13 Ceramic Clay Buying Tips: Know Your Clay Properties Before Ordering

Clay is the core raw material for ceramics, but ordering ceramic clay without understanding the properties of each type can lead to receiving materials unsuitable for the production process or desired results.

Therefore, before purchasing clay for sculpting, casting, or ceramic forming, it's crucial to understand the properties of the clay as much as possible. Choosing the right clay for the job will facilitate easier forming, reduce problems during production, and increase the likelihood of achieving the desired outcome.

Let's look at the 13 important properties you should check before buying ceramic clay.

1. Shrinkage

Both pre-firing and post-firing shrinkage should be checked. Form the clay to be tested into a piece and control the moisture content in each sample to be as consistent as possible. The test results depend on the clay's plasticity, fineness, and forming method.

If the clay has excessive shrinkage after drying, it may cause cracking problems in the piece during the drying process. Therefore, the shrinkage value of the clay is crucial information to consider before using it in ceramics.

2. Strength of the Clay

The strength of the piece should be checked at each stage: greenware, after drying, and after firing. The clay should be formed according to the actual production process required for use.

For greenware, strength can be tested with a strength testing machine. Dried pieces are fired before testing the MOR (Modulus of Rupture). For fired pieces, they are fired at the actual temperature before testing.

Clays with high greenware and dried strength often reflect good plasticity, making it easier to form large or intricately detailed pieces.

3. Water Absorption

Form the clay and fire it at the intended operating temperature. Then, weigh it before boiling it in water for about 5 hours and soaking it in water for another day. Finally, reweigh it to calculate the water absorption percentage.

Generally, each type of clay has different water absorption values, such as white clay, black clay, and red clay. Therefore, it should be chosen to suit the type of product and its intended use.

4. Thermal Expansion Coefficient

This property can be tested with a Dilatometer. The test results are displayed as a graph for analysis and calculation of the clay's thermal expansion coefficient.

Some types of clay with low thermal expansion coefficients can be used to produce cooking utensils, such as nabe pots, which are popular in Japan.

5. Residue Percentage

The test involves mixing clay with water and then sieving it through a 325 Mesh screen to observe the amount of material remaining on the screen.

If there is a high amount of residue on the screen, it may indicate that the clay from that source contains a significant amount of sand or rock particles. This information affects how the clay is used in different clay preparation and ceramic production processes.

6. Particle Size Distribution

The size and distribution of clay particles can be determined using a hydrometer or a Particle Size Distribution machine that employs laser technology for particle size analysis, displaying results as a graph. Particle size and fineness affect many properties, such as plasticity, greenware strength, fired strength, slip flow, and casting rate.

Therefore, if choosing clay for casting, special attention should be paid to the properties of particle size and distribution.

7. Moisture Content

Used to determine the amount of water present in the clay. A preliminary method is to weigh approximately 100 grams of clay, then completely dry the clay and weigh it again. Compare the pre- and post-drying weights to calculate the moisture percentage.

Moisture content affects storage, clay preparation, and forming efficiency, making it another factor to check before using the clay.

8. Casting Rate

For slip casting, the rate at which the wall of the piece forms over time during the casting process should be checked. Prepare the clay to be tested as a slip, then pour it into a plaster mold. After approximately 10–20 minutes, pour out the excess slip. Once the piece can be demolded, measure the thickness of the wall at the specified time intervals.

If the piece thickens well within an appropriate time, it indicates a good casting rate, which is suitable for production processes requiring control over time and piece thickness.

9. Fired Color

Form a clay sample and fire it at the temperature and firing atmosphere that matches the actual use. Then, compare the fired color with the original sample.

If precise control over the body color is desired, a colorimeter can be used to measure and compare values against established standards.

10. Plasticity

Generally, we often require clay with appropriate plasticity to facilitate easy forming and reduce problems of cracking or damage during handling.

However, clay with very high plasticity may have limitations in the production process, such as problems when mixed with a Mixer or Screen Feeder, as the clay body can easily stick to the machinery. It may also be more difficult to mix with other raw materials.

Therefore, when choosing clay, the plasticity should be considered to suit the production process, rather than always choosing the most plastic clay.

11. Slip Rheology

For applications using slip, its flow properties should be checked, considering viscosity and thixotropy, which are important factors for the slip's efficiency in the casting process.

If flow properties are controlled appropriately, it will lead to more efficient work with the slip and better quality control of the finished piece.

12. Organic Content in Clay

Clay with high organic content often has good plasticity and formability, but it can also have drawbacks, particularly the formation of black core within the piece.

This problem is more likely to occur in larger or thicker products, as well as in fast firing processes and clays containing flux. If the problem is severe, it can reduce fired strength and affect the surface quality of the glaze.

13. Deformation of the Clay Body

This can be tested by forming the clay according to the desired production process, such as pressing, casting, or extruding. Then, dry and fire it, placing the piece on a support at the actual operating temperature. Finally, check the curvature or deformation after firing.

If the piece exhibits significant curvature or deformation, it may indicate a lower ability to withstand high temperatures under load, potentially leading to deformation in large or heavy products.

Choosing the Right Clay Reduces Problems from the Start

When purchasing ceramic clay, one should not only consider color, body, or plasticity, but also evaluate the clay's properties to suit the type of work, forming process, firing temperature, and product application.

Whether it's clay for sculpting, casting, or ceramic production, understanding the raw material's properties before ordering will help reduce problems such as cracked, warped, or substandard pieces, and enable you to choose the right clay for the job more effectively.

Because selecting good raw materials from the outset not only simplifies the production process but is also a crucial factor in achieving beautiful and high-quality ceramic results as intended.