At the time of the clay dried, shrinkage occurs between 5% to 8% depending on the degree of plasticity. Although the shrinkage process takes place slowly, but raises problems of its own. At the time of the clay dry and then wetted, the clay will absorb an amount of water that will menyelaputi each particle. Plastic mass ready to be molded, clay require as many as 35 parts of water per 100 parts by weight.
In the drying process, water moves from inside the mass of clay through the pores to the surface and then evaporates into the air, then due to capillary attraction, moving water from the surface and in turn will evaporate into the air. Drying clay is always followed by the volume shrinkage. By the time the water layer in the form of the film surrounds the clay particles evaporates into the air, the particles become closer together, resulting in the entire mass shrinks.
And so on, this process occurs repeatedly until the water surrounding the clay particles evaporates, so that all particles will move closer together and result in shrinkage and mass becomes solid and strong. In such conditions, the clay drying process is considered complete.
For thin-walled ceramic objects, drying will be evenly distributed to all parts of the body. Conversely, if the wall thickness, cracks often occur in some parts, especially if the drying is done quickly. That is because the speed of the water leaving the surface as steam faster than the speed of moving water in the clay mass.
As a result, the surface will shrink first because pores will be narrowed compared to the inside so that the surface is not able to receive the vapor pressure of water moving out and resulted in objects being cracked. Many say the least dry shrinkage depends on the particle size and the amount of water that coats the particles.
For smooth berpartikel clay and many porous, dry shrinkage will be relatively large. In contrast to the coarse grained clay and slightly porous, dry shrinkage is relatively small. Dry shrinkage testing was done by calculating susutnya measuring line that has streaked on the test object or test object volume shrinkage specially constructed rectangular shaped slab.
Shrinkage Testing Process Dry Clay
1. Perform pengulian each clay formula that has been prepared for testing on the table casts, so the clay becomes homogeneous.
3. Trim clay tablets that have been made into clay slabs measuring 14 cm long, 4 cm wide and 1.5 cm thick〕 15 pieces for each clay formula.
Then give a code snippet clay tablets in accordance with the formula clay.
4. Scratches Draw a straight line on the surface of the slab has been cut along the 10 cm (100 mm) and mark on each end with a line of 2 cm, and then calculate the volume (14 x 4 x 1.5)
5. Perform drying of the specimen by means of aerated first. After a fairly dry, drying in the sun to dry.
6. Perform measurements on a straight line back scratch test specimens that have been dried, then calculate the percentage of clay shrinkage of the specimen using the line method or the method of calculating the volume using the following formula:
