and underground processes. Underground workings rarely go deeper than 100 feet underground. Because ball clays are plastic, they can be thrown and machined. To make them act like a liquid when water is introduced, they require a higher concentration of defloculents or chemicals. The colors produced after firing vary depending on the clay.
Carboniferous (The Carboniferous is a Paleozoic geologic era and system that stretches 60 million years from the end of the Devonian Period (358.9 Mya) to the start of the Permian Period (298.9 Mya).) fireclays make up the vast majority of British fireclays, which are mostly found in the Coal Measures.
The coal measures is a lithostratigraphical term for the Upper Carboniferous System’s coal-bearing section. The Upper Coal Measures Formation, Middle Coal Measures Formation, and Lower Coal Measures Formation are all part of the Coal Measures Group in the United Kingdom.
As a result, they have a fairly broad dispersion. They are either mined alongside coal or extracted through open cast processes. The concentration of silica varies between 40% and 80%. The percentage of alumina varies between 12 and 40%. High alumina fireclays are prized for their refractory properties, which make them ideal for making firebricks and other heat-resistant ceramics like saggars.
Aluminum oxide or Alumina is commonly referred to as alumina or Al2O3. Bauxite, an ore collected from soils in tropical and subtropical countries, is used to make alumina. The Bayer process, first established in 1887, is the most common method for extracting alumina from bauxite.
And a saggar in case you are wondering is a kiln-made piece of furniture. It’s a ceramic box-like container used to enclose or protect ceramics being fired within a kiln during the firing process.
Stoneware clays are flexible, silica-rich clays that vitrify (By exposing it to high heat, it can be transformed into glass or a glass-like body) quickly.
Fired Clay Plate Photo
This a photograph of the same clay plate picture above after firing in a kiln. The rich blues color came out looking very nice!