13 things to consider before buying clay for pottery
The indispensable material for ceramic work is clay. However, if you don’t have any idea about the properties of each type of clay, you might not get the results you want. This is something that potter must know before choosing raw material to work on. This means that
you can produce the work as desired.
- Clay shrinkagemust be observed before and after firing. The clay is casted to the approximate size of the finished work. Water absorption in greenware must be similar, depending on texture and the making process. High shrinkage after firing will result in cracks.
- Strength The clay will be examined after casting, drying and firing. The clay is shaped and casted. The clay body is calculated to find the strength value. After drying, the work is calculated for MOR value. High level of strength after drying means that the clay has high plasticity which is suited for large and more extreme shapes.
- Water absorption tests by weighing the fired product which is later boil in water for about 5 hours and let cool dry for a day. After that, the product is weighted again to calculate the percentage of water absorption of the clay. Normally, the clay has higher level of water absorption compared with ball clay.
- Thermal expansion coefficient- COE is checked by dilatometer. The results will be displayed in graphs. Clay with low COE can be used to produce earthen ware such as cooking clay pot which is often used in Japan.
- Percent retained the clay is stirred with water and shake on the no.325 mesh sieve. The clay with high percent retained means that there are residues of sand and stone found in the clay. This will have impacts on the production of the work.
- Particle size analysis and particle distribution uses hydrometer or Particle size distribution meter. The laser beam is used to calculate the particle. The results are plotted on the graph demonstrating plasticity and strength of the clay and the finished work after firing. In addition, the analysis also denotes rheology and casting rate of the clay.
- Percentage of humidity is conducted to calculate amount of water in clay. The simple method can be done by weighing 100 gram of clay and comparing it with the dry clay to determine the percentage of humidity in clay.
- Casting rate is to measure the thickness of the green ware processed through the slip casting method. The clay is put in the plaster mold for 10-20 minutes before pouring out the excess water. After removing the mold for 10 minutes the thickness of the product is measured again. Higher level of thickness means a good casting rate.
- Color reaction after firingThe clay is casted and fired before comparing with the finished product to examine the color. The color meter can be used if the color of the clay must be controlled.
- Plasticity Clay with high plasticity is needed to produce ceramic work to avoid cracks that might happen during moving. However, high plasticity might cause some problems when potter puts the mixture in the mixer and screen feeder. In addition, the machine might be gluey. It might be difficult to mix the clay with other additives.
- Rheology is measured by the tool designed for study the soft solids. Viscosity and Thixotropic values of the soil are examined.
- Organic matter Clay with high proportion of organic has high plasticity appropriate for casting. However, it will cause black core in the clay body, particularly the product with large size, thickness and processed through fast firing. High black core will result in low fire strength and affect the glazed surface.
- Warpage is to measure the clay processed through pressing, casting and extruding before drying and firing. The work is placed on the support at the specific temperature to measure warpage of the product. High warpage means that the clay has low capability to support its weight at high temperature. This will affect the large product which might be deformed during the process.